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Isotopic constraints on glacial/interglacial changes in the oceanic nitrogen budget
Author(s) -
Deutsch Curtis,
Sigman Daniel M.,
Thunell Robert C.,
Meckler Anele,
Haug Gerald H.
Publication year - 2004
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/2003gb002189
Subject(s) - denitrification , water column , interglacial , nitrate , glacial period , oceanography , geology , sediment , last glacial maximum , holocene , nitrogen , chemistry , geomorphology , organic chemistry
We investigate the response of the 15 N/ 14 N of oceanic nitrate to glacial/interglacial changes in the N budget, using a geochemical box model of the oceanic N cycle that includes N 2 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 N 2 fixation/denitrification. This response is compared to the available paleoceanographic data, which suggest an early deglacial maximum in nitrate 15 N/ 14 N in suboxic zones and no significant glacial‐to‐late Holocene change in global ocean nitrate 15 N/ 14 N. Consistent with the work of Brandes and Devol [2002], we find that the steady state 15 N/ 14 N 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 15 N/ 14 N 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 15 N/ 14 N. For this reason, modeling a realistic deglacial 15 N/ 14 N maximum further requires that the combined negative feedbacks from N 2 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.