Abstract

The depleted nitrogen isotope values observed in Devonian black shales have been ascribed to the dominant role of marine diazotrophic N fixation in providing new reactive N for the productivity that generated the organic matter preserved in these organic-rich facies. However, the emergence of substantial terrestrial ecosystems in the Middle to Late Devonian introduced an additional source of reactive N to shallow epicontinental seas. We examined three sites along a deepening transect of the Late Devonian Appalachian Basin (eastern United States) employing element ratios of C, N, and P, degree of pyritization, and stable isotope analyses of C, N, and S, in order to determine whether basinward changes in N biogeochemistry and δ15N reflect the dominant source of new reactive N. High degrees of pyritization and depleted δ34S indicate a persistently dysoxic to euxinic water column. Sediment N/P rises basinward, indicating that P was relatively more abundant than N in the water column with increasing distance from the shore. Nitrogen isotope values, however, are very consistently ∼0‰ at all three sites despite the evidence of distally increasing N limitation, suggesting that terrestrially derived reactive N was an important source of reactive N and that marine N fixation was not sufficient to address the stoichiometric deficit. Depleted δ15N values in these black shales do not provide a reliable record of diazotrophy; rather, they are a product of the reducing state of the water column and underlying sediments.

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