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Variations in the Indian summer monsoon (ISM) intensity during the last 16.7 ka have been studied using organic carbon (Corg), δ 15 N of sedimentary organic matter, CaCO3, sediment texture, δ 18 OC and Mg/Ca derived sea surface temperature, δ 18 O of sea water and sea surface salinity, in a 14 C-dated sediment core from the eastern Arabian Sea. The δ 18 O in water and planktonic foraminifera shells off the central west coast of India may be controlled by the ISM intensity as this area receives high precipitation and land runoff. Also, the Corg and CaCO3 contents of sediments and δ 15 N of sedimentary organic matter may be linked to ISM-induced productivity and denitrification. The results of the present study reveal that between 16 and 15.2 ka BP the ISM was weak with minor fluctuations and started intensifying around 15.2 ka BP, at the onset of the Bolling-Allerod (B-A) event. The B-A event is characterized by higher water column denitrification rates comparable to the present day. The ISM signatures observed in the δ 18 OC record of B-A event compare well with those from Timta cave of the western Himalayas and also the Asian summer monsoon signatures from the Hulu caves in China and warming signatures in Greenland Ice Sheet Project 2 (GISP2) suggesting atmospheric tele-connections through Intertropical Convergence Zone (ITCZ). The boundary between the Younger Dryas and the Holocene is discernible with small episodes of abrupt events of increased ISM intensity. This decrease in δ 18 OC values at ~ 11.8 ka BP is contemporary with June solar insolation maximum at 30° north and the increase in methane in the GISP2 ice core supporting episodes of warmer climate and increase in ISM intensity. The ISM seems to have been most stable between 7 and 5.6 ka BP. The core exhibits periodicity of 500 that is comparable to the Atlantic water formation and the Chinese monsoon.

Intensive observations in the northeast Indian Ocean (Bay of Bengal) during the presouthwest and northeast monsoon seasons of 1991 reveal that freshwater discharge from rivers of the Indian subcontinent exerts the dominant control over total carbon dioxide (TCO2) and pCO2 distributions in surface waters. Low pCO2 levels occur within the low-salinity zones, with a large area in the northwestern bay acting as a sink for atmospheric CO2. Only a part of the observed pCO2 variation can be accounted for by the effect of salinity, and biological production supported by external nutrient inputs in conjunction with strong thermohaline stratification may be more important in lowering surface water pCO2 by >100 μatm relative to that in the atmosphere. The pCO2 distribution is seasonally variable and appears to be controlled by the spreading of fresher waters by the prevailing surface circulation.

[1] Remotely sensed data are combined with shipboard measurements to investigate biogeochemical changes caused by a moderate tropical cyclone in the central Arabian Sea in December 1998. The sea surface temperature decreased by ∼4°C, whereas surface nitrate and chlorophyll concentrations increased by >5 μM and up to 4 mg m−3, respectively, over a large area affected by the cyclone. Nutrient enrichment in the surface layer of the cyclone-affected zone is estimated to have supported a new production of ∼4.2 Tg C, approximately 5% of the annual organic carbon export to the deep sea (beyond the continental margin) for the entire Arabian Sea. Entrainment of nitrous oxide from the thermocline led to more than doubling of its concentration in the mixed layer. The cyclone also resulted in an increase in nitrous oxide inventory within the oxygen minimum zone. Our results imply that, should there be an increase in the frequency and intensity of tropical cyclones as a result of global warming, as projected in some recent reports, carbon production and respiration, and redox processes within the oxygen minimum zones, such as the production of nitrous oxide through nitrification/denitrification, and of molecular nitrogen through denitrification/anaerobic ammonium oxidation, may be significantly impacted.

Extensive optical, physical, chemical, and biochemical measurements made simultaneously in the northwest Indian Ocean reveal the occurrence of an intermediate nepheloid layer (INL) invariably associated with the secondary nitrite maximum. Maxima in particulate protein and the activity of the respiratory electron transport system (ETS) are also found within the INL. Since the INL persists long distances from the continental margin with an offshore intensification, it may not be related to the transport of material resuspended along the continental margin. An apparent correlation of the INL with the previously reported subsurface maximum in bacterial abundance suggests that a local increase in the abundance of bacteria may be responsible for the increased turbidity. Positive correlations of the beam attenuation anomaly with nitrite and nitrate deficit suggest that most of these bacteria may be denitrifiers. The organic carbon demand within the denitrifying layer, computed from the observed ETS activity, appears to be severalfold higher than the sinking carbon fluxes to the denitrifying layer, requiring additional modes of supply of the biodegradable organic matter. It is proposed that a bacterial maximum could be maintained with efficient utilization of the dissolved organic matter within the denitrifying waters.