Author Posting. © Elsevier B.V., 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 54 (2007): 601-638, doi:10.1016/j.dsr2.2007.01.013.
This paper investigates ballasting and remineralization controls of carbon sedimentation
in the twilight zone (100-1000 m) of the Southern Ocean. Size-fractionated (〈1 μm, 1-51 μm,
〉51 μm) suspended particulate matter was collected by large volume in-situ filtration from the
upper 1000 m in the Subantarctic (55°S, 172°W) and Antarctic (66°S, 172°W) zones of the
Southern Ocean during the Southern Ocean Iron Experiment (SOFeX) in January-February 2002.
Particles were analyzed for major chemical constituents (POC, P, biogenic Si, CaCO3), and
digital and SEM image analyses of particles were used to aid in the interpretation of the chemical
Twilight zone waters at 66°S in the Antarctic had a steeper decrease in POC with depth
than at 55°S in the Subantarctic, with lower POC concentrations in all size fractions at 66°S than
at 55°S, despite up to an order of magnitude higher POC in surface waters at 66°S. The decay
length scale of 〉51 μm POC was significantly shorter in the upper twilight zone at 66°S (δe=26
m) compared to 55°S (δe=81 m).
Particles in the carbonate-producing 55°S did not have higher excess densities than
particles from the diatom-dominated 66°S, indicating that there was no direct ballast effect that
accounted for deeper POC penetration at 55°S. An indirect ballast effect due to differences in
particle packaging and porosities cannot be ruled out, however, as aggregate porosities were high
(~97%) and variable.
Image analyses point to the importance of particle loss rates from zooplankton grazing
and remineralization as determining factors for the difference in twilight zone POC concentrations at 55°S and 66°S, with stronger and more focused shallow remineralization at
66°S. At 66°S, an abundance of large (several mm long) fecal pellets from the surface to 150 m,
and almost total removal of large aggregates by 200 m, reflected the actions of a single or few
zooplankton species capable of grazing diatoms in the euphotic zone, coupled with a more
diverse particle feeding zooplankton community immediately below.
Surface waters with high biomass levels and high proportion of biomass in the large size
fraction were associated with low particle loading at depth, with all indications implying
conditions of low export. The 66°S region exhibits this “High Biomass, Low Export” (HBLE)
condition, with very high 〉51 μm POC concentrations at the surface (~2.1 μM POC), but low
concentrations below 200 m (〈0.07 μM POC). The 66°S region remained HBLE after iron
fertilization. Iron addition at 55°S caused a ten fold increase in 〉51 μm biomass concentrations
in the euphotic zone, bringing surface POC concentrations to levels found at 66°S (~3.8 μM),
and a concurrent decrease in POC concentrations below 200 m. The 55°S region, which began
with moderate levels of biomass and stronger particle export, transitioned to being HBLE after
iron fertilization. We propose that iron addition to already HBLE waters will not cause mass
sedimentation events. The stability of an iron-induced HBLE condition is unknown. Better
understanding of biological pump processes in non-HBLE Subantarctic waters is needed.
work was supported by the DOE Office of Science, Biological and Environmental Research
Program. Shiptime for SOFeX was funded by NSF.
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