The distribution of silicon isotopes along a meridional transect at 140°W longitude in the Eastern Equatorial Pacific was used to test the hypothesis that δ~(30)Si of silicic acid in surface waters should correlate with net silica production rates (gross silica production minus silica dissolution) rather than rates of gross silica production due to the opposing Si isotope fractionations associated with silica production and silica dissolution. Variations in δ~(30)Si appeared significantly correlated with net silica production rates in equatorial surface waters and not with gross production rates. Around the Equator, values of δ~(30)Si as low as deep water values occurred in the upper mesopelagic in a zone of net silica dissolution and high detrital biogenic silica content, where the release of low δ~(30)Si silicic acid from opal dissolution would be expected to decrease δ~(30)Si. The δ~(30)Si of the deep water at 140°W appears constant for depths >2000m and is similar to the deep water at 110°W. This study brings to light the importance of considering Si fractionation during diatom silica dissolution, the biological fractionation during silica production and physical factors such as currents and mixing with adjacent water masses when interpreting silicon isotope distributions.
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