Potential Sources Affecting Seawater Nd Isotopes in the Southwest Atlantic Ocean

Yingzhe Wu1, Steven L Goldstein1, Leopoldo Pena2, Alison E Hartman3,4, Micha J.A. Rijkenberg5 and Hein J W de Baar5, (1)Lamont Doherty Earth Observatory of Columbia Univeristy, Palisades, NY, United States, (2)University of Barcelona, GRC Geociències Marines, Dept. d'Estratigrafia, Paleontologia i Geociències Marines, Barcelona, Spain, (3)Hebrew University of Jerusalem, Institute of Earth Sciences, Jerusalem, Israel, (4)Interuniversity Institute of Marine Sciences, Eilat, Israel, (5)Royal Netherlands Institute for Sea Research, Den Burg, Netherlands
Abstract:
To better understand the reliability of εNd as a water mass tracer, 17 seawater profiles were sampled meridionally in the Southwest Atlantic (GEOTRACES GA02 Leg 3; RRS James Cook 057) and measured for εNd. Along the cruise track, there are potential sources that could add external Nd to seawater and disturb quasi-conservative behavior of εNd.

In South America, young continental crust in the south is potential source of high-εNd, and the Precambrian craton in the north is potential source of low εNd. This is reflected in decreasing εNd of continental shelf sediments from 55°S to 20°S (de Marhiques et al., Marine Geology 2008). εNd of seawater samples at 20m-250m depth also shows a decrease from ~50°S to ~20°S, indicating addition of Nd derived from South America at shallow depths. At intermediate and deep depths, the εNd section profile follows salinity and PO4* profiles.

The westerlies between ~60°S and ~40°S blow eolian dust off Patagonia that could impact seawater in this region with a high εNd signal (~ -1.9). Seawater εNd at 20m-250m in this region (-11.3 to -7.5) does not show dominance of such high εNd sources.

Oceanic volcanism is another potential source that could add Nd with high εNd-values to seawater. At stations on either side of the volcanic Rio Grande Rise, εNd-values are highly correlated with the water mass mixing proxies salinity and PO4* below 750m depth. Therefore seawater εNd near the Rise does not show impact from the volcanic components.

Nepheloid layers observed between ~49°S and ~42°S could exchange Nd with bottom seawater. εNd-values of bottom seawater at nearby stations are ~ -8.1, which is slightly higher than regional Circumpolar water values (εNd= -9.0 to -8.5). Therefore the small difference could be a result of the exchange processes between nepheloid layers and bottom seawater.

We plan to report REE data to further delineate the potential sources and processes that could impact seawater εNd.

Back to: Trace Elements and Isotopes at the Interfaces of the Atlantic Ocean I