Isotopic constraints on glacial/interglacial changes in the oceanic nitrogen budget

We investigate the response of the ¹⁵N/¹⁴N of oceanic nitrate to glacial/interglacial changes in the N budget, using a geochemical box model of the oceanic N cycle that includes N₂ fixation and denitrification in the sediments and suboxic water column. This model allows us to quantify the isotopic response of different oceanic nitrate pools to deglacial increases in water column and sedimentary denitrification, given a range of possible feedbacks between nitrate concentration and N₂ fixation/denitrification. This response is compared to the available paleoceanographic data, which suggest an early deglacial maximum in nitrate ¹⁵N/¹⁴N in suboxic zones and no significant glacial-to-late Holocene change in global ocean nitrate ¹⁵N/¹⁴N. Consistent with the work of Brandes and Devol [2002], we find that the steady state ¹⁵N/¹⁴N of oceanic nitrate is controlled primarily by the fraction of total denitrification that occurs in the water column. Therefore a deglacial peak in the ratio of water column-to-sediment denitrification, caused by either a strong feedback between water column denitrification and the N reservoir or by an increase in sediment denitrification due to sea level rise, can explain the observed deglacial ¹⁵N/¹⁴N maximum in sediments underlying water column denitrification zones. The total denitrification rate and the mean ocean nitrate concentration are also important determinants of steady state nitrate ¹⁵N/¹⁴N. For this reason, modeling a realistic deglacial ¹⁵N/¹⁴N maximum further requires that the combined negative feedbacks from N₂ fixation and denitrification are relatively strong, and N losses are relatively small. Our results suggest that the glacial oceanic N inventory was at most 30% greater than today's and probably less than 10% greater.