Oxygen isotopic variations in modern cetacean teeth and bones: implications for ecological, paleoecological, and paleoclimatic studies

Abstract

The oxygen isotope ratios (delta O-18) preserved in marine sediments have been widely used to reconstruct past ocean temperatures. However, there remain significant uncertainties associated with this method, owing to assumptions about the delta O-18 of ancient seawater which affects the temperature inferred from sediment delta O-18 records. In this study, oxygen isotope compositions of phosphate in teeth and bones from five different modern cetacean species, including sperm whale, pygmy sperm whale, short-finned pilot whale, killer whale, and Cuvier's beaked whale, and three fossil whales were determined. The data were used to assess whether the oxygen isotope ratios of biogenic phosphate (delta O-18(p)) from cetaceans are a reliable proxy for the oxygen isotopic composition of ocean water (delta O-18(w)). The delta O-18(p) values of modern cetaceans range from 15.5 parts per thousand to 21.3 parts per thousand, averaging (19.6 parts per thousand +/- 0.8 parts per thousand) (n = 136). Using a greatly expanded global cetacean delta O-18(p) dataset, the following regression equation is derived for cetaceans: delta O-18(w) = 0.95317 (+/- 0.03293) delta O-18(p) - 17.971 (+/- 0.605), r = 0.97253. The new equation, when applied to fossil teeth and bones, yielded reasonable estimates of ancient seawater delta O-18(w) values. Intra-tooth isotopic variations were observed within individual teeth. Among the selected species, the killer whale (O.orca) has the lowest delta O-18(p) values and the largest intra-tooth delta O-18(p) variation, reflecting its habitat preference and migratory behavior. The results show that oxygen isotope analysis of phosphate in cetacean teeth and dense ear bones provides a useful tool for reconstructing the oxygen isotopic composition of seawater and for examining environmental preferences (including migratory behavior) of both modern and ancient whales.