Late Pleistocene and the Holocene on the Atlantic coast of Tierra del Fuego: molluscan evidence

Quaternary International 233 (2011) 101e112

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Faunistic changes between the Middle/Late Pleistocene and the Holocene on the Atlantic coast of Tierra del Fuego: molluscan evidence Sandra Gordillo a, b, *, Federico. I. Isla c a

Centro de Investigaciones en Ciencias de la Tierra, Consejo Nacional de Investigaciones Científicas y Técnicas (CICTERRA, CONICET), Av. Vélez Sársfield 1611, X5016GCA Córdoba, Argentina b Centro de Investigaciones Paleobiológicas (CIPAL), Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Av. Vélez Sársfield 299, X5000JJC Córdoba, Argentina c Consejo Nacional de Investigaciones Científicas y Técnicas, Instituto de Geología de Costas y Cuaternario, Universidad Nacional de Mar del Plata, c. c. 722, 7600 Mar del Plata, Argentina

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 16 June 2010

The marine terraces located on the atlantic coast of Tierra del Fuego (Argentina) were previously assigned to different sea-level highstands from the Middle Pleistocene to the Holocene. This study is focused on the analysis of the mollusc assemblages recovered from those terraces. When comparing Middle/Late Pleistocene mollusc assemblages with adjacent Holocene/modern ones, ecological changes in the fauna were recognized. The Pleistocene assemblages are less diverse and are dominated by infaunal bivalves, whereas Holocene and modern assemblages are more diverse and are dominated by epifaunal taxa, with a higher presence of gastropods. However, on a biogeographic scale, the fact that Pleistocene taxa (with the probable exception of Retrotapes sp.) are living today in the region is attributed to the location of the study sites, at the centre of the Magellan biogeographic unit, with species far away from the edges of their ranges of distribution. Regional faunistic changes and the local disappearance of Retrotapes in post-glacial benthic communities in the area are therefore interpreted on the basis of regional and local causes, most probably associated to substrate changes, and not on a global trend related to large-scale patterns. Ó 2010 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction Among the animals living on the seafloor, molluscs are usually best preserved as marine fossils. In southern South America, Quaternary molluscs are the most common remains in marine sediments and the information derived from the comparison of modern and fossil marine mollusc communities can be used to understand the nature of environmental changes and to reconstruct paleocommunities, since they provide information about mode of life, substrata, water depth, salinity and temperature. Molluscs are also useful for dating and correlating deposits. In Argentina, extensive Pleistocene and Holocene marineterrace deposits are widely distributed over 3500 km of coastline facing the Atlantic Ocean. The mollusc fauna from the Río de la Plata

* Corresponding author at: Centro de Investigaciones Paleobiológicas (CIPAL), Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Av. Vélez Sársfield 299, X5000JJC Córdoba, Argentina. E-mail addresses: [email protected] (S. Gordillo), fi[email protected] (F.I. Isla). 1040-6182/$ e see front matter Ó 2010 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2010.06.006

to Patagonia has been studied by different authors (e.g., Farinati, 1985; Aguirre, 1990, 2003; Gordillo, 1998; Aguirre and Farinati, 2000), although little attention has been given to the mollusc fauna located on the Fuegian Atlantic coast, which was mentioned briefly by Feruglio (1950), and more recently by Gordillo (2006). In Tierra del Fuego, other studies centered on Quaternary molluscs have been performed along flooded piedmont valleys: i.e. the Beagle Channel (Gordillo, 1999; Gordillo et al., 2005a,b, 2008, 2010) and the western Magellan Strait (Cárdenas and Gordillo, 2009) coasts. Consequently, more research in southern South America is needed in order to complete our knowledge of the water-mass interchanges between the Southern Atlantic and Pacific oceans. There is still some controversy over the number of marine channels that operated even in historic times (Rabassa et al., 2009). Quaternary marine deposits along the Atlantic coast of Tierra del Fuego (Fig. 1) consist of a number of raised shorelines at different elevations, which represent different transgressiveeregressive phases; previous studies have provided data which permitted chronological reconstruction of these sequences (Feruglio, 1950; Rutter et al., 1989; Codignotto and Malumián, 1981; Isla and

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Fig. 1. Location map indicating Pleistocene and Holocene localities sampled along the Atlantic Fuegian coast. 1. La Sara. 2. Laguna Arcillosa. 3. Laguna de las Vueltas. 4. Río Chico. 5. San Pablo (See site references in Table 1). Panoramic views of Middle-Late Pleistocene and Holocene deposits: A. Middle Pleistocene, near Río Chico. B. Late Pleistocene, near Río Grande. C. Holocene, San Pablo locality.

Bujalesky, 2000, 2008; Bujalesky et al., 2001). Former ocean connections at the Fueguian Archipelago, are related to the timing of melting Pleistocene glaciers (Mc Culloch and Bentley, 1998; Coronato et al., 1999) and to environmental changes that occurred during the Holocene (Gordillo et al., 2005a,b). In this study, mollusc assemblages developed along the north Atlantic coast of Tierra del Fuego were characterized and analysed in order to quantify temporal trends in local and regional diversity and to understand their relationship with environmental changes during the Quaternary. Understanding how species and communities respond to climatic changes in sub Antarctic and Antarctic regions will help us

to prepare for environments.

future

changes

in

high-latitude

marine

2. Geological setting and sea-level highstands The geology of Tierra del Fuego has long been the subject of research (see Menichetti and Tassone, 2007, 2008; Hervé et al., 2008; Menichetti et al., 2008; Olivero and Malumián, 2008, and references therein). The Pleistocene to Holocene record has also been the subject of several studies that have focused on the recent Quaternary evolution of this remote southernmost South America area (Rabassa et al., 1986, 2000, Bujalesky, 1998, 2007).

S. Gordillo, F.I. Isla / Quaternary International 233 (2011) 101e112

The Tierra del Fuego Archipelago is the south-westernmost point of the South American plate, situated in the contact area with the Antarctic plate and the Scotia microplate. The northern Atlantic coast of Tierra del Fuego lies on a more stable setting, away from the Andean fold belt, and cannot be compared with the more tectonically active southern coast of Tierra del Fuego. However, tectonic activity cannot be entirely excluded from stable continental margins like the Patagonian Atlantic coast (Schellmann and Radtke, 2003). Due to its proximity to Antarctica, during Quaternary glaciations Tierra del Fuego was covered by continental ice sheets that drove ice towards the Atlantic, the Pacific and the Southern Ocean. Glacier fluctuations seem to reflect the climatic changes of the Southern Ocean and Antarctica from the Plio-Pleistocene onwards (Rabassa et al., 2000). Interglacials are represented by sea-level highstands that affected these coasts, although the Atlantic is mainly a trailing-edge coast. As mentioned above, it has also been affected by tectonic uplift that increased the altitude difference between the different Quaternary highstands. An estimation of an uplifting trend of 8e9 cm kyr
1 has been calculated for the last transgression (Guilderson et al., 2000). Under this scenario, glacio-isostatic uplift appears to be an important reason for much of the relative sea-level changes along the Patagonian Atlantic coast during the Quaternary. However, different local and regional factors, such as changes in tide and wave energy, also appear to have contributed to the present coastal formation (Schellmann and Radtke, 2003). Regional sea-level correlations should therefore be taken with caution.

103

Fig. 2. Global isotope curve showing glacials and interglacials over the last four climatic cycles. The different fossiliferous beds considered in this study correspond to different high sea-level stands: San Pablo and Río Chico (MIS 1), La Sara (MIS 5), Laguna de las Vueltas (MIS 9) and Laguna Arcillosa (MIS 11). RSL curve adapted from Waelbroeck et al. (2002).

necessary to confirm these interpretations of different oxygen isotopic stages. 4. Material and methods 4.1. Study area

3. Age and dating Several authors have provided information concerning the ages and altitudes of these marine highstands along the Atlantic Fuegian coast. Feruglio (1950) described the mollusc taxa associated to a raised beach at Río Grande and established the first pattern (i.e., a sequence of six e I, II, III, IV, V and VI e marine terraces) for correlating glacio-eustatic fluctuations along the Patagonian coast. According to this sequence, the Río Grande raised beach corresponds to the most modern marine terrace (terrace VI). The different Quaternary outcrops located along the Atlantic Fuegian coast have been dated by different methods (radiocarbon, ESR or U-series). The most accurate method depends on the age of the dated marine deposit, which makes correlations in a common temporal framework more difficult. Radiocarbon data from the La Sara Formation reported by Codignotto and Malumián (1981) and Codignotto (1983) gave ages from 31,900 to > 43,000 yr BP. Rutter et al. (1989) provided aminostratigraphy analysis of the La Sara raised beach and obtained a relatively high ratio of aspartic acid and leucine (i.e., D/L of 0.36) in comparison with other values from modern (0.03) and Holocene (0.10) beaches, thus suggesting, that the La Sara site is Late Pleistocene in age. U/Th datings yielded a Last Interglacial age (MIS 5) for the La Sara raised beach, and older interglacial ages for the Las Vueltas and La Arcillosa outcrops (Bujalesky et al., 2001). These outcrops were assigned to oxygen isotopic stages (Fig. 2). In relation to shorelines belonging to MIS 7, Rostami et al. (2000) provided direct evidence that MIS 7 may be missing or hidden in the area. Based on the U/Th and ESR dates from reworked mollusc shells, these authors postulated that it is likely that terraces of MIS 7 were eroded during MIS 5a and this material could have been reworked and/or overtopped during the Late Pleistocene sea-level highstands. Rostami et al. (2000) related this absence to a slow rate of tectonic uplift that would have been insufficient to isolate the terrace from reoccupation during the subsequent transgressive MIS 5 when sea-level rose higher than the previous interglacial (MIS 7). Unfortunately, geochronological research is still incipient in Tierra del Fuego and more work is

The Atlantic coast of Tierra del Fuego extends 330 km in a northwest-southeast direction. Presently, the area is exposed to high-energy waves generated in the South Atlantic Ocean, intense westerly winds and a macrotidal semi-diurnal regime (Bujalesky, 1998). The region was repeatedly flooded during Quaternary highstands, with ocean processes similar to those operating today. The criteria for selection of the studied localities were based on the existence of available information on geomorphology, sedimentology and geochronology. Five sites from the northern sector were selected where altitude and ages have been precisely determined (Table 1). Middle Pleistocene highstands: Two interglacials have been noted: Laguna Arcillosa and Laguna de las Vueltas (sites 2 and 3; Fig. 1), both located close to the joining of the Chico and Avilés rivers. The Laguna Arcillosa interglacial is located at an altitude of 23 m and Laguna de las Vueltas at 19 m (both over present storm berm; Bujalesky and Isla, 2006). Late Pleistocene highstand: La Sara (site 1; Fig. 1), composed of gravel and sand and located near the Estancia La Sara at an altitude of 14 m (above the present sea level), is considered a paleobarrier and is 14 km long and 2 km wide. The La Sara formation was defined as Late Pleistocene (Codignotto and Malumián, 1981) and is probably MIS 5. This marine terrace has been recognized in the city of Río Grande and close to the Cabo Peñas, and was assigned to the Sangamonian Interglacial (Isla and Bujalesky, 2000). Other remains of this barrier have been distinguished at latitudes of 53 590 S at Estancia Viamonte (Bujalesky and Isla, 2006). Holocene highstand: The Río Chico beach-ridge plain (site 4; Fig. 1) extends at an altitude of 7 m between the Chico River and small elongated coastal lagoons and ponds. Radiocarbon dates obtained from these deposits yielded an age of 4.620  0.07 kyr BP (Isla and Bujalesky, 2000; Bujalesky et al., 2001), while at Cabo San Pablo (site 5; Fig. 1), a marine deposit 2 m above present sea level gave a radiocarbon age of 0.47 kyr (Gordillo, 1992). Similar very modern radiocarbon ages of chenier ridges in San Sebastián Bay were related to storm effects within the bay and indicated

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Table 1 A synthesis of location and chronology of the different Quaternary marine highstands along the Atlantic coast of Tierra del Fuego. Localities (Fig. 1)

1. La Sara

Lat, long

53 300 S, 68 53 340 S, 68 050 W 020 W 14 29 Gravelly sands V

Altitude m a.s.l. Sediment Correlation with Feruglio’s terraces Age 14C BP

ESR age U-series

Oxygen isotope stages Highstands

2. La Arcillosa 3. Las Vueltas

Sands IV

4. Río Chico

5. San Pablo

53 340 S, 68 020 W 25

53 350 S, 68 010 W 7

54 170 S, 66 450 W 3

Gravelly sands IV

Gravelly sands VI

Sands

þ31,900 þ42,600 þ43,000 (Codignotto and Malumián, 1981) 0.36 (Rutter et al., 1989) 82 Uka (Bujalesky et al., 2001) 5

600e400 Uka (Bujalesky et al., 2001) 11

300 Uka (Bujalesky et al., 2001) 9 1

Late Pleistocene highstand

Middle Pleistocene highstand

Middle Holocene Pleistocene highstand highstand

VI

4620  70 470  90 (Isla and Gordillo Bujalesky, (1992) 2000)

1

Holocene highstand

Based on data by authors cited in the text. Location, altitude and age of the five sites studied. m a.s.l., meters above sea level; ESR, electron spin resonance.

a sequence that extends from 5616  282 to 509  41.5 years ago (Vilas et al., 1999).

4.2. Sampling Fossil samples were taken from previously-dated localities and studied from geological and geomorphological viewpoints. Handpicked samples and bulk samples were obtained from each selected outcrop. In the bulk sample (composed of sediment and fossils), shells and their fragments were sorted in the laboratory. This sample of approximately 0.05 m3 was treated as a quantitative random sample and in order to ensure that a single individual was not counted several times, only bivalve shells with a beak and gastropod shells with a spire were considered. In bivalves, the total number of specimens can be determined by averaging right and left valves. The hand-picked sample (a non-random sample) was collected to determine species composition. Gastropods and bivalves were identified at the lowest possible taxonomic level. Fossil specimens studied here were deposited in the Quaternary mollusc collection from Tierra del Fuego (TDF-Q), at the Centro de Investigaciones Paleobiológicas (CIPAL), Universidad Nacional de Córdoba, Argentina.

4.3. Taphonomic attributes Different taphonomic attributes (i.e., articulation, fragmentation, surface alteration, bioerosion and shell coloration) were recorded for each specimen from the random samples, and were then averaged over the entire sample. These signatures were observed in the field and/or in the laboratory.

Articulation refers to the presence of both valves (left and right) with no separation. It is evaluated using the relationship between articulated and disarticulated valves and is useful for evaluating transport from the original community. Fragmentation is associated with shell breakage. In general, it takes place within high-energy environments, such as beaches and tidal channels, as a consequence of impact with other shells, rocks and waves (Parsons and Brett, 1991), although it can be influenced by ecological interactions, like shell-breaking predation or bioturbation (Zuschin et al., 2003). The degree of fragmentation was estimated following a three-grade scale: whole shell (unbroken), broken shell (broken up to 50%) and fragment (more than 50% of the entire shell is absent). Articulation and fragmentation were calculated as proportions of composition in ternary plots using the PAST program (Hammer et al., 2005). Surface alteration is generally related to abrasive agents which produce the loss of surface ornamentation and shell details (Parsons and Brett, 1991). It was evaluated as presence or absence. Bioerosion refers to the alteration of shells through the activities of organisms, usually in search of food or shelter. It may take the form of boring, rasping, etching, breakage and abrasion of the shell. It was measured as presence or absence. Shell coloration was evaluated as shells that had kept their original color and those that had lost their color. Taphonomic categories: To analyze the percentage of fragmentation/surface alteration/bioerosion over the entire sample, four categories were considered: none, when no shell exhibits the attribute considered; low, when the shells that exhibit the attribute represent up to 10% of the sample; moderate, when the percentage of shells showing the attribute represents between 10 and 50% of the sample; and high, when this percentage is more than 50% of the sample. 4.4. Taxa The taphonomic analysis was applied to the most common species of bivalves: Mytilus chilensis, Aulacomya atra, Perumytilus purpuratus, Mulinia edulis and Retrotapes sp. A. atra (Molina): This species has a large elongated shell with a single broad tooth-like fold within the umbo of the left valve, and a corresponding groove in the right valve; externally it exhibits prominent growth lines crossed by strong radial ribs. M. chilensis (Hupé in Gay): This species has a large elongated shell with terminal, pointed beaks, small tooth-like folds within the umbos and exhibits growth lines only. P. purpuratus (Lamarck): This species has a large elongated shell with rounded, almost terminal beaks and several small, tooth-like crenulations within umbo. It is sculptured with strong radial cords and its inner margin is crenulated. Retrotapes sp.: This species has a large ovate shell and a hinge with 3 strong cardinal teeth in each valve. It is sculptured with regularly spaced, concentric lamellae, which become crowded in larger shells. The living venerid Retrotapes exalbidus extends from the coasts of Chiloé to the Buenos Aires province. Von Bertalanffy growth curves from specimens collected at the Beagle Channel indicate that ages between 40 and 60 years are very common. However, population age structure suggested that these communities have periods of poor recruitment lasting 20e40 years (Lomovasky et al., 2002). M. edulis (King and Broderip). This species has a large, roundedtrigonal inflated shell and a hinge with a ligament in a deep chondrophore; its sculpture is smooth with growth lines. For a more detailed morphological description of the shells of taxa see Reid and Osorio (2000).

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4.5. Ecological and paleobiogeographical data The ecological characterization of the fossil taxa is based on their living representatives. It includes preferred substratum, mode of life (habitat), locomotion and feeding-type. This data was used to reconstruct the structure of the mollusc assemblages represented at each individual site. An analysis of diversity (Simpson index), to quantify the taxonomical diversity of each site and to compare diversities between localities, was carried out using the PAST program (Hammer et al., 2005). Results were plotted as a dendrogram. The proportion of infaunal and epifaunal taxa at each site was also calculated and plotted and the biogeographic distribution pattern of these taxa was taken from papers by Dell (1971); Marincovich (1973); Brattström and Johansen (1983); Castellanos (1992); Castellanos and Landoni (1992); Valdovinos (1999) and Reid and Osorio (2000). A compilation of previous paleontological data for each species is also provided to define their paleobiogeographical ranges more precisely. This information is based on different studies, including Feruglio (1950); Herm (1969); Rabassa et al. (1986); Gordillo et al. (1992); Valdovinos (1996); Aguirre and Farinati (2000); Pastorino (2000); Aguirre (2003); Ortlieb et al. (2003); Rivadeneira and Carmona (2008); Cárdenas and Gordillo (2009) and Gordillo et al. (2010). A model of temporal trends in regional diversity (Roy, 2001) was also taken into account, supported by simulations and empirical data exploring the role of biogeography in constraining changes in taxa composition of Quaternary assemblages. 5. Results and discussion 5.1. Faunal composition and ecological characterization The faunal composition of Quaternary marine terraces consists mostly of bivalve and gastropod mollusc shells (Fig. 3). Although gastropods (10 species) outnumber bivalves (5 species) in diversity, bivalves are more abundant (Table 2). The higher richness of gastropods and the dominance of bivalves appear to be the result of natural ecological conditions of the region’s fauna. Holocene assemblages are dominated by mytilids (Mytilus, Aulacomya), while the venerid Retrotapes sp. dominates Pleistocene assemblages. In relation to Retrotapes, our material is quite different in shape and size from the living species of R. exalbidus, and as there is currently not enough information to evaluate the existence of morphological variations associated with ontogeny (as living R. exalbidus seems to have), our material is assigned to Retrotapes for the time being. The taxa collected are able to live in the intertidal and/or subtidal zones. Most of these taxa are members of the epifauna, with only two infaunal elements (Mulinia and Retrotapes). Regarding locomotion, some taxa live bysally attached (mytilids), others are burrowers (Mulinia, Retrotapes) and all the rest are vagile forms. With respect to feeding types, all the bivalves are suspension feeders, while gastropods are herbivorous or carnivorous. When comparing faunistic similarity, two main groups were distinguished: in the “Holocene” group, the Río Chico site is related to San Pablo, while in the “Pleistocene” group, the La Sara assemblage is closer to Las Vueltas than La Arcillosa (Fig. 4). Epifaunal taxa become dominant from Pleistocene to Holocene assemblages (Fig. 5). 5.2. Taphonomic features Bivalve shells from the different sites were all disarticulated, indicating transportation before burial (Fig. 6). Shell fragmentation was moderate to high, and surface alteration varied from low to high, suggesting different energetic conditions and periods of

105

exposure (Table 3). Most of the shells did not exhibit signs of bioerosion, and a low proportion of eroded shells contained holes produced by drilling gastropods. However, as a great number of shells were broken, this feature could have been underestimated. In the La Sara site, shells exhibit the highest grade of fragmentation and abrasion, resulting in rounded edges. They were most probably in a high-energy setting and may have been repeatedly reworked by waves. In the Las Vueltas site, shell fragmentation varies from moderate to high, indicating that shells may have been transported from their original habitats. In La Arcillosa and Río Chico, shells are better preserved and most of them have no abraded shell edges, indicating short transportation and no reworking. San Pablo mollusc assemblages represent the best state of bivalve preservation in the area studied (Fig. 6). Moderate fragmentation and low abrasion suggest that these shells were transported within their original habitat. Finally, analysis of shell coloration for each locality showed three different situations: complete preservation of the original color (shells from the San Pablo site); partial color degradation (shells from the Río Chico and La Arcillosa sites); and complete lack of pigments (white shells from the La Sara and Las Vueltas sites). The timing of color decay probably varied in response to taphonomic conditions. This assumption is based on the fact that exposure to both light and oxygen strongly degrades pigments, producing decay and loss of color, while a rapid burial may retard the process of degradation (Curry, 1999). Differences between sites could therefore be partly explained by taphonomic reasons. 5.3. Biogeographic and paleobiogeographic distribution All the species found in the studied deposits (except possibly Retrotapes sp.) are extant taxa, living in the Magellan Province, which extends from 42 S (Península Valdés) on the Atlantic coast to Isla Chiloé on the Pacific coast. Eight taxa are confined to the Magellan area and another six Magellan species extend their range of distribution to the adjacent faunistic provinces: i.e., on the Pacific side three species (M. chilensis, P. purpuratus and M. edulis) reach the Peruvian province; , while three other species (A. atra, Trophon geversianus eonly as fossils- and Pachysiphonaria lessoni) exhibit a widespread range of distribution extending northwards along the Magellan and along both adjacent provinces (Table 2). As fossils, most of these taxa were also collected along the Pacific and/or the Atlantic side of the Magellan province. Quaternary deposits from Tierra del Fuego, have been recorded in the south (Beagle Channel) and west (Magellan Strait), with the exception of Buccinanops squalidum which has only been recorded along the Atlantic Fuegian coast, in keeping with its present-day distribution. From a paleobiogeographic point of view, these records constitute a preliminary approach within the northern Tierra del Fuego coastline. Based on present knowledge, the mollusc fauna studied has, on the whole, a temperate-cool character; however, there are no significant variations in the geographic ranges of distribution of these taxa. When considering the conceptual and empirical model developed by Roy (2001), which predicts that only localities where most species are near the edges of biogeographic provinces would show large fluctuations in species composition (and richness) in response to changes in the ambient climatic conditions, it appears more plausible that the localities studied here, which are located at the center of the biogeographic Magellan unit, with species far away from the edges of their ranges, would show relatively stable diversity patterns, even during episodes of climatic change. Thus, changes in faunal composition may be more related to changes in the substratum, as a result of regional or local environmental changes, and not to a global trend related to large-scale patterns.

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Fig. 3. General faunal composition and preservation of the different Quaternary assemblages from the Fuegian Atlantic coast. 1, 2, 3, 4, 6. Mytilus chilensis Hupé. 1. La Arcillosa, Pleistocene, CEGH-UNC 23073 (L ¼ 32); 2. San Pablo, Holocene, CEGH-UNC 23062 (L ¼ 35); 3. Río Chico, Holocene, CEGH-UNC 23085 (L ¼ 32); 4. San Pablo, Holocene, CEGH-UNC 23056 (L ¼ 14, broken specimen); 6. San Pablo, Holocene, CEGH-UNC 23061 (L ¼ 22, broken specimen). 5, 9, 10. Perumytilus purpuratus (Lamarck). 5. San Pablo, Holocene, CEGH-UNC 23057 (L ¼ 11); 9. San Pablo, Holocene, CEGH-UNC 23060 (L ¼ 22, broken specimen); 10. San Pablo, Holocene, CEGH-UNC 23058 (L ¼ 13). 7, 8. Aulacomya atra (Molina). 7. Río Chico, Holocene, CEGH-UNC 23080 (L ¼ 50, broken specimen); 8. San Pablo, Holocene, CEGH-UNC 23059 (L ¼ 11). 11, 12, 13, 14. Retrotapes sp. 11. La Sara, Pleistocene, CEGH-UNC 22118 (L ¼ 78); 12. Las Vueltas, Pleistocene, CEGH-UNC 23079 (L ¼ 33, fragment with hinge); 13. La Sara, Pleistocene, CEGH-UNC 23077 (L ¼ 26, fragment with hinge); 14. La Arcillosa, Pleistocene, CEGH-UNC 23071 (L ¼ 32, broken specimen). 15, 16, 17, 18. Mulinia edulis (King and Broderip). 15. Río Chico, Holocene, CEGH-UNC 23084 (H ¼ 28, broken specimen); 16. La Arcillosa, Pleistocene, CEGH-UNC 23074 (H ¼ 23); 17. Río Chico, Holocene, CEGH-UNC 23083 (H ¼ 20, fragment with hinge); 18. La Arcillosa, Pleistocene, CEGH-UNC 23072 (H ¼ 24, fragment with hinge); 19. Nacella (P.) magellanica (Gmelin), Río Chico, Holocene, CEGH-UNC 23082 (W ¼ 36); 20, 21. Nacella deaurata (Gmelin). 20. San Pablo, Holocene, CEGH-UNC 23461 (W ¼ 30); 21. San Pablo, Holocene, CEGH-UNC 23064 (W ¼ 19); 22. Pareuthria plumbea (Philippi), San Pablo, Holocene, CEGH-UNC 23067 (L ¼ 20); 23. Trophon sp., Río Chico, Holocene, CEGH-UNC 23081 (L ¼ 45); 24. Xymenopsis muriciformis (King and Broderip), San Pablo, Holocene, CEGH-UNC 23068 (L ¼ 14); 25. Trophon sp., San Pablo, Holocene, CEGH-UNC 23065 (L ¼ 31); 26. Trophon sp., La Arcillosa, Pleistocene, CEGH-UNC 23075 (L ¼ 39); 27. Acanthina monodon (Solander), San Pablo, Holocene, CEGH-UNC 23066 (L ¼ 28); 28. Buccinanops squalidum (King and Broderip), La Arcillosa, Pleistocene, CEGH-UNC 23076 (L ¼ 30, broken specimen); 29. Volutidae, La Sara, Pleistocene, CEGH-UNC 23078 (L ¼ 59, eroded specimen); 30. Pachysiphonaria lessoni (Blainville) (W ¼ 8), San Pablo, Holocene, CEGH-UNC 23069; 31. Cirripedia, San Pablo, Holocene, CEGH-UNC 23070 (W ¼ 11). Dimensions (in mm). L ¼ length; W ¼ maximum width.

5.4. Paleoecological reconstruction Although each mollusc concentration can be time-averaged (Kidwell et al., 1986), the most constant and dominant taxa do represent valid indicators of past marine conditions. Two different

mollusc assemblages can be distinguished in the fossil record. These represent typical shallow benthic communities which developed within the Atlantic Fuegian marine realm during the Quaternary: The Retrotapes dominated assemblage (Fig. 7). This Pleistocene infralittoral assemblage is dominated by one main

Table 2 List of taxa recorded, main paleoecological features and paleobiogeographical patterns of distribution. Taxa/Species

Sampling sites (1e5)

Autoecology and habitat

1. La 2. La 3. Las 4. Río Sara Arcillosa Vueltas Chico

Gastropoda Nacella (P.) magellanica (Gmelin, 1791) Nacella (P.) deaurata (Gmelin, 1791) Pareuthria plumbea (Philippi, 1884) Buccinanops squalidum (King and Brod., 1832) Trophon geversianus (Pallas, 1769) Trophon sp. Xymenopsis muriciformis (King and Brod., 1832) Acanthina monodon (Solander, 1786) Odontocymbiola magellanica B (Gmelin, 1791) Pachysiphonaria lessoni (Blainville, 1824)

C

C

B

-

-

5. San Pablo

Substrate Life Locomotion Feeding CHILE habit type (Pacific Ocean)

Magellan Strait, Tierra del Fuego

Beagle Channel, Tierra del Fuego

Atlantic Ocean, Tierra del Fuego

PATAGONIA (Atlantic Ocean)

Geographical distribution

-

C

H

EP

Bys

SF

2, 9,11,12

12

4,13

1

1, 6, 7, 8

P, M, A

-

B

H H

EP EP

Bys Bys

SF SF

2, 5 2

12 12

4, 13 3, 4

1

1 1, 6, 7, 8

P, M P, M

S

IN

Bur

SF

2,11,12

12

4

1, 6, 7, 8

P, M

S

IN

Bur

SF

5,10

12

3, 4

1, 6, 7, 8

M

H

EP

Vag/Pas

BR

2

12

3, 4

1, 6, 7, 8

M

B

H

EP

Vag/Pas

BR

e

12

3, 4

1, 6, 7, 8

M

-

H, S

EP

Vag/Act

CAR

5

12

3, 4

1, 6, 7, 8

M

S

EP

Vag/Act

CAR

e

e

e

e

M

C

H, S

EP

Vag/Act

CAR

10

12

3, 4, 13

6, 7, 8

P, M, A

C

H, S H, S

EP EP

Vag/Act Vag/Act

CAR CAR

e

12

3, 4, 13

7

M

B

H

EP

Vag/Act

CAR

2

12

4

1, 6, 7

M

S

EP

Vag/Act

CAR

e

e

e

1, 6, 7

M

H

EP

Vag/Pas

BR

9

12

4

1, 6, 7, 8

P, M, A

C

B

B B

B

B

B B

1

1

1

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Bivalvia Aulacomya atra (Molina, 1782) Mytilus chilensis Hupé, 1854 Perumytilus purpuratus (Lamarck, 1819) Mulinia edulis (King and Brod., 1832) Retrotapes sp.* -

Other Quaternary fossil reports in South America

Abundance of sampling sites: black square (-): very common (10 specimens); black circle (C: common (between 4 and 9); white circle (B): scarce (3). Autoecology: H: hard; S: soft; EP: epifaunal; IN: infaunal; Bys: bysally attached; Bur: burrower; Vag: vagrant; Pas: passive locomotion; Act: active locomotion; SF: suspension feeder; BR: browser; CAR: carnivorous. Other Quaternary fossil reports: 1. Feruglio (1950); 2. Herm (1969); 3. Rabassa et al. (1986); 4. Gordillo et al. (1992); 5. Valdovinos (1996); 6. Aguirre and Farinati (2000); 7. Pastorino (2000); 8. Aguirre (2003); 9. Ortlieb et al. (2003); 10. Nielsen and Valdovinos (2008); 11. Rivadeneira and Carmona (2008); 12. Cárdenas and Gordillo (2009); 13. Gordillo et al. (2010). Geographical distribution: P. Peruvian Province, M. Magellanic Province, A. Argentinian Province. *Ecological features, geographical distribution and fossil records of Retrotapes exalbidus (Dillwyn, 1817).

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Fig. 4. Dendrogram of the localities on the basis of their taxa composition (Simpson index) showing two major associations: a Holocene group and a Pleistocene group where La Sara appears closely united with Las Vueltas.

infaunal element: the venerid Retrotapes, with secondary taxa including other suspension feeders (Mulinia and Mytilus) and different predatory gastropods (Trophon, Buccinanops, Odontocymbiola). The venerid R. exalbidus (also called Eurhomalea exalbida) is a typical Magellan species distributed along the Pacific coast from the Isla Chiloé (42 S) to the Beagle Channel (54 500 S) and from there along the Atlantic coast up to Península Valdés (42 S) and, in deeper waters up to southern Brazil (Carcelles, 1944; Rios, 1994). This infaunal bivalve is a shallow burrowing species which is able to live in soft substrates composed of fine to coarse sediment (i.e., coarse sand, gravel and pebbles). In Tierra del Fuego, R. exalbidus was found living at variable depths, mostly in the infralittoral (3e50 m depth), although it has also been found in the intertidal zone. It lives in the Beagle Channel and the Magellan Strait at temperatures of between 4.5 and 8.7 C (Urban and Tesch, 1996; Lomovasky et al., 2002; Ríos et al., 2003). However, there is no available data about the existence of a Retrotapes population living in shallow waters on the Atlantic coast of the Fuegian Archipelago. Considering the ecological requirements of these taxa and the available paleoenvironmental data, the original community might have developed associated to coarse sand bottoms in the infralittoral zone. When comparing this assemblage to the Holocene and

Fig. 5. Graph showing the percentage of infaunal and epifaunal taxa in each site. An increase of epifaunal taxa is noted from older to younger deposits.

modern assemblages, some differences are noticed: the Pleistocene assemblage exhibits lower diversity and is dominated by infaunal bivalves, while the Holocene assemblage is more diverse, dominated by epifaunal taxa, with higher presence of gastropods. Similar conclusions were reached from comparisons of Late Pleistocene and Holocene mollusc assemblages from Golfo San Jorge, Patagonia (Aguirre, 2003). The Mytilus intermixed assemblage (Fig. 8): The Holocene assemblage is more diverse, and dominated by sessile suspensionfeeder epifauna (mytilids) intermixed with some infaunal burrower elements (Mulinia), thus suggesting areas with soft substrates suitable for burrower clams. M. chilensis lives bysally attached to hard bottoms, forming clusters associated with other species (e.g., P. purpuratus, A. atra). Other epifaunal elements are different predatory gastropods (T. geversianus, Xymenopsis muriciformis, Acanthina monodon), the buccinid Pareuthria plumbea and a variety of limpets (e.g., Nacella spp., Pachysiphonaria). These taxa are typical of tidal flats and areas more exposed to highly unstable conditions with longer episodes of exposure. In southern Chile, M. chilensis and M. edulis exhibit a high degree of physiological plasticity (Velasco and Navarro, 2003). Reid and Osorio (2000) also mentioned a group of euryhaline taxa (Nacella magellanica, P. lessoni, M. chilensis, P. purpuratus, A. atra, M. edulis) that tolerate sharp salinity gradients along a fjord system in southern Chile. Thus, organisms of this assemblage tolerate large fluctuations of suspended particulate matter, mainly due to the action of winds and tides. 5.5. Paleoenvironmental comments For the Quaternary, different shallow benthic paleocommunities were recognized associated to different coastlines of Atlantic Tierra del Fuego. When comparing the Pleistocene deposits with those from the Holocene, or with the fauna living today in the region, a different faunal composition is evident. The Pleistocene association is characterized by infaunal elements typical of soft substrates, dominated by the venerid Retrotapes. During the Holocene, Retrotapes was regionally replaced by other taxa, mostly epifaunal, suggesting local environmental changes. Regarding living forms in this region, 25 different mollusc species (17 gastropods and 8 bivalves) were recognized from active beaches (i.e., tidal and storm berms) along the Atlantic Fuegian coast between Punta María (53 560 S) and Río Irigoyen (54 310 S; Isla et al., 2005), but Retrotapes shells were not found. In San Sebastián bay, northern Tierra del Fuego, A. atra dominates the inner shelf while M. edulis lives within the bay (Lopez Gappa and Sueiro, 2007). Present beach assemblages are dominated by rocky-bottom molluscs, suggesting that the species richness for each sampled site is locally controlled by energy dynamics spanning from dissipative to reflective beaches. Soft-bottom communities are clearly dominating the tidal flats of San Sebastián bay and are composed of Odontocymbiola magellanica, Adelomelon ancilla, Trophon geversianus, Natica atrocyanea, Buccinanops sp., Tegula sp., A. atra, Mytilus (as M. platensis) and Darina solenoides (Vilas et al., 1999). The exclusion of Retrotapes in this part of the Fuegian coast during the post-glacial might be related to physiological limitations of this species in its ability to tolerate broad fluctuations in concentration and quality of food available due primarily to events of resuspension of bottom sediment by the action of winds and tides. In northern Chile, Late Pleistocene molluscs (120 ka) were compared to Holocene and living ones in the area of Antofagasta, and ecological variations of the fauna were noticed. These changes were associated to local environmental conditions affecting coastal water masses and not to any significant climatic change (Ortlieb et al., 1994). North of Antofagasta, in Caleta Patillos, Rivadeneira

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Fig. 6. Ternary plots comparing articulation and fragmentation of different localities and showing variation among species used in this study. See explanations of taphonomic features in the text.

and Carmona (2008) also found differences in the species composition and attributed these changes to a recent alteration of the structure of local macrobenthic assemblages. In addition, in La Rinconada, north of Antofagasta, a warmer “thermally anomalous molluscan assemblage” was assigned for MIS 11 (Ortlieb et al., 1996), although this assemblage can be also explained by former paleolagoons which isolated these Panamic mollusc assemblages from the present cold waters of northern Chile and Peru.

It was also observed that the species richness of the Holocene Fuegian fauna considered in this study is consistently greater in diversity than that from the Pleistocene, which is characterized by very few species (3 gastropods and 3 bivalves), and large Retrotapes shells. Similar conclusions were reached by Aguirre (2003), who mentioned very low diversity and the large shells of the mactrid M. edulis in Late Pleistocene deposits from the Golfo San Jorge, further north in Patagonia.

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Table 3 Taphonomic attributes of the shells examined at the different localities. Taphonomic attributes

Pleistocene La Sara

La Arcillosa

Articulation Fragmentation Surface alteration Bioerosion Shell color

Absent High High

Absent High Moderate

Low Colorless (white)

Holocene Las Vueltas Río Chico

Absent High Moderate to high Low Low Discoloration Colorless (white)

Absent High Moderate

San Pablo Absent Moderate Low

None None Discoloration Original

With regard to diversity, it is important to mention that in Tierra del Fuego there is no latitudinal gradient as in Patagonia and that in the Beagle Channel, located in the south of the Tierra del Fuego Island, the existing high diversity is due to the habitat heterogeneity. Nevertheless, in this channel mollusc diversity during the Pleistocene is also lower than during the Holocene (Gordillo et al., 2005a,b, 2010). Regarding Retrotapes specimens sampled from coastal sequences, Gordillo (2006) stated that Pleistocene Retrotapes shells are thicker and larger than both Holocene and living specimens. The author explained these morphological differences by productivity conditions and/or changes in calcium concentrations associated with fully marine environments. Thus, the persistence of Retrotapes during the Pleistocene (Middle to Late Pleistocene highstands) reflects its ability to adapt during cold intervals (glaciations) between highstands. The spreading of other taxa (e.g., mytilids and patellids) and the restrictions in the distribution of Retrotapes during the Holocene seem to be more related to changes in environmental conditions (affecting the substrates) than to changes in sea temperature, although warmer conditions at the age of deposition are not excluded. Considering both assemblages, it is also plausible that since Pleistocene transgressions represent higher sea levels, the ancient bays were deeper with a dominance of suspended-feeder specimens in a rather stable soft bottom. As the Holocene sea-level fluctuation affected only a few meters, their deposits are dominated

Fig. 8. Reconstruction of a typical Holocene paleocommunity from the northeastern sector of Tierra del Fuego. 1. Mytilus chilensis; 2. Perumytilus purpuratus; 3. Aulacomya atra; 4. Mulinia edulis; 5. Trophon geversianus; 6. Xymenopsis muriciformis; 7. Pareuthria plumbea.

by gravel beaches and shallow bays. In this sense, present benthic communities are dominated by epifaunal specimens with suspended feeders tolerant to high-sediment concentrations. 6. Concluding remarks 1. Among all taphonomic features, fragmentation and surface alteration are the best indicatives of the energy of depositional environment. Four mollusc assemblages located to the north reflect high-energy environments, with the exception of the locality further south (San Pablo) which reflects lower energy. 2. Pleistocene coastal molluscs are characterized by infaunal assemblages dominated by the venerid Retrotapes sp. 3. Holocene assemblages are dominated by epifaunal communities. 4. Retrotapes remains today in the stable cold waters of the Magellan and Beagle channels. It is, therefore, the key species to understanding Fuegian connections which occurred during the Pleistocene between the Pacific and Atlantic oceans. 5. Substrate changes explain the replacement of one fauna by another. Acknowledgments

Fig. 7. Reconstruction of a typical Pleistocene paleocommunity from the northeastern coast of Tierra del Fuego. 1. Retrotapes sp.; 2. Odontocymbiola magellanica; 3. Buccinanops squalidum.

This study is a contribution to the projects PEI 6131 (National Research Council, CONICET) and ANPCyT ePICT 06-00468 (National Agency for the Promotion of Science and Technology). Many thanks to C. Ferrer (Estación Astronómica Río Grande, Universidad Nacional de La Plata) who provided logistic help, to G. Alsina who helped during the field work, and to C. E. Gómez for the illustrations. G. Bujalesky (CADIC, CONICET) and C. De Francesco (Universidad Nacional de Mar del Plata) provided information about the Quaternary geology of the Tierra del Fuego coastline and present-

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day mollusc communities, respectively. Thanks are also extended to M. Kowalewski (Virginia Polytechnic Institute and State University), to S. Nielsen (Christian-Albrechts-Universität zu Kiel), and to M. Rivadeneira (Universidad Católica del Norte) for very valuable suggestions on an early draft of this manuscript. C. Waelbroeck (Laboratoire des Sciences du Climat et de l’Environnement, CNRS) provided the RSL curve figure. References Aguirre, M.L., 1990. Holocene macrobenthic molluscan associations from northeastern Buenos Aires Province, Argentina. Quaternary of South America and Antarctica Peninsula 7, 161e195. Aguirre, M.L., 2003. Late Pleistocene and Holocene palaeonvironments in Golfo San Jorge, Patagonia: molluscan evidence. Marine Geology 194, 3e30. Aguirre, M., Farinati, E.A., 2000. Moluscos del Cuaternario marino de la Argentina. Boletín de la Academia Nacional de Ciencias 64, 235e333. Brattström, B., Johansen, A., 1983. 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