Hydrothermal sediments as a potential record of seawater Nd isotope compositions: The Rainbow vent site (36°14′N, Mid‐Atlantic Ridge)

Abstract

Geochemical compositions and Sr and Nd isotopes were measured in two cores collected ∼2 and 5 km from the Rainbow hydrothermal vent site on the Mid‐Atlantic Ridge. Overall, the cores record enrichments in Fe and other metals from hydrothermal fallout, but sequential dissolution of the sediments allows discrimination between a leach phase (easily leachable) and a residue phase (refractory). The oxy‐anion and transition metal distribution combined with rare earth element (REE) patterns suggest that (1) the leach fraction is a mixture of biogenic carbonate and hydrothermal Fe‐Mn oxy‐hydroxide with no significant contribution from detrital material and (2) >99.5% of the REE content of the leach fraction is of seawater origin. In addition, the leach fraction has an average 87Sr/86Sr ratio indistinguishable from modern seawater at 0.70916. Although we lack the ɛNd value of present‐day deep water at the Rainbow vent site, we believe that the REE budget of the leach fraction is predominantly of seawater origin. We suggest therefore that the leach fraction provides a record of local seawater ɛNd values. Nd isotope data from these cores span the period of 4–14 ka (14C ages) and yield ɛNd values for North East Atlantic Deep Water (NEADW) that are higher (−9.3 to −11.1) than those observed in the nearby Madeira Abyssal Plain from the same depth (−12.4 ± 0.9). This observation suggests that either the Iceland‐Scotland Overflow Water (ISOW) and Lower Deep Water contributions to the formation of NEADW are higher along the Mid‐Atlantic Ridge than in the surrounding basins or that the relative proportion of ISOW was higher during this period than is observed today. This study indicates that hydrothermal sediments have the potential to provide a higher‐resolution record of deep water ɛNd values, and hence deepwater circulation patterns in the oceans, than is possible from other types of sediments.