INTRODUCTION

Nitrogen is one of the important limiting nutrients in the ocean (Gruber, 2004; Redfield et al., 1963; Tyrell, 1999). The global carbon cycle, and, consequently, atmospheric CO₂ might be tightly coupled to the nitrogen cycle (Berger and Keir, 1984; Broecker, 1982), and therefore changes in the magnitude of the sinks and sources of fixed nitrogen in the oceans can significantly influence the global climate (Falkowski, 1997; Ganeshram et al., 1995; Sigman et al., 1999; Sigman and Boyle, 2000).

Biological nitrogen fixation, denitrification, and consumption of nitrate by phytoplankton, the major biological processes of the global nitrogen cycle, can each imprint a distinct isotopic signature (¹⁵N) on oceanic nitrate and on the phytoplankton that assimilate this nitrate as a nitrogen source (Altabet and Francois, 1994; Haug et al., 1998). Changes in ocean circulation and nutrient supply, which occur in response to changes in environmental conditions, affect the relative importance and spatial extent of the major pathways of the nitrogen cycle; these variations may be recorded in the isotopic ratio of marine phytoplankton, making ¹⁵N of organic matter buried in marine sediments a sensitive paleoceanographic proxy (Altabet et al., 1991; Altabet and McCarthy, 1985; Francois et al., 1992).

A critical question is whether diagenetic processes fractionate nitrogen isotopes in the buried organic matter, and, if they do, what is the sign and magnitude of this fractionation. A detailed review of previous work addressing this question is given in Prokopenko (2004). A brief summary is provided below. A few laboratory studies have directly addressed the effects of degradation on the isotopic composition of organic matter. The principal reaction in protein degradation involves peptide bond rupture by hydrolysis, which is the principal reaction in protein degradation. Silfer et al. (1992) experimentally demonstrated kinetic fractionation of nitrogen isotopes during abiotic peptide bond hydrolysis, leading to a 2–4 enrichment of ¹⁵N in the residual substrate. Macko and Estep (1984) and Macko et al. (1993, 1983, 1986) showed that both peptide bond hydrolysis and deamination result in the enrichment of ¹⁵N in the residual material with the fractionation factor of ~4. In a series of incubation experiments, Lehmann et al. (2002) showed that degradation of organic matter under aerobic conditions leaves the residual biomass enriched in ¹⁵N isotopes, whereas anoxic decomposition of organic matter results in depletion of ¹⁵N. He interpreted the latter case as evidence for bacterial growth, accompanied by the assimilation of ammonium into newly formed bacterial biomass because bacteria grown on ammonium as the sole nitrogen source produce biomass significantly depleted in ¹⁵N compared to the original substrate (Hoch et al., 1992).

These experimental findings were supported by field observations made a few years later. Sigman et al. (1999), working with the sediments from the Southern Ocean, and Sachs and Repeta (1999), examining the sapropels from the Mediterranean Sea, found evidence of diagenetic alteration of ¹⁵N in bulk sediment: a 2–5 positive shift relative to unaltered organic matter. On the other hand, based on their work in coastal sediments, Altabet et al. (1999) and Pride et al. (1999) argued that in the rapidly accumulating organic-rich sediments of the eastern tropical North Pacific, early diagenesis does not affect isotopic composition of sedimentary organic matter, if a significant fraction of original sedimentary organic matter is preserved.

To summarize, previous research has shown that under some geochemical conditions, bacterial degradation may lead to changes in nitrogen isotopic ratios of preserved organic matter. However, the degree and direction of isotopic fractionation in marine sediments, as well as the factors controlling them, remain poorly understood.

In this study, we address the problem of diagenetic fractionation of nitrogen isotopes by constructing isotopic mass balances for the sedimentary organic nitrogen and pore water ammonium, which is a major metabolic product of organic matter decomposition. If ammonium is not involved in other diagenetic reactions, its isotopic composition should reflect the isotopic composition of organic nitrogen plus fractionation associated with diagenesis. Cores retrieved during Ocean Drilling Program (ODP) expeditions provide a unique opportunity to evaluate the effect of diagenetic processes on the nitrogen isotopic composition of sedimentary organic matter on a time scale of hundreds of thousand years to million years. Here we present the results from two ODP Leg 201 sites, 1230 and 1227, which represent two geochemically distinct environments.