Riverine contribution of biogenic silica to the oceanic silica budget

Biological uptake of dissolved silicate (DSi) and formation of biogenic silica (BSi) during diatom growth modifies the form of Si carried from the continents to the world ocean. Significant concentrations of BSi, averaging 28.0 pmol L-l, are found in all sizes of rivers. The global contribution of BSi carried by rivers was estimated as 1.05 ? 0.20 Tmol Si year’. Combined with the global mean riverine DSi concentration of 150 pmol liter-‘, 16% of the gross riverine Si load is delivered to the world ocean as BSi. Most of this BSi would be remobilized by dissolution in marine environments. These results demonstrate that the contribution of biogenic silica carried in suspension by rivers is an important component in the world ocean Si budget that has not been recognized previously. The major inputs of Si to the world ocean are thought to be dissolved silicate (DSi) inputs carried by rivers from continental weathering (Treguer et al. 1995). Particulate inputs, primarily composed of clay minerals and debris from weathered rocks, have not been considered in global budgets to date; it is generally thought that the suspended mineral load does not dissolve and contribute to DSi inputs. Mineral silicates weather slowly at -6 X lOpI4 mol cm-* s-l (Lerman 1988) and dissolve slowly enough such that the Si contained as suspended particulate matter in rivers can be considered unavailable on the time scales at which biological processes such as DSi uptake and biogenic silica (BSi) dissolution occur. The amorphous silica contained in diatoms, however, dissolves five orders of magnitude faster than mineral silicates at -2 X 1O-9 mol cm-* s-l (Hurd 1983) and therefore has the potential to participate in the biogeochemical cycle at time scales relevant to biological processes. In the last 15 years techniques have become widely available to distinguish dissolvable amorphous Si contained in diatoms (e.g. BSi) from more refractory mineral silicates in particulate matter (Eggiman et al. 1980; DeMaster 1981; Mortlock and Froelich 1989; Muller and Schneider 1993). Until now, this information has not been available to evaluate the contribution of BSi transported by rivers to the world ocean Si budget. I have obtained such information from the literature and by measuring BSi concentrations directly in a number of large rivers. Samples for BSi were collected from rivers ranging in size from one of the world’s largest rivers, the Amazon, to small