Trace-metal covariation as a guide to water-mass conditions in ancient anoxic marine environments

Abstract

Patterns of sedimentary trace-metal variation can provide information not only about benthic redox conditions, but also about other water-mass properties in ancient marine depositional systems. Trace metals such as Mo, U, V, and Re display conservative concentration profiles in the global ocean but varying concentration profiles in modern anoxic silled basins (e.g., the Black Sea, Cariaco Basin, and Framvaren Fjord) as a consequence of enhanced sedimentary uptake, water-mass mixing, microbial cycling, and other processes. Because basin-specific patterns of aqueous trace-metal concentrations can be recorded by the sediment, chemostratigraphic studies of ancient anoxic marine facies have the potential to provide information about the degree of water-mass restriction as well as secular changes in aqueous chemistry and basin hydrography. In the Upper Pennsylvanian Hushpuckney and Stark shales of midcontinent North America, strong positive covariation among major trace metals supports extraction from a water mass of unmodified “normal” seawater chemistry with control of sedimentary trace-metal uptake primarily by benthic redox variation. This inference is consistent with unrestricted renewal of deep waters of the Late Pennsylvanian Midcontinent Sea via lateral advection of oxygen-deficient intermediate waters of the eastern tropical Panthalassic Ocean through a deep-water corridor in the Greater Permian Basin region. In contrast, trace metals in the Upper Devonian Ohio Shale of eastern North America exhibit divergent stratigraphic trends: relative to total organic carbon (TOC), V and Zn increase, Ni is constant, and Mo and U decrease upsection. This pattern records secular evolution of the aqueous chemistry of the silled Appalachian Basin in response to an increase in restriction of deep-water exchange. Basinal restriction intensified during the later stages of deposition of the Ohio Shale as a consequence of the onset of a major glacioeustatic regression that culminated at the Devonian-Carboniferous system boundary. Analysis of trace-metal patterns in other ancient anoxic marine systems has the potential to yield new insights regarding hydrographic variables such as rates of deep-water renewal and the degree of evolution of water-mass chemistry, in addition to information about paleoredox conditions.