Controls on N and P regeneration during organic matter (OM) decomposition, particularly the effects of oxygenation, remain a topic of debate. It has alternately been argued that: there is no preferential regeneration of either N or P relative to C under normal marine (i.e., oxic) conditions (Anderson and Sarmiento 1994); N is preferentially released under oscillating redox conditions (Aller, 1994); and P is preferentially released under anoxic conditions (Ingall and Jahnke, 1997). Upon release, remineralized nutrients may be retained in sediments (Fillipelli, 1997) or they may diffuse from sediments, becoming available for biological utilization, thereby increasing primary productivity and the potential delivery of OM to the sediments.

We hypothesized that P is preferentially released under anoxic depositional conditions, and N is preferentially released under oscillating redox depositional conditions. As a result, organic C/N/P ratios will be significantly elevated in sediments at the transition between oxic and anoxic conditions, and C/P ratios will be elevated under anoxic depositional conditions.

Four sediment cores were taken from the Orca Basin in 2003 under different depositional regimes, oxic, transitional, oscillating, and anoxic. The only factors that differ between cores taken from different parts of the basin are redox state and salinity.

Nitrogen Remineralization:

Last year, we reported on our measurements of the ratio of total organic carbon (C_org) to total nitrogen (N_tot) in these cores, which includes both organic N and inorganic N, most likely in the form of NH₃ either in pore waters or adsorbed onto clay minerals.  We have now measured C_org/N_org ratios as well. C_org/N_org ratios are near Redfield values of 8-10, suggesting no preferential degradation of N relative to C during OM remineralization, and no clear differences are noted between the various cores.  In contrast, C_org/N_total values, while mostly still near Redfield values, are significantly enriched in Core 7C (red laminated, from the transition zone with oscillating redox conditions) suggesting that this core preserves less inorganic N adsorbed to clays than the other cores.  Thus, the data suggest that under oscillating redox conditions, N_org is remineralized to inorganic N, but is not preserved in sediments through sorption to clay particles to the same extent that it is under either oxic or anoxic bottom water conditions.  Thus, it is not the remineralization of OM that is affected by the redox oscillations, but the preservation of inorganic N.

Phosphorus Remineralization:

The concentrations of sedimentary P species were determined following a sequential extraction procedure modified from Ruttenberg et al. (1992). The species determined include organic P, authigenic P (carbonate fluorapatite), P sorbed to Fe/Mn oxides & oxyhydroxides, and detrital mineral P.  C/P ratios are used to assess the potential for preferential remineralization of P compared to C under different redox conditions.  The general patterns between the different cores are the same for C_org/P_org and C_org/P_total (as well as N_org/P_org and N_org/P_total, not shown), with the highest C/P ratios in the anoxic core 5B, followed by the laminated transition core 2B, with red transition core 7C and oxic core 4b much lower, indicating that P is preferentially remineralized under anoxic relative to oxic conditions. C_org/P_oxide ratios, however, are highest in the laminated transitional core deposited under oscillating redox conditions.  The lack of preservation of P sorbed to oxides appears to be a major mechanism for the enhanced release of P from sediments under these conditions.

Conclusions:

P is preferentially remineralized relative to C or N under anoxic sedimentary conditions. Furthermore, both N and P are released preferentially from sediments relative to C under oscillating redox conditions compared to oxic conditions, though P to a greater degree.  The mechanism appears to be related to prevention of the sorption of inorganic P and N species to minerals (clays & oxides) in sediments under oscillating redox conditions. These results agree with previous studies of sedimentary systems of equivalent or older age (cf Ingall et al., 2005). In contrast, studies of water column and sediment trap OM remineralization show no preferential release of P under anoxic conditions (Benitez-Nelson et al., 2004). Thus, the timescale of study is important for the observation of preferential release of P during anoxic remineralization.  It appears that decades to centuries are required for observable differences in the remineralization of C, N, and P during diagenesis.

These results were presented at the 23^rd International Meeting on Organic Geochemistry, held in Torquay, England in September, 2007, and are currently the focus of a manuscript in preparation.  This ACS-PRF Type G grant was the first grant awarded to PI Werne as a new faculty member, and supported the research of 5 undergraduate students, 3 of whom are continuing in professional or graduate school.