versión On-line ISSN 0718-7106
AndGeo v.36 n.1 Santiago ene. 2009
Andean Geology 36 (1): 81-93. January, 2009
formerly Revista Geológica de Chile
Paleoenvironmental interpretation of late Quaternary molluscan assemblages from southern South América: A taphonomic comparison between the Strait of Magellan and the Beagle Channel
Interpretación paleoambiental de ensambles de moluscos del Cuaternario tardío del sur de América del Sur: una comparación tafonómica entre el Estrecho de Magallanes y el Canal Beagle
Javiera Cárdenas1,2, Sandra Gordillo3,4
1 Fundación Centro de Estudios del Cuaternario (CEQUA), Av. Bulnes 01855, Punta Arenas, Chile.
2 Centro de Ciencias Ambientales EULA-Chile, Universidad de Concepción, Casilla 160-C, Barrio Universitario s/n, Concepción, Chile. firstname.lastname@example.org
3 Centro de Investigaciones en Ciencias de la Tierra, Consejo Nacional de Investigaciones Científicas y Técnicas (CICTERRA, CONICET),Av. Vélez Sársfield 1611, X5016GCA, 5000, Córdoba, Argentina.
4 Centro de Investigaciones Paleobiológicas (CIPAL), Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Av. Vélez Sársfield 299, 5000, Córdoba, Argentina. email@example.com
ABSTRACT. This study analyzes the Holocene molluscan assemblage in raised marine terraces along the coasts of the Strait of Magellan and the Beagle Channel. A total of 33 gastropods and 12 bivalves were identified. They constitute the first mollusk listing for deposits in the Strait of Magellan. Taphonomic analysis suggests a high energy environment with epifaunal elements in the Strait of Magellan and a low energy environment with the predominance of infaunal elements in the Beagle Channel. All the collected species are living taxa suggesting that postglacial climatic variations were not enough to alter the composition of the communities since at least 6,000 yr BP in the southern Chile-Argentina región.
Keywords: Late Quaternary, Mollusks, Paleoecology Taphonomy Strait of Magellan, Beagle Channel, Chile, Argentina.
RESUMEN. Este trabajo analiza los ensambles de moluscos del Holoceno de las terrazas marinas presentes a lo largo de las costas del Estrecho de Magallanes y del Canal Beagle. Un total de 33 especies de gastrópodos y 12 de bivalvos fueron identificados. Ellos constituyen un primer listado de moluscos para los depósitos del Estrecho de Magallanes. El análisis tafonómico sugiere un ambiente de alta energía con elementos epifaunales en el Estrecho de Magallanes y un ambiente de baja energía con el predominio de elementos infaunales para el Canal Beagle. Todas las especies colectadas son taxa vivientes, lo que sugiere que las variaciones climáticas postglaciales no fueron suficientes para alterar la composición de las comunidades desde al menos 6.000 años AP en el extremo austral de Chile-Argentina.
Palabras claves: Cuaternario tardío, Moluscos, Paleoecología, Tafionomía, Estrecho de Magallanes, Canal Beagle, Chile, Argentina.
The southern tip of South América, Tierra del Fuego, experienced the combined effects of glaciations, changes in the sea level and interrupted connectivity between the Atlantic and the Pacific Ocean. Giventhat this área lies betweenboth oceans, it is a critical región for the evaluation of paleoenvironmental changes and the interpretation of faunal distributions inthe Magellan Región (Gordillo et al, 2008). Previous research in the Strait of Magellan and the Beagle Channel revealed that marine events during the Pleistocene and Holocene are represented by marine terraces with altitudinal differences resulting from glacio-isostatic effects and the neotectonic uplift which occurred after the Last Glacial Máximum (LGM) (Brambati et al, 1998;Rabassa et al., 2000).
Quaternary marine deposits containing mainly mollusks are extensively distributed along the coasts of the Strait of Magellan and the Beagle Channel (Rabassaeí al, 1986; Gordillo et al., 1992; Brambati et al, 1998). Previous studies along the southern Tierra del Fuego coastline show that mollusks are a key to reconstruct Holocene paleocom-munities (Gordillo, 1999) andto evalúate changes in faunal composition after the LGM (Gordillo et al, 2005). Interspecies variations existing in the fossil record are the result of a combination of life habits, intrinsic shell properties and habitat. In this way, taphonomic analyses may constitute a powerful tool for paleoenvironmental analyses of Quaternary marine deposits (Brandt, 1989; Nielsen and Funder, 2003).
The objective of this study is to compare the Holocene molluscan assemblages located along the Strait of Magellan and the Beagle Channel coastlines by means of taxonomic, paleoecologic and taphonomic analyses.
2. Materials and Methods
2.1. Study área
Strait of Magellan. The Strait of Magellan separates Patagonia from Tierra del Fuego. It was repeatedly occupied by outlet glaciers of an expanded southern Andean ice cap during successive Pleistocene glaciations (Porter et al, 1992; Kilian et al, 2007). Today, the strait is dominated by an indented rocky shoreline and characterized by semi-diurnal and heterogeneous tides with mean amplitudes ranging from 1.2 m on the west coast to 9 m in the Atlantic inlet of the east coast (Andrade, 1991).
Brambati et al. (1998) recognized four orders of marine terraces: I, II, III and IV order; with their age estimations based on radiometric dating of fossil macrofauna. The selected localities for paleontológica! samples correspond to five sites previously dated by Brambati et al. (1998): Bahía Porvenir (BP), Puerto del Hambre (PH), Punta Palo (PP), Punta Zeger (PZ) and Seno Otway (SO) (Table 1). Beagle Channel. The Beagle Channel is located in the seismically active tectonic área of the Fuegian Andes and was covered during the Last Glacial Máximum. This channel is a glacial Andean valley, occupied by the sea since 8,000 yr BP (Rabassa et al, 1986). It extends for 180 km long in a W-E trend linking the Atlantic and Pacific oceans along the southern coast of Tierra del Fuego. The northern Beagle Channel is characterized by a Holocene terrace system withat least 3 orders of terraces at 10-8 m, 6-4 mand 3-1.5 m (Gordillo etal, 1992). Today, the Beagle Channel, is dominated by an indented, rocky shoreline, with pocket gravel beaches. Tides are semi-diurnal with mean amplitudes of one meter (e.g., 1.1 m at Ushuaia) (Gordillo et al, 1992). The selected study localities for paleontológica! samples correspondió sites previously dated (Table 1): Alakush (AK), Bahía Brown (BB, two levéis), Harberton (HA), Isla Gable (IG), Lago Roca (LR) and Ushuaia (US).
FIG. 1. Map of the localities sampled and stratigraphic profiles from Strait of Magellan and Beagle Channel. BP: Bahía Porvenir; PH: Puerto del Hambre; PP: Punta Palo; PZ: Punta Zeger; SO: Seno Otway; AK: Alakush; BB: Bahía Brown; HA: Harberton; IG: Isla Gable; LR: Lago Roca; US: Ushuaia.
2.2. Material studied
2.2.1. Taxa composition andpaleoecology
Fossil mollusks were collected from selected terraces in the Strait of Magellan and the Beagle Channel. Three samples of 10 dm3 bulk of sediment andbiologic material were obtained from eachterra-ce; samples were sieved in the field using 1 mm and 0.05 mm sieves to reduce volume. All specimens were sorted, counted and identified to the species level inthe laboratory. Taxonomic and paleoecologic aspects of the mollusks were studied and examined. The autoecological characterization of the fossil taxa and their patterns of distribution (Le., substratum, depth and mode of life, nature of locomotion and feeding) were derived from comparison with the ecology of their current living counterparts. The identification and taxonomic status are based on Soot-Ryen (1959), Castellanos and Landoni (1988, 1993), Linse (1999), Valdovinos (1999), Reid and Osorio (2000), Osorio and Reid (2004). Specimens were deposited at the Centro de Investigaciones Paleobiológicas (CIPAL), Universidad Nacional de Córdoba, Argentina.
2.2.2. Taphonomic analyses
Taphons. Five filter feeder bivalve species were chosen for taphonomic analysis: Mytilus edulis chilensis (Hupé, 1854), Perumytilus purpuratus (Lamarck, 1819), Aulacomya atra (Molina, 1782), Tawera gayi (Hupé, 1854) and Retrotapes exalbidus (Dillwyn, 1817). This selection was based on the high abundance of these taxa, their ecological requirements and the distributional ranges that have been compiled and are common to both regions, thereby facilitating comparisons (Table 2).
Taphonomic attributes. Each specimen was characterized based on the main taphonomic features to reconstruct paleoenvironment conditions.
Type ofvalve (right, left): A significant concentration of one valve type indicates uneven hydrodynamic properties such as beaches or áreas with tidal influence (Parsons andBrett, 1991).
Disarticulation of bivalves: Articulated shells are associated with rapid burial or transpon capacity (Parsons and Brett, 1991).
Shell fragmentation: Breakage of skeletons is usually related to exposure to high energy environments resulting from wave action, tides or winds (Zuschin and Stanton, 2001).
Abrasión: The abrasión degree is associated to environmental energy, time of exposure andparticle size of abrasive agents (Parson and Brett, 1991).
Biotic interactions: This refers to different interactions with other organisms that remain in the shell, such as encrusted shells, borings, tunnels and drill holes made by predatory gastropods (Farinati et al, 2006).
Twelve marines terraces were considered for taphonomic analyses BPIII, PHIII, PPIII, PPIV, PZII, PZIII, AK, BBu, HA, IG, LR and US. Seno Otway (SOIII) and Bahía Brown (BB1) show a low species richness and there is no record of the bivalves selected for taphonomic analyses. Therefore these sites are considered only for analyses of composition and species richness (Table 3).
3.1. Taxa composition and paleoecology
A total of 2,727 specimens of mollusks were collected. The whole faunal content is composed of 45 mollusk species, 12 species of bivalves and 33 gastropods (Table 2).
The Magellan Strait has the highest species richness with a total of 43 species, while the deposits of the Beagle Channel have 22 species. Nineteenof the species are present inbotháreas: Fissurella radiosa, Nacella deurata, Nacella magellanica, Margarella violácea, Crepipatella dilatata, Calyptrea pileolus, Trophon geversianus, Xymenopsis muriciformis, Acanthina monodon, Pareuthria plúmbea, Siphonaria lessoni, Aulacomya atra, Mytilus edulis chilensis, Perumytilus purpuratus, Mulinia edulis, Prothotaca antigua, Retrotapes exalbidus, Tawera gayi and Hiatella solida (Table 2, Figure 2A). The sites with the greatest diversity are PHIII and PZIII, with 29 and 23 species respectively in the Strait of Magellan, and in the Beagle Channel the greater richness occurs inLR, with 16 species and HA with 14 (Figure 2B).
Of the 2,133 specimens collected inthe Strait of Magellan, 70% are gastropods; while in the Beagle Channel, 68% of the 594 specimens are bivalves. Organisms from the intertidal and subtidal zones are dominant in both áreas. Hard substrate and epybissate species dominate in the Strait of Magellan, while the Beagle Channel is dominated by species showing preference for soft substrate and an infaunal mode of life (Table 2). Many gastropods are phytophagous (60%), followed by carnivores (36%) with a minor proportion of filter feeders whereas all bivalves studied are filter feeders (Table 2).
3.2 Taphonomic analysis
Strait of Magellan terraces. Marine terraces located inthis área are mainly composed of epifaunal elements, characterized by poorly preserved shell remains. There is a difference inthe left/right proportion of valves in all sites, which suggests transport.
All mytilids (A. atra, P. purpuratus and M. edulis chilensis) analyzed were disarticulated, with highly fragmented and abraded valves (Table 3). Most venerids (T. gayi and R. exalbidus) were disarticulated, with modérate fragmentation and abrasión; the exceptions are PZII, PZIII terraces andBP site, where a few articulated specimens were collected. These infaunal species show traces of bioerosion on the valve surface, suchasborings attributedto gastropod predators like Naticidae and Muricidae. Beagle Channel terraces. Marine terraces located in this channel contain mainly infaunal bivalves. Most of these species are disarticulated and there is a modérate degree of fragmentation and abrasión
throughout the different locations. However, few articulated specimens of T. gayi collected on the AK, BB and HA terraces exhibit good preservation, modératefragmentationandmodérate orno abrasión in the valves. Tawera gayi and R. exalbidus show different levéis of bioerosion (high, modérate and scarce). Although there are differences in the type of valves (left/right) among sites, deposits of AK, HA, LR and US have at least one species with an equal number of right and left valves. In the LR, all species have the same number of opposite valves. Inaddition, the attributes of fragmentation, abrasión and biotic interaction indicate good preservation and minimal transport.
4.1. Taxa composition and paleoecology
Of a total of 45 species cited in this work, 42 are in the deposits of the Strait of Magellan and only 22 in the Beagle Channel, which implied greater richness in the Strait of Magellan. However, pre-vious studies of Gordillo (1999) and Gordillo et al. (1992, 2005) on deposits which also include other sites along the Beagle Channel indicate a total of 79 species for this channel, with Gastropoda (47 species) more diverse than Bivalvia (32 species). The species richness reported in these previous studies challenges a conclusión of greater diversity in the Strait of Magellan. Worksby Portería et al. (1984)and Di Gerónimo and Rosso (1996) only mention some Holocene paleontological mollusks and therefore provide a preliminary list for the Strait of Magellan deposits. Interms of relative abundance, the bivalves are the most representative, agreeing with the works mentioned above.
Differences in composition and paleoecology between the Strait of Magellan and the Beagle Channel are closely related to environmental factors such as substrate type, salinity and temperature (Fürsich and Flessa, 1987). Thus, changes in these factors may produce changes in the marine biota.
Global events, such as the climatic optimum, caused paleoenvironmental and geomorphologic changes in littoral zones that led to changes in as-semblages of mollusks (Lutaenko, 1993; Camus, 2001). For instance, for the Argentinian coastlines, Aguirre et al. (2005, 2006) described the shift of warm water mollusks to higher latitudes on Bonae-rensian coasts (south Atlantic) during the climatic optimum linked with a rise in the oceanic tempera-ture and salinity. The effect of this period is still an open debate for southern Patagonia, although changes in sea level in Tierra del Fuego (Porter et al, 1984; Rabassaeía/., 1986), as well as changes in moisture and temperature (Pendall et al, 2001; Kilian et al, 2007) have been documented. Results suggestthatthere are no changes inthe species composition since 6,000 years BP, which agrees with a previous work of Gordillo (1999) of deposits in the Beagle Channel. Likewise, all species analyzed in this work show a distribution that agrees with current references (Linse, 1999; Valdovinos, 1999; Osorio and Reid, 2004).
Although the diversity and stability of the communities have a cióse relationship to climatic variability (Walther et al, 2002), many taxa can tolérate small climate fluctuations or experience variations at the population level with morphome-tric and body size changes and relative abundance, thus showing an adaptation to different environ-ments (Stahl, 1996; Roy etal, 2001). For instance, Gordillo (1992) found morphometric differences between modern and fossil specimens of Tawera gayi from the Beagle Channel. Likewise, size and shape ofHiatella solida shells are potentially related to many associated parameters including the pri-mary influences of the substratum and temperature (Gordillo, 1995).
4.2. Taphonomic aspects
Environmental energy plays an important role in determining fossil preservation For instance, fragüe shells may remain intact in quiet waters for a long time prior to burial, and the damage they show is often of a nonmechanical nature (e.g., corrosión, bioerosion). High fragmentation and abrasión tends to occur in zones with high sedimentation rates indicative of high energy environments (Brett and Baird, 1986).
According to taphonomic analyses, two types of environment exist on the coasts of Tierra del Fuego, one of high energy inthe Strait of Magellan terraces and the other of low to modérate energy in the Beagle Channel. In the Strait of Magellan, the epifauna shows high fragmentation and abrasión, thus imply ing that they were subj ect to long exposu-re on the sea bottom before burial, and was affected by biotic and abiotic factors. It is also possible that these taxa experienced shell transpon in abrasive sediment due to currents in a high-energy setting such as a foreshore environment, and/or múltiple reworking episodes (Speyer and Brett, 1991). Inthe Beagle Channel, fossil assemblages have modérate fragmentation and abrasión, indicating a low to modérate energy environment with dominance of soft bottoms as sand or small gravel, especially in the LR where the preservation suggests minimal transpon and/or a minor influence of postmortem processes on the valves.
Despite the type of environment and external factors such as temperature, salinity and substrate, it is necessary to consider intrinsic factors such as biology, mode of life and mineralogy of the studied organisms to describe and interpret the processes that affect fossilization and the postmortem process (Lazo, 2004; Zuschinand Stanton, 2001). Epifaunal elements are thicker or have structures as byssus that give resistance and stability to organisms in conditions of high energy, but the long exposure to waves produced more fragmentation and abrasión of the shells, which creates a disadvantage in the preservation (e.g., Mytilus) (Zuschin and Stanton. 2001). Alternatively, the infaunal species support the results of other taphonomic works that describe the advantages of using infaunal bivalves, because they have a fast burial rate and the preservation is better than epifaunal organisms (Aguirre and Farinati, 1999; Nielsen and Funder, 2003). In this work, venerids show a good preservation in both zones of Tierra del Fuego and although abrasión and fragmentation in the deposits is modérate, this can be attributed to high bioerosion of their valves caused mainly by boring gastropods and/or encrus-ting elements on the surface of the shell (Zuschin and Stanton, 2001). In this regard, results show a relationship between the bioerosion level and the fragmentation in the deposits of the Strait of Magellan, Le., deposits withhighbioerosionalso show high fragmentation. Nonetheless, in Alakush and Ushuaia (Beagle Channel) this relationship does not exist and the high fragmentation of venerids could be associated to different postmortem processes.
Other relevant factors related to preservation are the distribution of trophic and life habit groups. Accordingto Gordillo and Aitken (2000), and to this study inthe Strait of Magellan and Beagle Channel, the infaunal suspensión feeders (bivalves) exhibit a high preservation potential, while carnivores and infaunal deposit feeders are a minor proportion of the Quaternary marine fossil records.
All fossil species collected in both zones correspond to mollusks occurring on modernbeaches, which suggest that postglacial climatic changes were not enough to produce variations in the composition of the benthic communities or the environ-mental conditions since at least 6,000 yrBP in Tierra del Fuego. The qualitative differences between the faunistic elements of both regions, however, could be the result of different types of habitat. The local ecological variations are caused by the occurrence of two substrate types that exist in the región: epifaunal elements associated to hard or rocky bottoms (Strait of Magellan) and infaunal elements associated to sandy bottoms (Beagle Channel).
The taphonomic attributes of five bivalve species, well represented in the Holocene littoral deposits of Tierra del Fuego, are useful indicators of paleoenvironmental conditions. In particular, venerids show a higher preservation potential than mytilids, and their infaunal life cycle and their fast burial rate makes them more reliable to use in further taphonomic analyses.
Integration of taxonomic, ecologic and taphonomic analyses do not show any significant change, either in the molluscan composition or in the energy conditions of both áreas since 6,000 years BP. Future research needs to consider stable isotopes (δ13C y δ18O) and morphometric analyses of the valves to evalúate changes in paleotemperature and other paleoenvironmental conditions during the Holocene, and their potential effects at the species level.
We are grateful to K. Linse (British Antarctic Sur-vey), C. Valdovinos (Universidad de Concepción) and S. Nielsen (GeoForschungsZentrum Potsdam) for their valuable reviews of the manuscript. This work was partly supported by CONICET grant PIP 6323/05, Fundación CEQUA and Unidad de Sistemas Acuáticos Centro EULA, Universidad de Concepción.
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Manuscript received: November 11, 2007; accepted: October 17, 2008