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Basinward nitrogen limitation demonstrates role of terrestrial nitrogen and redox control of delta 15N in a Late Devonian black shale

Michael L. Tuite and Stephen A. Macko
Basinward nitrogen limitation demonstrates role of terrestrial nitrogen and redox control of delta 15N in a Late Devonian black shale
Geology (Boulder) (August 2013) 41 (10): 1079-1082

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 delta (super 15) N reflect the dominant source of new reactive N. High degrees of pyritization and depleted delta (super 34) S 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 approximately 0 per mil 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 delta (super 15) N 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.


ISSN: 0091-7613
EISSN: 1943-2682
Coden: GLGYBA
Serial Title: Geology (Boulder)
Serial Volume: 41
Serial Issue: 10
Title: Basinward nitrogen limitation demonstrates role of terrestrial nitrogen and redox control of delta 15N in a Late Devonian black shale
Affiliation: University of Virginia, Department of Environmental Sciences, Charlottesville, VA, United States
Pages: 1079-1082
Published: 20130812
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 44
Accession Number: 2013-070789
Categories: Isotope geochemistryStratigraphy
Document Type: Serial
Bibliographic Level: Analytic
Annotation: GSA Data Repository item 2013299
Illustration Description: illus. incl. 1 table, sketch map
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201342
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