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The 14C measurements in the Arabian Sea and the Bay of Bengal during the late 1990s offer a way to assess the temporal changes in the inventories of bomb-14C and its penetration into the ocean, in two decades since GEOSECS expeditions (1977-1978). The mean penetration depth of bomb radiocarbon during GEOSECS (1977-1978) was 270 m, which increased by ∼40% to 381 m in 1994-1998. The small changes in bomb-14C inventories, significant increase in the mean penetration depths and lowering of the surface Δ14C values in the northern Indian Ocean indicate the temporal variation of bomb-14C in two decades is mainly through downward transfer through mixing with deeper waters. The observed bomb-14C inventory in the northern Indian Ocean agrees with numerical model simulated values, except at the equatorial Indian Ocean. The high bomb-14C inventory at the equator can be attributed to lateral advection of 14C-enriched waters from the Pacific Ocean through the Indonesian archipelago. The air-sea CO2 exchange rates in the northern Indian Ocean calculated from the bomb-14C inventories range from ∼7 mol m-2 yr-1 (in the northern Bay of Bengal) to 20 mol m-2 yr-1 (in the equatorial Indian Ocean). Net sea-air flux of CO2 estimated for the northern Indian Ocean between 0° and 25°N is ∼104 ± 30 TgC yr-1. The Bay of Bengal is a net sink of atmospheric CO2 (∼-1 ± 0.4 TgC yr-1), while the Arabian Sea is a source of CO2 (∼69 ± 21 TgC yr-1). [ABSTRACT FROM AUTHOR]

Abstract: The Andaman Basin is an enclosed region in the northeastern Indian Ocean with its deep water below ∼1800 m almost isolated with respect to horizontal ventilation by the Andaman–Nicobar Islands separating it from the Bay of Bengal. The physical and chemical properties including radiocarbon (14C) measured at two stations of the Andaman Basin show negligible variation with depth in the waters below 1300 m, indicating a well-mixed water mass. This study attempts to derive the mixing rates for deep waters of the Andaman Basin. Model calculations based on 14C profile measurements indicate rapid vertical mixing (vertical advection velocity, w >200 m year−1) in waters deeper than 1800 m of the basin. For a basin with deep water thickness of 1000 m below 1800 m, deduced mixing rate of >200 m year−1 translates to mixing time of <5 years. As shown for other regions, the possible mechanism responsible for such high vertical mixing rates could be the internal waves generated from tidal currents flowing through rough topography. In addition, Andaman Basin is underlain with a young crust and is known for its high heat flow, which also could contribute to the high vertical mixing. [Copyright &y& Elsevier]

Dissolved organic carbon (DOC) plays an important role in the oceanic carbon cycle, with its distribution and variation primarily controlled by in situ productivity. The vertical and spatial distribution of DOC measured during two cruises has been investigated to decipher the processes controlling DOC in the Bay of Bengal (BoB). The terrestrial DOC flux to the BoB constitutes a significant fraction of the total DOC and has implications for the carbon budget of the northern Indian Ocean. High concentrations of DOC were observed in the northern BoB, which is primarily due to high flux of terrestrially derived fresh water from various rivers draining to the BoB. Compared to surface DOC concentration of the world oceans, BoB surface water generally has a higher DOC concentration (75–100 μM), which can be attributed to the high DOC content of the riverine flux. Stratification of surface water helps to maintain DOC content due to inhibited vertical mixing in the BoB as a result of the fresh water lens. High subsurface DOC concentration is due to the remineralisation of biogenic sinking particles brought along with the sediments. Subsurface DOC concentrations in the BoB exhibit a conspicuous decreasing trend southwards due to reduced influence of riverine biogenic flux. This study demonstrates that the average contribution of apparent oxygen utilization (AOU) to the DOC remineralisation in the BoB was ~18%. [ABSTRACT FROM AUTHOR]

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°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 ∼0.6 to ∼152.5 μmol/kg, whereas the δ 30 Si data cover a range +1.2‰ to +3.6‰. The depth profiles of dissolved Si show generally lower values in the surface increasing with depth, whereas the pattern reverses in the case of δ 30 Si. These vertical distribution patterns of Si and δ 30 Si 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 ∼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 δ 30 Si values of ∼1.34 ± 0.10‰ 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 ∼100 m of the BoB seems to suggest that meso-scale eddies frequently occurring during non-monsoon periods can supply at the most ∼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 (∼1.3 ± 0.5 × 10 11 mol/year) from the G–B river system and SGD has been calculated based on the distributions of dissolved Si concentration and δ 30 Si 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. [ABSTRACT FROM AUTHOR]

Abstract: Dissolved barium concentrations in water column of the Bay of Bengal along the 87°E transect (~6°N to ~21°N) have been measured to track the dispersion of its large influx from the Ganga–Brahmaputra river system and the outflow to the equatorial Indian Ocean. A typical barium concentration–depth profile shows relatively higher Ba concentrations in surface waters (depth ≤5m) followed by a minimum in the depth interval ~50–150m and an increase at greater depth. The barium concentrations in surface waters (depth≤5m) of the Bay of Bengal vary from ~35nmol/kg at the southernmost station 0806 to ~113nmol/kg close to mouth of the Hooghly estuary (station 0816). The Ba data in upper layers (depth ≤100m), excluding the very high Ba at station 0816, generally show a strong and significant inverse correlation with salinity (R2 =0.75; P<0.0001). This indicates the southward flow of dissolved Ba from the Ganga–Brahmaputra river system that also includes its contributions by particle release and submarine groundwater discharge. The subsurface Ba minimum found in this study are ubiquitous and most probably a result of Ba uptake on settling particulates. On the other hand, the Ba concentrations in deep waters (depth >500m) is controlled dominantly by water mixing as suggested by a very strong and significant inverse correlation with salinity (R2 >0.95; P<0.0001). Exceptions to this conservative behavior are the “hot-spots” of dissolved Ba in bottom waters, which are probably resulted by the dissolution of sediments at and/or below the sediment–water interface. Attempts were made to budget the Ba abundance in the Bay of Bengal using a two box model approach; surface (top ~100m) and deep waters (below ~100m). Under the steady state the annual Ba influx from the Ganga–Brahmaputra river system seems to be balanced through its removal via sinking particulates as a result there is no lateral outflow of dissolved Ba from the Ganga–Brahmaputra to the equatorial Indian Ocean through top ~100m of the Bay of Bengal. Most of this sinking particulate Ba (~95%) is regenerated again in the lower box, preferentially in the intermediate waters ~100–500m. Therefore, frequently ventilated intermediate waters of the Bay of Bengal, receiving a large input of dissolved Ba through particle remineralization can be a significant source of dissolved Ba to the Indian Ocean. [Copyright &y& Elsevier]

Abstract: The dissolved concentrations of redox sensitive elements, U, Mo and Re have been measured in the water column of the Bay of Bengal (BoB) to track their distributions and behavior in its suboxic layers at the intermediate depths. The dissolved oxygen content in the intermediate waters (200–800m) north of ~11°N decreases to values as low as ~3μmol/ℓ. The distribution of U, Mo and Re in the surface waters along the 87°E transect shows a north–south gradient with significant correlation with salinity, underscoring the dominant influence of the Ganga–Brahmaputra (G–B) Rivers in controlling the geochemical budgets of these three elements. The concentrations of these three elements in surface and deep waters of the BoB are governed by salinity suggestive of their conservative behavior. On a macroscale, there is no discernable removal of U, Mo, or Re from the suboxic layer in the BoB water column, consistent with earlier studies in the suboxic zones of other oceanic regions. Further, their conservative behavior in the bay, suggest that the annual delivery of ~109 tons of suspended matter rich in organic matter by G–B and other rivers do not have any measurable impact in determining their distributions. This study suggests that the suboxic conditions and high particulate flux prevailing in the water column of the Bay of Bengal do not affect the conservative behavior of U, Mo and Re. The dissolved Re concentrations in the upper layers in the central BoB seem to be marginally higher compared to that expected based on their salinities. This seems to be contributed from the Godavari River, which drains into the BoB and has dissolved Re three times higher than seawater. The higher Re concentrations in the central Bay of Bengal suggest that mixing process by local gyre and/or eddies in the BoB transports riverine dissolved material to ~400km offshore. More detailed transects of Re from the Godavari coast to central bay are needed to confirm this. [Copyright &y& Elsevier]

Abstract: In this paper, we have assessed the temporal, spatial and depth related variation of suspended particulate organic matter (POM) in the Bay of Bengal. For this purpose, suspended particulate matter (SPM) samples were collected from eight depths (2 to 1000 m) at 9 locations in the Bay of Bengal during July/August 2001 (Southwest monsoon, SWM), September/October 2002 (Fall Inter-monsoon, FIM), and March/April 2003 (Spring Inter-monsoon, SPIM). The SPM samples were analyzed for chlorophyll a (Chl a), particulate organic carbon (POC) and total particulate nitrogen (TPN). The surface concentrations of POC varied from 4.3 to 11.1 µM C, 3.1 to 10.9 µM C, and 4.3 to 9.0 µM C during SWM, FIM, and SPIM, respectively. The levels of SPM, and the concentrations of Chl a, and POC were relatively higher in the offshore compared to the near shore stations, especially during SWM. The POC concentration and its contribution to the SPM were higher in the surface waters, and both decreased with increasing water column depth at most of the stations. The observed decrease with depth of POC indicates heterotrophic uptake and/or dilution by inorganic material, poor in POC. This was also reflected in the C/N ratio which generally increased with water column depth. The relatively low C/N and POC/Chl a ratios, and high Chl a and % POC during the SWM and FIM indicate the presence of relatively fresh POM in the Bay during these two seasons. The observed seasonal differences in the quality of POM appears to be governed by river run-off and the physical forces such as eddies which pump nutrients into the surface waters thereby enhancing biological production. [Copyright &y& Elsevier]