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Dissolution kinetics of biogenic silica from the water column to the sediments

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Citation:
Rickert, D. , Schlüter, M. and Wallmann, K. (2002): Dissolution kinetics of biogenic silica from the water column to the sediments , Geochimica et cosmochimica acta, 66 , pp. 439-455 .
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Abstract:

Stirred flow-through experiments were conducted for the first time with planktonic biogenic silica(BSi). We investigated the dissolution kinetics of uncleaned and chemically cleaned BSi collected in oceansurface water, sediment traps, and sediments from the Norwegian Sea, the Southern Ocean, and the ArabianSea. The solubility at 2°C is rather constant (1000 to 1200 M). The dissolution rates are, however, highlyvariable, declining with water depth, and phytoplankton reactivity is two to three orders of magnitude higherthan pure siliceous oozes. The reactivity decrease correlates well with an increase in the integrated peakintensity ratios of Si-O-Si/Si-OH measured by Fourier transform infrared (FTIR) spectroscopy. The removalof organic or inorganic coatings enhance the reactivity by at least an order of magnitude. Atomic Al/Si ratiosof 0.03 to 0.08 in sedimentary diatom frustules decrease significantly to 0.02 as a result of removal ofinorganic coatings and detritals present. Near equilibrium, the dissolution rates exhibit a linear dependence onthe degree of undersaturation. At higher degrees of undersaturation that is, at low concentrations of dissolvedsilica the dissolution rates of uncleaned samples define a nonlinear trend.The nonlinear kinetics imply that the dissolution of natural BSi is strongly accelerated in silica-depletedsurface waters. The FTIR results suggest that internal condensation reactions reduce the amount of surfacereaction sites and are partly responsible for the reactivity decrease with depth. The high content of Al insedimentary BSi is likely caused by precipitation of dissolved silica with Al dissolved from minerals insediment. Nonbiogenic silica as coatings or detritals are partly responsible for the solubility and reactivitydecrease of BSi in sediments. One order of magnitude different rate constants measured in Norwegian Sea andSouthern Ocean sediment trap material support the so-called opal paradox that is, high BSi accumulationrates in sediments in spite of low BSi production rates in surface waters of the Southern Ocean.

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