HR: 1340h
AN: PP53B-1395    [Abstracts]
TI: $^{15}$N depleted nitrogen isotope values in Cretaceous black shales: paleoceanographic event or diagenesis.
AU: * Junium, C K
EM: cjunium@geosc.psu.edu
AF: The Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AU: Arthur, M A
EM: arthur@geosc.psu.edu
AF: The Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AB: Nitrogen isotopic values of bulk sediment samples in black shales are almost exclusively near 0 $\permil$ and C/N ratios are high (20-35). Sequential extraction of exchangeable and non-exchangeable N fractions demonstrates that the inorganic N fraction is negligible and bulk sediment data reflect the organic N fraction. The trend in \delta$$^{15}$N and C/N has been observed in numerous localities and depositional environments in the mid-Cretaceous (Demarara Rise, DSDP Sites 367, 603B and 530, the Cretaceous Western Interior Seaway, Wunstorf, Germany, and Bahloul, Tunisia) as well as the Toarcian of England and in Quaternary Mediterranean Sapropels. Three explanations are considered: 1) That primary production during black shale deposition was dominated by a unique community composed of nitrogen-fixing cyanobacteria; or 2) utilization of a $^{15}$N depleted ammonium source by another set of biota; or 3) that the values are the result of diagenetic loss of N prevailing in C$_{org}$-rich strata with a low capacity for N adsorption. The depleted nitrogen isotopic values suggest that nitrogen fixation or utilization of a depleted nitrogen source (e.g. ammonium) may have been important which is plausible in consideration of nitrogen deficiencies that might characterize widespread deep-water anoxia. Secular variations in nitrogen isotope values across the Cenomanian-Turonian Boundary of ODP Site 1261, Demarara Rise, show a shift from -1 to -3$\permil$. One interpretation of the origin of the excursion could be that a fraction of the organic matter was produced utilizing a $^{15}$N depleted ammonium source, assuming that the nitrogen isotopic composition of fixed oceanic nitrogen does not change. However, it should be noted that large (greater than 1$\permil$) variations in nitrogen isotope values are observed above and below the Cenomanian-Turonian Boundary Event and have no known paleoceanographic forcing. The variations in nitrogen isotope values are matched by antithetic variations in C/N indicating diagenetic N-loss that could be interpreted as the primary control on the nitrogen isotope variations. The high C/N ratios probably result from selective removal of N-enriched compounds in the water column and during burial diagenesis. Clay-poor, C$_{org}$-rich sediments have a low sorptive capacity, allowing diffusion of dissolved N out of the C$_{org}$-rich sediments resulting in higher C/N ratios. Pore water ammonium concentrations are mM, indicating low sorption and high rates of diffusion into overlying strata. Coupled C/N and N-isotope variations occur by the removal of a $^{15}N$ enriched fraction. Protein degradation has been suggested to result in negative isotopic shifts but would not result in large increases in C/N and the isotopic shifts are limited by the protein richness of the primary organic matter and the internal isotopic heterogeneity of the organic matter pools. Although we favor an explanation that involves a primary signature, diagenesis cannot be excluded in low $\delta$$^{15}$N values.
DE: 4805 Biogeochemical cycles (1615)
DE: 4870 Stable isotopes
DE: 4267 Paleoceanography
DE: 4802 Anoxic environments
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting