Silica Cycling and the Role of Diatoms in the Biological Pump of the Southern Ocean

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This work is one of forty-four projects that are collaborating in the Southern Ocean Experiment of the Joint Global Ocean Flux Study. The Southern Ocean Experiment is a three-year effort south of the Antarctic Polar Frontal Zone to track the flow of carbon through its organic and inorganic pathways from the air-ocean interface through the entire water column into the bottom sediment. JGOFS itself is a decadal study of the processes controlling the time-varying fluxes of carbon and associated biogenic elements in the ocean with the objective of observing changes in ocean biogeochemical cycles in relation to climatic change. The experiment will make use of the RVIB Nathaniel B. Palmer and the R/V Thompson. This component concerns the general topic of removal of organic carbon from the surface layers of the ocean, and deals specifically with diatoms, a form of siliceous phytoplankton, as a major component of the biological pump of the Southern Ocean. Diatoms are known to be responsible for the majority of primary production in the Polar Frontal Zone (PFZ) and along the retreating ice edge in the Ross Sea. Strong correlations between the fluxes of diatomaceous silica and organic carbon indicate that diatoms are also the autotrophic source of much of the organic matter exported from the surface waters of the Southern Ocean. The role of diatoms in the biological pump may be especially important in the PFZ, since nearly all of the silica flux in the Southern Ocean occurs beneath the PFZ with the result that this region and the waters to the south comprise the largest area of modern siliceous sediment accumulation in the world. Surprisingly, that immense opal accumulation is supported by very low rates of primary production near the surface. Within the water column and upper sediments the regional cycles of carbon and silica are decoupled to a degree that does not occur elsewhere, with the result that opal rich, but organic poor, sediments are forming throughout much of the abyssal Southern Ocean. The observational program is designed to reveal the fate of the silica produced and help constrain the contribution of diatoms to carbon fixation and export in the region. Two newly developed tracer methods will be used. The first involves the use of the newly available radioisotope of silicon, 32Si (half life 134 y), to measure silica production rates. The second is a new method for measuring natural variations in silicon isotopes within diatoms and seawater that may be useful as a proxy for level of paleo-silica production. The insights gained regarding the magnitude, fate of biogenic silica, and the factors controlling silica cycling and diatom productivity will help evaluate the role of diatoms in the biogeochemical cycling of elements in the region and explain the mechanisms producing high accumulation rates of diatom silica in the Southern Ocean.