CHRONOLOGY OF ANTHROPOGENIC RHENIUM AS DERIVED FROM BLACK CORAL RECORDS, GULF OF MEXICO

The concentration and the distribution of redox sensitive elements in sediment, such as rhenium (Re), have been used to investigate paleo-redox conditions. The utility and application of Re as a paleo-redox indicator requires a better understanding of the geochemical behavior, such as the sources, transport, and sinks. Natural sources of Re include weathering of organic-rich shales in which Re is easily mobilized, and delivered to the oceans primarily by rivers. Recent examination of major world rivers suggests that Re enrichment is due to human activity. An anthropogenic enhancement of Re delivery to modern oceans might be expected given high Re concentrations in fossil fuels. However, chronological variations of anthropogenic Re accumulated in sediments cannot be accurately determined due to its redox sensitive behavior.

Biogenic archives can be used as historical recorders of trace metal pollution and variability in marine environments. Uptake by marine organisms plays a critical role in the sequestration, transport and transformation of trace metals in the ocean. As some of the longest living marine organisms, deep-sea corals have an ability to serve as paleo-recorders of the marine environment. Using newly developed LA-ICP-MS methods to measure Re concentrations in black corals collected from the Gulf of Mexico, we present Re variability over the last millennia. Rhenium concentrations in the black coral skeleton, composed of chitin and protein, are elevated in the last 150-200 years, coincident with the rise in coal production in the US in the late 1800s. The Mississippi River Watershed encompasses two of the major US coal producing regions and because of the high mobility of Re in oxic waters, Re released to the environment can be easily leached from ash fallout and carried to the rivers. With the Mississippi River draining at least 40% of the contiguous US, the Mississippi River is a natural conduit for Re flux to the Gulf of Mexico. We found that the enriched Re coral signal is coupled to increased coral nitrogen isotope values. The synchronous rise in Re and δ¹⁵N is indicative of increased fluvial input from the adjacent watershed. Therefore, activities in the Mississippi watershed including those associated with industry and agriculture are captured in the black coral skeleton record.