Relationship of nitrogen isotope fractionation to phytoplankton size and iron availability during the Southern Ocean Iron RElease Experiment (SOIREE)
The 15N composition of sediments has been used as a proxy for nitrate utilization in surface waters to assess the role of Southern Ocean export production in glacial/interglacial changes in atmospheric CO2 concentration. Interpretation has relied on a temporally constant isotope effect (�) associate...
| Main Authors: | , , , |
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| Other Authors: | |
| Format: | Text |
| Language: | English |
| Subjects: | |
| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.419.7340 http://www.aslo.org/lo/toc/vol_48/issue_3/1058.pdf |
| Summary: | The 15N composition of sediments has been used as a proxy for nitrate utilization in surface waters to assess the role of Southern Ocean export production in glacial/interglacial changes in atmospheric CO2 concentration. Interpretation has relied on a temporally constant isotope effect (�) associated with uptake and assimilation of nitrate by phytoplankton. To investigate the reliability of this approach, we examined the relationships between the 15N compositions of dissolved nitrate, bulk and size-fractionated (200, 70, 20, 5, 1 �m) suspended particulate organic nitrogen (PON), and sinking particles obtained from sediment traps during the Southern Ocean iron release experiment (SOIREE). We found variations in phytoplankton nitrogen isotopic compositions with both cell size and iron availability. �15NPON increased by �2 ‰ with increasing size, both within and outside the ironenriched patch. In comparison to unfertilized waters, �15NPON within the iron-fertilized patch was a further 3–4‰ higher in those size fractions dominated by large diatoms (20–70, 70–200 �m). We speculate that this iron response might result from (1) variation in � of nitrate utilization or (2) an iron-stimulated shift from ammoniumbased to nitrate-based production. Comparing the �15N of the large diatom–dominated size fractions to the �15N of nitrate suggests relatively low � values of 4–5‰, in contrast to estimated values of 7–10 ‰ from seasonal |
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