Sr and Nd isotope systematics in fish teeth

Abstract

High concentrations of Sr and Nd in fish teeth apatite (up to 2000 and 3800 ppm. respectively) make them relatively impervious to diagenetic overprints and allow high-precision analysis of 87Sr/ 86 Sr and 143Nd/ 144Nd of very small amounts of material. 87Sr/ 86Sr ratios of 14 Cenozoic samples (< 55 Ma) from the Atlantic, Indian, and Pacific Oceans and the Caribbean Sea are close to the seawater value at the time of formation, suggesting their use for 87Sr/ 86Sr stratigraphic dating. A compilation of published 143Nd/ 144Nd data from recent Mn-nodules and marine FeMn deposits shows that the North-Central Pacific Ocean and the major portion of the Atlantic Ocean display distinctly different, narrow ranges in isotopic compositions. Between these two ares, the 143Nd/ 144Nd of Mn-nodules vary systematically towards intermediate compositions in the Southern Oceans, reflecting the communication between the Atlantic and Pacific Oceans through the eastward moving Antarctic Circumpolar Current. The 143Nd/ 144Nd of the 2–55 Ma old fish teeth analyzed are typically close to the range of 143Nd/ 144Nd in Recent Mn-nodules and seawater for the respective ocean and, thus, indicate no significant change through this time period. The 143Nd/ 144Nd of fish teeth from the North Pacific Ocean are highest during periods of high aeolian sedimentation. Since such dust probably introduces Nd with a low 143Nd/ 144Nd , it is unlikely that the fish teeth contain any significant amounts of sediment-derived REE's. Our data suggest that fish teeth probably obtained their REE signature from seawater during early diagenesis, not from later diagenetic overprints. For this reason they may be used to study the communication between the paleo-oceans. The 143Nd/ 144Nd of Caribbean Sea fish teeth is nearly constant through the past 8 Ma, precluding major influx of waters from the Pacific or the Atlantic Oceans, despite the evidence for faunal communication through the Isthmus of Panama prior to the Pliocene. The 143Nd/ 144Nd of 33 Ma old fish teeth from the Giant Piston Core, deposited at near equatorial paleolatitudes, suggest that the Central and North Pacific Ocean did not receive sufficient Atlantic-derived REE's to alter its 143Nd/ 144Nd . For this reason, a large-scale equatorial current through the Isthmus of Panama appears to be unlikely.