Tracing dust input to the Mid-Atlantic Ridge between 14°45′N and 36°14′N: Geochemical and Sr isotope study

Abstract

A geochemical and Sr isotope study was carried out on sediment cores collected from the Logatchev hydrothermal vent site (14°45′N) on the Mid-Atlantic ridge, with the aim of identifying aeolian inputs into the overlying water column. The Logatchev vent site lies beneath the NE trade winds, which transport large amounts of detrital material across the North Atlantic. Sequential dissolution of the sediments has been used to discriminate the detrital fraction from other types of sediment. REE patterns for the acetic acid fraction at the Logatchev site record contributions from biogenic carbonate (seawater-like REE pattern) and oxy-hydroxides, which exhibit positive Eu and negative Ce anomalies, typical of hydrothermal origin. The Logatchev site leach fractions have similar major element compositions to terrigenous Al–Fe–Mn oxides, whose REE patterns show strong positive Ce and negative Eu anomalies that are comparable to those of “preformed” Fe–Mn oxides of continental origin. Based on major element and REE data and 87Sr/ 86Sr ratios (0.7197–0.7236), the residue fractions at the Logatchev site contain a significant contribution from aeolian sources, whose chemical composition is virtually identical to oligotrophic settling particles in the Equatorial Atlantic Ocean. In comparison, residues from the Rainbow hydrothermal site (36°14′N), have geochemical composition controlled by hydrothermal anhydrite [Chavagnac V., German C.R., Milton J.A., Palmer M.R., 2005. Sources of REE in sediment cores from the Rainbow vent site (36°14′N, MAR). Chem. Geol. 216, 329–352.]. They are characterised by 87Sr/ 86Sr ratios of 0.7087–0.7093, which are significantly less radiogenic than those of the Saharan dust. Hence, the data suggest (1) a significant contribution of aeolian material to the sediments at 14°45′N that is not present at 36°14′N, (2) that this material is most likely of North-African origin, and (3) that this source of aeolian material has remained relatively constant over the last 13 kyr.