SUMMARY

Nitrogen isotopic composition of pore water ammonium and N_org were examined at Sites 1234 and 1235. Mass balance calculations suggest that ~10%–30% of organic matter buried below the 1.45-mcd horizon is degraded between this interval and 42 mcd (Site 1235) to 62 mcd (Site 1234). The ¹⁵N of ammonium released in the processes of degradation closely resembles the isotopic composition of _org at these sites. The similarity between the pore water ammonium and N_org ¹⁵N observed through the sediment column at Sites 1234 and 1235 implies that no significant isotopic fractionation is associated with decomposition of organic matter in these sediments on the timescale of hundreds of thousands of years. Two factors may have determined the absence of isotopic fractionation:

1. A high degree of organic matter preservation due to rapid sediment accumulation rates at both sites.
2. The predominantly marine nature of sedimentary organic matter at both sites, with only a small fraction of terrestrial organic material.