HR: 0830h
AN: PP41B-0841    [PDF]
TI: Organic matter diagenesis and nitrogen isotopes in Cretaceous black shales ODP Leg 207, Demarara Rise.
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:
AF: The Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AU: ODP Leg 207, S
EM: malone@odpmail.tamu.edu
AF: The Ocean Drilling Program, 1000 Discovery Drive, College Station, TX 77845 United States
AB: Excellent recovery of Cretaceous black shales on Ocean Drilling Program Leg 207, Demarara Rise has provided a unique opportunity to study organic matter diagenesis. Interpretation of paleoceanographic signals requires a sound understanding and assessment of changes to sediment chemistry and proxies. The 90m thick sequence of black shales recovered show evidence of continuing microbial activity and it is highly likely that these reactions have imparted chemical change on paleoceanographic proxies. Nitrogen isotopes are good tracers of such changes due such mircrobially mediated reactions as denitrification and deamination of organic nitrogen, and its subsequent conversion to N$_{2}$ and ammonia respectively. Nitrogen isotope data are reported for a 1.8 m high resolution, 18 sample section from Site 1260B (426.2-428 meters composite depth). Carbonate free and exchangeable ammonia-extracted (2M KCl) $\delta^{15}$N pairs were run for each sample. Additionally, $\delta^{13}$C$_{org}$, $\delta^{13}$C$_{carb}$, $\delta^{18}$O$_{carb}$, ethylene glycol monoethylether surface area (EGME SA), and C and N elemental data have been obtained. The samples have very high TOC % (10-15 wt.%) and average C/N ratios of 25. Values average -1.13 $\permil$ for $\delta^{15}$N$_{bulk}$, and -0.97 $\permil$ for $\delta^{15}$N$_{KCl}$,. The high C/N ratios and depleted $\delta^{15}$N, are typical for black shales but are significantly different from modern marine OM. The +0.16 per mil average difference between $\delta^{15}$N$_{bulk}$ and $\delta^{15}$N$_{KCl}$ indicates sub-sediment microbial degradation and $\delta^{15}$N, depleted ammonia production. Samples characterized by significant differences between $\delta^{15}$N$_{bulk}$ and $\delta^{15}$N$_{KCl}$ also have elevated C/N ratios, supporting a loss of nitrogen as ammonia. EGME SA show a positive correlation with $\delta^{15}$N (r$^{2}$ =.73) with $\delta^{15}$N$_{bulk}$ that may be the result of a preferential sorption of relatively 15N enriched ammonia on mineral surfaces. EGME SA values range between 3-3.5 m$_{2}$ /g. These values are relatively low despite the very high TOC% implying a nominal role of sorbative protective processes in OM preservation in this depositional setting.
DE: 1045 Low-temperature geochemistry
DE: 1055 Organic geochemistry
DE: 1615 Biogeochemical processes (4805)
DE: 4267 Paleoceanography
DE: 4870 Stable isotopes
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting