DIFFERENTIAL MINERALIZATION OF N AND P AND THE REDOX STATE OF LATE DEVONIAN EUXINIC BASINS

Late Devonian epicontinental basins were characterized by the episodic deposition of organic rich sediments beneath a euxinic water column. Although the ultimate cause of these episodes of elevated primary productivity remains contentious, the proximal biogeochemical mechanism that maintained high rates of export production over geological time spans was the mineralization and recycling of the macronutrients nitrogen and phosphorus. This study examines a series of Late Devonian black shale intervals in an effort to determine the distinct roles of N and P in controlling productivity and the redox state of epeiric seas utilizing elemental ratios and isotopic analyses of C, N, and S. Sediment ratios of organic C to N and P are a measure of the degree to which organic N and P have been preferentially remineralized relative to C and one another. Sediment N:P represents the inverse of relative N and P availability in the water column. Biochemical constraints on the degradation of N- and P-containing organic compounds result in distinctly different patterns of mineralization and nutrient availability. The black shales examined are characterized by C:P values that vary positively with TOC in contrast to C:N values that are nearly constant across a range of TOC values. This relationship suggests an increasing N limitation on autotroph growth as productivity increased and also suggests that in situ N fixation was not a sufficient source of new N to alleviate the imbalance in N and P bioavailability. This supports the proposition that the primary source of new N for production was the outwelling flux of fixed N from newly emerging forest ecosystems.