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Late Pleistocene Paleoclimatology: Foraminifera & Tephrochronology in Gulf of Mexico
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QCATERNARY RESEAR(‘II Late 2. 3X-09 ( 1972) Pleistocene Paleoclimatoiogy, stratigraphy and Gulf JAMES P. KENNETT Foraminiferal Tephrochronology, of Mexico AND PAUL Bio- Western HUDDLESTUN The distribution of planktonic foraminifera has been studied in 28 piston cores of Late Pleistocene age from the western Gulf of Mexico. Detailed correlation between the cores has been made possible by a high degree of similarity of frequency changes within several species ; by coiling direction changes within Globorotalia trmcatulinoides; by a datum level representing the near extinction of Globorotaliu wxardii flexuosa, and Globorotaloides hexagona at the end of the last interglacial; by three distinct volcanic ash horizons, and by calcium carbonate dissolution effects at distinct intervals. Almost all species demonstrate distinct frequency oscillations that are correlatable between cores. A high proportion of these are clearly related to paleoclimatic oscillations and reflect rapidly changing water-mass conditions within the Gulf of Mexico during the latest Pleistocene. No interval appears to have been represented by stable environmental conditions. Causes of frequency changes within several other species are not clearly related to inferred paleoenvironmental changes. High similarity of faunas exists at all times between the northwest and southwest Gulf of Mexico, reflecting similarity of water-mass conditions over a wide latitudinal range. High sedimentation rates, which average between 10 cm/1000 years and 30 cm/1000 years, have enabled a detailed paleoclimatic curve to be established for the last 200,ooO years. Three interglacials and two glacials are recognized. Distinct foraminiferal assemblages have enabled definition of I8 zones most of which are related to paleoclimatic changes. Most intense toolings occurred at the end of the penultimate glaciation (zone W) and during the middle of the Wisconsin glaciation (zone Y). The Gulf of Mexico curve is somewhat similar to those of other regions based on ratios, except that, despite close control, no intense cooling is apparent 01s~01, near the end of the last glaciation. The most sensitive warm water indicators are the G. menardii complex and Pulleniatina obliquiloculata; the most sensitive cooler water forms are Globorotalia inpata. and Globigerina falcouensis. Several species have intermediate temperature tolerances. Much climatic information is lost when only the G. pnenardii complex is utilized in climatic studies, because these forms are essentially absent during glacial episodes. The most distinct fauna1 change associated with the Holocene warming does not coincide with the Z-Y boundary defined by the first consistent occurrence of G. wenardii but occurs slightly earlier during the latest Wisconsin. The X zone, based on the consistent occurrence of the G. nwuardii group is of shorter duration in Gulf of Mexico cores than in cores from the central Caribbean Sea and the equatorial Atlantic Ocean. Three major volcanic ash horizons coincide with climatic toolings; almost immediately preceding the last interglacial (135,000 years B.P.) ; towards the end of i Graduate a Department School of Oceanography, of Geology, Florida Universityof State Univer-sity. Rhode Island, Tallahassee, 38 0 1972 by Academic All rights Press, Inc. of reproduction in any form reserved. Kingston, FL 32306. RI 02881. WESTERN GULF OF MEXICO LATE PLEISTOCENE 39 the last interglacial (end of zone X; 90-95,000 years B.P.), and in the earliest part of the last glaciation (75,000 years B.P.). A drastic reduction of G. menardii fiexuosa and G. Izexagona coincides with the middle ash horizon. IXTRODUCTION Much of the knowledge about Late Cenozoic paleoclimatic changes has been gained through the study of marine stratigraphic sequences.The degree of resolution that. may be obtained from these studies is largely determined by rates of sedimentation throughout the sequence.The Gulf of Mexico represents an ideal area for the detailed study of Pleistocene planktonic foraminiferal relations and paleoclimates becausesedimentation rates are considerably higher than in most oceanic areas and yet planktonic foraminifera are often abundant in the relatively shallow depths typical of the Gulf (maximum depth 3800 m). We have carried out quantitative studies of planktonic foraminifera in 28 piston cores fro?, the western Gulf of Mexico collected du$g cruises of the USNS Kane (US Nays1 Oceanographic Office) (Fig. 1; Table 1). Our examination of material from a total of 46 cores throughout the Gulf of Mexico showed that Late Pleistocene sediments in the eastern area are largely devoid of planktonic foraminifera compared with the western region where rich assemblages commonly span entire cores. Although average core lengths are approximately 8 m (Table 1 ), the majority of the cores do not penetrate beyond the last glaciation, and hence much potential exists for highly detailed biostratigraphic and paleoclimatic studies of the late Pleistocene. The primary objectives of our study were to establish detailed correlations between cores utilizing as many parameters as possible ; to establish frequency distribution patterns and relations among planktonic foraminiferal speciesand to determine if such patterns correlate between cores despite changing sedimentation rates, and to establish a paleoclimatic curve for the latest Pleistocene. : ! LOUISIANA 3 FIG. 1. Location of cores used in this study. 40 KENNETT AND The study of Pleistocene paleotemperatures within the marine realm has been largely based on changes in oxygen isotope ratios (Emiliani, 195.5, 1971), and changing frequencies of planktonic microfossils. Almost all microfossil analyses have utilized planktonic foraminifera, although other groups have recently been playing an increasingly important role such as the Radiolaria (Hays, 1967; Huddlestun, 1972) ; the calcareous nannofossils (McIntyre and Jantzen, 1969 ; Geitzenauer, 1972)) and the silicoflagellates (Jendrzejewski and Zarillo, in press). Various methods have been applied to the study of planktonic foraminifera, each of which has different advantages and disTABLE DATA HUDDLESTUK advantages. Several workers have based their paleoclimatic interpretations on interrelations among certain groups of species with distinct temperature tolerances (Parker, 1955, 1958; Phleger et al., 1953) ; others have utilized one or two species that apparently reflect extreme temperature ranges (Ericson and Wollin, 1956, 1970; Beard, 1969) ; others use ratios of warm to cool species (Lidz, 1966) ; while others utilize various statistical techniques to derive a single paleoclimatic curve based on fluctuations of a high proportion of planktonic foraminiferal species within the assemblages (Ruddiman, 1971; McIntyre et al., in press ; Imbrie and Kipp, 1971; Lynts, 1971). 1 FOR CORES Core Latitude Longitude Water depth (14 87 91 92 93 94 97 99 102 108 23O32.7’ 23”27.0’ 24Y4.1 24”04.8’ 24O14.7’ 2Y37.0’ 2Y53.0 27O34.0’ 26O.59.7’ 91O29.8’ 93”ll.O’ 94”20.5’ 93O12.0’ 92W.O 93O12.0’ 92O23.0’ 93Y2.0’ 94O18.7’ 3685 3760 37.50 3762 3749 3408 2388 379 1796 111 114 115 120 124 125 127 129 131 135 136 138 139 141 143 144 146 147 150 26O19.0’ 2Y29.9’ 2459.9 23O17.9’ 23Y5.9’ 21”50.0’ 21”18.0’ 20”56.7’ 20”11.2’ 19”33.5’ 19”.58.9’ 20”31.5’ 20”30.0’ 21O23.7’ 21O54.0’ 22O40.3’ 23O22.9’ 2.Y42.5’ 26”28.0’ 93O18.3’ 9Y23.5’ 9YOO.9 96”10.8’ 96O45.0’ 96Y8.0’ 94O23.0’ 9YO5.7 9Y59.3 93O17.9’ 93”lS.O’ 93”13.5’ 92O37.0’ 93O27.9’ 93O20.8’ 93O13.0’ 94”2.5.8’ 9.5”56.0’ 91O28.0’ 2450 1626 3576 2537 1827 1245 3360 3108 2001 580 1215 1711 2462 3169 3393 3720 3755 1086 2127 Physiographic province Sigsbee Plain Sigsbee Plain Sigsbee Plain Sigsbee Plain Sigsbee Plain Sigsbee Plain Sigsbee Rise Texas Continental Slope Texas-Louisiana Continental Slope Texas Continental Slope Louisiana Continental Slope Sigsbee Rise Mexican Ridges Mexican Continental Slope Mexican Ridges Vera Cruz Tongue Vera Cruz Tongue Mexican Ridge Tabasco Knolls Tabasco Knolls Tabasco Knolls Campeche Trough Tabasco Knolls Tabasco Knolls Sigsbee Abyssal Plain Sigsbee Abyssal Plain Texas Continental Slope Sigsbee Scarp Core length (cm) 817 965 876 927 1122 835 880 600 780 610 465 447 920 754 710 757 666 788 822 767 820 880 917 884 714 910 373 199 WESTERN. GULF OF MEX.ICO Valuable new nonmarine approaches to the analysis of Late Pleistocene paleoclimatic hiStory are offered by the oxygen isotope analysis of polar ice sheets (Dansgaard et al., 1966 ; Epstein et al., 1970)) and of speleothems (Hendy and Wilson, 1968). No previous investigations in the Gulf of Mexico have attempted to establish detailed relationships among planktonic foraminiferal species and to determine a detailed paleoclimatic curve for the Late Pleistocene. Previous paleoclimatic curves based on cores from the Gulf of Mexico are highly generalized and mostly based on few species. Phleger (1951, 1955) recognized general paleoclimatic changes in short cores from the Gulf of Mexico based on fluctuations between a warm and a cool planktonic foraminiferal group. Ewing et al. (1958) presented semiquantitative data for 18 planktonic foraminiferal species in 10 cores and generalized curves for 40 cores from the central and northern regions of the Gulf of Mexico. These curves are based on relations between G. waenardii and G. inflata, as were curves presented by Beard (1969), and delineate only gross climatic changes. As pointed out by Ruddiman ( 1971)) this method is semiquantitative and disregards over 90% of the foraminiferal information available in each sample. Ewing et al. (1958) recorded a maximum of three interglacial stages and two glacial episodes within the cores studied. Oxygen isotope analysis has been carried out for only three cores within the Gulf of Mexico (Sackett and Rankin, 1970 ; Emiliani, 1955). Sackett and Rankin (1970) consider that the S1*O range indicates a temperature range of about 5°C for the latest Pleistocene within the Gulf, which is similar to the range within the Caribbean. METHODS Approximately taken at intervals down the cores, 15 g of sediment were of approximately 20 cm were weighed dry, and LATE -41 PLEISTOCENE washed over a 63-,p mesh Tyler screen. The dry residue was sieved on a 175-,p mesh Tyler screen and split into a representative fraction of approximately 300 specimens using a modified Otto microsplitter. Frequency counts were made of the split to determine species frequency and coiling ratios of Globorotalia truncatulinoides. Distributions of volcanic ash and pteropods were noted. The >175,JL fraction was chosen for counting because it can be examined rapidly and is regarded as most representative for determination of climatic change within the Gulf of Mexico. It was determined after experimentation that larger mesh sizes cause the loss of important small species, while smaller mesh sizes separated faunas that are overwhelmed by small species such as Globigerina quinqueloba, Globigerina rubescens, Turborotalita humilis, Globorotalia scitula, Globigerinita uvula, and Globigerinita glutinata. As a result, even during distinctly warm water intervals, the more characteristic warm water forms were undesirably deleted. In addition, the smaller fractions presented greater taxonomic problems and slowed the work. We recognized 28 species or forms of planktonic foraminifera (Plate 1) . OCEANOGRAPHIC SETTINGS Physical oceanographic characteristics of the Gulf of Mexico have recently been summarized by Nowlin (1971), Leipper ( 1970), and Greiner (1970). The primary circulation pattern within the Gulf consists of subtropical underwater (O-200 m) derived from the Caribbean, which forms a clockwise loop current flowing from the Yucatan Channel to the Florida Strait in the eastern Gulf. In the western Gulf, no strong semipermanent currents occur (Nowlin, 1971; Leipper, 1970) and a well-defined winter flow contrasts with a highly variable summer pattern. Surface water temperatures within the Gulf are distinctly different between winter and summer. At the peak of the summer 42 KENNETT AND HUDDLESTUN FIG. 1. Globorofnlia vtrunrdii (d’Orbigny), Kane 131, 3-5 cm, x.33. FIGS. 2 and 3. Globovofaliu rrzetzardii fumidn (Btdy), Kane 131, 22-24 cm, Fig. 2, X.19. FIG. 4. Glol~orofalia mrna~dii fle.t-I~OSU (Koch), Kane 131, 682-684 cm. X43. FIG. 5. Pullerziafirra ohliqzClocu/afa (Parker and Jones), Kane 131. 3-5 cm, ~68. FIG. 6. Glollorofaloides hcrcago~tn (Natland), Kane 131, 722-724 cm, X 144. FIG. 7. Glohigrrirroidcs strcrztlifcr (Brady), Kane 60, 5 cm, X61. FIG. 8. Spltaevoiditwlla dehiscens (Parker and Jones), Kane 60, 5 cm, ~61. FIG. 9. Glol~igrrinoidcs con,g/obatus (Brady), Kane 131, 176-178 cm, x65. ITi:. 3, x.5 WESTERN GULF OF MEXICO season so much local heating has occurred, that the entire northwestern Caribbean and Gulf are at nearly the same temperature (approximately 29°C). In contrast, the common winter pattern is basically one of eastwest trending isotherms reflecting increasing temperatures toward the equator. Mean surface water temperatures during the winter (January) range from about 25°C in the south to 17°C in the north (Greiner, 1970 ; Leipper, 1970) reflecting the 10 degrees of latitude between these regions. Surface salinities average about 36.25?&, although much lower salinities (32-35%,,) over the northern continental shelves reflect the effects of influx of fresh river waters. The core of subtropical underwater has relatively high salinities (36-36.75%,) reflecting its source area in the Caribbean Sea, The bathymetry and submarine regional geomorphology for the Gulf of Mexico have been discussed by Uchupi (1967), and Bergantino (1971). Most of the cores examined in the present study were collected from the Sigsbee Abyssal Plain and the continental slope of Mexico, Texas, and Louisiana. Llate Pleistocene sediments from these areas have been described by Bouma (1971), Ewing et al. (1958), and Huang and Goode11 ( 1971). Such studies have shown that the Mississippi Cone is by far the most active accumulating province, followed by the Abyssal Plain and the eastern Campeche Bay area. In almost all papers concerned with sediment transport in the deep Gulf, turbidity currents have been considered the major depositional agent. Huang and Goode11 (1971) however, have FIG. FIG. FIG. FIG. FIG. FIG. FIG. FIG. FIG. LATE 43 PLEISTOCENE presented evidence from the eastern Mississippi Cone, that low-flow regime currents have played a dominant role in bottom sediment transportation because turbidity current deposits are known to complicate biostratigraphic records. CORRELATION BETWEEN CORES Detailed correlation between cores has been made possible by utilization of the following parameters in general order of importance : 1. Species frequency patterns of planktonic foraminifera 2. Disappearance of Globorotalia vnenardii flexuosa and Globorotaloides hexagona 3. Coiling directtion changes in Globorotalia truncatulinoides 4. Volcanic ash horizons 5. Pteropod distribution 6. Dissolution effects of foraminiferal faunas Frequency Patterns The most important method for correlation between the cores is by comparison of frequency patterns exhibited by the species. Frequency distributions of eight of the most abundant species are shown for three traverses of cores (Figs. 2-5). The most valuable forms for correlation are those that occur in highest frequencies, although the distribution of several less common species such as Globigerinoides conglobatus, Orbulina universa, G. hexagona, and Globigerina digitata are also’ valuable. Although it is difficult to visually compare frequency oscillations of several speciessimultaneously, the 10. Globorotalia truncatulinoides (d’orbigny), Kane 131, 3-5 cm, X50. 11. “Globigerina” dutertrei d’orbigny, Kane 60, 5 cm, X75. 12. Globigerinoides mber (d’orbigny), Kane 60, 5 cm, X75. 13. Globigerina falconensis Blow, Kane 131, 176-178 cm, Xl&I. 14. Globorotalia influta (d’orbigny), Kane 131, 176-178 cm, X72. 15. Orbulina &versa d’orbigny, Kane 131, 3-5 cm, X66. 16. Hastigerina aequilateralis (Brady), Kane 131, 3-5 cm, X125. 17. Globorotalia crassaformis (Galloway and Wissler), Kane 67, 100 cm, 18. Globigerina calida Parker, Kane 131, 3-5 cm, X97. X55. 44 KENNE’I-T AKD method has three major advantages over those that either ignore the fluctuations of most species or attempt solely to integrate fluctuations of several species into one “climatic curve.” First, the comparison of fluctuations of several species supplies highly detailed correlations which in themselves serve as a basis for better understanding of paleoenvironmental changes. Second, interaction of the various species supplies greater insight into the factors that may be controlling the frequency changes. Third, in paleoclimatic analysis, each species is not given equal weight as is the case in determination of “total faunal” curves. Considerable time, however, is required for the satisfactory correlation of species frequency patterns between cores. In the initial phases of this study, patterns in several cores did not appear to resemble each other. However, it was discovered that sedimentation rates vary dramatically within individual cores and between cores, and that the presence of disconformities and highly compressed core sections strongly control 1 124 A' 50 I1 FIG. which Figs. 2. Locationof species 3-5. frequency $144 \ three traverses of cores patterns are shown for in HUDDLESTUN the detailed character of the curves. Once these complications are recognized, distinct patterns emerge, and speciesfluctuations are remarkably similar between cores. Zones :: In the tropical to subtropical Atlantic, Ericson and Wollin (1965, 1968) have utilized a semiquantitative evaluation of the relative abundance of one group of related species or subspecies,the G. ntenardii complex. A sequence of zones has been established in frequency oscillations of the G. menardii complex and have been designated Q-Z in order of decreasing age. These zones reflect alternating warm and cold intervals and are broadly supported by oxygen isotope curves for approximately the last 150,000 years but diverge for earlier intervals (Emiliani, 1966; Ericson and Wollin, 1968). The importance of this zonal scheme is that it forms a valuable framework for correlation (Ericson and Wollin, 1968 ; Ruddiman, 1971; Imbrie and Kipp, 1971). In the Gulf of Mexico cores studied by us, zones V-Z are represented (Figs. 6 and 7). In keeping with the system used by Ericson and Wollin, we have distinguished zones V, X, and Z on the basis of the consistent presence of the G. ,Ilzenardii complex, and zones W and Y on the virtual absence of the group. Initially we subdivided the G. Ilzenardii complex into three subspecies( G. ntenardii wtenardii, G. wenardii fuwsida, and G. fjzenardii pen-uosa) but as they appear to react as a group, we have treated them as such. The biostratigraphic record in the Gulf of Mexico (Figs. S-10) is clearly related to climatically controlled ocean temperature oscillations. Three warm episodes and two cold episodesthat correlate closely with zones V-Z (Figs. 6 and 7) are recognized. We infer that these represent three late Pleistocene interglacials and two glacials. In addition we have recognized 18 distinct 3 Zone implying is here used in a biostratigraphic no time relationships. sense, K-131 K-125' FIG. 3. Downcore zonal designations, intercore miniferal species for traverse A-A’. Distribution Core length indicated in meters. A correlations, of volcanic and frequency ash is indicated. K-127 oscillations The V-Z (in percent of fauna) of several important planktonic zonation has been defined by Ericson and Wollin fora(1968). P wl KENNETT AND HUDDLESTUN WESTERN kJLF OF MEXICO LATE PLEISTOCENE 48 KENNETT AND HUDDLESTUN G.MENARDII,G.TUMIDA, & G.FLEXUOSA k1% 0 50 K-12~ 0 : FIG. 6. Percent abundance of the Globorotulia menardii complex in seven cores from the Gulf of Mexico. These species provide the basic V-W-X-Y-Z zonations used for correlations. Arrows represent last appearances in cores of G. menardii jkxuosa. Core lengths indicated in meters. planktonic foraminiferal zones within the Gulf of Mexico late Pleistocene sequence ranging from the upper part of zone V to the present day. It has been necessary to establish these subzones in order to describe the planktonic foraminiferal sequence in detail and to facilitate detailed correlation between cores. The subzones have been establishedas subdivisions of the V-Z zonation, with two subzones recognizable within W ; five subzones within X ; eight subzones within Y, and two subzoneswithin Z (Figs. 3-S). A subzonal classification in combination with the V-Z zonation has been used for three reasons: first, boundaries between the V-Z zones as defined (Ericson and 0K-13( : 0K-I24 ! 1 --_ 2 2 3 3 A4 G MENARDII, GTUMIDA, & GFLEXUOSA tm5 0 5a 2 3 4 ---i WA ox) k9. 0 7 I ---I FIG. 7. Percent abundance of the Globorotalia menardii complex in seven cores from the Gulf of Mexico. These species provide the basic X-Y-Z zonations used for correlations. Arrows represent last appearances in cores of G. rnenardii flexuosu. Dotted horizon in K-144 is barren. WE~TERN'G~LF OF MEXICO Wollin, 1968) are essentially objective and are based on the consistent presence or absence of the G. menardii complex ; second, the zonation of Ericson and Wollin is wellestablished, widely used (Ericson and Wollin, 1968 ; Ruddiman, 1971; Imbrie and Kipp, 1971), and at least for the last 150,000 years, closely approximates the glacial and interglacial stages ; third, a subdivision of Ericson and Wollin’s zonation is more easily remembered and utilized than a new one which might be numbered from 1 to 18. It is important to note that at this time the zonation is considered only to reflect planktonic foraminiferal changes within the LATE PLEISTOCENE 49 Gulf of Mexico sequence. For instance, the X zone, based on the consistent occurrence of the G. menardii group is of shorter duration in Gulf of Mexico cores than in cores from the central Caribbean Sea and the equatorial Atlantic Ocean. This is because the G. menardii group disappeared in the Gulf of Mexico earlier near the end of the last interglacial than in the other areas. In core P 6304-9 from the Central Caribbean Sea (Emiliani, 1969), the G. menardii group (including G. lrcenardii flexuosa) ranges upward in abundance to near the boundary between ages 5 and 4 of Emiliani (1966). Thus, the X-Y boundary is best placed at FIG. 8. Correlationsof coiling ratio changes of Globorotuli~ truncatdinoides between seven cores in the Gulf of Mexico. 100% left-coiling is to the left of each column; 100% right is to the right. Correlation lines are zonal boundaries based on changes within a high proportion of the planktonic foraminifera. 50 KENNETT AND HUDDLESTUN s N K-114 K-120 K-124 K-125 K-127 K-129 K-131 FIG. 9. Correlation of zones in a north-south traverse of cores in the far westernpart of the Western Gulf of Mexico. Volcanic ash horizons are indicated. Core lengthsindicatedin meters. this level. However, in the Gulf of Mexico cores, the G. menardii group disappears as a consistent element significantly lower than this within the later part of age 5 (Emiliani, 1966). The X-Y boundary is therefore diachronous between these areas. An earlier reduction of the G. menardii group in Gulf of Mexico faunas near the end of the last interglacial is considered to reflect slightly cooler conditions in the region at this time. Although each zone represents a distinct fauna1 assemblage,in most casesthe boundaries between the zones are indistinct and arbitrarily placed at gradational fauna1 changes. Several boundaries, however, are both abrupt and very distinct. The clearest of these occur between zones V and W; between X and Y, and between subzones Y 1 and Y 2. The Y 1-Y 2 boundary is even more distinct than the Y-Z boundary, which is distinguished primarily on the first consistent appearance of G. menardii. The environmental significance of the boundaries will be discussedlater. Extinction of G. menardii flexuosa and G. hexagona The only irreversable biostratigraphic changes that occur within the planktonic foraminifera in the sequenceare the extinctions of G. menardii flexuosa and G. hexagona within Y 7. Both forms consistently occur in the interglacial zones V and X and are absent in the glacial zone W. At the X-Y boundary, a drastic reduction occurs in both, forming a distinct and valuable datum level in the cores (Figs. 6 and 7). This sudden reduction has been previously noted by Ericson and Wollin (1968) and Ewing et al. (1958) in tropical-subtropical Atlantic cores. Examination of cores with high sedimentation rates in the lower part of Y has revealed, however, that a complete extinction of the two forms does not take place at the X-Y boundary, but that they reappear fleetingly and rarely in a few cores within zone Y 7. Their occurrence within Y 7 is not considered to reflect reworking because of association with a relatively warm water fauna. The final occurrence of the two forms in Y 7 is too spasmodicto be of much value in correlation. In the Pacific, G. hexagona has survived until the Recent (Natland, 1938; Parker, 1967). Furthermore, G. menardii j7exaloSais still a member of the living planktonic fauna in the northern Indian Ocean (Be, 1970). Additional biostratigraphic control is WESTERN. GULF OF MEXICO LATE PLEISTOCENE 51 G. INFLATA Fro. 10. Percent abundance of Gtoborotalia iwflata in seven cores from the Gulf of Mexico. Note preferred distribution in zones Y and W. Arrows reoresent last appearances in cores of Globorofaliamenardii flexuosa. offered by distinct differences in the character of the G. menardii plexus throughout the sequence. In zone X, G. menardii flexuosa is more dominant, G. menardii menardii is the next most important form, and G. menardii tumida is of least importance. In zone Z, G. menardii menardii is the dominant form, with G. menardii tumida much reduced, and G. menardii flexuosa absent. Coiling in Globorotalia truncatulinoides Coiling direction changes in G. truncatulinoides have previously been shown to be of value in correlation between late Pleistocene cores (Ericson and Wollin, 1968) including the Gulf of Mexico area (Ewing et al., 1958). Right-coiling forms of G. truncatulinoides are by far the most important in Gulf of Mexico cores. In several intervals the species is represented entirely by rightcoiling forms, while in other intervals, leftcoiling forms occur in varying frequencies (Fig. 8). Peaks formed by increases in the percentage of left-coiling forms correlate rather closely between cores and support correlations based on frequency distributions (Fig. 8). Differences in the shape of leftcoiling peaks are mainly due to changes in : sedimentation rates between cores. Intervals almost exclusively represented by right-coiling forms are as follows : much of Z 1 ; lower Y 2; upper Y 3; middle Y 5 ; and much of X and W. Left-coiling peaks occur in the following intervals: at the Z 1-Z 2 boundary ; in Y 1 and upper Y 2 ; Y 4 ; uppermost Y 5 ; Y 6 ; lowermost Y 7, and in lower X (Fig. 8). The most conspicuous peaks coincide with subzonesY 4 and Y 6. Volcanic Ash Distribution Three conspicuoushorizons of volcanic ash occur in the late Pleistocene sequenceof the western Gulf of Mexico (Fig. 9) and form valuable datum planes. These horizons occur in the upper part of W 1 ; in Y 8, and in Y 6. Planktonic foraminifera occur throughout the ash horizons, although they are highly diluted in megascopically distinct layers. The W 1 ash horizon is the thickest, although it was penetrated by only one core (Kane 129). In this core, a megascopically distinct layer is approximately 40 cm thick with abundant ash scattered 10 cm above and below the layer. The ash horizon occuring in Y 8 is thickest 52 KENNETT AND HUDDLESTUN FAUNAL CHARACTERISTlCS 01; in Kane 131 where a megascopically distinct THE ZONES layer is al)out 25 cm thick and abundant ash is scattered throughout a horizon 90 cm in Characteristics of the planktonic foraminitotal thickness (from 435 cm to 525 cm). feral assemblagesthat distinguish the 18 In cores further north and east, the ash zones (Figs. 3-5) described for the late horizon becomes thinner. In Kane 127, the Pleistocene sequence in the Gulf of Mexico ash horizon is 40 cm thick with a distinct are described below in order of decreasing layer 10 cm thick, while in Kane 124 and age. General characteristics of zones W-Z, Kane 120 the ash is spread over about 20 and of the boundaries between these zones are cm. Based on these changes in thickness, the also described. volcanic ash almost certainly was derived V-W Boundary from the Mexico City-Veracruz area of Mexico (Fig. 1) . This ash horizon is equivaThis boundary is marked by the very lent to that described by Ewing et al. (1958) abrupt upward declines in the G. menardii as occurring at the X-Y boundary and hav- plexus, G. hexagona, and Pulleniatina obliing widespread distribution in the western quiloculata, all of which are characteristic of and central Gulf of Mexico. the V zone. The upper ash horizon is concentrated primarily in Y 6, although in two cores Gerteral Characteristics of W (Kane 131 and Kane 124) which contain Zone W is distinguished by the general a highly compressed Y 6 subzone, the ash paucity of warmer water planktonic foraminialso occurs in the base of Y 5 (Fig. 9). fera such as G. menardii, P. obliquiloculata, The ash horizon is a widespread as the and G. hesngona, particularly in the upper middle ash horizon, but does not form a part, and by the presence of cooler water megascopically distinct layer in any of the forms such as Globorotaliu injlata, and cores. Globigerina falconensis. G. truncatulinoides Scattered volcanic ash also occurs throughforms relatively high percentages. In general, out X in cores from the southwest Gulf of G. inflata varies inversely with “Globigerind Mexico, especially in Kane 131 which is thus dutertrei, with G. dutertrei very important in assumed to be closest to the source area the lower part and G. inflata in the upper (Fig. 1). part. The three distinct ash horizons are inferred to reflect major volcanic activity in W ,3 Subzone the central Mexican region close to the A characteristic association occurs between beginning and end of the last interglacial G. dutertrei, G. crassaforwais, G. truncatu(X), and during the early Wisconsin Gla- linoides, and Globigerinoides conglobatus, ciation (lower Y) . with very low frequencies of P. obliquiloculata, and G. menardii. At the W 2-W 1 Pteropod Distribution boundary, an abrupt upward increase occurs The general distribution of pteropods is in G. inflata and an abrupt decrease in G. also Of value in intercore correlation. Ptero- dutertrei in addition to the complete reducpods are abundant in the Z 2, Y 1, and Y 2 tion of the warmer water forms that occur subzones. They are most abundant in Y 1 rarely in W 2. and in the upper part of Y 2. This distribuW 1 Subzonc tion supports observations of Chen (1966) This zone is characterized by an associawho examined pteropod distribution in Gulf of Mexico cores. tion in high frequencies of G. in&&a, and WESTERN G. truncatulinoides. “Orbulina also an important element. GULF &Versa” OF MEXICO is W-X Boundary This boundary is marked by the following fauna1 changes in rapid upward succession: 1. the abrupt disappearance of G. inflata and a decrease in G. trwncatulinoides; an increase in G. dutertrei, G. cras.safor+ni.s, and G. conglobatus; 2. the abrupt appearance of the G. vnenardii plexus, P. obliquiloculata, and G. hexagona. General characteristics of X X is characterized by the consistent OCcurrence of the G. rnenardii complex, in addition to other warm water forms such as P. obliquiloculata, and G. hexagona. Cool water forms such as G. inflata, and G. falconensis are much less important than in W. G. truncatulinoides, G. dutertrei, and G. crassaformis are generally prominent. Within the G. rtzenardii plexus, G. vnenardii menardii is the most important form in the lower part of X, while G. +nenardii flexuosa, and G. menardii tuwtida are the most important forms in the upper part. Five zones are recognizable within X. Three of these (X 5, X 3, and X 1) have moderate to high frequencies of the G. menardii complex, G. he.ragona and to a lesser extent P. obliquiloculata and very low to virtual absence of G. inflata. The other two subzones (X 4 and X 2) are marked by a decrease in G. menardii and an increase in G. inflata. X 5 Subzone The fauna in this zone is distinguished by relatively low frequencies of the G. menardii complex and a significant increase in G. crassaformis. X 4 Subzone This zone is marked by the virtual disappearance of the G. menardii complex, P. obliquiloculata, and G. hexagona, while G. inflata increases dramatically and briefly. In addition, G. crassafornzis decreases in fre- LATE PLEISTOCENE 53 luency and a significant increase occurs in left-coiling G. truncatulinoides. X 3 Subzone Highest frequencies in the G. menardii complex within X occurs at this interval. Relatively high frequencies also occur in P. obliquiloculata, G. hexagona, G. conglobatus, and in right-coiling G. truncatulinoides. Globorotalia injlata disappears in this interval and G. falconensis is drastically reduced. A slight increase occurs in G. crassaf ormis. X 2 Subzone Within this subzone, yet another drastic reduction in frequencies takes place within the G. vnenardii complex, P. obliquiloculata, and G. conglobatus. Although reduced in frequencies, these three forms maintain higher percentages than in X 4. Globorotaloides hexagona is absent in this zone and G. truncatulinoides reduced in frequencies. Globorotalia inflata appears in low frequencies in two cores (Kane 129 and Kane 120) although this form is not present in the same interval in other cores. Globigerina dutertrei forms a peak within the subzone and abruptly diminishes near the top. X 1 Subzone In this subzone significant increases occur in the G. menardii complex ; slight increases occur in P. obliquiloculata, G. conglobatus, G. dutertrei, G. crassafor&s, G. trusratulinoides, and G. hexagona; and G. inflata disappears. X-Y Boundary By definition, this boundary is based on the severe reduction of the G. menardii complex, so that it is no longer a consistent element of the fauna. Several other species are severely reduced, forming one of the most conspicuous fauna1 boundaries in the sequence. 54 General C‘hracteristics KENNETT AND of Y Y is distinguished by high frequencies of the cool water forms G. inflata, and G. falconensis, while the G. menardii complex and P. obliquiloculata are characteristically absent. Low frequencies of G. crassaforvnis occur in the lower part of Y and high frequencies in the upper part. Sharp fluctuations occur in G. sacculifera throughout the zone and in G. dutertrei in the middle part of the zone. Globigerina dutertrei forms very low frequencies in the lower and upper parts, while G. ruber forms high frequencies during these intervals. Y 8 Subzone This subzone in all cases is very narrow (l&20 cm in average thickness) and its base is marked by one of the most dramatic fauna1 changes in the sequence. The G. menardii conzplex and G. hexagona are absent, while G. truncatulinoides, G. conglobatus, G. dutertrei, and G. crassafornzis are strongly diminished. Three speciesare consistently important within Y 8, these being G. ruber, 0. universa, and Globigerinella aequilateralis. Pulleniatina obliquiloculata, G. sacculifera, and possibly G. falconensis do not significantly change in frequencies. Hastigerina pelagica appears in very low frequencies in a number of cores. The upper boundary of Y 8 is marked by the abrupt and dramatic increase in G. dutertrei, and an abrupt increase in G. crassafor&s followed immediately by an abrupt increase in G. inflata. HUDDLESTUN tively high frequencies of G. irzflata and almost exclusively right-coiling forms of G. truncatuhoides. The upper part contains much reduced frequencies of G. inflata and higher frequencies of G. truncatrrlinoides of which left-coiling forms are important. In three cores (Kane 114, Kane 127, and Kane 131)) G. menardii nlenardii, G. menardii fEe.ruosa, and G. hexagona appear in very low frequencies in the upper part of Y 7 associated with a slight increase in P. obliquiloculata. The upper boundary of Y 7 is marked by a dramatic increase in G. infiata from relatively low frequencies typical of Y 7 to a very important element in Y 6. An equally dramatic upward decrease in G. dutertrei also occurs and G. CrassaforvMisis reduced upward near the boundary. Y 6 Subrolle This subzone is equivalent to age 4 of Emiliani (1966). In compressed sections Y 6 is marked by consistently high frequencies of G. inflata and G. Tuber, moderately high frequencies of G.,falconensis, and the virtual absenceof any warm water forms other than very low frequencies of G: conglobatus. Both G. dutertrei and G. crassaformis are drastically reduced, and P. obliquiloculata disappears within the lower part of Y 6. G. digitata and 0. universa are consistently present and a peak occurs within left-coiling G. truncatulinodies. In cores with extended sections, the large single high frequency peak of G. inflata is split into two by a drastic reduction in G. inflata. Associated with this reduction is an increase in G. truncatulinoides and G. ruber and a slight increase in Y 7 Subzone G. digitata. The upper boundary of Y 6 is The Y 7 assemblage typically contains marked by an abrupt and dramatic reaphigh frequencies of G. dutertrei and mod- pearance of G. dutertrei and the simultaneous erate to low frequencies of P. obliquiloculata, abrupt decline of G. inflata. In addition, a and G. crassaforunis, without the G. me- slight decreaseoccurs in G. falconensis in a aardii complex, and G. hexagona. Cool wa- few cores. The percentage of right-coiling G. ter faunas such as G. injkzta, and G. fal- truncatulinoides increases to approximately conensis may or may not be present. More .lOO% and, G. conglobatus is virtually elimispecifically, the lower part contains rela- nated at the boundary. WESTERN GULF OF MEXICO Y 5 Subzone The base of Y 5 is equivalent to the boundary between ages 3 and 4 of Emiliani (1966). Immediately above the Y 6-Y 5 boundary a sharp increase in G. sacculifera occurs to form a peak within the subzone. A slight but significant increase also occurs in G. crussaformis. The Y 5 assemblage typically contains low frequencies of G. inflata, moderate frequencies of G. falconensis, moderately high but variable frequencies of G. dutertrei and low but consistent percentages of G. crassaformis. Globorotalia truncatulinoides is almost entirely right-coiling throughout except in the uppermost part, where a slight increase in left-coiling forms occurs. The upper boundary of Y 5 is marked by a sharp increase in frequency of G. inflata, and G. falconensis. In addition, an abrupt upward coiling switch in G. truncatulinodies occurs from dominantly right to dominantly left-coiling. Y 4 Subzone This is a very narrow subzone containing an assemblage marked by high frequencies of G. inflata and G. falconensis. Globigerina bulloides appears briefly in low but significant frequencies ; G. crassaformis is virtually absent and G. truncatulinoides is dominated by left-coiling forms. The upper boundary of Y 4 is marked by a decrease in frequencies of G. falconensis and/or G. inflata and an abrupt increase in G. sacculifera which is generally followed by an abrupt increase in G. dutertrei. A return to dominantly rightcoiling forms of G. truncatulinodies takes place near the boundary. This boundary is less well-defined than most others. Y 3 Subzone This subzone is marked by moderately high, although highly variable frequencies of G. sacculifera and G. dutertrei. High frequencies occur in G. inflata, and G. falconensis and moderately high frequencies in G. crassaformis. Globigerinoides conglobatus LATE PLEISTOCENE 55 persistently forms low peaks in the middle of Y 3 and G. digitata is persistent in low frequencies. The lower part of Y 3 contains relatively high percentages of left-coiling G. truncatulinoides, whereas the upper part has particularly high percentages of right-coiling forms. The upper boundary of Y 3 is marked by an abrupt increase in G. sacculifera and distinct increases in G. crassaformis, and G. irtflata. Y 2 Subzone This subzone is approximately equivalent to age 2 of Emiliani (1966). Three parts can be distinguished as follows: 1. A narrow lower part marked by high frequencies of G. inflata, relatively high frequencies of G. crassaformis and a virtual absence of warm water forms. 2. A middle part, containing a consistent, moderate to low amplitude peak of G. sacculifera accompanied by G. conglobatus and highly decreased frequencies of G. in@ata. In four cores (Kane 143, Kane 138, Kane 97, and Kane 125) low frequencies of G. menardii, G. menardii tumida, and P. obliquiloculata occur within this interval. 3. An upper part, marked by continued reduction in G. inflata. The most important feature of this interval is the very high frequencies of G. crassaformis. In addition, a distinct upward increase occurs in G. ruber. Warm water forms are essentially absent. The upper boundary of Y 2 is marked by a distinct fauna1 change that includes a consistent, uniform increase in G. dutertrei, a distinct increase in G. crassaformis and G. ruber, and the last consistent occurrence of G. inflata. In a few cores, G. menardii and G. menardii tumida appear in low frequencies at the boundary. Y 1 Subzone This subzone is typically marked by high frequencies of G. ruber and G. dutertrei, the virtual absence of G. inflata, and lower frequencies of G. crassaformis. During this in- YJ KENNETT AND HUDDLESTUN water forms. -1 reduction in frequency occurs in G. sacclllifera a.wl G. dztteutwi. Globorotalia trmcath~oidcs is typically made up entirely of right-coiling forms and G. con<$ohatus is a consistent element. In the few cores where tops were available, Y-Z Boundary a slight decrease occurs in frequencies of G. The Y-Z boundary by definition is based menardii in addition to a slight increase in on the first consistent occurrence of the G. left-coiling G. tmncatulinoides. menardii plexus (G. menardii and G. menardii twnida) . In addition, P. obliquiloculata PALEOCLIMATIC HISTORY increases markedly upward, a consistent inMethods of Analysis crease occurs in G. crassaformis, a decrease The rich planktonic foraminiferal faunas occurs in G. ruber, and G. truncatulinoides begins to increase in frequency upward from in the cores (Plate 1) have enableda detailed examination of the late Pleistocene history. this boundary. The first step in our analysis was to deterGeneral characteristics of Z mine those forms that consistently and disThe fauna within Z typically contains tinctly reflect the gross paleotemperature consistent frequencies of warm water forms changes that occurred between glacial and interglacial episodes. As in previous investisuch as the G. vnenardii complex and P. obligations of Pleistocene paleotemperatures of quiloculata, and an absence of cool water forms such as G. inflata and G. falconensis. the Gulf of Mexico (Ewing et al., 1958; Beard, 1969)) we find that the consistent Z 2 Subzone occurrence of the G. menardii complex most The fauna of Z 2 typically contains high clearly marks the interglacial episodes (Figs. frequencies of G. crassaforwais, G. dutertrei, 6 and 7) while the occurrence of G. inflata G. sacculifera and moderate to low frequen- most clearly marks the glacial episodes(Figs. cies of G. falconensis, P. obliquiloculata, G. 10 and 11). Furthermore, these two forms mena.rdii, G. menardii tamida, and G. trunshow consistently reciprocal oscillations withcatulinoides. The upper boundary of Z 2 is in the cores, with oscillations of G. ntenardii generally marked by the upward, abrupt reflecting paleotemperature changes within virtual elimination of G. crassaformis and a the interglacials, and oscillations of G. infEata decrease in G. falconensis. In addition, rnod- reflecting paleotemperatures throughout but erate increases in frequencies occur in G. especially in the glacial episodes where G. menardii menardii, G. menardii tumida, P. menardii is absent (Figs. 3-5). obliqltiloculata, and G. truncatulinoides to In addition to the gross relations demonform an important constituent of the fauna. strated by these two forms, a detailed examFurthermore, a persistent, distinct increase ination of the frequency oscillations of a in left-coiling G. tvuncatulinoides occurs at high proportion of the species has enabled the boundary. US to group them into five categories having different temperature characteristics as folZ 1 Subzone lows : warm water forms ; marginally warm This subzone is marked by high frequen- water forms ; marginally cool water forms ; cies of G. menardi, G. nzenardii tumida, P. and forms with no apparent temperature obliqztiloculata, G. truncatulinoides, and G. preferences. Frequency fluctuations between rubev and by the virtual absence of cooler coolest and warmest species are, in most C. saccztlifera begins to form consistently high frequencies that continue to the present day. Warm water forms such as G. menardii, G. aaenavdii ttinzida, and P. obliquiloculata are typically absent. terval, WESTERN GULF OF MEXICO part, consistent and clear cut, and form the basis for the interpretation of possible paleotemperature relations among the other forms. Detailed frequency changeswithin the marginally cool and warm categories are often difficult to explain in terms of environmental change. Wavtn-relater forges. This group includes the G. +nelzardii complex (Figs. 6 and 7), P. obliquiloculata, G. hexagona, and S. dehiscens. These forms have previously been regarded as distinctly warm water forms (Berger, 1969 ; Ruddiman, 1971; Emiliani, 1969). Globorotaloides hexagona consistently occurs with G. wzeenardii, although rarely forms more than a few percent of the assemblage. S. delziscens is very rare in Gulf of Mexico cores and when present occurs in association with warmer water forms. This speciesis regarded by Emiliani (1969) as a highly restricted warm-water form. Frequency changes occurring with P. obliquiloculata (Fig. 12) are somewhat similar to those occurring in the G. wzenardii complex, although in several intervals P. obliqztiloculata occurs with cooler water forms such as G. inflata (Figs. 10 and 11). For instance, P. obliquiloculata occurs at the LATE 57 PLEISTOCENE base and top of Y where G. wzenardii is absent, reflecting intermediate temperatures at these levels. This speciesthus appears to have a slightly greater tolerance to cooler conditions than the G. wzenardii complex. Marginally warn+water ~OYSZS.This group includes G. truncatulinoides, G. dzttertrei (Fig. 13), G. sacculifera (Fig. 14), and G. conglobatus. These species generally in- creased in frequencies during warm water intervals in the Gulf of Mexico. Both G. truncatulinoides and G. dutertrei have been regarded as cool-water forms by Ruddiman ( 1971). The other two species were regarded by Ruddiman (1971) as warm-water forms. It is also important to note that Ewing et al. (1958) considered G. trmcatulinoides as a cool-water speciesin their study of Gulf of Mexico cores. The extent that factors other than temperature have controlled the distribution of these forms is unknown at this time. Ruddiman (1971), and Jones (1967) for instance have pointed out that G. dutertrei responds sensitively to salinity, preferring lower values and specifically avoiding high salinities. Marginally cool-water forums. This group includes G. crassaformis which reacheshigh- G. INFLATA K-144 K97 FIG. 11. Percent abundance of Globorotualia ilzfiata in seven cores from the Gulf of Mexico. Note preferred distribution in zone Y. Arrows represent last appearances of Globorotalia menardii flexuosa. KENNETT AND HUDDLESTUN F! OBLIQUILOCULATA 31 50 K-129 0 xl It127 0 K-124 iw5 50 0 50 0 WC 50 0: : i-_--- FIG. 1.2.Percent abundance of Pul1eniatin.a obliqzcilocrk~atta est frequencies during moderately cool intervals and is generally absent during the coldest intervals (Fig. 15). This species (as G. puncticulata) was also considered a cold species by Ruddiman (1971). Cool-water forms. This group includes four species: G. inflata, G. falconensis, G. digitata, and G. bulloides. As previously mentioned, G. inflata responds inversely to the G. menardii complex. Globigerina falconensis reaches highest frequencies during in seven cores from the Gulf of Mexico. intervals in which G. inflata is most conspicuous, although it also occurs during intervals where G. injlata is absent (Fig. 16). Globigerina bulloides is a rare speciesoccurring only within the coldest intervals where it appears to grade morphologically into G. falconensis. Globigerina digitata is also a rare species and consistently occurs during the coldest intervals of Y. Forms with no distinct temperature relations. This group includes G. ruber and G, DUTERTREI 3 50 KW 0 XI --. 2 3 ---II FIG. 13. Percent abundance of “Globigerina” dutertrei in seven Gulf 4 of Mexico cores. WESTERN GULF OF MEXICO LATE 59 PLEISTOCENE G.SACCULIFERA K-135 K-136 K-138 K-143 0 K-144 .-2 2 3 3 3 4 4 4 5 5 5 6 6 6 6 7 7 7 7 6 7 ,’ i ! 8 8’ , 8! 8 \ \\ ,’ 14. Percent distribution K-108 1 -_‘- FIG. K-97 5 of Globigerinoides .__-____.---_________., sacculifera “0. &versa.” Globigerinoides ruber forms approximately 40% of the assemblage, although percentages range from 15 to 80% (Fig. 17). Distinct peaks formed by these oscillations represent strong criteria for correlation between the cores. Intervals with a distinct reduction in G. ruber in most cases correspond with distinct increases in G. dutertrei. As G. ruber is one of the most solution susceptible speciesand G. dutertrei in seven cores from the Gulf of Mexico. is a highly solution resistant form (Berger, 1969 ; Ruddiman, 1971) , these intervals almost certainly reflect marked dissolution of the planktonic foraminiferal assemblages. Conspicuous test breakage and frosting supports this conclusion. The few high frequency peaks of G. ruber appear to have little relation with fluctuations of the more temperature sensitive species. Consistently high frequency peaks of “0. G. CRASSAFORMIS 0 ml5 tm 0 ! 5 1 Z -- _-- ---. kl42 0 5 1 ‘---_ 2 3 4 Y 5 7 5 7 ! i x ‘,\I ‘. FIG. ‘\_ - -------- 15. Percent distribution __-- -- 4’ of Globovotalia \\ ‘\ i i .________________________ ----- i crassaformis in seven cores from the Gulf of Mexico. 60 KENNETT .+ND HUDDLESTUN G. FALCONENSIS FIG. 16. Percent distribution of Globigerina falronensis &versa” occur in the upper part of W 1 ; in Y 8 ; in Y 6, and in the upper part of Y 2. These peaks also represent strong correlation criteria between cores. As in G. ruber, however, this form does not demonstrate distinct relations with the more temperature sensitive species. Late Pleistocene Paleoclinzatic Curve A broadly based paleoclimatic technique should incorporate all relevant information. in seven cores from the Gulf of Mexico. Ruddiman (1971) combined a warm and a cold fauna1 group into a single plot, resulting in a “total faunal” curve. iis pointed out by Ruddiman, this method has the disadvantage of considering all species as equally “warm” or “cold when gradations exist. We also attempted to construct a total fauna1 curve basedon the “warm” and “cool” groups distinguished by Ruddiman ( 1971) . Resulting curves, however, had little similarity with curves established by other methods. G. RUBER FIG. is the 17. Percent distributionof dominant species in the Globigerinoides assemblages. ruber in seven cores from the Gulf of Mexico. This WESTERN GULF OF MEXICO This results from the fact that the paleotemperature designations of several species by Ruddiman (1971) differ from ours. For instance, G. truncatulinoides and G. dutertrei were regarded by us as marginally warm species, but by Ruddiman (1971) as “cool” species. Furthermore, G. rubw was classified by Ruddiman ( 1971) as a warm species in equatorial cores. In the Gulf of Mexico cores this species does not appear to react strongly to paleotemperature fluctuations and because it is the dominant species, this different interpretation will obviously influence the nature of a “total faunal’ curve. We have however, constructed curves which show relations between the coolest and warmest species (Fig. 18). The dominant cool water forms affecting the curves are G. inflata and G. falconensis, while the high frequency peaks of warm water forms are mainly controlled by increases ’ in the G. LATE 61 PLEISTOCENE wenardii complex and P. obliquiloculata. A relatively close relationship is demonstrated between the frequencies of distinctly warm and cold forms, the V-Z zonation of Ericson and Wollin (1968) and assumed interglacial-glacial oscillations. A much more detailed paleoclimatic curve has been constructed by determining relative frequencies of both the assumed temperature sensitive and marginally temperature sensitive forms (Fig. 19). The amplitudes of the peaks and troughs on the curve have been determined by evaluation of average relative frequencies of temperature sensitive forms in all the cores. Cores containing very high sedimentation rates in certain levels supplied the more detailed paleoclimatic information. Adjustments were made in the interpretation of relative frequencies of certain species in those assemblages obviously affected by calcium carbon- r&Y * ” Z ‘\ i Y X ---‘- w -v A FIG. 18. Cumulative percent of cool water planktonic foraminifera (left columns) and warm water planktonic foraminifera (right columns) in five cores from the Gulf of Mexico. Note dominance of warm water forms in zones V, X, and Z (interglacial stages) and of cool water forms in zones W and Y (glacial stages). Arrows indicate last appearance of Globorotalia menurdii flexzlosa. The cool water curve is a sum of the frequenciesof Globorotalia inflata, Globigerina falconensis, Globigerina bulloides, and Globorotalia scitzlla. The warm water curve is a sum of the frequencies of Globorotalia menardii menardii, Globorotalia menardii tumida, Globorotalk menardii flexuosa, Pulleniatinu obliquilocz*lata, Candeina ptitida, Sphaeroidinellu dehiscens, and Globorotaloides hexaqonn. 62 KENNETT AND HUDDLESTUN ate dissolution. The consistency of intercore Y 2 and Y 3 which are only partly related to paleoclimatic change (‘Fig. 19). The nature of the paleoclimatic changes associated with each zone is discussedin the following section. Relatively warm-water conditions areassociated with V as indicated by the high frequencies of the G. menardii complex, G. hexagona, and P. obliquiloculata and the general absence of cool water forms. Subzone W 2 is cooler than T’ because of higher percentages of G. injkta and G. crassaformis, and much reduced frequencies of G. menardii and P. obliquilomlata. On the other hand, it is warmer than W 1 because of very low but persistent frequencies of G. menardii and/or P. obliqzkloculata. Sphaeroidinelia dehiscens is also spasmod- frequency patterns was used as the basis for the paleoclimatic interpretation of each level. Sections in only five cores contain frequency patterns with little resemblance to other cores. More specifically, no correlation was possible in Kane 111 becauseof highly anomalous frequency patterns ; correlation was difficult in the middle part of Kane 108 ; and the frequency patterns of X in Kane 124, Kane 125 and Kane 120 are somewhat generalized and of little use in formulating the paleoclimatic model. Relations between Zones and Paleoclimatic History All foraminiferal zones closely approximate distinct paleoclimatic events except for EMILIANI 1971 ENDY 8 WILSON 1968 w’%G5gGT A GULF OF MEXICO g-i 20 ; T E w 40 xi z a 60 ” :_: 80’8 0°C 25T -25 -35 (%Q) -45 FIG. 19. Comparison of Late Pleistocene paleoclimatic curves. In the curves, cold is to the left and warm ‘is to the right. The paleoclimatic curve and zonation to the left are based on cores from the western Gulf of Mexico (this study). Consistent specimens of the Globorotalia mena& complex appear to the right of the vertical line. Emil&i (1971) curve based on oxygen isotopes in Caribbean cores ; Hendy and Wilson (1968) curve based on oxygen isotopes of New Zealand speleothems ; Ewing et al. (1958) curve based on presence and absence of the G. memwdiz’ plexus which also provide the basic V-W-X-Y-Z zonations used in partial correlations; Dansgaard et al. (1%9) curve is based on oxygen isotopes of a Greenland ice-core. The late Pleistocene succession of Illinois (Willman and Frye, 1970) is plotted for reference to a continental glacial and interglacial sequence. Time scales of both Broecker and Van Donk (1970)) and Rona and Emil&i (1969) are plotted. WESTERN GULF OF MEXICO ically present. Subzone W 1 is cooler than W 2 because of the virtual absence of warmwater forms, and a distinct increase in frequencies of G. injlata and decrease in G. crassaforvnis. Three warm-water peaks occur within X, these being associated with X 5, X 3, and X 1. Subzone X 5 is the coolest of the three peaks as shown by low frequencies of G. rnenardii and G. inputa, and moderate frequencies of G. crassaformis. Subzone X 3 is the warmest of the three, because it contains the highest frequencies of G. menardii, P. obliquiloculata, and G. hexagona. The rare warm-water form, S. dehiscens, is present. Furthermore, the coldest form, G. inflata is entirely absent and the marginally cool form. G. crassaforks, is moderately abundant, Subzone X 1 is similar, although slightly cooler than X 3 as indicated by the presence in low frequencies of G. inputa. Two relatively cool-water intervals occur within X, coinciding with subzones X 4 and X 2. Subzone X 4 is the cooler as shown by the virtual absence of G. menardii and fairly high frequencies of G. inj-lata. The slightly warmer conditions of X 2 are indicated by the persistent occurrence in low frequencies of G. unenardii, despite the presence in a few cores of relatively high frequencies of G. inflata. Subzone Y 8 is represented by a very unusual fauna strongly dominated by G. ruber, and with unusually high frequencies of 0. universa and Hastigerina aequilateralis. All other species are reduced to low frequencies. This is the only interval that H. pelagica was observed in the sequence.Preservation of the foraminifera is excellent with essentially no dissolution effects on the assemblage. Neither the distinctively warm or cold forms are present. We interpret this subzone as reflecting a sudden cooling at the end of X resulting in the elimination of the warm forms and yet a delayed responsein the appearance of the cold forms. LATE PLEISTOCENE 63 Subzone Y 7 is the warmest peak within Y as indicated by the consistent occurrence throughout of relatively high frequencies of P. obliquiloculata, and the spasmodic and rare occurrence in the upper part of the G. fnenardii complex and G. hen-agona. Cooler forms are also present, with G. inflata forming a prominent peak in the early part. Subzone 6 is an extremely cold interval as indicated by very high frequencies of G. inflata, moderately high frequencies of G. falconensis, and an absence of all warm water forms including P. obliquiloculata. Conditions were also apparently too cold for G. crassaformis which is either absent or occurs in very low frequencies. A slight warming occurs within Y 5 as indicated by a decrease to moderate frequencies in G. inflata, a slight decrease in G. falconensis, and general increases in G. crassaformis and G. sacculifera. The coldest interval within Y is associated with Y 4 as indicated by very high frequencies of both G. inj-lata and G. falconensis, very low frequencies of G. crassafornlis, and a dominance of left-coiling G. truncatulinoides. Subzone Y 3 represents a generally warmer interval than Y 4 as indicated by a decrease in G. inflata and/or G. falconensis, and a slight increase in G. crassaformis. Furthermore, G. sacculifera forms three peaks within the subzone, two of which are prominent in the lower and upper part and an additional lower amplitude peak near the middle. The relative importance of both G. inflata and G. falconensis indicate that this interval is still relatively cool. Subzone Y 2 contains three relatively minor climatic intervals. The lower part reflects a slight upward cooling from Y 3 as demonstrated by an increase in G. inflata and G. crassaforwzis. This is followed by a slight warming in the middle of the subzone as distinguished primarily by a promin- 64 KENNETT AND ent peak of G. snccltlifcru and a decrease in G. ,inj&~fn. In addition, the G. menardii complex and P. obliquiloculata occur spasmodicthis interval indicating ally throughout warmer conditions. The upper part of Y 2 is slightly cooler than the middle part as shown by lower frequencies of G. saccuZifera and a complete absence of P. obliquiloculata and the G. 9nenardii complex. From subzone Y 1 to the Recent, the faunas indicate that temperatures increased in a steplike pattern. From Y 1 upward, G. inflata is virtually absent, reflecting the warmer conditions. Within Y 1, G. lnenardii sporadically occurs and G. crassaforwzis is reduced in abundance. At the top of Y 1, P. obliquiloculata becomes a persistent element of the assemblage. Warmer temperatures within Z 2 are reflected by the first consistent appearance of the G. lnenardii complex and increased frequencies of G. sacculifera. Further increase in temperature to Z 1 is reflected by severe reductions in G. crac-saformis and G. falconensis, and relatively abrupt increases in the G. lnenardii complex and P. obliquiloculata. Environmental Significance rection in G. truncatulinoides of Coiling Di- Seven intervals of increased left-coiling forms of G. truncatulinoides occur within the sequence. A distinct relationship occurs between increases in left-coiling G. truncatulinoides and the coolest intervals associated with subzones Y 6 and Y 4. It is at these levels that G. truncatulinoides has strongest left-coiling tendencies. In other intervals the relations between temperature and coiling direction are not clear. For instance W 1 contains 100% right-coiling forms of G. truncatulinoides and yet this interval appears to be as cool as Y 6 and Y 4. Thus leftcoiling trends are associated in part with cool intervals and right-coiling forms in part with the warmest intervals. In several intervals, however, paleoenvironmental relations to coiling direction changes within G. truncatulinoides are not clear. Our interpretation of HUDDLESTUN at least some relations between coiling direction and climatic change is opposite that of Wollin et ~2. (1971a, b) in North Atlantic cores. It is possible, however, that coiling direction changes within the Pleistocene are distinctly different in different parts of the Atlantic Ocean. This is supported by the fact that present-day North Atlantic coiling direction patterns in G. truncatzdinoides cut indiscriminantly across strong nutrient, salinity, and temperature gradients (Ericson et al., 1954). CHRONOLOGY No direct age dates were available for the suite of western Gulf of Mexico cores studied. We have, however, placed the paleoclimatic curve in a chronological framework by correlation with climatic curves established for Caribbean cores which have been dated by the 231Pa and 230Th methods by Broecker and Ku (1969), and Broecker and Van Donk (1970). Ages applied by these workers to the late Pleistocene climatic curve differ from those of Rona and Emiliani (1969) who argue for a time scale shorter by 25%. Dating of the U-V boundary by extrapolation of sedimentation rates from the Brunhes-Matuyama boundary (Ericson and Wollin, 1968) strongly supports the longer time scale, and we have, therefore, accepted it in this paper. The maximum age of our climatic sequence is 190,000 years B.P. (Fig. 19). The most conspicuous climatic episodes occurring in Caribbean cores also occur in the cores from the Gulf of Mexico (Fig. 19) and thus we have dated these indirectly. Details of the climatic curve that could not he identified in curves from other areas were dated by extrapolation of sedimentation rates from the dated horizons. RATES OF SEDIMENTATION Sedimentation rates generally range from 10 cm/1000 years to 20 cm/1000 years, and are often highly variable within cores and between cores. As a result, correlation is WESTERN GULF OF MEXICO quite difficult without the establishment of detailed planktonic foraminiferal trends. For example, in Kane 120, approximately 50% of the thickness of Y (275 cm) is represented by Y 6-Y 8 (Fig. 3) ; whereas in Kane 138 these subzones are represented by about 1 m of sediment which is only about 15% of Y (Fig. 4). Both cores contain a continuous section throughout Y. These differences in sedimentation rates are highlighted in Fig. 18 by the difference in vertical distribution of warm-water foraminifera in the lower part of Y. Sedimentation rates are higher than 30 cm/1000 years in some core sections. For instance, in Kane 127 a sedimentation rate of 12 cm/1000 years exists for the base of Y 8 to the top of Y 6 ; and in Kane 120, a rate of 15 cm/1000 years exists for the same interval. In contrast, Kane 138 has a rate of less than 1 cm/1000 years for the same interval. In general, rates of sedimentation are much lower in the western part of the area associated with the continental slope of Mexico and Texas (Fig. 9) compared with the eastern part of the area associated with the Sigsbee Abyssal Plain and Gulf of Campeche (Fig. 20). However, even within each area, rates of sedimentation may be highly variable between zones. LATE 65 PLEISTOCENE A general increase in sedimentation rates occurs northward from the Gulf of Campeche across the Sigsbee Abyssal Plain to the base of the Sigsbee Escarpment (Fig. 20) and is related to the increased influence of the Mississippi Cone. Huang and Goode11 ( 1971) have determined average sedimentation rates of 30 cm/1000 years in the eastern part of the Mississippi Cone. They also determined that deeper silty layers represent deposition during low stands of sea level and were deposited faster (34 cm/1000 years) than the upper foraminiferal clays (22 cm/1000 years). Surprisingly little reworking is evident in the carbonate-rich cores examined in this study. A reworked horizon occurs within zone Z 2 in Kane 143 and Kane 141. The influx of relatively deep water benthonic foraminifera within this interval suggests derivation from a local source. Furthermore, the planktonic foraminiferal faunas of the reworked interval are typical of Y 3, suggesting little mixing during transportation of the original zonally restricted material. COMPARISON CLIMATIC WITH OTHER SEQUENCES Late Pleistocene climatic sequences have been determined from deep-sea sedimentary cores of various oceans; from ice-cores 3. N. K-97 K-144 K-143 K-139 K-133 K-136 K-133 FIG. 20. Correlation of zones in a north-south traverse of cores in the eastern part of the western Gulf of Mexico. Note steady increase in thickness of zones in northern direction. 66 KENNETT AND drilled from the polar ice caps; and from speleothemsfrom New Zealand caves. Widespread use of the “C. 231Pa, and 23”Tll methods for dating have enabled correlations to be carried out among these various sequences. A close similarity is evident between the Gulf of Mexico paleoclimatic curve and that determined by Emiliani (1966, 1971) for the Caribbean based on oxygen isotopes (Fig. 19). General correlations exist between age 6 of Emiliani and W 1; age 5 and X 5-Y 7; age 4 and Y 6; age 3 and Y 5- Y 3 ; age 2 and Y 2 ; and age 1 and Y 1-Z 1. The warmest intervals represented in both curves are associated with X 3 and Z 1 ; while both curves show particularly cool intervals associatedwith W 1 and Y 6. Cool intervals occurring in the Gulf of Slexico, but not represented on the Caribbean curve, are associatedwith Y 8 and Y 4. On the other hand, a distinct cooling occurs during age 2 of Emiliani (1971) which is not apparent in the Gulf of Mexico sequence, despite very high sedimentation rates in several of the cores examined. Broecker and Van Donk ( 1970) have pointed out that most of the isotope fluctuations show abrupt terminations of glacial events and slow growth of glacial conditions. This characteristic sawtooth pattern is not apparent in the Late Pleistocene curve of the Gulf of Mexico. Imbrie and Kipp (1971) also have establishlished paleotemperature curves that show no evidence of a sawtooth pattern and have suggested that the divergence in curves resulted from the fact that isotopic fluctuations primarily reflect ice volume. and that ice volume is thus not necessarily in phase with climatic change. The paleotemperature curves for a Caribbean core (V 12-122) presented by Tmbrie and Kipp (1971) shows quite close agreement with an isotopic curve determined by Broecker and Van Donk (1970) for the same core. Although a general similarity exists with the Gulf of Mexico curve, the HUDDLESTUN curve based on 1; 12-122 does not contain the details of the former, and diverges significantly from the idealized curve of Emiliani (1966 1. especially within zone X. The validity of using the Cloborotalia menavdii plexus as a reliable indicator of Pleistocene paleotemperatures has been questioned by Lidz (1966). Our data shows however that variations in frequency of the G. menardii plexus represent the most sensitive and reliable indicator of paleotemperature oscillations within interglacial episodes where these forms occur. As a result, close agreement exists between the major climatic fluctuations within the late Pleistocene sequence of the Gulf of Mexico and the climatic inferences based on the V-Z zonation previously determined on cores from the Gulf of Mexico (Ewing et al., 19SS), and the Caribbean (Ericson and Wollin, 1968) based on fluctuations within the G. menardii plexus. A close similarity also exists between our curve and that of an equatorial Atlantic core (180-73) based on analysis of the “total planktonic foraminiferal” assemblage (McIntyre et al.. 1972). Resemblance between the two curves is particularly striking from W 1 to Y 5. However, as in comparison with the Caribbean curve, correlations are more tenuous from Y 4 to Y 1. In particular, a distinct cooling at the end of Y is absent in the Gulf of Mexico sequence. McIntyre et al. (1972 1 have dated an intense cooling assumedto correlate with Y 6 as 72,000 years B.P. This is close to the 75,000 year age determined by Broecker and Van Donk (1970) for the sameclimatic event in Caribbean cores. An oxygen isotope curve from the last interglacial to the Recent determined on speleothemsfrom the Waitomo Cave, New Zealand (Hendy and Wilson, 1968) generallv resemblesthe paleotemperature curve from the Gulf of Mexico except in the middle part of Y (Fig. 19). .An oxygen isotope curve established for the Antarctic ice sheet WESTERN GULF OF MEXICO at Byrd Station (Epstein et al., 1970) is also similar to ours, although ages determined for the oscillations are different. Comparison of the amplitude and position of the oscillations suggests that a significant cooling at 70,000 years B.P. correlates with the boundary between X 1 and Y 8 ; a warming at 60,000 years corresponds to the warming during Y 7 ; a warm interval between 45,000 and 36,000 years B.P. corresponds with Y 5 and a warming at 15,000 years with the Y 2-Y 1 boundary. Correlation with the other warm water peaks is more difficult. If it is assummed that the chronological sequence of oscillations is the same, the sequences represent strong evidence of near synchronism of late Pleistocene climatic changes in the Northern and Southern Hemispheres. An isotopic curve derived from deep ice-cores at Camp Century, Greenland (Dansgaard et al., 1969) rather strongly resembles the Byrd Station core during the interval from 75,000 years and 10,000 years B.P., and in turn with the Gulf of Mexico curve. In summary, there exists substantial similarity of the Gulf of Mexico paleoclimatic curve with those of Emiliani (1971), and McIntyre et al. (1972) for the Caribbean and equatorial Atlantic areas. Significant differences occur, however, during the interval from 50,000 years to 20,000 years B.P. General agreement exists between our curve and those of Imbrie and Kipp (1971) ; Hendy and Wilson (1968) ; Dansgaard et aZ. ( 1969) ; and Epstein et al. (1970), although significant differences occur in the details. CONCLUSIONS 1. Planktonic foraminiferal frequency fluctuations in 28 piston cores of high sedimentation rates from the western Gulf of Mexico have enabled a detailed biostratigraphy and paleoclimatic history to be established for the last 200,000 years. A maximum of three interglacials and two glacials is clearly delineated (zones V-Z). LATE PLEISTOCENE 67 2. Distinct foraminiferal assemblages have enabled definition of 18 subzones, most of which are related to paleoclimatic changes, and which have enabled high resolution correlation between the cores. Most species demonstrate distinct frequency oscillations that are consistent between cores. In part these are clearly related to paleo-oceanographic changes associated with paleoclimatic cold-warm changes ; other oscillations are difficult to explain in terms of environmental change. 3. The most warm-water sensitive forms are the Globorotalia menardii complex and Pulleniatina obliquiloculata. The most useful cool-water indicators are Globorotaliu inflata and Globigerina falconensis. Several species reflect marginally warm or marginally cool conditions. 4. The most distinct warm-water intervals occurred near the beginning of the last interglacial and in the Holocene. Most intense toolings occurred at the end of the penultimate glaciation (W), and in the middle of the Wisconsin Glaciation (Y) . 5. The extinctions of Globorotalia menardii flexuosa and Globorotaloides hexagona occur near the base of Y, although a drastic reduction in these forms coincides with the boundary between the last interglacial and the last glacial (X-Y boundary). 6. Coiling direction changes within Globorotalia truncatulinoides, which provide valuable additional correlation criteria, are not consistently related to apparent paleoenvironmental changes, although the two most distinct intervals of left-coiling forms correspond with particularly cool climatic intervals. 7. Three major volcanic ash horizons occur within the sequence and provide additional correlation criteria. The lowest and largest ash horizon immediately precedes the last interglacial. The middle and second largest ash horizon coincides with the X-Y boundary. The upper ash horizon occurs within the lower part of the Wisconsin. c;s KENNET’I‘ .\NI) All three ash layers correspond with climatic cooling. 8. Substantial similarity exists between the late Pleistocene paleoclimatic curves of the Gulf of Mexico, and paleoclimatic curves for the Caribbean and equatorial Atlantic based on oxygen isotopes and planktonic foraminiferal assemblages. A dramatic cooling reported by several workers to occur at the end of the last glaciation is not apparent in Gulf of Mexico cores despite high resolution made possible by high sedimentation rates over this interval. ACKNOWLEDGMENTS We thank Dr. Charles Holmes and Dr. Henry Berryhill, U.S. Geological Survey, for their valuable assistance in obtaining Kane samples. Lianne Armstrong and Rosemarie Raymond drafted the figures, and Dennis Cassidy and Donald Scales carried out the photographic work. Technical assistance by Charlotte Brunner, and secretarial assistance by Leonore Allen is greatly appreciated. The scanning electron photographs were taken at the University of Rhode Island and Florida State University. The scanning electron microscope facility at the University of Rhode Island was established with the assistance of a grant from the National Science Foundation (Oceanography Division ; GA-28905). This research was supported by National Science Foundation Grant GA-25175. 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