Late Miocene; Western Amazonia/Brazil)
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Journal of South American Earth Sciences 42 (2013) 216e241 Contents lists available at SciVerse ScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames Ostracods (Crustacea) and their palaeoenvironmental implication for the Solimões Formation (Late Miocene; Western Amazonia/Brazil) Martin Gross a,*, Maria Ines Ramos b, Marco Caporaletti c, Werner E. Piller c a Department for Geology and Palaeontology, Universalmuseum Joanneum, Weinzöttlstrasse 16, 8045 Graz, Austria b Coordenação de Ciências da Terra e Ecologia, Museu Paraense Emílio Goeldi, Avenida Perimetral, s/n Terra Firme, Belém-PA 66077-830, Brazil c Institute for Earth Sciences, Karl-Franzens-University, Heinrichstrasse 26, 8010 Graz, Austria article info abstract Article history: Western Amazonia’s landscape and biota were shaped by an enormous wetland during the Miocene Received 5 March 2012 epoch. Among the most discussed topics of this ecosystem range the question on the transitory influx of Accepted 5 October 2012 marine waters. Inter alia the occurrence of typically brackish water associated ostracods is repeatedly consulted to infer elevated salinities or even marine ingressions. The taxonomical investigation of Keywords: ostracod faunas derived from the upper part of the Solimões Formation (Eirunepé; W-Brazil) documents Western Amazonia a moderately diverse assemblage (19 species). A wealth of freshwater ostracods (mainly Cytheridella, Late Miocene Penthesilenula) was found co-occurring with taxa (chiefly Cyprideis) usually related to marginal marine Solimões Formation d18 d13 Palaeoenvironments settings today. The observed faunal compositions as well as constantly very light O- and C-values Ostracoda obtained by measuring both, the freshwater and brackish water ostracod group, refer to entirely freshwater conditions. These results corroborate with previous sedimentological and palaeontological observations, which proposed a fluvial depositional system for this part of western Amazonia during the Late Miocene. We demonstrate that some endemic, “brackish” water ostracods (i.e., Cyprideis)havebeen effectively adapted to freshwater conditions. Thus, their occurrence is no univocal evidence for the influence of brackish or marine waters in western Amazonia during the Miocene. Ó 2012 Elsevier Ltd. Open access under CC BY-NC-ND license. 1. Introduction and bivalves have been studied intensively during the last years and revealed (aside essential palaeoenvironmental and bio- Today, lowland Amazonia is famous for its biodiversity, which stratigraphical results) prime examples for speciation events obviously root in pre-Quaternary times (e.g., Haffer, 2008; Hoorn related to a variety of ecological factors. Thus, fundamental ques- et al., 2010a; Jaramillo et al., 2010; Wesselingh et al., 2010). The tions of evolutionary biology are touched based on the fossil record evolution of the modern Amazon system and its precursors is (e.g., Wesselingh, 2007; Anderson et al., 2010). Comparably, ostra- synoptically summarised recently (Hoorn and Wesselingh, 2010; cods experienced an extensive radiation during the “Pebas phase” Hoorn et al., 2010a). However, it remains heavily disputed by e.g., (especially the genus Cyprideis; e.g., Whatley et al., 1998), which is, Latrubesse et al. (2007, 2010). Nevertheless, from a palaeobiological however, less well documented and understood. point of view the proposed Middle to (early) Late Miocene “Pebas Studies on Neogene ostracods from the “Pebas system”, started system” (Hoorn et al., 2010a) is of outstanding interest. Albeit of its with the fundamental work of Purper (1977, 1979), followed by still debated nature (mega-lake, e.g., Wesselingh et al., 2002; mega- contributions of Sheppard and Bate (1980), Purper and Pinto (1983, wetland, e.g., Hoorn et al., 2010b; mega-fan (partly), e.g., Latrubesse 1985), Purper and Ornellas (1991) and Swain (1998). Later, the et al., 2010) and chronology, this vast wetland (w1 million km2; comprehensive research of Muñoz-Torres et al. (1998), Whatley Fig. 1a) shaped western Amazonia’s landscapes and life for several et al. (1998, 2000) significantly improved ostracod taxonomy and millions of years. Among aquatic biota it holds a spectacularly stimulated an initial ostracod based biozonation as well as phylo- diverse, largely endemic mollusc and ostracod fauna. Gastropods genetic hypotheses. Additional publications come from Ramos (2006), Celestino and Ramos (2007), Ramos et al. (2009). Lately, the state of the art in ostracodological research is reviewed by * Corresponding author. Tel.: þ43 316 8017 9733; fax: þ43 316 8017 9671. Wesselingh and Ramos (2010). E-mail addresses: [email protected] (M. Gross), mramos@ museu-goeldi.br (M.I. Ramos), [email protected] (M. Caporaletti), The current paper investigates ostracod faunas originating from [email protected] (W.E. Piller). a region, which is suggested to be situated at the edge of the “Pebas 0895-9811 Ó 2012 Elsevier Ltd. Open access under CC BY-NC-ND license. http://dx.doi.org/10.1016/j.jsames.2012.10.002 M. Gross et al. / Journal of South American Earth Sciences 42 (2013) 216e241 217 Fig. 1. Location of the study area around Eirunepé (western Amazonia). (a) Schematic illustration of the maximal extent of the “Pebas system” (¼ hatched area) during the Middle Miocene after Wesselingh (2008). (b) Location of investigated outcrops along the cut banks of the Juruá and Tarauacá River. system” e both in terms of palaeogeography and time (Eirunepé (1977), Purper (1979), Hoorn (1994), Latrubesse et al. (1997, area, western Brazil; Wesselingh et al., 2006a; Wesselingh and 2010), Hoorn et al. (2010b) and Silva-Caminha et al. (2010). Ramos, 2010; Figueiredo, 2012; Fig. 1a, b). We aim to provide a profound taxonomical base, which is crucial for forthcoming palaeoecological, biostratigraphical and phylogenetic research. Our 3. Materials and methods systematic evaluation is supplemented by compilations of the autecology as well as by detailed illustrations, inclusively of For micropalaeontological investigations bulk samples (w1e2kg) different sexes and ontogenetic stages. Ostracodological results of were taken from all outcrops under investigation (Gross et al., 2011). the present study are complementary to an earlier sedimentolog- 500 g of dried sediment (40 C, 24 h) were washed by using diluted fl ical analysis, which documented an aggrading uvial system as hydrogen superoxide for disintegration through standard sieves depositional environment for the examined outcrops (Gross et al., (H2O2:H2O ¼ 1:5; 63/125/250/500 mm). Wet sieve residuals were 2011). washed with ethanol (70%) before drying (40 C, 24 h). Residuals 250 mm were picked out completely and subject of 2. Geological setting detailed taxonomic investigations. From the 125 mm sieve-residual 0.2 g/sample were picked, which contained mainly, hardly distin- The sampled sections are situated along the river banks of the guishable juvenile and/or fragmented ostracod valves. Therefore Juruá and Tarauacá River, NE respectively SE of Eirunepé (state of the following taxa are not further differentiated within this fraction Amazonia; Solimões Basin (Jandiatuba Sub-Basin); e.g., Caputo, (for details see ESM 1): Cypria sp. and Physocypria sp. are counted as 1991; Wanderley-Filho et al., 2010; Fig. 1a, b). Aside Quaternary Cypria/Physocypria. Cyprideis graciosa, Cyprideis longispina, Cypri- deposits (alluvium, terraces) the studied outcrops (Remanso, deis pebasae, C. aff. pebasae, Cyprideis sp. 1 and 2 are summarised Aquidabã, Morada Nova, Pau D’Alho, Torre da Lua, Barro Branco) under Cyprideis “ornate” as well as Cyprideis aff. machadoi and expose sediments of the upper part of the Solimões Formation Cyprideis ?olivencai are subsumed under Cyprideis “smooth”. (Del’ Arco et al., 1977; Maia et al., 1977; Paz et al., in press). Prior to stable isotope analyses (d18O, d13C; 48 measurements) Detailed sedimentological descriptions of these locations and ostracod valves were additionally washed with distilled water in an interpretations are already presented in Gross et al. (2011) to ultrasonic bath and rinsed in ethanol finally. Adults and juveniles which we refer here. (A-1 instars) of 5 species were measured: C. pebasae, C. graciosa, The more than 1000 m thick Solimões Fm. covers most of Cytheridella danielopoli, Penthesilenula olivencae (only adults), western Amazonia and comprises peliticesandy alternations, Rhadinocytherura amazonensis (only adults). The number of valves lignitic intercalation as well as paleosols. Diverging views on in its required for analyses (w50 mg) varied between 2 and 7. For analyses definition, its stratigraphical and geographical range as well of its a Thermo-Finnigan Kiel II automated reaction system and depositional environments clearly mirror the controversially a Thermo-Finnigan Delta Plus isotope-ratio mass spectrometer debated history of Amazonia through Neogene times. In-depth were used (University of Graz; standard deviation ¼ 0.1& relative reviews of the Solimões Fm. are provided by e.g., Del’ Arco et al. to NBS-19; results in per mille relative to VPDB). 218 M. Gross et al. / Journal of South American Earth Sciences 42 (2013) 216e241 4. Results Juveniles of Vestalenula Rossetti & Martens, 1998 have antero- and posteroventral teeth in left valves (which are absent in adults) 4.1. Systematic palaeontology and no external posteroventral keel in right valves (which is diag- nostic for adults; e.g., Artheau, 2007;