[1] Using batch cultures, the 15 N/ 14 N (hereafter d 15 N) of diatom-bound organic matter was measured and compared to the d 15 N of total diatom biomass during the progressive consumption of a nitrate pool in four polar diatom species (Fragilariopsis cylindrus, Fragilariopsis kerguelensis, Pseudo-nitzschia seriata, and Thalassiosira nordenskioeldii) and one temperate species (Thalassiosira aestivalis). In general, the d 15 N of the dissolved nitrate in seawater was greater than that of the biomass, which was greater than that of the diatom-bound N. Rayleigh-type relationships were observed, allowing for estimation of the isotope effect (") for each species, with a range from 1.0‰ to 14.0‰ across all species. For all cultured strains, the (2) a low-d 15 N component of the N that we measure as diatom frustule-bound is lost during early diagenesis. Given documented assemblage changes, our culture data for relevant species do not suggest that the higher d 15 N DB observed in the Antarctic during ice ages can be explained by species related changes in the sedimentary bulk-to-diatom-bound isotopic difference. Future work on the diatom-bound material in cultured diatoms grown under in situ nutrient conditions, analysis of sediment trap and net tow material, and frustule dissolution experiments will more completely assess this paleoproxy.

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The Southern Ocean plays a critical role in the air-sea CO 2 balance through biological and physical mechanisms. Vertical supply of deep waters returns nutrients and CO 2 to the surface and stimulates phytoplankton growth. Photosynthesis in the Southern Ocean is limited by iron and only a fraction of the carbon and nutrients that return to the surface are consumed for potential sequestration in the deep sea. Here we present the most spatially extensive data set of silicon and nitrogen isotope measurements from diatom frustules to date to examine the controls on nutrient drawdown during the last glacial period and across the glacial termination in both the Antarctic and Subantarctic zones. The new data confirm existing views that differing silicon and nitrate consumption patterns in the Antarctic zone are likely the result, at least in part, of iron addition during the last glacial maximum (LGM). However, earlier in the glacial, a more coordinated response in the two proxy records, with both reflecting enhanced consumption during episodes of increased iron accumulation and export production, implies a different system response than observed for the LGM. A collapse of the expected equatorward gradient in silicon isotope values and contraction of the nitrogen isotope gradient during the deglaciation suggests that nutrient supply increased not only in the Antarctic Zone, but also in the Subantarctic, perhaps due to enhanced deep mixing locally. Enhanced deep water ventilation across the Southern Ocean likely increased the nutrient content of mode waters during the deglaciation.

Ground-truthing the stable isotope ratio of diatom frustule-bound organic nitrogen (N) as a paleoceanographic proxy of phytoplankton nutrient consumption calls for studies of modern diatoms from cultures and the field. This work has been hindered by the lack of a method to prepare fresh diatom material, which has significant geochemical differences from sedimentary fossil diatom, for analysis. We tested and compared the performance of five oxidative treatment protocols in cleaning fresh diatom frustules of external organic material for isotopic analysis of diatom frustule-bound N. We applied each treatment to diatom material from a T. weissflogii culture and measured the diatom-bound N content and isotopic composition (d 15 N) of samples. A favored treatment is described that includes a multi-step, progressive, oxidative cleaning. The results suggest that the application of H 2 O 2 to fresh biogenic opal for more than 20 min increased the accessibility of the N within the opal matrix to subsequent chemical treatment, an effect not observed for diatom frustules from deep ocean sediments. We attribute this effect to structural alteration and dissolution of the diatom opal by the H 2 O 2 , consistent with previous observations regarding the high reactivity of fresh diatom opal and its subsequent chemical hardening in the sedimentary environment. In addition, the presence of high levels of organic matter upon treatment with H 2 O 2 may exacerbate the chemical alteration of frustules. We find that the d 15 N of diatom-bound N is ~3 ‰ higher than that of the bulk culture biomass, consistent with expectations from surface sediment and down-core measurements.

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