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Abstract

Constraining variations in marine N(2)-fixation over glacial-interglacial timescales is crucial for determining the role of the marine nitrogen cycle in modifying ocean productivity and climate, yet paleo-records from N2-fixation regions are sparse. Here we present new nitrogen isotope (delta(15)N) records of bulk sediment and foraminifera test-bound (FB) nitrogen extending back to the last ice age from the oligotrophic Gulf of Mexico (GOM). Previous studies indicate a substantial terrestrial input during the last ice age and early deglacial, for which we attempt to correct the bulk sediment delta(15)N using its observed relationship with the C/N ratio. Both corrected bulk and FB-delta(15)N reveal a substantial glacial-to-Holocene decrease of delta(15)N toward Holocene values of around 2.5 parts per thousand, similar to observations from the Caribbean. This delta(15)N change is most likely due to a glacial-to-Holocene increase in regional N(2)-fixation. A deglacial peak in the FB-delta(15)N of thermocline dwelling foraminifera Orbulina universa probably reflects a whole ocean increase in the delta(15)N of nitrate during deglaciation. The delta(15)N of the surface dwelling foraminifera Globigerinoides ruber and the corrected bulk delta(15)N show little sign of this deglacial peak, both decreasing from last glacial values much earlier than does the delta(15)N of O. universa; this may indicate that G. ruber and bulk N reflect the euphotic zone signal of an early local increase in N(2)-fixation. Our results add to the evidence that, during the last ice age, the larger iron input from dust did not lead to enhanced N(2)-fixation in this region. Rather, the glacial-to-Holocene decrease in delta(15)N is best explained by a response of N(2)-fixation within the Atlantic to the deglacial increase in global ocean denitrification.