The geochemical behavior and isotopic composition of Hg in a mid-Pleistocene western Mediterranean sapropel

The concentrations of mercury (Hg) and other trace metals (Ni, Cu, Zn, Mo, Ba, Re, U) and the Hg isotopic composition were examined across a dramatic redox and productivity transition in a mid-Pleistocene Mediterranean Sea sapropel sequence. Characteristic trace metal enrichment in organic-rich layers was observed, with organic-rich sapropel layers ranging in Hg concentration from 314 to 488 ng/g (avg = 385), with an average enrichment in Hg by a factor of 5.9 compared to organic-poor background sediments, which range from 39 to 94 ng/g Hg (avg = 66). Comparison of seawater concentrations and sapropel accumulations of trace metals suggests that organic matter quantitatively delivers Hg to the seafloor. Near complete scavenging of Hg from the water column renders the sapropel Hg isotopic composition representative of mid-Pleistocene Mediterranean seawater. Sapropels have an average delta(202)Hg value of -0.91 parts per thousand +/- 0.15 parts per thousand. (n = 5, 1 SD) and Delta(199)Hg value of 0.11 parts per thousand +/- 0.03 parts per thousand (n = 5, 1 SD). Background sediments have an average 8202 Hg of -0.76 parts per thousand +/- 0.16 parts per thousand (n = 5, 1 SD) and Delta(199)Hg of 0.05 parts per thousand +/- 0.01 parts per thousand. (n = 5, 1 SD), which is indistinguishable from the sapropel values. We suggest that the sapropel isotopic composition is most representative of the mid-Pleistocene Tyrrhenian Sea. (C) 2008 Elsevier Ltd. All rights reserved.