Oxygen isotope compositions of phosphate from Middle Miocene–Early Pliocene marine vertebrates of Peru
Introduction
Quantitative reconstitutions of temperatures and oxygen isotope compositions of past seawater (δ18Osw) mainly rely on the oxygen isotope analysis of skeletal phosphate (δ18Op) and carbonate (δ18Oc) secreted by marine organisms. As the oxygen isotope fractionation between these biominerals and water is temperature-dependent, the δ18Op or δ18Oc value of aquatic ectothermic organisms (such as marine invertebrates and most fish) both reflect environmental temperature and water composition whereas endothermic organisms (marine mammals) provide δ18Ow estimates. Temporal variations in both temperature and δ18Ow value of sea or fresh waters can be tracked by analysing the phosphatic tissues of both coexisting marine mammals and fish or reptiles and fish (Barrick et al., 1993, Lécuyer et al., 1996, Barrick et al., 1999) using the fractionation equations established between phosphate and water for cetaceans (δ18Op = 0.773 δ18Ow + 17.8; Yoshida and Miyazaki, 1991), fish (T °C = 113.3 − 4.38 (δ18Op − δ18Ow); Kolodny et al., 1983), turtles (δ18Ow = 1.01 δ18Op − 22.3; Barrick et al., 1999) or crocodilians (δ18Ow = 0.82 δ18Op − 19.13; Amiot et al., 2007). It is noteworthy that using cetaceans is not possible for periods older than the Eocene when these marine mammals appeared (Fordyce, 1994).
The Miocene and Pliocene were periods of great changes in the Earth's global climatic regime marked by the development of polar ice-caps and by the progressive global cooling that followed the Middle Miocene Climatic Optimum (see Zachos et al., 2001 for a review). Oxygen isotope compositions of phosphatic remains from coexisting cetaceans and fish have been used as proxies of thermal changes and ice volume fluctuations (Barrick et al., 1992, Barrick et al., 1993). Surprising conclusions were drawn from their study of fossil samples recovered from Miocene deposits of the Chesapeake Bay (North America, Atlantic coast). Assuming that the studied samples were not diagenetically altered, Barrick et al. (1992) applied the oxygen isotope fractionation equation established for modern cetaceans (Yoshida and Miyazaki, 1991) to Miocene porpoises and whales. They obtained unrealistically high δ18Ow values ranging from + 2 to + 4.7‰. They also observed a positive correlation between estimated marine temperatures and δ18Ow values, meaning that warmer marine conditions prevailed during polar glaciations and conversely cooler temperatures during ice-cap melting. Moreover, the inferred variations in δ18Ow values suggest larger volumes of polar ice involved during the Miocene growth and decay stage than previously suspected (Haq et al., 1987, Miller et al., 1991). These results raised several questions concerning the meaning of the oxygen isotope compositions recorded in Miocene cetacean bones from Chesapeake Bay. Do they reflect global palaeoenvironmental conditions or local ones (Barrick et al., 1992, Barrick et al., 1993)? To which extent is the equation established by Yoshida and Miyazaki (1991) applicable to any fossil cetacean given the excessively high δ18Ow values estimated from the δ18Op values of their Miocene counterparts?
The fossil sites of the Pisco Formation (Peru, Pacific coast) have yielded rich and well-preserved marine vertebrate fauna of Miocene and Pliocene ages that include cetaceans (porpoises, whales), pinnipeds (seals), birds (among others, penguins), turtles, crocodiles and sharks. Fossil remains of these animals having various ecologies and physiologies have been analysed for their oxygen isotope contents in order to estimate the variations in marine temperatures and ice volume changes as well as to search for answers to questions raised by the results provided by Barrick et al. (1993)'s study.
Section snippets
Geological settings
The Pisco Formation consists of marine Neogene deposits located along the southern coast of Peru, and is known for its abundant marine vertebrate fauna. The Pisco Formation extends about 300 km from the city of Pisco south to Yauca (Fig. 1) and is about 640 m thick (Brand et al., 2003). Its geology and palaeoecology in the Sacaco area were studied by Muizon and DeVries (1985). The age of the sediments is constrained by vertebrate and molluscan biostratigraphies and radioisotopic dating. Samples
Sample collection
Phosphatic remains of marine vertebrates (whales, dolphins, seals, penguins, marine sloths, turtles, crocodilians and sharks) recovered from five fossil sites of the Pisco Formation were collected, cleaned and analysed for their oxygen isotope compositions. Except for small teeth for which bulk analyses were performed as well as two aquatic sloth teeth covered with durodentine, enamel was preferentially selected and sampled with a microdrill. Cortical regions of various bones which are the most
Analytical techniques
Measurements of oxygen isotope ratios of apatite consist of isolating phosphate ions using acid dissolution and anion-exchange resin, according to a protocol derived from the original method published by Crowson et al. (1991) and slightly modified by Lécuyer et al. (1993). Silver phosphate is quantitatively precipitated in a thermostatic bath set at a temperature of 70 °C. After filtration, washing with double deionised water, and drying at 50 °C, 8 mg of Ag3PO4 are mixed with 0.5 mg of pure
Results
Oxygen isotope measurements of tooth and bone phosphate are reported in Table 1. The whole δ18O dataset ranges from 17.5‰ to 23.1‰. For each of the five localities sampled, mean δ18Op values of teeth and bones for each taxonomic group are plotted against their relative age in Fig. 2. Significant isotopic differences are observed at any given locality between the various groups of vertebrates, with ranges from 18.2‰ to 21.4‰ for marine mammals, 19.5‰ to 21.5‰ for marine birds and 20.9‰ to 23.1‰
Pristine oxygen isotope conservation of fossil apatite
Secondary precipitation of apatite and isotopic exchange during microbially-mediated reactions may scramble the primary isotopic signal (Blake et al., 1997, Zazzo et al., 2004). However, apatite crystals that make up tooth enamel are large and densely packed, and isotopic exchange under inorganic conditions has little effect on the oxygen isotope composition of phosphates even at geological time scales (Kolodny et al., 1983, Lécuyer et al., 1999). Although no method is available to demonstrate
Conclusion
The study of the oxygen isotope compositions of marine vertebrates from Miocene and Pliocene localities of the Pisco formation leads to the following conclusions:
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Coexisting marine mammals, reptiles, birds and fish have different δ18Op values as a result of different ecologies and physiologies. Fish and reptiles tend to have higher δ18Op values than mammals and birds due to their lower body temperature, and birds have higher δ18Op values than marine mammals due to their higher metabolic rates.
Acknowledgements
The authors would like to thank E. Frey from the Staatliches Museum für Naturkunde Karlsruhe, Germany (SMNK) for providing samples of the Miocene dolphin Tursiops oligodon. Thomas Tütken and an anonymous reviewer provided critical reviews that greatly helped us to improve the manuscript. U. G. was supported by a fellowship in the Feodor-Lynen-Program by the Alexander von Humboldt Foundation.
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- 1
Now at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, #142 XiZhiMenWai DaJie, Beijing 100044, China.
- 2
Now at the Natural History Museum Vienna, Geological–Paleontological Department, Burgring 7, A-1010 Vienna, Austria.
- 3
Also at Institut Universitaire de France.