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- Title
Exploring interacting influences on the silicon isotopic composition of the surface ocean: a case study from the Kerguelen Plateau.
- Authors
Coffineau, N.; De La Rocha, C. L.; Pondaven, P.
- Abstract
This study presents 6 new water column profiles of the silicon isotopic composition (δ30 Si) of dissolved silicon (DSi) from the Atlantic and the Indian sectors of the Southern Ocean and a variable depth box model of silica cycling in the mixed layer constructed to illuminate the evolution of surface ocean δ30Si over the full course of a year. In keeping with previous observations, δ30 Si values ranged from +1.9 to +2.4 ‰ in the mixed layer (ML), +1.2 to +1.7‰ in Winter Water (WW), and +0.9 to +1.4‰ in Circumpolar Deep Water (CDW). These data also confirmed the occurrence of diminished values for ML δ30 Si at low DSi concentrations in early austral autumn on the Kerguelen Plateau. The box model was used to investigate whether these low, post-growing season values of δ30 Si were related to input of DSi to the ML from basalt weathering, biogenic silica dissolution (with or without isotopic fractionation), the onset of winter mixing, or some combination of the three. Basalt weathering and fractionation during biogenic silica dissolution could both lower ML δ30Si below what would be expected from the extent of biological uptake of DSi. However, the key driver of the early autumn decrease in δ30Si appears to be the switch from bloom growth (with net removal of DSi and net accumulation of biogenic silica (BSi) biomass) to steady state growth (when slow but continuing production of BSi prevented significant net increase in DSi concen- trations with diffusive input of DSi from WW but not decrease in ML δ30 Si towards WW values). Lastly, fractionation during dissolution had only a negligible effect on the δ30 Si of BSi exported throughout the course of the year, implying that seasonal changes in export efficiency (e.g., favoring the export of bloom BSi vs. the export of BSi produced during other times of the year) strongly influence the δ30Si of BSi accumulating in marine sediments. Altogether, these results suggest that as a paleoceanographic proxy, δ30Si may more reflect the dominant mode of production of the BSi that is exported (i.e. bloom vs. steady state growth) rather than strictly the extent of DSi utilization by diatoms.
- Subjects
ANTARCTIC Ocean; KERGUELEN Plateau; SILICON isotopes; OCEANOGRAPHY; CASE studies; PALEOCEANOGRAPHY
- Publication
Biogeosciences Discussions, 2013, Vol 10, Issue 7, p11405
- ISSN
1810-6277
- Publication type
Article
- DOI
10.5194/bgd-10-11405-2013