Limits on conservative behavior of Nd isotopes in seawater assessed from analysis of fish teeth from Pacific core tops

作者：Horikawa Keiji^*; Martin Ellen E; Asahara Yoshihiro; Sagawa Takuya

来源：Earth and Planetary Science Letters, 2011, 310(1-2): 119-130.

DOI：10.1016/j.epsl.2011.07.018

• neodymium isotopes
• fish teeth
• NPDW
• LCDW
• deep-water circulation
• Pacific Ocean

Neodymium (Nd) isotopes have become an accepted water mass tracer for paleoceanographic reconstructions; however, high rates of radiogenic Nd input from volcanic islands/arc's in the western equatorial Pacific (WEP) and the subarctic North Pacific (SANP) can significantly alter the isotopic composition of ambient seawaters, leading to questions about the conservative nature of Nd isotopes in the Pacific. Here we present the first comprehensive study of Nd isotopes preserved in core-top fossil fish teeth/debris from 31 sites in the Pacific to define the vertical and horizontal distributions of seawater Nd isotopes and to test the reliability of Nd isotopes as a water mass tracer in the Pacific. We found that Nd isotopic values of northward-flowing Lower Circumpolar Deep Water (LCDW) are significantly modified in the western-central equatorial Pacific and the SANP as a result of local radiogenic inputs from young circum-Pacific island arcs. Consequently, 'modified LCDW' with similar to-5 epsilon(Nd) is created in the central equatorial Pacific, while recirculated North Pacific Deep Water (NPDW) acquires a radiogenic Nd isotope value (similar to-2 epsilon(Nd)) in the SANP. Nd isotopic ratios of fish teeth/debris in the central North Pacific plotted against salinity (or phosphate concentration) from the overlying bottom water produce a mixing envelope between modified LCDW and NPDW, indicating that these Nd isotopic values represent the southern- and northern-sourced end member values for the central North Pacific and that mixing of these water masses defines vertical profiles of Nd isotopes in this region. In the eastern equatorial Pacific (EEP) three water masses (modified LCDW, NPDW(SANP), and NPDW(western Pacific)) combine to produce Nd isotopic values that reflect this conservative mixture. Therefore, Nd isotopes behave as a conservative water mass tracer in the central North Pacific and the EEP, but high rates of lithogenic inputs from young circum-Pacific island arcs compromise the conservative behavior of Nd isotopes in the WEP and SANP.

• 出版日期2011-10-1