The effect of upwelling on the distribution and stable isotope composition of Globigerina bulloides and Globigerinoides ruber (planktic foraminifera) in modem surface waters of the NW Arabian Sea

Hydrographic changes in the NW Arabian Sea are mainly controlled by the monsoon system. This results in a strong seasonal and vertical gradient in surface water properties, such as temperature, nutrients, carbonate chemistry and the isotopic composition of dissolved inorganic carbon (delta(13)C(DIC)). Living specimens of the planktic foraminifer species Globigerina bulloides and Globigerinoides ruber, were collected using depth stratified plankton tows during the SW monsoon upwelling period in August 1992 and the NE monsoon non-upwelling period in March 1993. We compare their distribution and the stable isotope composition to the seawater properties of the two contrasting monsoon seasons. The oxygen isotope composition of the shells (delta(18)O(shell)) and vertical shell concentration profiles indicate that the depth habitat for both species is shallower during upwelling (SW monsoon period) than during non-upwelling (NE monsoon period). The calcification temperatures suggest that most of the calcite is precipitated at a depth level just below the deep chlorophyll maximum (DCM), however above the main thermocline. Consequently, the average calcification temperature of G. ruber and G. bulloides is lower than the sea surface temperature by 1.7 +/- 0.8 and 1.3 +/- 0.9 degreesC, respectively. The carbon isotope composition of the shells (delta(13)C(shell)) of both species differs from the in situ delta(13)C(DIC) found at the calcification depths of the specimens. The observed offset between the delta(13)C(shell) and the ambient delta(13)C(DIC) results from (1) metabolic/ontogenetic effects, (2) the carbonate chemistry of the seawater and, for symbiotic G. ruber, (3) the possible effect of symbionts or symbiont activity. Ontogenetic effects produce size trends in Deltadelta(13) Cshell-DIC and Deltadelta(18)O(shell-w): large shells of G. bulloides (250-355mum) are 0.33 parts per thousand (delta(13)C) and 0.23 parts per thousand (delta(18)O) higher compared to smaller ones (150-250 mum). For G. ruber, this is 0.39 parts per thousand (delta(13)C) and 0.17 parts per thousand (delta(18)O). Our field study shows that the delta(13)C(shell) decreases as a result of lower delta(13)C(DIC) values in upwelled waters, while the effects of the carbonate system and/or temperature act in an opposite direction and increase the delta(13)C(shell) as a result lower [CO32-] (or pH) values and/or lower temperature. The Deltadelta(13)C(shell-DIC) [CO32-] slopes from our field data are close to those reported literature from laboratory culture experiments. Since seawater carbonate chemistry affects the delta(13)C(shell) in an opposite sense, and often with a larger magnitude, than the change related to productivity (i.e. delta(13)C(DIC)), higher delta(13)C(shell) values may be expected during periods of upwelling. (C) 2002 Elsevier Science B.V. All rights reserved.