Constraining the Sources of Neodymium in the North Pacific Ocean
Abstract:The high Nd isotope ratios of North Pacific Deep Water (NPDW, εNd ~ -4) have been difficult to reconcile with the eolian inputs as reflected in surface waters (e.g. Jones et al. 2008 EPSL), which have much lower εNd (~ -10), indicating potential addition of volcanic component. In order to constrain the Nd sources in the North Pacific, we measured εNd on seawater from 5 stations across the North Pacific sampled by the INOPEX Cruise (2009).
Below ~ 2000m, measured εNd values in the North Pacific match previously published data and correspond with NPDW values. Easternmost Sta. 32 (45.5°N 158.5°W) shows higher εNd (~ -3) than corresponding potential density (ρ) layers of other stations. This may indicate addition of a high-εNd source from Aleutians-Kamchatka volcanic margin along the transport path of Lower Circumpolar Deep Water (LCDW) from westernmost Sta. 44 (39.8°N 152.4°E) to Sta. 32. As a result, NPDW at Sta. 32, formed from the LCDW, also shows higher εNd than the other stations.
Above ~ 2000m, samples at Sta. 5 (52.7°N 164.9°E) near the Aleutian-Kamchatka intersection have higher εNd than corresponding density layers of other stations, consistent with higher contributions from regional volcanoes. From the surface to ρ = 27.17 (250 - 750m, depending on location), a peak of εNd ~ -1.4 at ~ 100m (ρ = 26.56) at Sta. 5 coincides with a peak of dissolved 232Th, which indicates high lithogenic input from the Aleutian-Kamchatka margin (Hayes et al. 2014 EPSL). Thus the εNd peak likely reflects the remineralization of volcanic ash. εNd values of Sta. 32, 39 (38.0°N 164.5°E), 41 (38.4°N 160.3°E), and 44 generally increase with increasing density over this interval. Since the surface εNd at these stations (-4.8 to -4.0) indicates eolian dust input from the Asian deserts, an explanation for the increasing εNd is that the addition of a high-εNd volcanic source prevails over the remineralization of eolian desert dusts.
Our results are consistent with contributions to the high εNd in North Pacific seawater from volcanic margins. [Nd] and REE will be analyzed to further investigate the Nd sources.