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Shallow seas sediment interface study

The importance of the coastal ocean and river-dominated depositional environments as sinks of nutrients such as silica is uncertain. Studies in tropical deltaic systems (i.e. Amazon delta) have shown that biogenic silica particles upon deposition to the sediments undergo rapid conversion to cation-rich alumoninosilicate phases. The reactions that lead to the formation of new cation-rich aluminosilicate phases are considered as potentially important for the global elemental budgets of major cations (K, Mg) , minor elements (F) and nutrients (Si).

During the cation uptake reactions, there is a return of CO2 from the oceans to the atmosphere, a feedback mechanism that returns CO2 consumed during silicate weathering on land. Thus these reactions are known as "reverse weathering reactions." As part of ORFOIS we have investigated the role of biogenic silica as a substrate for the formation of newly formed aluminosilicate phases in the Ganges-Brahmaputra and Mississippi deltaic systems. Operational analytical methods, both standard and modified, were used for the determination of reactive silica content in these sediments.

Fresh, unaltered biogenic silica (BSi) is measured with the most generally accepted operational method that utilises the time-dependent release pattern of dissolved Si from sediment exposed to 1% Na2CO3 (~0.1 M) at 85°C over ~ 2 - 10 hrs. For the measurement of altered biogenic silica particles and other authigenic aluminosilicate phases in addition to fresh biogenic silica, a two-step operational leach method is used. First, a mild acid digestion (0.1N HCl for 18 hours, room T) is used in order to remove metal coatings, poorly crystalline authigenic minerals, and to activate biogenic silica surfaces for subsequent alkaline dissolution, prior to leaching in alkaline solution.

Subsequently the sample is analysed with the time-dependent release alkaline method described before. On the basis of area-weighted sedimentation rate patterns and the corresponding reactive silica concentration in the two deltaic systems we were able to determine the unaltered biogenic silica and the total reactive silica (i.e. biogenic silica + converted biogenic silica+silica in newly formed aluminosilicates) stored in these systems. Extrapolation of the existing results for the Amazon and the newly acquired results for the Mississippi and Ganges-Brachmaputra to the global coastal ocean show that deltaic systems are significantly more important sinks of riverine dissolved silica than previously thought. In addition observations and chemical analyses on newly formed cation rich aluminosilicate phases on siliceous substrates were obtained with electron microscopy in an attempt to estimate on the importance of these reactions as a CO2 source to the atmosphere on geologic time scales.

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Reported by

Hellenic Center for Marine Research
46.7 KM Atens-Sounio Avenue
19013 Anavyssos