Paleoceanographic applications of trace-metal concentration data

Recent studies have identified a range of new applications of trace-metal concentration data in the analysis of paleoceanographic systems. In restricted anoxic marine systems, trace-metal/TOC ratios can provide insight into the degree of watermass restriction and estimates of deepwater renewal times. In such systems, secular changes in sediment trace-metal ratios may provide evidence of the chemical evolution of basinal deepwaters in response to differential rates of trace-metal removal to the sediment. The degree of deepwater restriction in silled basins is generally controlled by eustatic elevations, with higher (lower) sea levels resulting in lesser (greater) watermass restriction. Short-term (ca. million-year) drawdown of the trace-metal inventory of seawater has occurred repeatedly in conjunction with oceanic anoxic events, which result in elevated rates of trace-metal removal to widespread anoxic facies. Long-term (eon-scale) changes of the trace-metal composition of seawater have occurred in response to secular changes in atmospheric–oceanic redox conditions and their effects on trace-metal cycling. Caution must be exercised in evaluating trace-metal patterns in paleomarine systems, however: (1) hydrographic analyses based on trace-metal/TOC relationships can be undertaken only on systems that had anoxic deepwaters, and (2) the influence of redox variation on trace-metal accumulation patterns outweighs that of hydrographic factors in some paleomarine systems.