This study addresses the problem of diagenetic fractionation of 15N in sedimentary organic matter by constructing isotopic mass balances for the sedimentary nitrogen and pore water ammonium at two Ocean Drilling Program (ODP) sites, 1227 and 1230. At Site 1230, ammonium production flux integrated through the sedimentary column indicates that >60% of organic matter is lost to decomposition. The 15N of pore water ammonium is <0.7 different from that of the sedimentary organic matter, which implies that very little isotopic fractionation is associated with degradation of organic matter at this site. The constant 15N of the solid-phase sedimentary nitrogen through the whole profile supports this conclusion. Atomic C/N ratios (9–12) indicate that organic matter at this site is primarily of marine origin. At Site 1227, the sedimentary organic matter appears to be a mixture of terrestrial and marine components. Ammonium is ~4 heavier than the organic matter. The observed isotopic enrichment of pore water ammonium relative to the sedimentary nitrogen might indicate either the preferential decomposition of isotopically heavier marine fraction of the organic matter, or possibly, a nonsteady-state condition of the ammonium concentration and 15N profiles. Interpretation of the results at Site 1227 is further complicated by the contribution of ammonium with 15N of ~4 that is diffusing upward from Miocene brines.
1Prokopenko, M.G., Hammond, D.E., Spivack, A., and Stott, L., 2006. Impact of long-term diagenesis on 15N of organic matter in marine sediments: Sites 1227 and 1230. In Jørgensen, B.B., D'Hondt, S.L., and Miller, D.J. (Eds.), Proc. ODP, Sci. Results, 201 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/201_SR/117/117.htm>. [Cited YYYY-MM-DD]
2Department of Earth Sciences, University of Southern California, 3651 University Avenue, Los Angeles CA 90089-0740, USA.
3Present address: Department of Geosciences, Princeton University, Guyot Hall, Washington Road, Princeton NJ 08544, USA. email@example.com
4Graduate School of Oceanography, University of Rhode Island, Bay Campus, South Ferry Road, Narragansett RI 02882, USA.
Initial receipt: 30 August 2004
Acceptance: 24 July 2006
Web publication: 1 September 2006