Poster de conférence
Résumé : The nitrogen isotope composition (δ15N) of microfossil-bound N recovered from Southern Ocean sediment cores provides a measure of the degree of nitrate consumption in the high-latitude ocean and has been used to formulate and test hypotheses on fluctuations in atmospheric CO2 over glacial cycles. The δ15N of the nitrate supply and the amplitude of the isotope discrimination associated with nitrate assimilation (i.e., its isotope effect) are two key parameters required to estimate the degree nitrate consumption in the past. It has been suggested that the isotope effect correlates with mixed layer depth, driven by a physiological response of phytoplankton to light availability, which introduces complexity to the interpretation of sedimentary records. However, most of these estimates derive from acidified water samples, in which volatile nitrite may have been lost during storage. Nitrite in Antarctic Zone surface waters has been observed to have extremely low δ15N (i.e., < -70‰), consistent with the expression of an equilibrium N isotope effect between nitrate and nitrite. Moreover, the occurrence of nitrate-nitrite conversion is associated with deep mixed layers, complicating interpretations linking isotope effect with mixed layer depth. Overall, the δ15N of nitrate+nitrite is likely more representative of the N available for consumption than the nitrate-only pool. We revisit the variability of the isotope effect in the Antarctic Zone using the relationship between nitrate concentration and the δ15N of both nitrate+nitrite and nitrate-only in the Atlantic, Indian, and Pacific sectors of the Antarctic Zone. The isotope effect estimated using nitrate+nitrite δ15N is relatively constant (5.6 ± 0.9‰) and shows no relationship with mixed layer depth. Values of the isotope effects derived using nitrate-only δ15N are higher and more variable (8.6 ± 1.9‰), consistent with an artifact from nitrate-nitrite interconversion. The strong relationship between nitrate concentration and nitrate+nitrite δ15N closely matches the Rayleigh fractionation trends (R2 = 0.85, p-value < 0.001). By running sensitivity tests on a diatom-N bound δ15N record, we infer that during the past two glacial-interglacial cycles, surface nitrate concentration in the Antarctic Zone was close to modern conditions (~ 24 µmol l-1) during interglacials and decreased to < 10-15 µmol l-1 during glacials.

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