Deglacial nitrogen isotope changes in the Gulf of Mexico: Evidence from bulk sedimentary and foraminifera‐bound nitrogen in Orca Basin sediments

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 N 2 ‐fixation regions are sparse. Here we present new nitrogen isotope ( δ 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 δ 15 N using its observed relationship with the C/N ratio. Both corrected bulk and FB‐ δ 15 N reveal a substantial glacial‐to‐Holocene decrease of δ 15 N toward Holocene values of around 2.5 ‰, similar to observations from the Caribbean. This δ 15 N change is most likely due to a glacial‐to‐Holocene increase in regional N 2 ‐fixation. A deglacial peak in the FB‐ δ 15 N of thermocline dwelling foraminifera Orbulina universa probably reflects a whole ocean increase in the δ 15 N of nitrate during deglaciation. The δ 15 N of the surface dwelling foraminifera Globigerinoides ruber and the corrected bulk δ 15 N show little sign of this deglacial peak, both decreasing from last glacial values much earlier than does the δ 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 δ 15 N is best explained by a response of N 2 ‐fixation within the Atlantic to the deglacial increase in global ocean denitrification.