Paleoenvironment of the Toropí Formation (Upper Pleistocene), Corrientes province (Mesopotamian region, Argentina): A phytolith approach

Abstract

Two Pleistocene sedimentary units are recognized in the Corrientes province, Argentina: the Toropí and Yupoí formations. These sediments have proven productive for fossil vertebrates, but few macrofossils have been recovered. To remedy this situation, plant silica (phytoliths) were extracted from the sediments, enabling a direct comparison of paleovegetation data and information based on previous study of vertebrates. The studied samples (n = 28) come from two profiles from the Toropí Stream (28°36′S; 59°02′W), near Bella Vista, Corrientes, from which two Quaternary mammals, Lestodon (Xenarthra) and Hippocamelus (Artiodactyla), have previously been excavated. All samples were productive and contained both non-plant biogenic silica (diatoms, sponge spicular, chrysophyte cysts) and phytoliths. Phytolith assemblages were dominated by morphotypes diagnostic of grasses, in particular C₃ pooids, C₄ chloridoids, and C₃ or C₄ panicoids (or related grasses in the PACMAD clade). Rare phytoliths of palms and other woody or herbaceous dicotyledonous angiosperms were also present. This combination of C₃ and C₄ grasses, and rare palms and other forest indicators, indicates grass-dominated habitats with groves with palms and other trees/shrubs along rivers, growing under a relatively warm and dry climate. This mixed plant community reflects shifting biogeographic affinity with the Chaco-Pampean plain and inter-tropical regions, respectively, linked to the frequent climatic-environmental fluctuations during the Late Pleistocene.

Introduction

During most of the Cenozoic, South America was an island continent, lacking any terrestrial connections to other continental land masses. This long-lasting geographical isolation resulted in a highly endemic vertebrate fauna and flora that cannot be easily compared to other continents (Simpson, 1980; Ricklefs, 2002; Morley, 2003) This biota, in particular the fauna, was significantly restructured during the Great American Biotic Interchange (GABI), which peaked around 3 Ma with the establishment of a land bridge between North and South America; Coates and Obando, 1996. During this mass-dispersal event, mammals (and birds) from North America immigrated and displaced, or filled niches left vacant by South American endemics (Marshall et al., 1982; Stehli and Webb, 1985; Weir et al., 2009). Following immigration, mammals and other biota underwent significant diversification (Webb, 1976; Marshall et al., 1982; Stehli and Webb, 1985; Dacosta and Klicka, 2008). The Pleistocene (c. 2.6–0.011 Ma) represents the ecological aftermath of this intense transformation of South America ecosystems in the context of increasingly fluctuating climates as Earth transitioned into the last Ice Age.

Many recent multidisciplinary studies have sought to reconstruct paleoclimate, paleoenvironment, and faunal biogeography during this ecologically important time. A classic area of focus is the current Mesopotamian region in Argentina (Carlini et al., 2008; Orfeo et al., 2009). The interpretations of paleoenvironmental context in which the Quaternary biota developed, together with detailed faunal analyses performed in the area (Scillato-Yané et al., 1998, 2005; Zurita and Lutz, 2002; Tonni, 2004; Carlini et al., 2003, 2004, 2008; Gasparini and Zurita, 2005; Ferrero and Noriega, 2007; Zurita and Ferrero, 2009, among others) suggest that the northern part of the current Mesopotamian region in Argentina had a biological connection to the southern part of Brazil and western Uruguay, particularly during the wet and warm pulses of the late Pleistocene (Carlini et al., 2003, 2004; Francia and Carlini, 2009). In the past, most of the knowledge of paleoenvironment has come from the study of vertebrates, whereas paleobotanical work has been limited (see Orfeo et al., 2009). The Pleistocene deposits in Mesopotamia consist of two fossil-rich lithostratigraphic units: the Toropí and Yupoí formations, both representing floodplain sedimentation. Extracted biosilica (phytoliths) from the Toropí and Yupoí formations from a site in the province of Corrientes constitutes the first fossil record of plants in this region. The main objective of this contribution is to analyze these phytolith assemblages to provide a paleoenvironmental interpretation for the late Pleistocene of northeastern Argentina that can be compared to the faunal record in this region.

The Mesopotamian region of Argentina covers 200,000 km² between the current Paraná and Uruguay rivers, and includes the territories of Misiones, Corrientes and Entre Ríos provinces (Aceñolaza, 2007). Within Mesopotamia, the Corrientes province covers an area of 88,800 km² (Fig. 1). Geomorphologically, the province belongs to a vast sedimentary basin that constitutes a part of ancient shelf relief.

Within the Corrientes province, the area surrounding the town of Bella Vista has a climate described as wet subtropical, characterized by relatively equable temperatures and regular rains throughout the year (Fig. 1). The Mean Annual Temperature (MAT) is 20.9 °C. Soils in the region range from poorly developed over modern sediments to better developed with the formation of A–B–C horizons on top of older material, resulting in Entisols (the least developed soils, on sandy parent material), Mollisols and Alfisols (well developed soils that typically reflect grasslands and forest, respectively) (Herbst and Santa Cruz, 1999). Phytogeographically, the town of Bella Vista is in the Chaco Eastern District (Cabrera, 1976), part of the Mesopotamian parkland. This can be described as a savanna parkland, with mesophyllous, xerophilous and xero-halophilous forests in terrace levels on the river banks and topographic highs, mesophyllous to hygrophilous savannas and abundant weeds in topographic lows, sub-concave plains and sandy ridges, Syagrus palm groves in sandy hillocks, a marshy and aquatic vegetation complex, Copernicia palm groves and xero-halophilous forests in large depressions of old creeks (Carnevali, 1994).

The geological history of the Quaternary deposits in the province of Corrientes has historically been controversial. Álvarez (1974) initially recognized only one unit for the area, the Yupoí Formation. Based on re-study of fossil vertebrates, minerals from the sand fraction, and clay analysis, Herbst and Álvarez (1977) and Herbst and Santa Cruz (1985) later acknowledged two successive units, the Toropí Formation and the Yupoí Formation, but kept the chronology suggested by Álvarez (1974). More recently, Iriondo (1996) suggested combining the units into a “Toropí/Yupoí Formation” and correlating this new unit to the Hernandarias Formation (Middle Pleistocene, between 0.8 and 1.3 Ma). In contrast, a new analysis of fossil mammals in the area (Scillato-Yané et al., 1998) indicates an age for the sequence (Toropí Formation/Yupoí Formation) comparable to that of the ‘Lujanense’ sensu lato, that is, Middle Pleistocene–Early Holocene (Cione and Tonni, 1995). Finally, recent dating analyses using Optically Stimulated Luminescence (OSL) of the deposits have resulted in ages between 58 ka (for the Toropí Formation) and 36 ka (for the Yupoí Formation), “Lujanense’ sensu strict (Late Pleistocene–Early Holocene) (Tonni et al., 2005; Tonni, 2007; Carlini et al., 2008). Lithologically, the Toropí Formation is composed of clayey sand, sandy limestone, and sandy clay. The Yupoí Formation consists of pelitic sandstone with variable portions of sandy limestone and sandy clay (Herbst and Santa Cruz, 1999). These units represent floodplain deposits, and are broadly distributed, covering a large part of the western and eastern sections of the Corrientes province along the Paraná and Uruguay rivers, respectively (Herbst and Álvarez, 1975). These Pleistocene units unconformably overlie sandy deposits of the Ituzaingó Formation (Late Miocene–Pliocene), which consist of mineralogically unsorted sands and sporadic, but distinct finer-grained deposits. Both units have broad distributions in Corrientes along the slopes of the Paraná River and distributaries (the Ambrosio Stream, the Santa Lucía River), as well as in some localities along the banks of the Uruguay River and its tributaries, such as the Miriñay (Herbst and Santa Cruz, 1985).

The Toropí and Yupoí formations are both highly fossiliferous (see Herbst, 1971; Iriondo, 1973; Herbst and Álvarez, 1975), and preserve an important and varied fauna of mega- and micro-mammals. The Pleistocene fauna, which has been collected primarily from the outcrops on both margins of the Toropí Colonia Progreso Stream, Department of Bella Vista, is the most diverse fauna found in a single location (see Álvarez, 1974; Scillato-Yané et al., 1998; Zurita and Lutz, 2002; Alcaraz and Carlini, 2003; Carlini et al., 2004; Alcaraz and Francia, 2010; Francia et al., 2010). Invertebrate faunas (pelecypod molluscs, Morton and Jalfin, 1987; Herbst and Santa Cruz, 1999; Morton, 2004) that have recently been described add to the knowledge of the animal communities represented in this unit.

The Late Pleistocene vertebrate faunal record in the province of Corrientes show clear compositional changes through time, linked to fluctuations in climate. Specifically, the changes in diversity are consistent with pulses of colder and arid to semiarid climate (presence of Stegomastodon platensis, Dolichotis sp., Pampatherium typum, Neosclerocalyptus paskoensis, Glyptodon reticulatus, etc.), alternating with wetter and warmer climate (presence of Holmesina paulacoutoi, Tapirus sp., Stegomastodon waringi, Euphracthus aff. sexcintus, Boa constrictor). In this context, species of lineages with more tropical affinities in the ‘Lujanense’ of the Corrientes province are H. paulacoutoi, S. waringi and Tapirus. Scillato-Yané et al. (2005) associate H. paulacoutoi with a wet environment. Tapirus is also usually linked to wet to transitional or high altitude forests, particularly near stable bodies of water and fluvial–lacustrine environments (Eisenberg, 1997; Parera, 2002; Ferrero and Noriega, 2007). Finally, S. waringi has a clearly tropical distribution (Alberdi et al., 2002). In addition, the great diversity of cervids, including the two rather large Antifer ultra and Antifer ensenadensis (Alcaraz and Zurita, 2004; Alcaraz and Francia, 2010), suggests diverse environments, with both open areas and trees/shrubs.

Most paleoenvironmental analyses of Pleistocene faunas performed in the Mesopotamian region were conducted on material from the Corrientes and Entre Ríos provinces (Alcaraz and Carlini, 2003; Carlini et al., 2003, 2004, 2008; Noriega et al., 2004; Gasparini and Zurita, 2005; Mino-Boilini et al., 2006; Ferrero et al., 2007; Zurita and Ferrero, 2009; Francia et al., 2009, 2010; Miño-Boilini and Carlini, 2009; Alcaraz and Francia, 2010; Mino-Boilini et al., 2010). Only a few have addressed more in-depth interpretations of paleoclimatology and paleoenvironments (Tonni, 2009). These studies found that Quaternary sediments of the Corrientes province likely reflect successive, frequent environmental fluctuations, resulting in biogeographic ties alternating between the Chaco-Pampean plain and intertropical areas.

The paleobotany of the Cenozoic Mesopotamia is summarized by Anzótegui and Lutz (1985, 1988) and Barreda et al. (2007). The macrofloral record, restricted to a site at the base of the Yupoí Formation in the town of Empedrado, Corrientes, consists of impressions of stems and branches with nodes and internodes with affinities to the current genus Equisetum (Lutz and Gallego, 2001). Within Mesopotamia, several studies of phytolith assemblages have been conducted on Neogene deposits of the province of Entre Ríos, namely the marine, middle Miocene Paraná Formation in the Paraná River basin (Zucol and Brea, 2000a, 2000b), the continental, early Pleistocene deposits of the Alvear Formation (Zucol and Brea, 2001, 2005, in press), and the late Pleistocene–Holocene Tezanos Pinto Formation (Kröling et al., 2005; Erra et al., 2006; Erra, 2010a,b, 2011; Erra et al., 2011). In the Uruguay River basin, late Pleistocene phytolith assemblages from a site in the “Parque Nacional El Palmar” of the El Palmar Formation have been analyzed (Zucol and Brea, 2001; Brea et al., 2001a,b; Zucol et al., 2005). In contrast, no work on phytoliths has so far been carried out in the Corrientes province and, given the lack of macrofossils and palynomorphs in the deposits, very little independent evidence for vegetation for this area exists. The goal of this paper is to supply such a paleobotanical record for the late Quaternary of the mid-latitudes of Mesopotamian Argentina and compare it to the associated faunal record, as well as to phytolith assemblages of equivalent age from other formations.