Paleoecological significance of Late Quaternary molluscan faunas of the Bahia San Blas area, Argentina

Accepted Manuscript Paleoecological significance of Late Quaternary molluscan faunas of the Bahia San Blas area, Argentina M.P. Charó, S. Gordillo, E.E. Fucks PII:

S1040-6182(12)03386-1

DOI:

10.1016/j.quaint.2012.12.019

Reference:

JQI 3591

To appear in:

Quaternary International

Received Date: 10 November 2012 Revised Date:

14 December 2012

Accepted Date: 15 December 2012

Please cite this article as: Charó, M.P, Gordillo, S, Fucks, E.E, Paleoecological significance of Late Quaternary molluscan faunas of the Bahia San Blas area, Argentina, Quaternary International (2013), doi: 10.1016/j.quaint.2012.12.019. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Paleoecological significance of Late Quaternary molluscan faunas of the Bahia San Blas area, Argentina

a

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Charó, M. P. a ‫ ;٭‬Gordillo, S. b, Fucks, E. E. c

- Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, Argentina.

b

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CONICET.

- Centro de investigaciones en Ciencias de la Tierra (CICTERRA, CONICET-UNC),

c

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Córdoba, Argentina.

- Facultad de Ciencias Naturales y Museo y Ciencias Agrarias y Forestales-LATYR-

UNLP, Argentina.

* Corresponding autor. E-mail addresses: [email protected] (M. Charó),

Abstract

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[email protected] (S. Gordillo), [email protected] (E. Fucks).

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Late Quaternary marine deposits in the area of Bahía San Blas (Buenos Aires, Argentina) form beach ridges and tidal plains bearing remains of organisms, mainly mollusc shells.

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These mollusc assemblages were characterized, and their changes assessed both over time, and related to local environmental factors. Twenty-six sites were analyzed, eleven Pleistocene, seven Holocene and eight modern sites. Fifty taxa (27 bivalves and 23 gastropods) were recorded for the area, with marine species predominant over estuarine. Within gastropods, epifaunal species and carnivores are predominant, with a lower proportion of herbivores and filter feeders, while bivalves are mostly infaunal and suspension feeders. Pleistocene associations are characterized by the high quantity of

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molluscs in rocky environments, and those of the Holocene by the presence of the warmwater Crassostrea rhizophorae, which is currently displaced to lower latitudes. Within the modern associations four taxa, Mesodesma mactroides, Solen tehuelchus, Barnea lamellosa

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and Crassostrea gigas (an introduced species), are not recorded as fossils, but Mesodesma mactroides is found in Holocene shell middens from the area. According to multivariate analysis, the sites were divided into two groups: high energy deposits (storm ridges) and

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low energy deposits (tidal plains), regardless of their age. Based on these results, mollusc variations in the area are thought to be related in part to changes in temperature that took

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place since the Pleistocene(e.g. Crassostrea rhizophorae); but mostly associated with the presence of sub-environments of different energy levels. This would favor the presence of different taxa in accordance to their ecological requirements.

1. Introduction

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Key words: Molluscs, Pleistocene, Holocene, Paleoecology, Argentina.

The Quaternary is characterized worldwide by important climate oscillations, with

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extremes represented by glacial and interglacial periods resulting from temperature variations that caused marked changes in sea level (e.g. Rohling et al., 2003). In coastal

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areas, transgressive events have been registered by a sequence of erosion forms (coastal terraces and paleocliffs) and beach deposits that, for different reasons, have been protected from degradation processes and are therefore an important testimony of climate changes that have occurred in most recent geologic times (Shackleton, 1987). In the Patagonian coastal region, these deposits can be found both along the coastline, and well inland. This gives evidence of the magnitude of the transgressive events, and of the geomorphologic characteristics and isostatic variations. The most extreme sea levels are

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clearly seen at various sites along the coast, although due to the continental rise (Fucks et al., 2009; Pedoja et al., 2011) lower levels can also be seen above the current sea level in several sites. On a larger scale, Pleistocene coastal fossil deposits are known worldwide,

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from the North American Pacific coast (e.g. Roy et al., 1998; Parham et al., 2007), to Mexico (e.g. Ortlieb, 1991; Mush et al., 1992, 2002; De Diego – Forbis et al., 2004), Bermuda (Hearty, 2002), Japan (e.g. Kim et al., 1999; Kitamura et al., 2000), Australia

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(Murray – Wallace and Belperio, 1991; Murray-Wallace et al., 2000), and Europe (e.g. Lario et al., 1993; Zazo, 1999; Zazo et al., 2003a; Dumas et al., 2005). Along the South

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American coast, deposits of different sea levels related to transgressive events can be seen in Brazil (e.g. Caruso et al., 2000; Barreto et al., 2002), Uruguay (e.g. Martínez et al., 2001; Goso Aguilar, 2006), Chile (Quezada et al., 2007), and Argentina (e.g. Cionchi, 1987; Codignotto et al., 1988; Codignotto and Aguirre, 1993; Schellmann, 1998; Isla et al.,

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2000; Rostami et al., 2000; Schellmann and Radtke, 2000, 2003; Weiler 2000; Bujaleski and Isla, 2006; Isla and Bujalesky, 2008; Pedoja et al., 2011). The most representative fauna of these Quaternary marine deposits are the molluscs, in

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particular gastropods and bivalves, and their preservation makes them very useful as proxy paleoclimatic and paleoenvironmental records. Analysis of these taxa provides information

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on different environmental parameters such as temperature, salinity and substrate, thus leading to an interpretation of the paleoenvironment and the paleo-communities that existed in the different regions throughout time. Recent studies have also used this information as paleoenvironmental parameters and for paleoclimatic studies such as atmospheric and oceanographic changes (e.g. Ortlieb et al., 1994; Martínez et al., 1997, 2001; Maasch et al., 2001; Ragaini et al., 2002; Aguirre et al., 2006; Cardenas and Gordillo, 2009; Rabassa et al., 2009; Jones et al., 2010; Rojas and Urteaga, 2011).

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During the last interglacial the MIS 5e (MIS= Marine Isotope Stage), which occurred between 130± 2 and 119 ± 2 ka (Hearty et al., 2007), sea level rose at least twice (e.g. Zazo et al., 2003ab; Tuccimei et al., 2006). In this period, surface sea temperatures (SST) were

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approximately 2 C° above the present temperature (Mc Culloch and Esat, 2000; MurrayWallace et al., 2000; Rohling et al, 2008), and sea level reached approximately five to six meters above the present sea level (Shackleton, 1987; Neumann and Hearty, 1996).

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Records worldwide show evidence of warm water in benthic marine molluscs, as well as changes in their geographical distribution (Cuerda et al. 1991; Chaar and Farinati, 1988;

Urteaga, 2011, among others).

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Muhs et al. 2002; Zazo et al., 2003b; Martinez et al., 2001; Zazo et al., 2010; Rojas and

A warm event was detected in the Mid-Holocene (8700-7800 BP) of the Northern Hemisphere, known as the Climatic Optimum or Hypsithermal, in which the SST and

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humidity were higher than today (e.g. Funder and Weidick, 1991; Salvigsen et al., 1991; Hjort et al., 1995; Rohling and Rijk, 1999; Yuan et al., 2011). Some events have also been recorded in the Southern Hemisphere (e.g. Cohen et al., 1992; Lutaenko, 1993; Aguirre,

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1990, 1993b, 2002; Gordillo et al, 2005). The main purpose of this study is to characterize molluscan associations of Bahía San Blas during this period, and to assess their changes

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over time and related to local environmental factors.

1.1.

Study area

The study area is located in the south of the Buenos Aires Province, extending from Isla Jabalí (40°36'S; 62°11'W) to Faro Segunda Barranca (40º46´S; 62º16´W; Lighthouse Segunda Barranca) and from the coastline to National Route 3 (Figure 1). The most characteristic geomorphologic features are beach ridges and low, gravel plains. The beach

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ridges may be grouped according to different transgressive episodes. The lower ones (3-5 m.a.s.l.) are grey gravels and sands with low-angle cross bedding, four meters of maximum thickness. They are small active cliffs and topographical ledges among the different

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groupings that can even be considered as beach strand plains due to their quantity and disposition (Fucks et al., 2012). Holocene deposits can be seen in large number at Isla Jabalí, where they can be geomorphologically divided into two well defined groups. To the

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east, they are flat and composed of brown-grey silty sand; the lower section is finely stratified, while the upper is more homogenous and browner. The western area has a

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significant amount of ridges very close together and are either parallel, transverse or at an oblique angle to each other (Fucks et al. 2011, Fucks et al., 2012). Inland, behind the Holocene deposits, there is a second level of beach ridges (8-10 m), less pronounced and either parallel to the coast or at an oblique angle. These ridges are made up

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of friable gravel, with nodules, and correspond to the Late Pleistocene transgressive event (MIS 5e). They extend north to south and are cut off in the north by the Jabalí Stream and in the south by the Walker Stream (Fucks et al., 2012). These ridges are parallel to the coast

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to Faro Segunda Barranca, where a gravel and sand beach is observed on the coastal platform of the Río Negro Formation. The cliffs are 10 m maximum height, and consist of

2).

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sand and silt at the base, and well-stratified gravel in the middle and upper sections (Figure

Above these, up to near 35 m.a.s.l., gentle plains with outcrops of long, circular elevations of gravel highly cemented with calcium carbonate have been observed. In all of these morphologies, mollusc remains and/or flattened clasts indicate their littoral origin (Fucks et al., 2012).

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The development of numerous deflation basins occupied by salt flats (Barrancoso, Grande, La Salinita, Salina del Inglés, among others) is one of the most common features of the continental area. The highest concentration of salts is normally found in the lowest areas,

(Trebino, 1987, Del Blanco et al., 2005).

Geological background

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1.2.

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with sodium chloride predominating, and clay and alkaline sulphates in lower quantities

Witte (1916) was the first to study the area and to describe the geology of the San Blas

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coastal deposits. He defined five evolution stages (I-V) represented in different sections of the coast, and noted that the sediments forming the deposits corresponded to Stages III (Pleistocene), IV and V (Holocene).

Ambrosini (1984) and Trebino (1987) analyzed the geomorphology of Bahía San Blas area,

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and described Pleistocene, Holocene and modern geofeatures. Trebino (1987) recognized three levels of marine terraces, I and II from the Pleistocene and III for the Holocene. Terraces I and II are located in Rincón de Walker, and III in Isla Jabalí, and are a result of

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successive accumulation of beach strands and spits (Echeverría et al., 2006). The Holocene deposits, located at an altitude of 3 m a.s.l., were previously dated by Trebino (1987) at

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2170±110, 3450±110, 3650±80 (LP=2434), 4100±95, 5370±110, 2320±80, and 3450±80 C BP.

Pleistocene beach ridges, located to the west of the Jabalí Stream, were dated at 28,400± 800 and 29,120±970 14C BP (Trebino, 1987) and 30,780±1650 BP Weiler (2000). These are considered to be the minimum ages, correlating with the Latest Interglacial event (MIS 5e). Schnack et al. (2005) considered that Witte’s (1916) I to III stages were equivalent to Trebino’s (1987) level I and II Pleistocene terraces, whereas the Holocene deposits

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corresponding to Witte’s (1916) stage IV would be equivalent to Trebino’s (1987) level III terrace. The most recent geochronological research carried out in the Faro Segunda Barranca area

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used ESR and amino acid racemization dating techniques. The base of the cliff (site 24) was dated at 102 and 108 ka, while the upper part of the cliff (site 23) oscillated between 94, 79, and 72 ka. Although all the dating was carried out using the ESR method, the D/L

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ratio of aspartic acid and leucine could link this site to an older transgressive event (Rutter et al., 1989, 1990). These last sequences belong to the MIS 5e stratigraphic unit, which is

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equivalent to Feruglio’s terrace V (1959) and to the Belgranense (Ameghino, 1889) or Puente Pascua Formation (Fucks et al., 2005).

Fucks et al. (2012) reinterpreted the stratigraphic sequences of this area, assigning a minimum of four transgressive cycles. Beach ridges, as well as beach strand plains and tidal

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plains with maximum altitudes of 6 m a.s.l. and very clear morphologies are present from the coast to the present day continent, particularly in Isla Jabalí. Above them, at altitudes of 8 to 10 m a.s.l., clear ridges could be probably related to MIS 5e, although less than the

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previous ridges, and that increase gradually to altitudes of over 30 m a.s.l. These could have

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been originated in two ≥9 transgressive events (Table 1).

1.3.

Paleontological background

The first studies on marine molluscs in the Buenos Aires Province were mainly of a geological character. Frenguelli (1928, 1950) contributed greatly to the understanding of Quaternary marine fauna. Pioneer studies from around this period were also carried out by d’Orbigny (1834-1847, 1842-1844), Darwin (1846), Ameghino (1898, 1908), Ihering (1907) and Wahnish (1939), among others. The first mention of systematic mollusc studies

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appeared in contemporary research (Carcelles 1944; Camacho, 1966; Castellanos, 1967, 1979, 1981). Among the most recent studies on Quaternary molluscan assemblages in the littoral deposits of the Buenos Aires Province, contributions from Farinati and Camacho

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(1980), Farinati (1985, 1994), Chaar et al. (1992), Aguirre (1990, 1993a-c, 2002), Aguirre and Whatley (1995), Aguirre and Farinati (2000), Aguirre and Fucks (2004), and Fucks et al. (2005), among others, are noteworthy. Some recent studies of Quaternary deposits of

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Bahía San Blas and Faro Segunda Barranca mention molluscs, but they are mainly

Fucks et al., 2012).

2. Material and methods

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geomorphological and geochronological research (Trebino, 1987; Rutter et al., 1989, 1990;

Twenty six sites were analyzed in the study area: eleven Pleistocene, seven Holocene, and

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eight modern (Table 2). The samples collected from the Pleistocene and Holocene deposits (1 dm3) were exposed to running water using sieves of three different size: 2.80 mm, 1.40 mm and 0.080 mm. A sequence of washing and drying on paper was then followed. The

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modern samples were taken in a 1 m x 1 m quadrant on transects perpendicular to the coastline. The molluscs were collected by hand and were then sorted using a 2.80 mm

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sieve.

Each piece of biogenic content that was captured by the sieves was identified and labelled; first, the material was sorted, using a magnifying glass when necessary, and then it was divided into taxa by comparing it with catalogs and current specific systematic studies. For each sampled site, the Shannon Index (H´) was calculated through an X-Y graph and the ecological parameters of gastropods and bivalves were compared for each period. In order to analyze the degree of similarity between sites, cluster analyses were carried out

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with R software, version 2.15.0 (vegan package) (Oksanen, 2011), and the UPGMA method was used to group the faunal associations according to the Bray – Curtis Index. In this case, a matrix was used for each site to show the abundance of different species. A

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Correspondence Analysis (CA) was then carried out to observe the relationship between sites. The ecological information on the taxa is based on living specimens and also on bibliographical sources (Lasta et al., 1998; Bastida et al., 2007; Balech and Ehrlich, 2008;

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Rosenberg, 2009).

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3. Results

Taxonomic and paleoecological composition

Twenty six sites were analyzed in Bahía San Blas, including Pleistocene, Holocene and modern deposits, in which 50 taxa (N=388 shells) were recorded (27 bivalves and 23

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gastropods) (Figures 3 and 4) (Tables 3 and 4). According to Shannon’s diversity index (H´), the modern sites have greater diversity, with values ranging between 2.3 and 1.5, compared to Holocene sites which show lower values, between 0.22 and 0.86, with the

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exception of sites 11 (H´=2.28) and 14 (H´=2.28) that belong to beach ridges. Pleistocene sites show values between 0.29 and 1.98, with the highest values at sites 20 (H´= 2.09) and

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22 (H´= 2.52) (Figure 5).

The faunal composition of bivalves and gastropods shows that marine species predominate over estuarine species (e.g. Heleobia australis). They mainly are species with active locomotion as epifaunal gastropods and infaunal bivalves. Bivalves from Pleistocene deposits are mainly rocky sediment and filter-feeders, whereas for the Holocene and modern, sandy sediment and suspension feeders are predominant. Among gastropods, the epifaunal type, common in sandy sediments, is predominant, with filter feeders mainly

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present in Pleistocene deposits and carnivores in Holocene-modern ones (Figure 6) (Tables 5 and 6). In the cluster analysis, Holocene bivalves were more similar to modern than to Pleistocene

modern ones.

The Pleistocene of Bahía San Blas

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3.2.

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ones. However, Pleistocene gastropods were similar to Holocene ones, and both differ from

Eleven sites were analyzed for the Pleistocene, in which 33 taxa were recorded (17 bivalves

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and 16 gastropods). Five of the eleven sites were studied from a lithological and paleoecological point of view. Site 25, is located approximately 50 km from the village of Bahía San Blas. This outcrop is composed of 2.5 m of gravel sediments with a sandy silt matrix and a homogeneous grey colour (Figure 9) (Fucks et al., 2012). The mollusc fauna is

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composed of Pitar rostratus, Glycymeris longior (bivalves), Buccinanops cochlidium, and Olivancillaria urceus (gastropods).

The second Pleistocene deposit is site 22, near Salina del Inglés, 26.6 km from the village

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of Bahía San Blas. It is one of the main evaporite deposits in the area, ovoid-shaped, 5.5 km long E-W direction and 4.8 km wide, and composed of silty sediments, brown to green,

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with gypsum crystals and mollusc shells (Etcheverría et al., 2006). At the western end there is a selenite gypsum layer nearly 50 cm thick composed of slightly carbonatic, brown clay mud (Del Blanco et al., 2005). In this site, a 50 cm unexposed profile was found, with articulated Pitar rostratus in life position (Figure 10). Nineteen taxa were observed (10 bivalves and 9 gastropods) in a good state of preservation and with gypsum precipitation on the shells. Among the most abundant were Pitar rostratus (bivalve), Olivancillaria urceus,

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Tegula patagonica and Zidona dufresnei (gastropods), micro-molluscs Nucula nucleus, Carditamera plata, Corbula patagonica (bivalve), and Heleobia australis (gastropod). Sites 23 and 24 correspond to the Pleistocene of Faro Segunda Barranca. This outcrop has

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two levels: the lower level (site 24) comprises 2 m of yellow-brown sandy silt sediments with boulders. Mollusc fauna includes Tegula patagonica, Bostrycapulus odites, Urosalpinx cala, Parvanachis

isabellei, Buccinanops cochlidium and Buccinanops

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globulosus (gastropods), and Mytilus edulis and Brachidontes rodriguezi (bivalves) (Figure 11). The upper level (site 23) is composed of clast-supported gravel, with sand and

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conglomerated sand strata on the surface. The overall colour is grey, with clearly defined parallel to low-angle cross bedding, showing a slight southern inclination and partial clustering in some strata. Fossil remains include Tegula patagonica, Bostrycapulus odites, Crepidula sp., Olivella tehuelcha (gastropods) and Pitar rostratus, Mytilus edulis,

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Brachidontes rodriguezi, Aequipecten tehuelchus, Ostreola equestris, Ostrea puelchana and Pitar rostratus (bivalves) (Figure 12).

A Pleistocene ridge at an altitude of 33 m.a.s.l., 10 km from the village of Cardenal

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Cagliero (40°39'18.10" S / 62°44'53.66" W), was analyzed (site 26). It is a quarry of clustered gravel probably older that MIS 5e (Fucks et al., 2012). Pitar rostratus (bivalve)

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and Zidona dufresnei (gastropod) were found.

3.3.

The Holocene of Bahía San Blas

Large quantities of diverse gastropods and bivalves were found in the Holocene beach ridges. Seven sites were analyzed and 31 taxa were collected (18 bivalves and 13 gastropods) in a good state of preservation, in some cases articulated. All Holocene deposits are located at Isla Jabalí, and are represented by two geo-features: beach ridges

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(Figure 13) and tidal plains (Figure 14). Beach ridges (sites 11 and 14) deposits are well stratified gravels (Figure 10B a, c) and sands (Figure 10B b, d; C) with shell remains. Among the most common taxa are Pitar rostratus, Amiantis purpurata, Glycymeris

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longior, Ostrea equestris (bivalves) and Tegula patagonica, Bostrycapulus odites, Buccinanops globulosus (gastropods). In the tidal plains (sites 9, 10, 12, 13, and 15), which are lower energy environments, Heleobia australis (gastropod) and Corbula patagonica

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and Tagelus plebeius (bivalves) are common, and in most of these deposits, articulated

3.4.

Modern fauna

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Tagelus plebeius in life position can be observed (Figure 12).

Beaches of Bahía San Blas are made of boulders and have a very pronounced slope, thus forming a reflective beach (Trebino, 1987; Codignotto, 1997; Cuadrado and Gomez, 2010)

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with mollusc remains (Figure 15).

At the Isla Jabalí beaches, six sites were analyzed and 24 taxa were found (10 gastropods and 14 bivalves) including Bostrycapulus odites (gastropods) and Brachidontes rodriguezi,

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Ostrea equestris, Pitar rostratus, Amiantis purpurata and Crassostrea gigas (bivalves), being particularly noteworthy Mesodesma mactroides, Solen tehuelchus, Barnea lamellosa

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and Crassostrea gigas. At the beaches of Faro Segunda Barranca, two sites were analyzed and 16 taxa were recorded (10 gastropods and 6 bivalves), being the most abundant Bostrycapulus odites, Trophon varians and Tegula patagonica (gastropods), and Ostreola equestris (bivalve).

3.5.

Mollusc associations

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In order to assess the degree of similarity (or dissimilarity) among the set of samples from both areas, the UPGMA method was used, together with the Bray-Curtis Index. Two large sample groups, A and B, were defined according to the cluster analysis. Group A mainly

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contains Pleistocene and modern sites, with the exception of two Holocene sites. All belong to high energy environments. This group is divided into four sub-groups. Sub-group A1 stands out for having three Pleistocene sites and three modern sites, including all the sites

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of Faro Segunda Barranca. Sub-group A2 contains three Pleistocene sites that belong to the continental part of Bahía San Blas, of which two (sites 25 and 26) are located at the highest

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altitudes. Sub-group A3 gathers three Pleistocene and four modern sites from Isla Jabalí, but did not include site 16. Group B gathers only Holocene sites, all of which belong to low-energy environments (Figure 16).

According to the Correspondence Analysis (CA), the sites can be divided into groups A and

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B. Group A is composed of continental Pleistocene sites and modern sites of Isla Jabalí. Group B comprises all Holocene sites in low-energy environments. In the upper region of the graph, the sites located at the top, 22 and 15, correspond to the two Pleistocene sites

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considered to be older than MIS 5e. Site 17 is located in the lower region of the graph, close to group A and sites 7 and 8, which belong to modern Faro Segunda Barranca sites

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(Figure 17).

4. Discussion

For the Pleistocene, 33 taxa were found (17 bivalves and 16 gastropods), mostly represented by the gastropods Brachidontes rodriguezi, Mytilus edulis, Ostreola equestris, Ostrea puelchana, Pitar rostratus, and Amiantis purpurata, and the gastropod Buccinanops globulosus. For the Holocene deposits, 31 taxa were registered (18 bivalves and 13

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gastropods), with the predominant bivalves being Pitar rostratus and Corbula patagonica, and the gastropods Heleobia australis, Tegula patagonica, and Zidona dufresnei. In general, the Pleistocene and the Holocene molluscs of Bahía San Blas show no significant

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differences in composition. However, the presence of the bivalve Crassostrea rhizophorae in the late Pleistocene (MIS 5e) is a clear sign of warmer conditions. This is also seen in the Pleistocene deposits of Bahía Blanca, 285 km farther south (Chaar et al., 1992). In contrast,

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no warm climate molluscs, currently present in lower latitudes, were recorded during the Holocene. In this regard, the Holocene fauna from Bahía San Blas contrasts with the results

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of previous studies in the northeast region of the province of Buenos Aires, and in Bahía Blanca (Aguirre, 1993b; Aguirre and Whatley 1995; Farinati 1985), in which larger amounts of gastropods and bivalves were reported compared to the Pleistocene record (MIS 5e). The abovementioned studies relate those differences to the Climatic Optimum or

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Hypsithermal (mid-Holocene), a paleoclimatic event that occurred in the Northern Hemisphere, with higher SST and warmer conditions compared to the present (e.g. Lutaenko, 1993; Hjort et al., 1995). However, the results of this study area do coincide with

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other research carried out along Central Patagonia (Pastorino 1991, 1994, 2000; Aguirre 2003; Aguirre et al. 2005, 2006) in which no record of faunal changes related to the

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Hypsithermal were found.

Within the Holocene fauna, the presence of articulated Tagelus plebeius in life position is worthy of mention. Tagelus plebeius is an infaunal eurohaline species, commonly found in large quantities in Holocene deposits in intertidal areas of estuaries on the Atlantic coast, from North Carolina (34º N) to San Matías Gulf (41ºS) (i.e. Gutiérrez and Iribarne, 1999; Bushek et al., 2008). In the province of Buenos Aires, episodes of mass mortality have

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been recorded (Schnack et al., 1982; Farinati et al., 1992; Golfieri et al., 1998; Iribarne et al., 1998; De Francesco and Zarate, 2001). In addition, Mesodesma mactroides, one of the most common modern species in the Bahía

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San Blas area, that lives today on sandy beaches from 23°S (Rio de Janeiro, Brazil) to near 40ºS (Isla Jabalí) (i.e. Rios 1994; Fiori and Morsan, 2004; Fiori and Defeo, 2006), was not found in the Holocene ridges analyzed in this study, despite being mentioned in an

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unspecified area for this period by Aguirre and Farinati (2000). In Holocene archaeological sites of the Pampean littoral, Mesodesma mactroides was not found to be consumed by

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hunter–gatherer aborigines (Bonomo, 2007). Nevertheless, in Holocene shell middens of the coastal region of Bahía San Blas, this species has been mentioned as amongst the most abundant (Zubimendi, 2007; Aldazabal et al., 2010). Hence, although M. mactroides was not found in this study, it has been present in the region, probably with a discontinuous

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distribution along the coast of the Buenos Aires province. Similarly, Solen tehuelchus and Barnea lamellosa have not been found in the studied ridges, but have been recorded in other Holocene deposits of the Buenos Aires Province between Sierra Grande and Bahia

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Blanca (Aguirre and Fucks, 2004).

The appearance of Crassostrea gigas on the current coast of Isla Jabalí is the result of

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anthropic modification. This Japanese species was brought to Argentina and introduced into the area of Bahía Anegada (39º50´S /40º 40´S) in 1981 for farming purposes, and in 2005 the most southern natural population in the country was recorded in El Condor beach resort (41ºS).

5. Conclusions

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For the Quaternary of Bahía San Blas, 50 taxa were recorded (27 bivalves and 23 gastropods) in a good state of preservation, and in some cases, articulated. The highest diversity indexes correspond mainly to modern sites, followed by Pleistocene sites.

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Holocene sites have the lowest diversity, except for two. Marine species of bivalves and gastropods prevail over estuarine species, being mainly free locomotion species as epifaunal gastropods and infaunal bivalves.

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Among bivalves, rocky sediment and filter-feeder bivalves prevail in Pleistocene deposits, whereas sandy sediment and suspension feeders are predominant in Holocene and modern

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deposits. On the other hand, epifaunal gastropods typical of sandy sediments and filter feeders are predominant in Pleistocene deposits, whereas mobile carnivores prevail in Holocene and modern deposits. It remains unclear if these changes in the predominance of different trophic groups is related to the presence of different sub-environments within the

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study area or are due to regional changes over time.

Two mollusc associations were defined in Bahía San Blas (A and B) depending on the type of environment: high energy (storm range) or low energy (tidal plains), and not on their

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age. Sites 22 and 25 are different from the rest of the Pleistocene sites: the former due to its species richness, and the latter for being one of the oldest, along with site 26. Both sites

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yielded the largest numbers of Pitar rostratus. During the Pleistocene, warmer conditions favoured the entrance of Crassostrea rhizophorae, currently inhabiting lower latitudes. The large number of articulated Tagelus plebeius in life position indicates a low-energy environment of fine sediments and silty sand, best seen in Holocene deposits. For the Holocene, no warm water taxa currently living in lower latitudes was found. Modern sites from Faro Segunda cluster with Pleistocene sites of Bahía San Blas, but separate from modern sites of Isla Jabalí in the Correspondence Analysis (CA). This

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difference is due to the large number of Trophon varians in Faro Segunda Barranca beaches. Four modern taxa were not found in either Pleistocene or Holocene deposits: Mesodesma

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mactroides, Solen tehuelchus, Barnea lamellosa and Crassostrea gigas. However, although Mesodesma mactroides was not found in this study, it has been reported in archeological sites (Holocene shell middens) in the south of Bahía San Blas. Meanwhile, Crassostrea

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gigas was introduced into the Argentine seas of Bahía Anegada and has proliferated over the last few years along the southern coast of the Buenos Aires Province. From the 50 taxa

recorded since the Pleistocene.

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found in Quaternary deposits in the study area, 14 (8 bivalves and 6 gastropods) are

The last marine transgressive events have been recorded in Bahía San Blas area. MIS 1 and MIS 5e (last interglacial) are well represented, while other older events, MIS 7 and/or MIS

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9, are not so clearly represented. The ≥MIS 7 events are represented by elongated geofeatures, characterized by scarce marine fauna and maximum altitudes of 30 m a.s.l. The Late Pleistocene (MIS 5e) is well developed inland, where typical outcrops can be

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found within maximum altitudes of 10 m a.s.l. MIS 1 is recorded exclusively on Isla Jabalí, with continuous beach strands, beach ridges

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and shallow tidal plains at maximum altitudes of 6 m a.s.l. as the most typical geofeatures. For both MIS 1 and MIS 5e, the characteristic marine molluscs are bivalves and gastropods. Among these, Crassostrea rhizophorae, today living at lower latitudes, is an excellent indicator of warmer conditions during the Late Pleistocene (MIS 5e). Nevertheless, as a whole, molluscs do not denote warmer environments in the study area that could be related to the global event known as the Climatic Optimum, or Hypsithermal. Based on the quantitative and qualitative analysis changes of Pleistocene and Holocene molluscs, are

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mostly due to the presence of sub-environments and local, rather than global, environmental changes that affect ecological parameters.

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Acknowledgments

The authors thank Lic. Florencia Pisano (FCNYM, CONICET) for her collaboration during fieldwork; Prof. Mario Giaconi (FCNYM, UNLP, CONICET) for logistic support, Dr.

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Mikel Zudimendi (UNLP, CONICET), who provided the bibliography which enriched this study in an interdisciplinary way and Dra. Cecilia Deschamps for help in the editing of the

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final version.

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Pleistocene

Stage IV: Marine deposits 1.50 m higher than current height. Stage III: Marine coastal banks with a sea level approx. 10 m higher than in modern times. Old coastlines are clearly visible.

Erosive features: terraces Level III: Poorly represented; can be seen in a discontinuous along Walker Stream. Corresponds to more modern terraces with altitudes of 3 m.a.s.l.

Accumulation features Section B: All marine accumulation cut off by marine erosion or interdigitation.

Level II: Rows of beach strands corresponding to old coastlines at 10 and 11 m.a.s.l.

Section A: Located west of Jabali Stream and to the south of the Guanaco Stream. Long, wide beach ridges located between 9 and 10 m.a.s.l.

Level I: The oldest: located on old boulder deposits between 12 and 14 m.a.s.l.

Schnack et al. (2005)

Fucks et al. (2012)

Witte’s (1916) Stage IV equivalent to Trebino’s terrace level III.

Represented by beach ridges, beach strand plains and tidal plains with maximum altitudes of 5 m.a.s.l.

Witte’s (1916) Stages I to III – equivalent to Trebino’s (1987) terrace levels I and II.

Beach ridges at 8 – 10 m.a.s.l.

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Bahia San Blas

Stage V: Modern beach marine deposits.

Trebino (1987)

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Holocene

Witte (1916)

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Marine deposits

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Table 1. Geomorphological characterization of the studied sites located in the San Blas area.

ACCEPTED MANUSCRIPT Site s

modern

1

40°31'40.80"S; 62°19'5.40"O

Beach

0

2

40°32'9.32"S; 62°18'47.16"O 40°32'20.12"S; 62°15'22.08"O

Beach

0

Beach

0

4

40°33'15.68"S; 62°13'28.38"O

Beach

5

40°34'11.70"S; 62°11'49.86"O

Beach

6

40°35'35.92"S; 62°10'15.78"O 40°46'30.80"S; 62°16'18.50"O

Beach

0

Beach

0

7

Altitude (m.a.m.s.l.)

0 0

8

40°46'32.91"S ;62°16'19.95"O

Beach

0

9

40°32'9.43"S; 62°17'22.55"O

Tidal plain deposits

1

10

40°34'14.33"S; 62°15'19.45"O

Tidal plain deposits

1

11

40°34'9.30"S; 62°13'57.80"O

Beach ridge made of sandy sediments and containing clasts Tidal plain deposits

4

Tidal plain deposits

5

Beach ridge made of sandy sediments and containing clasts Tidal plain deposits

4

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Holocene

12

40°34'23.59"S; 62°14'11.71"O 40°34'56.20"S; 62°14'11.43"O

EP

13

3

14

40°34'51.91"S; 62°13'7.14"O

15

40°37'9.00"S ; 62°12'49.20"O

16

40°33'58.50"S; 62°19'7.30"O

Beach ridge made of sandy sediments

9

17

40°34'6.70"S; 62°18'52.12"O

Beach ridge made of sandy sediments

7

18

40°35'9.90"S; 62°17'28.80"O

Beach ridge made of sandy sediments

7

AC C Pleistocene

Geomorphology

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3

Coordinates (LatLong)

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Age

M AN U

Bahía San Blas

5

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40°35'19.40"S; 62°17'5.80"O

Beach ridge made of sandy sediments

4

20

40°35'47.35"S; 62°19'11.93"O

Beach ridge made of sandy sediments

5

21

40°36'6.17"S; 62°16'10.55"O

Beach ridge made of sandy sediments

5

22

40°40'49.99"S; 62°30'6.60"O

Profile 50 cm from the surface

2

23

40°46'24.02"S; 62°16'17.24"O

Beach ridge with layers of sandy sediments and clasts

8

24

40°46'34.56"S; 62°16'24.50"O

Fine sediment facies at the base of the profile

10

25

40°27'32.77"S; 62°47'57.66"O

32

26

40°35'30.36"S; 62°49'52.91"O

Beach ridge with sandy sediments containing clasts and rough gravel Quarry with clustered gravel

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Table 2. Description of sampled localities

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19

33

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H

X

X X

X X X X X X X X X X X

M

X X X

X X X

X

X

X

X X

X X X X

X

X X

M AN U

X X

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P

Nucula (N.) nucleus ( Linné, 1758) Ennucula grayi (d´Orbigny, 1846) Glycymeris (G.) longior ( Sowerby,1832) Mytilus (M.) edulis Linné, 1758 Brachidontes (B.) rodriguezi ( d´Orbigny, 1846) Aequipecten tehuelchus (d'Orbigny, 1842) Plicatula gibbosa Lamarck, 1801 Ostreola equestris (Say, 1834) Crassostrea rhizophorae (Guilding, 1828) Ostrea puelchana d´Orbigny 1841 Crassostrea gigas (Thunberg, 1793) Diplodonta (D.) patagonica ( d´Orbigny, 1842) Diplodonta (F.) vilardeboana ( d´Orbigny, 1846) Carditamera plata (Inhering, 1907) Trachycardium muricatum (Linné, 1758) Mactra isabelleana d´Orbigny, 1846 Mactra guidoi Signorelli & Scarabino Mulinia edulis (King & Broderip, 1831) Mesodesma mactroides (Reeve, 1854) Solen tehuelchus (Hanley, 1842) Macoma (P.) uruguayensis (Smith, 1885) Tagelus (T.) plebeius (Ligthfood,1786) Pitar (P.) rostratus (Philippi, 1844) Amiantis purpurata ( Lamarck, 1856) Corbula (C.) patagonica d´Orbigny, 1846 Corbula (C.) lyoni Pilsbry, 1897 Barnea lamellosa ( d´Orbigny, 1846)

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BIVALVIA

X

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X X X

X X X X X X

X X X

X

Table 3. Bivalves from the Quaternary (P=Pleistocene, H=Holocene and M=modern) marine deposits in the

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EP

Bahía San Blas area.

P X X X X

X X X X

X X

H

M

X

X X X

X X

X X

X

X

X

X

X

X X

X X X

X

X X X X

X X X

M AN U

Diodora (D.) patagonica ( d´Orbigny, 1841) Lucapinella henseli (Martens, 1900) Tegula (A.) patagonica (d´Orbigny, 1835) Calliostoma coppingeri ( Smith, 1880) Heleobia australis ( d´Orbigny, 1835) Bostrycapulus odites (Collin, 2005) Crepidula argentina Simone, Pastorino & Penchaszadeh, 2000 Notocochlis isabelleana (d´Orbigny, 1840) Trophon varians (d´Orbigny, 1841) Urosalpinx cala (Pilsbry, 1897) Zidona dufresni ( Donovan, 1823) Adelomelon (P.) brasiliana (Lamarck, 1811) Odontocymbiola magallanica (Gmelin, 1791) Olivella (O.) tehuelcha ( Dúclos, 1835) Olivancillaria urceus ( Röding, 1798) Olivancillaria carcellesi Klappenbach, 1965 Marginella martini Petit, 1853 Buccinanops moniliferum ( Kiener, 1834) Buccinanops cochlidium (Dilwyn, 1817) Buccinanops globulosum (Kiener, 1834) Buccinanops uruguayense (Pilsbry,1897) Parvanachis isabellei (d'Orbigny, 1839)

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GASTROPODS

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X X X X

X

Table 4. Gastropods from the Quaternary (P=Pleistocene, H=Holocene and M=modern) marine deposits of

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EP

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the San Blas bay area.

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Salinity

Life habit

Depth (m)

Substrate

Trophic Type

Distribution area

Nucula (N.) nucleus ( Linné, 1758) Ennucula grayi (d´Orbigny,1846) Glycymeris (G.) longior ( Sowerby,1832) Mytilus (M.) edulis Linné, 1758 Brachidontes (B.) rodriguezi ( d´Orbigny, 1846) Aequipecten tehuelchus (d'Orbigny, 1842) Plicatula gibbosa Lamarck, 1801 Ostreola equestris (Say, 1834) Crassostrea rhizophorae (Guilding, 1828) Ostrea puelchana d´Orbigny 1841 Crassostrea gigas (Thunberg, 1793) Diplodonta (D.) patagonica ( d´Orbigny, 1842) Diplodonta (F.) vilardeboana ( d´Orbigny, 1846) Carditamera plata (Ihering, 1907) Trachycardium muricatum (Linné, 1758) Mactra isabelleana d´Orbigny, 1846 Mactra guidoi Signorelli &Scarabino Mulinia edulis (King & Broderip, 1831) Mesodesma mactroides (Reeve, 1854) Solen tehuelchus (Hanley, 1842) Macoma (P.) uruguayensis (Smith, 1885) Tagelus (T.) plebeius (Ligthfood,1786) Pitar (P.) rostratus (Philippi, 1844) Amiantis purpurata ( Lamarck, 1856) Corbula (C.) patagonica d´Orbigny, 1846 Corbula (C.) lyoni Pilsbry, 1897 Barnea lamellosa ( d´Orbigny, 1846)

E E E P-E E E E P-E P-E P-E E E E E E P-E P-E E E E E P E E E E E

I I I Ep Ep Ep Ce Ce Ce Ce Ce I I I I I I I I I I I I I I I I

0 - 200 5 - 1850 10 - 75 0 - 50 0 - 25 10 - 120 0 - 120 0 - 80 0 - 50 0 - 70 0 - 40 36 - 102 25 - 77 17 - 70 0-11 0 - 25 0 -25 0 - 30 0 - 20 10-18 18 - 70 0 - 10 10 -100 0 - 20 15 - 90 11 - 67 15 - 150

S S S H H M H H H H H S S S S S S S S S S S S S S S H

D D Sf Sf Sf Sf Sf C C C Ff Sf Sf Sf Sf Sf Sf Sf Sf Sf D Sf Sf Sf Sf Sf Sf

23°S - 53.5°S 22.93°S - 55.5°S 10°S - 42°S 68°N - 55.5°S 34°S - 42°S 21°S - 53°S 35.3°N - 34°S‫٭‬ 37°N - 42°S 21.4°N - 35°S 22°S - 42°S Cosmopolitan 21°S - 42.58°S 21°S - 42°S 23°S - 39°S‫٭‬ 35°N - 42°S 23°S - 42°S 34°S - 42°S 52.7°S - 55.5°S ‫٭‬ 23°S - 41°S 23°S - 39°S‫٭‬ 29°S - 39°S‫٭‬ 42°N - 54°S 22°S - 38.7°S‫٭‬ 19°S - 43°S 23°S - 43°S 19°S - 43°S 34°S - 43°S

SC

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EP

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BIVALVIA

Table 5. Ecological requirements and distribution of bivalves: Ep = epifaunal, I = infaunal, Ce=Cemented; H = hard, S = soft; C= carnivorous, D = detritivorous, H= herbivore, Sf = suspension feeder; O= oligohaline (3-8 ‰), M= mesohaline (8-18 ‰), P= polyhaline (18-30 ‰), E= euhaline (> 30-35 ‰).‫ ٭‬taxa found in the studied area.

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Depth

Diodora (D.) patagonica ( d´Orbigny, 1841) Lucapinella henseli (Martens, 1900) Tegula (A.) patagonica (d´Orbigny, 1835) Calliostoma coppingeri (E. A. Smith, 1880) Heleobia australis ( d´Orbigny, 1835) Bostrycapulus odites (Collin, 2005) Crepidula argentina Simone, Pastorino &Penchaszadeh, 2000 Notocochlis isabelleana (d´Orbigny, 1840) Trophon varians (d´Orbigny, 1841) Urosalpinx cala (Pilsbry, 1897) Zidona dufresni ( Donovan, 1823) Adelomelon (P.) brasiliana (Lamarck, 1811) Odontocymbiola magallanica (Gmelin, 1791) Olivella (O.) tehuelcha ( Dúclos, 1835) Olivancillaria urceus ( Röding, 1798) Olivancillaria carcellesi Klappenbach, 1965 Marginella martini Petit, 1853 Buccinanops moniliferum ( Kiener, 1834) Buccinanops cochlidium (Dilwyn, 1817) Buccinanops globulosum (Kiener, 1834) Buccinanops uruguayense (Pilsbry,1897) Anachis isabellei (d'Orbigny, 1839) Siphonaria lessoni (Blainville, 1824)

E E E E O, P, M E E

Ep Ep Ep Ep Ep Ep Ep

0 - 15 0 - 55 0 – 57 13 - 86 0 – 60 0 – 46 30 – 50

E E E E E E E E E E E E E

I Ep Ep Ep Ep Ep Ep Ep Ep Ep Ep Ep Ep Ep Ep Ep

0 – 113 0 - 50 28 - 28 10 - 90 8 - 70 10 – 200 15 – 57 5 – 50 0 – 22 10 – 80 0 – 50 5 - 66 0–6 15 - 45 10 – 65 0

TE D

EP

E E

Substrate Trophic Type H H H S M H H

He He He He He Sf Sf

S H H S S M S S S S S S S S C H

C C C C C C C C C C C C C C C He

Distribution area

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Life habit

SC

Salinit y

M AN U

GASTROPODS

11°N - 45°S 23°S - 53°S 23°S - 54°S 30ºS-44.21ºS 24°S - 41°S 25°S - 45.8°S

38°S - 41.03°S 22.4°S - 42.58°S 32ºS-40ºS 32°S - 41°S 23°S - 42°S 23°S - 52°S 35°S - 55.2°S 23.69°S - 43°S 19°S - 42°S 23°S - 42.5°S 22.93°S - 42°S 35°N - 42°S 23°S - 42.58°S 35°S - 46°S 24°S - 42°S 30°S - 54°S 32°S - 55.22°S

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Table 6. Ecological requirements and distribution of gastropods: Ep = epifaunal, I = infaunal; H = hard, S = soft; C= carnivorous, D = detritivorous, He= herbivore, Sf = suspension feeder; O= oligohaline (3-8 ‰), M= mesohaline (8-18 ‰); P= polihaline (18-30 ‰); E= euhaline (> 30-35 ‰). *Range of distribution with the upper limit in higher latitudes.