Precipitation of Nd isotopes in planktonic foraminiferal shells: Approaches from sediment trap and core-top samples in the South China Sea
Abstract
Nd isotopic composition of uncleaned planktonic foraminifera was interpreted as a representative of bottom water Nd signature and subsequently was extensively applied to reconstruct past water mass changes in the ocean in various time scales. This interpretation, however, has been challenged recently, because the acquisition of Nd isotopes by planktonic foraminiferal shells and associated authigenic phases through water column is not well constrained. In this study, we present elemental geochemistry of uncleaned planktonic foraminifera obtained from sediment traps deployed in the northern South China Sea and from adjacent core-top sediments to better understand the potential source of Nd signal recorded in the planktonic foraminifera. Our study shows that Nd/Ca ratio together with REE concentrations of planktonic foraminifera increased with depths from 24 nmol/mol at 500 m to 805 nmol/mol at the bottom, and are notably lower than the Nd/Ca ratio of 1749 nmol/mol in core-top planktonic foraminifera. The Nd/Ca ratio does not indicate a significant correlation to the Mn/Ca ratio of live planktonic foraminifera. This result is in contrast to previous studies that Fe-Mn phases are the main carriers of Nd in planktonic foraminifera. In addition, time-series variations of Nd/Ca ratio obtained from the upper layer (500 m) and the bottom layer (3820 m) present a remarkable enriched Nd/Ca ratio in winter compared to that in summer. Nevertheless, no apparent change of Nd/Ca ratio in the intermediate layer (2070 m) was observed throughout the year. Our results indicate that the precipitation of Nd isotope into planktonic foraminifera could take place at the deep water in addition to at/below the sediment-water interface. The observed enrichment of Nd in winter is probably associated with temporally increased sediment flux that potentially enhanced the release of element ions from sediments to seawater in the South China Sea.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMPP11A2004W
- Keywords:
-
- 1039 Alteration and weathering processes;
- GEOCHEMISTRYDE: 1050 Marine geochemistry;
- GEOCHEMISTRYDE: 4805 Biogeochemical cycles;
- processes;
- and modeling;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICALDE: 4924 Geochemical tracers;
- PALEOCEANOGRAPHY