SUMMARY

Our results from Site 1230 indicate that the isotopic composition of pore water ammonium differs from the ¹⁵N of sedimentary organic matter by <1, pointing toward absence of significant isotopic fractionation during organic matter diagenesis. At this site, the sedimentary organic matter is of predominantly marine origin, as suggested by atomic C/N ratios of 9–11. These findings confirm the hypothesis put forth by Altabet et al. (1999), who postulated that in rapidly accumulating organic-rich coastal sediments, organic matter decomposes without significant isotopic fractionation. A similar conclusion was reached based on the results obtained from the sediments collected at Sites 1234 and 1235 along the Chile margin during the ODP Leg 202 (Prokopenko et al., in press), where no or little isotopic fractionation between N_org and pore water ammonium was observed. In those sediments, the sedimentary organic matter was found to be of marine origin as well. All three sites were characterized by rapid sediment accumulation rates, so that the sedimentary columns represent 150–200 k.y. of depositional history. The constant (±1) ¹⁵N of sedimentary N_org through depth at these sites indicates little variation in the isotopic composition of organic matter deposited during this time interval.

At Site 1227, the interpretation of the ¹⁵N of ammonium profile is complicated by several factors: addition of isotopically different ammonium from the brine and variability in concentrations and isotopic composition of deposited organic matter through time, which possibly led to a nonsteady-state condition of the current ammonium profile.

If the ammonium profile at Site 1227 is in steady state, then an effective fractionation of ~4 exists between decomposing organic matter and released ammonium, most likely due to preferential decomposition of the marine fraction of sedimentary organic matter that contains both marine and terrestrial components, as suggested by Sweeney and Kaplan (1980).

However, it appears that at Site 1227 several factors have varied through time. These factors include the weight percent of N, its lability, and its isotopic composition. If these changes have occurred in the recent past, the present profile of ammonium concentrations and its ¹⁵N might have not reached a new steady state. The time-dependent variation in ¹⁵N of the organic matter deposited on the ocean floor would result in a mixing pattern between ammonium released in the upper few meters of the sediment column and ammonium diffusing from below. The mixing between two end-members with different isotopic compositions would lead to the apparent isotopic enrichment of ammonium relative to the organic matter at depth.

Synthesizing results from Sites 1227 and 1230, we can conclude that no significant fractionation is associated with diagenesis of marine organic matter deposited at the ocean margins, such as at Site 1230. When a significant fraction of a terrestrial component is present, preferential decomposition of the marine fraction, which is usually enriched in ¹⁵N isotopes relative to terrestrial organic matter may lead to an overall decrease in ¹⁵N of bulk sediments downcore. However, further work in a less complex system than Site 1227 is required to confirm this conclusion.