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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Palaeogeography, Palaeoclimatology, Palaeoecology 280 (2009) 480–488 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo Early Miocene subtropical water temperatures in the southeast Pacific Sven N. Nielsen ⁎,1, Johannes Glodny Deutsches GeoForschungsZentrum (GFZ), Telegrafenberg, 14473 Potsdam, Germany article info abstract Article history: Cenozoic climate of western South America is strongly controlled by features like Andean uplift and the Received 14 May 2009 Humboldt Current. The first strontium isotope age data from central and southern Chile provide a latest Accepted 24 June 2009 Oligocene to late early Miocene age for classic warm-water mollusk faunas reaching as far south as 45°S. Available online 3 July 2009 Comparison with the biogeography of congeneric living species indicates that sea surface temperatures off central and southern Chile during that time were at least 5 °C higher than today; i.e., minimum annual mean Keywords: sea surface temperatures for Darwin's Navidad fauna at 34°S are estimated as 20 °C. As expected, the number Strontium isotope stratigraphy Climate of tropical taxa decreases towards the south but several are still present as far south as 45°S. The ages scatter Mollusks relatively broadly between ~24 and ~16 Ma, partly even within individual localities. Shallow-water and Neogene deeper-water faunas are revealed to have similar ages. When considered in light of convincing Chile micropaleontological evidence for late Miocene to early Pliocene depositional ages, the Sr isotope data support a hypothesis that the mollusk fauna is reworked. © 2009 Elsevier B.V. All rights reserved. 1. Introduction localities and also test the hypothesis that the lesser known deep-water mollusk fauna of these formations might be contemporaneous with the Neogene sediments potentially recording paleoenvironmental con- microfossil biostratigraphic markers and therefore younger than the ditions are known from a limited number of basins along the Chilean shallow-water mollusk fauna. coast (Herm, 1969; Martínez-Pardo, 1990; Encinas et al., 2006; Le Roux et All of the specimens reported herein come from central to southern al., 2006). The classic mollusk fauna from the Navidad, Ranquil and Lacui Chilean localities (Figs. 1 and 2). The Navidad, Ranquil, and Lacui Formations of central Chile (Fig. 1), known since Darwin's (1846) voyage formations are coeval (Finger et al., 2007) and have been synonymised on the Beagle, contains a number of species indicating a near-shore under the name Navidad Formation (Martínez-Pardo, 1990). However, environment and tropical to subtropical water temperatures (Groves and the regional formation names are maintained here and the term Nielsen, 2003; Nielsen et al., 2004). Correlation of molluscan taxa from Navidad Formation is used in its restricted original sense (Darwin,1846; the Navidad Formation with Cenozoic faunas from Peru resulted in the Encinas et al., 2006). The rich molluscan fauna of the three units, proposal of a latest Oligocene to middle Miocene age for the Navidad especially the Navidad Formation, has been the subject of numerous fauna (DeVries and Frassinetti, 2003). Recent micropaleontological work studies (e.g., Tavera, 1979; Philippi, 1887; Frassinetti and Covacevich, (Finger et al., 2007), however, reveals that the Navidad, Ranquil and Lacui 1993; Nielsen et al., 2004; Nielsen, 2005; Nielsen and Frassinetti, 2007a). Formations are all late Miocene to early Pliocene in age and were While depositional ages younger than the molluscan fauna have been deposited at bathyal depths. Ages based on planktic foraminifera are proposed based on planktic foraminifera (Finger et al., 2007), additional between 4.5 and 11 Ma (Zones N19a–N16a) for most of the localities planktic foraminifera seem contemporaneous with the mollusks. (Finger et al., 2007); the investigated localities of the Navidad Formation are the same as those that yielded the apparently older mollusks of 2. Geologic setting and sampling sites DeVries and Frassinetti (2003). The current explanation for the age discrepancy is that yet unknown older strata were reworked and The geology of most of the investigated areas is still poorly described. displaced to greater depths (Finger et al., 2007). Here we test this All deposits are clastic sediments and most are situated at today's coastal hypothesis and check for the alternative possibility that some mollusk bluffs. Only from the Navidad Formation a detailed study exists (Encinas, species survived much longer in Chile than in Peru. We analyze the 2006; Encinas et al., 2006), which suggests deposition on the strontium isotope signatures of several mollusks from these same continental slope, based on sedimentary structures, trace fossils and microfossils. The deposits of the Ranquil Formation were briefly described by Tavera (1942) and García (1968), but no detailed ⁎ Corresponding author. Fax: +49 431 880 5557. sedimentological description is available. Similarly, the deposits of the E-mail address: [email protected] (S.N. Nielsen). 1 fl Present address: Institut für Geowissenschaften, Christian-Albrechts-Universität zu Lacui Formation were brie y described in the geological map of Antinao Kiel, Ludewig-Meyn-Str. 10, 24118 Kiel, Germany. et al. (2000) but detailed investigations are wanting. The deposits of 0031-0182/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2009.06.035 Author's personal copy S.N. Nielsen, J. Glodny / Palaeogeography, Palaeoclimatology, Palaeoecology 280 (2009) 480–488 481 Fig. 1. Study areas in central Chile and geographic ranges for some early Miocene warm-water mollusks and proposed approximate sea surface temperatures (SST). Modern annual average SSTs (in parentheses) from Ocean Climate Laboratory (1999).1,Aturia cubaensis; 2, Architectonicidae; 3, Sinum subglobosum;4,Olivancillaria claneophila;5,Lamprodomina dimidiata;6,Distorsio ringens;7,Terebra undulifera;8,Xenophora paulinae;9,Zonaria frassinettii;10,Ficus spp.; 11, Strombus medinae;12,Lisanerita chilensis. Open star in 8 marks a shell fragment possibly belonging to this species. See Fig. 2 for detailed maps and sampled localities. both Ranquil and Navidad formations are very similar to those of the about 2 m above the base of the coastal cliffs about 1 km north of the Navidad Formation and a similar depositional history is inferred here. No village of Matanzas, that consist of more than 30 m of alternating detailed sedimentological or other geological information was ever sandstones and siltstones (Frassinetti and Covacevich, 1981; Nielsen, published about the islands of the Chonos Archipelago (see Fig. 1). 2005). Punta Alta (PTA) is situated at the coastal cliff just below the Finger et al. (2007) reported late Miocene to early Pliocene road between Navidad and Matanzas. The fossiliferous lens consists of planktonic foraminifera from the Navidad, Ranquil, and Lacui forma- grey siltstone (Frassinetti and Covacevich, 1982). North of the Rapel tions, supporting the data published by Ibaraki (1992) from Navidad River, to the north of a granite outcrop, fallen blocks of brownish grey and Finger et al. (2003) from all three areas. These young ages are sandstone were sampled (RAP), that probably were derived from highly reliable because the used index species are very different from different beds within the cliff (Nielsen et al., 2004). The locality name their predecessors. However, Finger et al. (2007) also correlated the Punta Perro has been used for sandstone at the base of the coastal cliff mollusk fauna with well dated late Oligocene to middle Miocene immediately south of the mouth of the Rapel River (Covacevich and occurrences in Peru (see also DeVries and Frassinetti, 2003) and Frassinetti, 1986). Finger et al. (2007) distinguished several localities concluded that the older mollusk fauna was reworked during the at Punta Perro and separated grey siltstone of the coastal platform latest Miocene to early Pliocene. Reconstructions of the tectonic (PPP) from overlying orange-brown to yellowish sandstone (PPN). history of the Navidad and Arauco areas (Melnick and Echtler, 2006; The latter coincides with the locality of Covacevich and Frassinetti Encinas et al., 2008; Farías et al., 2008) support a history of tectonic (1986). downwarp and uplift as outlined by Finger et al. (2007). All of these localities coincide with the coastal cliffs described by Our localities are the same as those of Finger et al. (2007) and Tavera (1979) as belonging to the Navidad Member, the basal member locality data including GPS points (Table 1), detailed strontium data of the Navidad Formation in his classification, which he considered as for individual samples and localities