Trace element indicators of increased primary production and decreased water-column ventilation during deposition of latest Pliocene sapropels at five locations across the Mediterranean Sea

Abstract

Compositions of productivity- and redox-sensitive trace-metals in two Upper Pliocene sapropel sequences from western Mediterranean ODP Sites 974 and 975 and eastern Mediterranean Sites 964, 969, and 967 provide information about the conditions leading to sapropel deposition. Except at Site 975, closest to the Atlantic Ocean, all sapropels display elevated element/Al ratios for Ba, Re, Cd, Mo, U, Cr, Cu, V, and Zn. High total organic carbon (TOC) concentrations and Ba/Al ratios point to increased primary productivity during sapropel deposition. Enrichments of redox-sensitive trace elements and elevated Re/Mo ratios indicate water-column oxygen depletion during sapropel formation. Anoxia likely was caused by increased fluxes of organic matter to the seafloor that would have enhanced oxygen demand at depth. One of the sapropel sequences investigated in this study, insolation-cycle 178, contains a zone having low TOC, low Ba/Al ratios, and low trace-metal contents. These features identify this interval as an interruption of the increased productivity associated with sapropels and of the sub-oxic/anoxic conditions that are fully-developed in the upper half of this sapropel layer. Significant differences in trace-metal enrichments between the eastern and western Mediterranean indicate that the eastern basin was a focal point for sapropel initiation. In contrast, Site 975 appears to have been heavily influenced by the Atlantic Ocean during i-cycles 178 and 180 and was not strongly influenced by the paleoceanographic changes affecting the rest of the Mediterranean.