Rixen, Tim, Guptha, M.V.S. and Ittekkot, V. (2005) Deep ocean fluxes and their link to surface ocean processes and the biological pump. Progress in Oceanography, 65 (2-4). pp. 240-259. DOI https://doi.org/10.1016/j.pocean.2005.03.006.

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Abstract

Intense studies of upper and deep ocean processes were carried out in the Northwestern Indian Ocean (Arabian Sea) within the framework of JGOFS and related projects in order to improve our understanding of the marine carbon cycle and the ocean’s role as a reservoir for atmospheric CO2. The results show a pronounced monsoon-driven seasonality with enhanced organic carbon fluxes into the deep-sea during the SW Monsoon and during the early and late NE Monsoon north of 10°N. The productivity is mainly regulated by inputs of nutrients from subsurface waters into the euphotic zone via upwelling and mixed layer-deepening. Deep mixing introduces light limitation by carrying photoautotrophic organisms below the euphotic zone during the peak of the NE Monsoon. Nevertheless, deep mixing and strong upwelling during the SW Monsoon provide an ecological advantage for diatoms over other photoautotrophic organisms by increasing the silica concentrations in the euphotic zone. When silica concentrations fall below 2 μmol l−1, diatoms lose their dominance in the plankton community. During diatom-dominated blooms, the biological pathway of uptake of CO2 (the biological pump) appears to be more efficient than during blooms of other organisms, as indicated by organic carbon to carbonate carbon (rain) ratios. Due to the seasonal alternation of diatom and non-diatom dominated exports, spatial variations of the annual mean rain ratios are hardly discernible along the main JGOFS transect.

Data-based estimates of the annual mean impact of the biological pump on the fCO2 in the surface water suggest that the biological pump reduces the increase of fCO2 in the surface water caused by intrusion of CO2-enriched subsurface water by ∼50–70%. The remaining 30 to 50% are attributed to CO2 emissions into the atmosphere. Rain ratios up to 60% higher in river-influenced areas off Pakistan and in the Bay of Bengal than in the open Arabian Sea imply that riverine silica inputs can further enhance the impact of the biological pump on the fCO2 in the surface water by supporting diatom blooms. Consequently, it is assumed that reduced river discharges caused by the damming of major rivers increase CO2 emission by lowering silica inputs to the Arabian Sea; this mechanism probably operates in other regions of the world ocean also.

Document Type:	Article
Programme Area:	UNSPECIFIED
Research affiliation:	Biogeochemistry and Geology > Carbon and Nutrient Cycling
Refereed:	Yes
Open Access Journal?:	No
DOI:	https://doi.org/10.1016/j.pocean.2005.03.006
ISSN:	00796611
Date Deposited:	20 Mar 2020 10:53
Last Modified:	01 Oct 2020 13:01
URI:	http://cris.leibniz-zmt.de/id/eprint/3676

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