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

Abstract

INTRODUCTIONThe biogeochemistry of silicon (Si) is the focus of considerable contemporary attention by geochemists, paleoceanographers and paleoclimatologists. Biologists are showing revived interest in the Si cycle in relation to diatom productivity as it's very strong role in marine productivity (e.g. Smetacek, 1999) and effect on ocean-atmosphere processes becomes apparent, following a period in which the focus was almost exclusively on smaller and smaller planktonic organisms. The objective of this communication is to review the role of biological processes in the formation of the ocean Si environment first at the broad scale and then in more detail in two special settings, the equatorial Pacific and the Southern Ocean. In doing so, we will follow the evolution of a set of nitrogen (N) based models, that conclude with inclusion of Si and it's impact on global carbon cycles and paleoclimatology. We will examine the differences between the oceanic Si and N cycle, the existence of areas of unusually low silicate relative to nitrate concentrations and how we think they are formed. The concept of the Pacific equatorial upwelling area as a Si limited chemostatlike system and the model complexity necessary to describe its functioning will be presented. SUMMARY: 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 CO 2 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 pCO 2 . The deficiency in source concentrations of Si(OH) 4 results from low Si(OH) 4 :NO 3 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 pCO 2 . 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.

Opal burial in the equatorial Atlantic Ocean over the last 30 ka: Implications for glacial‐interglacial changes in the ocean silicon cycle

“…In the Amazon plume area, dissolved Si concentrations are as high as 13 mM, and in this area diatoms account for 29% of primary productivity, compared with 3% at open ocean Western Equatorial Atlantic (WEA) stations where concentrations of Si are as low as 0.2 mM [Shipe et al, 2006]. This supports the work of Dugdale and Wilkerson [2001], which showed in a culture study that diatoms can outcompete other phytoplankton for nitrate as long as there is enough available silicic acid. Therefore if SO-derived water upwelled at the equator during glacial periods containing significantly more dissolved Si (and/or a higher Si:N ratio) than today, then diatoms would have been at a competitive advantage over other taxa for nutrients such as nitrate.…”

“…[42] Increased silicic acid availability might be expected to have the largest impact on diatom productivity (and therefore also on coccolithophorid productivity) in areas rich in other nutrients [Dugdale and Wilkerson, 2001], namely the upwelling zone in the eastern equatorial Atlantic. While records of CaCO 3 accumulation and productivity in the eastern part of the basin do not consistently show a glacial decrease in CaCO 3 productivity, some do show such a trend, and most records in the western Atlantic also indicate decreased glacial CaCO 3 production.…”

Control of silica production by iron and silicic acid during the Southern Ocean Iron Experiment (SOFeX)

“…This creates a linkage between the Antarctic and Subantarctic where nutrient dynamics in the south set the initial nutrient content of the water delivered to the north. The preferential Si depletion known to occur between the SACCF and the APF thus likely contributes to the low-Si high-nitrate character of the modern-day APFZ and Subantarctic (Dugdale et al 1995;Pondaven et al 2000Brzezinski et al 2003. These linkages would cause any perturbations to nutrient depletion ratios occurring to the south of the APF during glacial periods to have significant effects to the north in the Subantarctic.…”

Eastern Equatorial Pacific productivity and related-CO 2 changes since the last glacial period

“…S1). The increase in silicate availability in the EEP would favor diatom production over coccolithophores, which cannot compete with diatoms when there is enough SiðOHÞ 4 available (8)(9)(10). This causes a reduction in calcite production, affecting the CaCO 3 to organic carbon rain ratio to the deep ocean and ultimately, a lowering of atmospheric CO 2 of 40 to 50 ppm (5,11).…”