Effect of deep Southwestern Subtropical Atlantic Ocean circulation on the biogeochemistry of mercury during the last two glacial/interglacial cycles

Abstract

During glacial/interglacial cycles, changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) modified the intermediate and deep-water mass proportions and high latitude productivity in the Atlantic Ocean. These factors influence the distribution and geochemical partitioning of trace metals in the ocean. Mercury is a redox and productivity-sensitive trace metal, making it a potential proxy of paleoenvironmental changes. Therefore, this work examines the effect of Atlantic Ocean circulation changes during the last two glacial/interglacial cycles on the biogeochemistry of Hg. For this, a high-resolution record of the total Hg concentration was determined in core GL-1090 collected from the Southwestern Subtropical Atlantic that represents the last 185 thousand years. During the reported glacial/interglacial cycles, Hg showed a distinct trend throughout Marine Isotope Stages with higher concentrations during periods of enhanced penetration of northern component water into the southwestern Atlantic. This is supported by the similarity of mercury variability with benthic foraminifera δ13C, suggesting a strong influence of deep ocean circulation on the availability and accumulation of this metal in deep-sea sediments. Mercury geochemistry and particle scavenging were correlated with organic matter (OM) input at the core site. We also noted that mercury responded to redox variation in sediment after Termination II, which can be explained by the increase in deep ocean ventilation due to AMOC strengthening. This hypothesis was confirmed by the antiphase behavior of Hg and Total Organic Carbon when compared with Mn/Al ratios and CaCO3. Our work, therefore, allows for a better understanding of the processes leading to long-term mercury removal to sediments.