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The nitrogen isotopic composition of organic matter contained within diatom frustules of different sediment size fractions from the Guaymas Basin, Gulf of California, suggests that multiple nutrient cycling processes can be examined from a single sediment sample. The diatom-bound δ15N values from the greater and less than 63 μm size fractions were analyzed from six intervals of core MD02–2517, each representing ∼30–40 years. The diatom-bound δ15N values are compared to bulk sedimentary δ15N values, weight percent biogenic opal, and weight percent organic carbon. Bulk sedimentary δ15N values range from 10‰ to 12.5‰ and diatom-bound δ15N values of the 63 μm fraction range from 1‰ to 7‰. The best explanation for the low diatom δ15N values in the >63 μm fraction is that the large diatoms grew at the base of the euphotic zone in excess nitrate and under low light conditions. Large diatom δ15N values correlate significantly with bulk sedimentary δ15N values (r = 0.68, 0.82), indicating that large diatoms, such as Thalassiothrix longissima, are an important component of export production fueled by a nonupwelling source of nitrogen. The >63 μm diatoms are estimated to contribute between 3% and 15% of the total nitrogen in the sediments based upon an assumed Si-to-N ratio of 1.2.

[1] Below its sill depth, the Santa Barbara Basin (SBB) is commonly suboxic ([O2] ∼ 3 μM), with only brief periods of ventilation. Associated with development of suboxia, the concentration of nitrate decreases with depth into the basin without an associated decrease in phosphate, indicating that a substantial fraction of the nitrate supplied to the basin is removed by denitrification. Coincident with the decrease in nitrate concentration across the “redoxcline” (the interface between oxic and suboxic waters) within the SBB, there is an increase in the 15N/14N of that nitrate, as would be anticipated from the isotopic fractionation associated with denitrification. However, the increase in 15N/14N of nitrate is much smaller than occurs in the open eastern tropical North Pacific (ETNP) for a comparable amount of nitrate loss. Both the concentrations of N species within the basin and measurements of nitrate 18O/16O suggest that the lower-than-expected 15N enrichment in the suboxic SBB involves denitrification, rather than being due to some unknown source of low-15N/14N N to the deep SBB. Calculations with a range of models of nitrate supply and consumption indicate that the degree of nitrate consumption in the basin is too small for differences in water circulation to explain the isotopic differences between the Santa Barbara Basin and the open ETNP. Previous studies indicate that the isotope effect of sedimentary denitrification is negligible due to nitrate diffusion in sediment pore waters. Thus we infer that the small magnitude of the isotopic enrichment of SBB water column nitrate is due to the importance of sedimentary denitrification within the basin. Assuming that water column and sedimentary denitrification have isotope effects of 25 and 1.5 per mil, respectively, our results suggest that sedimentary denitrification accounts for more than 75% of the nitrate loss within the suboxic SBB.