ELEMENTAL PROXIES FOR HYDROGRAPHIC CONDITIONS IN RESTRICTED MARINE SYSTEMS

Patterns of trace metal-TOC covariation have potential to reveal changes in hydrographic conditions in marine paleoenvironments, especially when watermasses are restricted. In modern anoxic silled basins, sedimentary Mo and TOC covary positively, yielding correlation-line slopes m from ~4.5 to 45 (x 10^-4)*. Smaller values of m are associated with greater restriction of the subpycnoclinal watermass, as reflected in deepwater renewal times t_dw). This relationship exists because restriction reduces aqueous Mo concentrations ([Mo]_aq) owing to removal of Mo to the sediment at a rate faster than its resupply through deepwater renewal events. Sedimentary Mo covaries with TOC because it is controlled by the availability of organic substrates for Mo uptake as well as by [Mo]_aq. Organic-rich facies of Middle Devonian to Early Carboniferous age from the North American craton also exhibit significant Mo-TOC covariation, with m ranging from ~2 to 65 (x 10^-4)^#. Systematic patterns of geographic and stratigraphic variation in m suggest control by slowly evolving watermass characteristics of the Devono-Carboniferous North American Seaway. Based on modern analogs, deepwaters of this seaway had [Mo]_aq ranging from <20% to >70% that of modern seawater, and t_dw ranging from <100 y to >1000 y. Trace metal-TOC relationships in anoxic black shales of the Late Pennsylvanian Midcontinent Sea (LPMS) exhibit a completely different pattern of covariation, one suggesting strongly dynamic fluctuations in eustasy and redox conditions^†. The dynamic character of the LPMS is counterintuitive in view of its nearly landlocked setting and geochemical evidence of sulfidic deepwater conditions. The resolution of this conundrum is that LPMS deepwaters were renewed by lateral advection of oxygen-deficient waters from below the elevated pycnocline on the eastern margin of tropical Panthalassa. *Algeo, T.J., Lyons, T.W., 2006, Mo–TOC covariation in modern anoxic marine environments: Implications for analysis of paleoredox and -hydrographic conditions Paleoceanography, 21, PA1016, doi:10.1029/2004PA001112. ^#Algeo, T.J., Lyons, T.W., Blakey, R.C., Over, D.J., in press, Hydrographic conditions of the Devono-Carboniferous North American Seaway inferred from sedimentary Mo-TOC relationships Palaeo-3. ^†Algeo, T.J., Rowe, H., Schwark, L., Hower, J.C., Heckel, P.H., in prep., Palaeo-3.