Sources and fates of silicon in the ocean: the role of diatoms in the climate and glacial cycles

Abstract

Diatoms with their fast growth rates and obligate requirement for Si have a unique relationship to the oceanic Si cycle with the potential for controlling the nutrient and CO₂ environment of large important areas of the ocean. The new production of diatoms based on both new nitrogen and Si sources is described using a Si-pump based upon the differential regeneration of the two elements. This approach, applied to the eastern equatorial Pacific, showed diatoms to respond as in a Si-limited chemostat, to the low source Si(OH)₄ in the Equatorial UnderCurrent. Increased Si(OH)₄ results in increased diatom productivity, suppression of non-diatom populations and decreased surface pCO₂. The deficiency in source concentrations of Si(OH)₄ results from low Si(OH)₄:NO₃ water originating in the vicinity of the Antarctic Polar Front, a consequence of the extraordinary trapping of Si by the Southern Ocean. In glacial periods this trapping is reduced several fold and likely results in increased Si(OH)₄ export to the north, and increased Si(OH)₄ production and deposition at the equatorial Pacific which can be expected to reduce surface pCO₂. The connections between the eastern equatorial Pacific export production and Southern Ocean Si trapping may provide a major biogeochemical feedback system with implications for contemporary and paleoclimatology.