Marine Sediments Attributed to Marine Isotope Stage 3 in the

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SOIBELZON ET AL.
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Marine Sediments Attributed to Marine Isotope
Stage 3 in the Southeastern Buenos Aires Province,
Argentina
Eduardo P. Tonni, Jorge E. Carbonari, and Roberto Huarte
➤ Keywords: Marine Isotope Stage 3, marine sediments, Buenos Aires
Province, Argentina
Approximately 200 m south of the mouth of Arroyo La Tigra (38° 20′ 55″ S
and 57° 59′ 28″ W, Mar del Sur town, General Alvarado County, Buenos Aires
Province), marine sediments crop out about 6 m above sea level. The stratigraphic unit, about 1 m thick, is composed of green yellowish silty-clayey
sands, slightly compacted. It contains shells in life position of Tagelus plebeius
(Lightfoot), a euhaline bivalve species that is a common inhabitant of estuaries and coastal lagoons of Argentina (Olivier et al. 1972). The Tagelus plebeius
shells are associated with shells of Heleobia australis (d’Orbigny), a gastropod of
wide range of salinity (mesohaline to euhaline, ca. 10–35‰), as well as isolated
osteoderms of the extinct megamammal Glyptodon sp.
Ameghino (1908) described this outcrop as “una capa marina que por la
naturaleza del terreno y la posición de los moluscos que contiene indica una playa
tranquila y fangosa” [“ a marine layer that by the nature of the terrain and the
position of shellfishes it contains indicates a peaceful and muddy beach”]
(Ameghino 1908:384). Ameghino later states that “representa la transgresión
marina que he llamado belgranense” [“represents the marine transgression that I
have called belgranense”] (Ameghino 1908:385). While all subsequent authors
recognized the “Belgranense” as a marine deposit, more recently it was considered that under this name were included transgressive events developed during
Eduardo P. Tonni, División Paleontología Vertebrados, Museo de La Plata, Paseo del Bosque s/n,
B1900FWA La Plata, Argentina; e-mail: [email protected]
Jorge E. Carbonari and Roberto Huarte, Laboratorio de Tritio y Radiocarbono (LATYR, CIGCONICET), Museo de La Plata, Paseo del Bosque s/n, B1900FWA La Plata, Argentina; e-mails:
[email protected]
[email protected]
CRP 27, 2010
TONNI ET AL.
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different warm episodes of the Pleistocene (e.g., Marine Isotope Stage (MIS) 11
and MIS 5e; see Cione et al. 2002; Pardiñas et al. 1996, Verzi et al. 2004).
In Mar del Sur two samples of shells of Tagelus plebeius were obtained for
radiocarbon dating. Sample LP-2124 gave a conventional radiocarbon age of
25,700 ± 800 RCYBP, corrected by δ13C at 0 ± 2 . This sample is stratigraphically about 30 cm above sample LP-1757, which gave a conventional
radiocarbon age of 33,780 ± 1200 RCYBP, also corrected for δ13C at 0 ± 2 .
This radiocarbon period corresponds to the end of MIS 3. During MIS 3
there were abrupt warmings in the Northern Hemisphere (DansgaardOeschger events; see Huber et al. 2006; Van Meerbeeck et al. 2008), which
were also manifested in the Southern Hemisphere (Bae et al. 2003).
Hodgson et al. (2009) described East Antarctica marine sediments at 8
m.a.s.l. According to radiocarbon dates these sediments were attributed to a
rise in sea level that was developed in the radiocarbon period 26,650 ± 220 and
28,750 ± 300 RCYBP (i.e., the end of MIS 3). Previously, Cortelezzi (1977),
Weiler et al. (1987), González and Ravizza (1987), and Weiler and González
(1988) described coastal marine sediments in Buenos Aires Province attributed to the last Interstadial between 25,000–38,000 RCYBP. Rabassa (1983)
described beach marine sediments from the Antarctic Peninsula considered
to be isostatically raised and attributed to a partial glacier recession. Shells of
the mollusk Laternula elliptica (King and Boderip) found in life position gave
an age of 34,115 ± 1110 RCYBP (Hv-11002), which also corresponds to the end
of MIS 3.
Martinez et al. (2001) described marine sediments in Puerto de Nueva
Palmira (Colonia County, Uruguay, 33° 52′ S - 58° 25′ W) and provided radiocarbon dates. These dates were from shells of Mactra isabelleana (d’Orbigny)
(31,000 ± 1,200 RCYBP; LP-738) and Anomalocardia brasiliana (Gmelin)
(34,600 ± 2000 RCYBP; LP-730). The dated shells of Anomalocardia brasiliana
had a 100% aragonitic composition, which are discarded alterations that may
have affected the age obtained. In Mactra isabelleana, the composition is
equally (50%) calcite and aragonite. According to Kennedy et al. (1969), the
Mactridae have completely aragonitic shells, so the sample LP-738 has been
affected, but this does not seem to have influenced the age obtained. In this
case it is possible that it had been a partial crystallographic conversion of
aragonite/calcite, and not a replacement of calcite of different ages. However,
Martinez et al. (2001) consider these radiocarbon dates as minimum ages,
concluding that the transgression which gave origin to the deposit must be
correlated with the last interglacial (MIS 5e). From our point of view, however,
these ages are finite, statistically distinguishable from the dating limit of the
method. Consequently, they represent an event that occurred near the end of
MIS 3 and not during MIS 5e.
The dates reported here for marine sediments of Mar del Sur provide new
evidence about the abrupt warming that occurred during MIS 3, causing rising
sea levels, probably not significant but still geologically expressed.
The authors thank Agencia Nacional de Promoción Científica y Tecnológica, Comisión de
Investigaciones Científicas de la Provincia de Buenos Aires, Consejo Nacional de Investigaciones
Científicas y Técnicas, and Universidad Nacional de La Plata for financial support.
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