This document summarizes research on a marine deposit located near Villalonga Channel in southern Buenos Aires Province, Argentina. The deposit contains two levels: a basal level composed solely of the bivalve Ostrea stentina dated to 43,000 years ago, and a bottom level with diverse marine fauna dated to 28,700-38,500 years ago. The presence of O. stentina, which has a predominantly tropical distribution, suggests the basal level may correlate with the warm Marine Isotope Stage 11 around 400,000 years ago. This would indicate warmer sea surface temperatures in the study area during the Middle-Late Pleistocene.

1. OSTREA STENTINA PAYRAUDEAU, 1826 FOUND IN A MARINE
DEPOSIT OF MIDDLE-LATE PLEISTOCENE IN THE SOUTH OF
BUENOS AIRES, ARGENTINA
CHARÓ MELISA PAOLA 1,2,3 ,ACEÑOLAZA GUILLERMO2,3, AND
CAVALLOTTO, J. L.1
1. División Geología y Geofísica Marina, Servicio de Hidrografía Naval, Av. Montes de Oca 2124, C1270ABW Buenos Aires,
Argentina.
2. CONICET
3. Instituto Superior de Correlación Geológica (INSUGEO /CONICET-UNT) Miguel Lillo 205, 4000 Tucumán, Argentina.

2. INTRODUCTION
Marine deposits representing the Quaternary events are observed along the coastal
area of the Southwestern Atlantic Ocean. Buenos Aires Province is situated in a tectonic
stable area. MIS 5e ( ̴120 Ka) and MIS 1 (11.7 Ka) are represented by marine
deposits in the south of Buenos Aires Province, with exception of MIS 3 (60 - 27 Ka).
Littoral deposits of Pleistocene are limited in the northern of Buenos Aires Province.
While these type of deposits are found with well conserved fossil marine fauna in
Anegada area in the south of Buenos Aires province (Figure 2). The most representative
marine deposits are aged MIS 5e . They are characterized by the presence of
subtropical - tropical species which indicate warmer conditions. MIS5e are registered all
over the world, with sea surface temperatures (SST) about 2ºC warmer than today.
Figure 2. Area of study. A) Bahia Anegada area,
south of Buenos Aires Province, Argentina.
N Figure 1. Middle and Late Pleistocene sea level changes ( adapted from
Murray-Wallace and Woodroffe, 2014).
The different interglacial and glacial events are registered from
Quaternary and they are described by Marine Isotopic Stages (MIS).
These events alternated cold and warm periods in the Earth´s
paleoclimate and they were registered using oxygen isotope data that
reflects changes in temperature derived from deep sea core samples
(Figure 1). Some stages, in particular MIS 5 (130 - 71 Ka), are divided
into sub-stages, such as MIS 5a, with 5 a, c, and e being warm and b
and d cold.
The main objective of this investigation is to analyze a marine deposit and to describe its
marina fauna near Villalonga Channel in the south of Buenos Aires Province, Argentina.

3. MATERIALS AND METHODS
Littoral ridge of Villalonga site (39º59´38¨S/62º22´6.02¨W) is composed by a high abundance of fossil marine shells,
especially gastropods and bivalves. The samples ( 1dm3) were exposed to running water using sieves of three different
sizes: 2.80 mm, 1.40mm and 0.080 mm. The identification of species was made with catalogs and specific systematic
papers.
Figure 3. A littoral ridge near Villalonga Channel. The basal level (A) of Ostrea,
with 1m thick, and the bottom level (B) which is a deposit of Pleistocene (MIS 5e).
This deposit is divided into two units, with 11 m height m.a s.l.
The basal level (A) is made of gravel sediments with parallel
stratification, and cemented by calcium carbonate (Fig.3 A
level). This level is only composed by Ostrea stentina
Payraudeau, 1826, with 1 m thick. The valves of Ostrea are
articulated.
The bottom level (B) is composed of a clast-supported gravel
with low-angle cross stratification (Figure 3 level B) . Fourteen
species (7 bivalves and 7 gastropods) are identified with well
preservation. The most abundant species are: Glycymeris
longior(Sowerby,1832), Ostrea puelchana d’ Orbigny, 1841,
Pitar rostratus ( Philippi, 1844), Amiantis purpurata (Lamarck,
1856) which are bivalves, and Tegula patagonica (d’ Orbigny,
1835), Heleobia australis (d’ Orbigny, 1835), Bostrycapulus
odites (Collin 2005)and Crepidula argentina Lamarck, 1822
which are gastropods.

4. RESULTS
This marine deposit was associated with marine intertidal deposits that
correlated with MIS 7 ( 237 - 190 Ka ) in San Antonio Oeste area in the
north of San Matias Gulf. Both of them are located at a lower height
than other interglacial events.
In this study, the basal level is dated at 43 Ka by 14C (Weiler, 2000).
The method of 14C is obtained with minimum ages. This data suggests
that this deposit is correlated with MIS 3 and it is associated with a
deposit older than MIS 7 because MIS 3 is not represented in marine
deposits on the continental shelf of Argentina. This interglacial event is
characterized by lower temperatures than today. Several studies
indicate that sea level during MIS 3 would have oscillated between –
80 m and -20 m height m.a. s.l. (Gasparini et al., 2016) (Figure 5). The
bottom level is dated at 28.7 and 38.5 Ka by 14C and it is correlated
with MIS 5e .The benthonic marine fauna of B level is similar than other
marine deposits of Anegada bay and San Blas area in the south of
Buenos Aires Province (Charo, 2014).
The marine fauna of Villalonga site is distinguished by the only
presence of Ostrea stentina. This specie is a marine bivalve mollusc in
the Ostreidae family.
This bivalve is small and it has oval or round valves. It is characterized
by a strong crenellation in the left valve (Fig.4a, c and d) and flatness
in the right valve (Fig.4b), with the teeth fitting into each other.
Ostrea stentina lives embedded in the shallow rocks. It is distributed in
the Mediterranean area, and in the Atlantic Ocean.
a b c d
Figure 4. Ostrea stentina Payraudeau,1826 found in A level. Scale 1cm.
Figure 5. It is shown different sea levels of MIS 3 (adapted from
Gasparini et al., 2016).

5. DISCUSSION AND CONCLUSIONS
Reference
Bowen, 2010. Sea level ∼400 000 years ago (MIS 11): analogue for present and future sea-level? Climate of the Past, 6: 19–29.
Charo, M.P.,2014. Caracterización paleoambiental y paleodiversidad malacológica en los depósitos marinos cuaternarios del norte patagónico (Sur de Buenos Aires y Norte de Río Negro).. Tesis doctoral, Facultad de Ciencias Naturales y Museo,
Universidad La Plata, 306p.
Gasparini, G., Rabassa, J., Deschamps, C. and Tonni, E. ,2016. Marine Isotope Stage 3 in Southern South America, 60 KA B.P.-30 KA B.P.. Editorial springer. doi10.1007/978-3-319-40000-6.
Murray-Wallace, C. and Woodroffe, C. ,2014. Pleistocene sea-level changes. In Quaternary Sea-Level Changes: A Global Perspective (pp. 256-319). Cambridge: Cambridge University Press. doi:10.1017/CBO9781139024440.007.
Rojas, A., Zaffaroni, J. C., Martínez, S., 2018. New molluscan records and palaeoecology of the Late Pleistocene marine assemblage from La Coronilla (Rocha, Uruguay). Journal of Sedimentary Environments, 3(4): 220-233.
Weiler, N. E., 2000. Evolución de los depósitos litorales en Bahía Anegada, Provincia de Buenos Aires, durante el Cuaternario tardío. Tesis Doctoral, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, 184p
The basal level of Villalonga is located at a lower height than MIS 7 and MIS 5e. Therefore, this deposit may indicate
older marine deposits of Pleistocene as MIS 11. This Interglacial event was the warmest of recent interglacial events, and it
was characterized by stronger thermohaline circulation, and extension in the geographical range of coral reefs.
MIS 11 is dated between 424.6-392 Ka( Murray-Wallace and Woodroffe,2014), where the averaged position of the sea
level was at 1.5 m ± 3 height m.a. s.l. (Bowen, 2010).
Along the South Atlantic coast, Ostrea stentina is found with tropical marine molluscs (Rojas et al., 2018) in marine deposits
of Pleistocene in the Coronilla Formation, Uruguay and these deposits are correlated with MIS 5e.
O. stentina is found for the first time in littoral ridges of Pleistocene in the coast of Buenos Aires Province, Argentina. The
geographical range is a predominant tropical distribution influenced by Brazilian Current. Finally, the presence of Ostrea
stentina in a marine deposit of Villalonga suggests that the SST was higher than MIS 5e SST. This tropical specie was
moved to lower latitudes when the superficial temperatures of sea decreased.
We conclude that the marine deposit of Ostrea stentina may indicate the existence of environmental changes in the south
of Buenos Aires province during Middle-Late Pleistocene.