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 N₂-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 (δ¹⁵N) 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 δ¹⁵N using its observed relationship with the C/N ratio. Both corrected bulk and FB- δ¹⁵N reveal a substantial glacial-to-Holocene decrease of δ¹⁵N toward Holocene values of around 2.5 %, similar to observations from the Caribbean. This δ¹⁵N change is most likely due to a glacial-to-Holocene increase in regional N₂-fixation. A deglacial peak in the FB-δ¹⁵N of thermocline dwelling foraminifera Orbulina universa probably reflects a whole ocean increase in the δ¹⁵N of nitrate during deglaciation. The δ¹⁵N of the surface dwelling foraminifera Globigerinoides ruber and the corrected bulk δ¹⁵N show little sign of this deglacial peak, both decreasing from last glacial values much earlier than does the δ¹⁵N 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 δ¹⁵N is best explained by a response of N ₂-fixation within the Atlantic to the deglacial increase in global ocean denitrification.