David Nelson and Mark Brzezinski; Oregon State University and UC-Santa Barbara

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

Diatoms are a major component of the biological pump of the Southern Ocean. They 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 diatom opal 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. About 90% of the opal flux in the Southern Ocean occurs beneath the PFZ with the result that region and the waters to the south comprise the largest area of modern siliceous sediment accumulation in the world, accounting for at least 50% of the global removal of Si from the ocean. Surprisingly, that immense opal accumulation is supported by very low rates of primary production that average <40 g C m^-2 y^-1. The regional cycles of carbon and silica within the water column and upper sediments 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. Thus, essentially complete degradation of particulate organic carbon occurs in the deep waters with little permanent burial of organic carbon in the sediments. In contrast, the preservation of opal is unusually efficient in the Southern Ocean with about 20% of surface silica production being preserved beneath the PFZ compared to the global average of about 3%.

We propose to examine silica production and the rate of silica dissolution in the surface waters of the PFZ. Measurements will 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 paleosilica 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.