Nutrient Utilization During the Last Glacial Maximum, Evidence From a New Diatom-bound N Isotope Method

Bulk sedimentary and diatom-bound N isotope data from the Southern Ocean are widely held as a key piece of evidence for a change in nutrient status in the Antarctic across the glacial-interglacial transition. Previously published sedimentary N isotope results, showing a decrease in nitrate utilization into the Holocene (Francois et al., 1997; Sigman et al., 1999; Crosta and Shemesh, 2002), are considered a potential indication of the high latitude ocean's role in glacial-interglacial CO₂ cycles. Microfossil-bound organic material, by virtue of its physical protection from the environment, is likely to be less vulnerable to isotopic alteration by diagenesis during sinking and burial. A new method for diatom-bound N isotopic analysis, which combines a wet chemical oxidation with the "denitrifier method" for nitrate isotopic analysis (Sigman et al., 2001), was developed in an attempt to reduce sample size so as to expand the utility of the diatom-bound proxy beyond opal-rich sedimentary settings. However, new results from Antarctic sediment cores contrast with those that have been published previously (Sigman et al., 1999; Crosta and Shemesh, 2002). There is no change in the diatom-bound N isotopes between the last glacial and the Holocene in the Atlantic and a 2‰ change in the Indian Ocean, ∼2‰ less than the shift documented by Sigman et al., (1999). These data suggest no change in the degree of nitrate utilization in the Atlantic sector and a small increase in the Indian sector of the Antarctic. The significance of these results and the role for the biological pump in glacial-interglacial CO₂ change will be discussed.