[1] Nitrogen isotopes are an important tool for evaluating past biogeochemical cycling from the paleoceanographic record. However, bulk sedimentary nitrogen isotope ratios, which can be determined routinely and at minimal cost, may be altered during burial and early sedimentary diagenesis, particularly outside of continental margin settings. The causes and detailed mechanisms of isotopic alteration are still under investigation. Case studies of the Mediterranean and South China Seas underscore the complexities of investigating isotopic alteration. In an effort to evaluate the evidence for alteration of the sedimentary N isotopic signal and try to quantify the net effect, we have compiled and compared data demonstrating alteration from the published literature. A >100 point comparison of sediment trap and surface sedimentary nitrogen isotope values demonstrates that, at sites located off of the continental margins, an increase in sediment 15 N/ 14 N occurs during early burial, likely at the seafloor. The extent of isotopic alteration appears to be a function of water depth. Depth-related differences in oxygen exposure time at the seafloor are likely the dominant control on the extent of N isotopic alteration. Moreover, the compiled data suggest that the degree of alteration is likely to be uniform through time at most sites so that bulk sedimentary isotope records likely provide a good means for evaluating relative changes in the global N cycle.Citation: Robinson, R. S., et al. (2012), A review of nitrogen isotopic alteration in marine sediments, Paleoceanography, 27, PA4203,

Abstract. Fixed nitrogen (N) loss to biogenic N 2 in intense oceanic O 2 minimum zones (OMZ) accounts for a large fraction of the global N sink and is an essential control on the ocean's N-budget. However, major uncertainties exist regarding microbial pathways as well as net impact on the magnitude of N-loss and the ocean's overall N-budget. Here we report the discovery of a N-loss hotspot in the Peru OMZ associated with a coastally trapped mesoscale eddy that is marked by an extreme N-deficit matched by biogenic N 2 production, high NO − 2 levels, and the highest isotope enrichments observed so far in OMZ's for the residual NO − 3 . High sea surface chlorophyll in seaward flowing streamers provides evidence for offshore eddy transport of highly productive, inshore water. Resulting pulses in the downward flux of particles likely stimulated heterotrophic dissimilatory NO − 3 reduction and subsequent production of biogenic N 2 within the OMZ. A shallower biogenic N 2 maximum within the oxycline is likely a feature advected by the eddy streamer from the shelf. Eddy-associated temporal-spatial heterogeneity of N-loss, mediated by a local succession of microbial processes, may explain inconsistencies observed among prior studies. Similar transient enhancements of N-loss likely occur within all other major OMZ's exerting a major influence on global ocean N and N isotope budgets.

We investigate the influence of chloride concentration on the performance of the chemical reduction method for measurement of the nitrogen isotopic ratio (δ 15 N) in NO 3 -in natural waters (McIlvin and Altabet, 2005). In this method, NO 3 -is first reduced to NO 2 -using activated cadmium metal, with further reduction to N 2 O using sodium azide in an acetic acid buffer. N 2 O is introduced into an isotope ratio mass spectrometer (IRMS) for isotopic measurement. Previously, it was recognized that the presence of halides was necessary for the speed and efficiency of the second step but not thought to be important for the first step. Whereas quantitative Cd reduction of NO 3 -to NO 2 -had been noted for seawater samples, here we report, for freshwater and low-salinity (S < 30) samples, a variable conversion efficiency (both under-and overreduction were observed) and significant variation in δ 15 N determination. Addition of 5 M NaCl to all samples resulted in rapid (<4 h) and quantitative (>99%) reduction of NO 3 -to NO 2 -as well as stable δ 15N values that closely matched expected values for standards (within 0.3‰ of standard value). The positive effect of NaCl is likely due to a decrease in free Cd 2+ produced over the course of the reaction due to formation of CdCl 2 .*Corresponding author: E-mail: eryabenko@ifm-geomar.de AcknowledgmentsThe authors thank Frank Malien, Annette Kock, and Gert Petrick for technical assistance. The work was supported by the DFG-funded "Future Ocean" Excellence Cluster and Sonderforschungsbereich 754 "Climate-Biogeochemistry Interactions in the Tropical Ocean." Limnol. Oceanogr.: Methods 7, 2009, 545-552 © 2009, by the American Society of Limnology and Oceanography, Inc. LIMNOLOGY and OCEANOGRAPHY: METHODSdeveloped by McIlvin and Altabet (2005), which uses azide for quantitative nitrite conversion to N 2 O for the isotopic analysis of seawater and freshwater. The method allows separate analysis of nitrite without interference from the isotopic signature of nitrate and has a standard deviation of less than 0.2‰ for δ 15 N in nitrate samples ranging in concentration from 40 to 0.5 µM. We refer to the McIlvin and Altabet method as MA (2005).For NO 3 -concentration measurements (e.g., by an autoanalyzer), a quantitative (100%) reduction is not necessary, because NO 3 -standards are run under exactly the same conditions as the samples, so that sample concentrations are corrected for any N analysis, on the other hand, a quantitative (100%) reduction of NO 3 -is essential to avoid potentially large and variable isotopic fractionation.Application of the MA (2005) method at IFM-GEOMAR includes analysis of samples collected from the Atlantic and Pacific Oceans and the Baltic Sea covering a wide range in salinity (equivalent to 0-0.5 M NaCl). A salinity effect on NO 3 -reduction yields has been discussed in several articles, which examined the effect of Cd column methods for NO 3 -concentration measurements (Gal et al. 2004, Nydahl 1976. For example, Nydahl (1976) argued: "Conside...

Abstract. Fixed nitrogen (N) loss to biogenic N2 in intense oceanic O2 minimum zones (OMZ) accounts for a large fraction of the global N sink and is an essential control on the ocean's N budget. However, major uncertainties exist regarding microbial pathways as well as net impact on the magnitude of N-loss and the ocean's overall N budget. Here we report the discovery of a N-loss hotspot in the Peru OMZ associated with a coastally trapped mesoscale eddy that is marked by an extreme N deficit matched by biogenic N2 production, high NO2− levels, and the highest isotope enrichments observed so far in OMZ's for the residual NO3−. High sea surface chlorophyll (SSC) in seaward flowing streamers provides evidence for offshore eddy transport of highly productive, inshore water. Resulting pulses in the downward flux of particles likely stimulated heterotrophic dissimilatory NO3− reduction and subsequent production of biogenic N2. The associated temporal/spatial heterogeneity of N-loss, mediated by a local succession of microbial processes, may explain inconsistencies observed among prior studies. Similar transient enhancements of N-loss likely occur within all other major OMZ's exerting a major influence on global ocean N and N isotope budgets.

We present new data for the stable isotope ratio of inorganic nitrogen species from the contrasting oxygen minimum zones (OMZs) of the Eastern Tropical North Atlantic, south of Cape Verde, and the Eastern Tropical South Pacific off Peru. Differences in minimum oxygen concentration and corresponding N-cycle processes for the two OMZs are reflected in strongly contrasting &delta;¹⁵N distributions. Pacific surface waters are marked by strongly positive values for &delta;¹⁵N-NO₃^&minus; reflecting fractionation associated with subsurface N-loss and partial NO₃^&minus; utilization. This contrasts with negative values in NO₃^&minus; depleted surface waters of the Atlantic which are lower than can be explained by N supply via N₂ fixation. We suggest the negative values reflect inputs of nitrate, possibly transient, associated with deposition of Saharan dust. Strong signals of N-loss processes in the subsurface Pacific OMZ are evident in the isotope and N₂O data, both of which are compatible with a contribution of canonical denitrification to overall N-loss. However the apparent N isotope fractionation factor observed is relatively low (&epsilon;_d = 11.4 &permil;) suggesting an effect of influence from denitrification in sediments. Identical positive correlation of N₂O vs. AOU for waters with oxygen concentrations [O₂] > 50 μmol l⁻¹ in both regions reflect a nitrification source. Sharp decrease in N₂O concentrations is observed in the Pacific OMZ due to denitrification under oxygen concentrations O₂ < 5 μmol l⁻¹