Early diagenesis in differing depositional environments: The response of transition metals in pore water

Abstract

The cycling of Fe, Mn, Ni, Co, Cu, Cr, V, and Mo during early diagenesis was investigated in sediments from five different depositional environments in the California Borderland. Dissolved O 2, NO 3 −, NO 2 −, and SO 4 − were also measured at each site to establish the position of redox boundaries pertinent to this study. Sites were chosen to allow the comparison of several parameters of potential importance to the cycling of these metals: bottom water O 2 concentration; sediment redox conditions; cycling of metal oxide carrier phases and the relative contribution of biogenic and terrigenous material to the detrital metal flux. At 10 μM oxygen and above the sequence of terminal electron acceptor utilization was typical of pelagic sediments, differing only in scale. Under these conditions the recycling of Mn oxides resulted in an enriched layer near the interface. At bottom water 0 2 concentrations < 5 μM Mn oxides are not recycled. Under these conditions Fe oxides and SO 4 2− are important oxidants. The depth of the zone of manganese oxidation with respect to the interface affects the efficiency of metal scavenging by manganese oxides. Trapping and recycling of Ni and Co with Mn oxides results in remobilization from reducing sediments and enrichment in oxic sediments. Scavenging of these metals is favored by a deep oxic zone in the sediments. In contrast, Cr, V, and Mo appear to be transported to the sediments as reduced species and are released from the sediments by oxidation. The source of the reduced species is assumed to be biogenic material. The accumulation of these metals is favored by reducing conditions in the sediments. Cu is enriched in the sediments by transport with detrital biogenic material, followed by adsorption onto sediment solids. Cu is released at the interface and rapidly removed onto the solids at all sites except the one pelagic site, at the base of the slope. The magnitude of Cu released in the slowly accumulating pelagic sediment exceeds the Cu binding capacity of the solids, resulting in pore water Cu concentrations in excess of 100 nM. In general, metal cycling associated with the early diagenesis of sediments was observed to decouple transport processes from burial processes for the transition metals measured in this study.