The Future of Trace-Metal Proxies for Paleoceanographic Analysis
High concentrations of redox-sensitive trace metals in sediments have long been accepted as evidence of reducing conditions in ancient depositional systems. While assessment of paleo-redox conditions continues to represent an important application, recent studies have shown that trace-metal data can provide insight into other aspects of ancient marine systems as well. Work in modern restricted-marine environments has demonstrated that the concentrations of trace metals in the watermass can change as a function of removal to the sediment under conditions of limited deepwater renewal (Algeo and Lyons, 2006), providing a basis for assessment of the degree of watermass restriction in ancient anoxic marine systems (Algeo et al., 2007; McArthur et al., 2008; Rowe et al., 2008). As a consequence of variable rates of removal to the sediment, the ratios of trace metals in the deep watermass of restricted-marine systems may evolve away from that of seawater, and this signal, too, can be transferred to the sediment (Algeo and Maynard, 2008; Tribovillard et al., in prep.). At longer time scales, the trace-metal chemistry of the global ocean may evolve in response to sustained redox-driven changes in source and sink fluxes (Algeo, 2004; Scott et al., 2008), and such long-term changes may have implications for the evolution of life (Anbar and Knoll, 2002). Molybdenum has received the most attention to date owing to its comparatively high concentration in seawater and low concentration in the detrital fraction of sediments, resulting in large enrichment factors (EF = Xtot/(Xtot-Xauth)). Yet normalization to purely detrital elements (Al, Ti, or Zr) can permit estimation of the authigenic fraction of other redox-sensitive trace metals (Murray et al., 1994; Perkins et al., 2008), potentially allowing an assessment of the fluxes of multiple trace-metal proxies within a single depositional system. Trace-metal isotopes also have potential as paleoceanographic tools (Anbar and Rouxel, 2007). δ98Mo has been shown to reflect benthic redox conditions (Poulson et al., 2006) and, possibly, the rate of cycling of organic carbon in continent-margin sediments (Siebert et al., 2006). Despite its radioactive character, uranium is amenable to use as a tracer in sedimentary environments owing to the long half lives of its principal isotopes (Weyer et al., 2008). Both isotopic systems have potential for documenting the long-term redox evolution of the Earth (Siebert et al., 2005).
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009