Spatial and temporal distribution of heavy metal concentrations and enrichment in the southern Gulf of Mexico.

Trace element (As, Co, Cr, Cu, Ni, Pb, V and Zn) enrichment trends during the past century, were assessed in thirteen 210Pb-dated sediment cores from the southern Gulf of Mexico, with the purpose to evaluate the impact on the environment, and potentially on public health, of the offshore oil industry and of oil spills such as that of the Ixtoc1 well blowout in 1979. The trace element composition was quite homogeneous among cores; and the pre-industrial concentrations of Ba, Cr, Cu and Ni are naturally high in the region, as to reach levels of potential ecological concern. The influence of multiple and simultaneous processes (e.g. industrial activities, natural seeps, fluvial discharges) on the trace element concentrations is difficult to disentangle. Some cores suggested long-term preservation of putative oil spill traces, although it was not possible to attribute their origin. The Al-normalized redox element ratios, and the crude oil contamination ratio, suggested that these events occurred along almost four decades, and that the traces attributed to the Ixtoc1 spill were comparable to background conditions, most likely owing to active natural oil seeps in the area. In most cases there was a trend towards a lowering in the supply of trace elements; this might be associated with environmental controls in the region since the 1980s. This study highlights the relevance of using dated environmental archives to reconstruct the historical trends of trace metal contamination in areas where long-term environmental studies are scarce.