Nitrogen isotopic evidence for a poleward decrease in surface nitrate within the ice age Antarctic

Surface sediment diatom-bound delta N-15 along a latitudinal transect of 170 degrees W shows a previously unobserved increase to the South of the Antarctic Polar Front. The southward delta N-15 increase is best explained by the combination of two changes toward the South, a decrease in the isotope effect of nitrate assimilation (epsilon) and an increase in the degree of nitrate consumption, both associated with shoaling of the mixed layer into the seasonal ice zone (SIZ). New downcore records show high amplitude changes in diatom-bound delta N-15 during the last ice age, with intervals of higher delta N-15, including the last glacial maximum, the transition between marine isotope stages 5 and 4, and marine isotope stage 6, while other intervals are similar in delta N-15 to interglacial sediments. Variation in the range of 0-3 parts per thousand, as seen in previously published records, may be entirely due to changes in e. However, the observed magnitude of the change of 4-10 parts per thousand in the three new records and the locations of these records relative to the modern meridional gradient in mixed layer depth appear to require increased nitrate consumption to explain the high-delta N-15 intervals. The new sites are near the modern Southern Antarctic Circumpolar Current Front (SACCF), and one of the sites has been shown to be associated with sporadic summer sea ice during the LGM. As with other Antarctic sites, the available proxy data suggest that they were characterized by lower export production. Based on these and other observations, we propose that the weak southward nitrate decrease in the modern Antarctic surface was a fully developed "nutrient front" in the glacial Antarctic, associated with the SACCF. Both modern ocean and paleoceanographic work is needed to test this hypothesis, which would have major implications for atmospheric CO2 (C) 2008 Published by Elsevier Ltd.