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Abstract

The concentration of dissolved Si and its isotope composition are measured in the Bay of Bengal (BoB) region of the northern Indian Ocean; the isotope data are the first data set from the northern Indian Ocean. The measurements are made in eight depth profiles closely along the 87 degrees E transect (GIO1 section of the international GEOTRACES program) and in a few samples from the northern shelf of the bay. Dissolved Si in the water column varies from similar to 0.6 to similar to 152.5 mu mol/kg, whereas the delta Si-30 data cover a range +1.2 parts per thousand to +3.6 parts per thousand. The depth profiles of dissolved Si show generally lower values in the surface increasing with depth, whereas the pattern reverses in the case of delta Si-30. These vertical distribution patterns of Si and d30Si are similar to those reported in other oceanic regions and suggestive of the significant role of biological processes in governing Si biogeochemistry in the upper layers (top similar to 1500 m). In contrast, dissolved Si in near surface waters of the northern shelf and the southernmost station is exceptionally high. These results indicate a continental supply of dissolved Si from the Ganga-Brahmaputra river system (G-B) and submarine groundwater discharge (SGD) to the shelf region, and an intrusion of high salinity waters from the Arabian Sea in the southern bay. The delta Si-30 values of similar to 1.34 +/- 0.10 parts per thousand for deep/bottom waters of the BoB (depth > 1500 m) are similar to those reported for the deep Southern Ocean and indicate the dominant control of water mass mixing. The dissolved Si concentrations in the bottom waters of the BoB are generally higher than those of the water mass endmembers, which suggest the need for an additional source of Si; in situ particle dissolution and/or benthic release in the central bay seem to be the potential candidate. The annual Si budget in the top similar to 100 m of the BoB seems to suggest that meso-scale eddies frequently occurring during non-monsoon periods can supply at the most similar to 2.6 g Si/m(2)/year, which is about 33% of the Si requirement to support new production in the bay. The supply of dissolved Si (similar to 1.3 +/- 0.5 x 10(11) mol/year) from the G-B river system and SGD has been calculated based on the distributions of dissolved Si concentration and delta Si-30 in the northern shelf waters. A comparison of this supply with the reported Si flux upstream of the estuarine zone indicates about 40% removal of dissolved Si in the G-B estuary. The mass balance of Si isotopes in the deep waters indicates that the dissolution of diatoms is the main cause of excess Si in the bay. (C) 2014 Elsevier Ltd. All rights reserved.