Insights into nitrogen cycling in the western Gulf of California from the nitrogen isotopic composition of diatom-bound organic matter

Abstract

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.