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

Abstract

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 ( δ 13C 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 ( δ 18O 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 °C, respectively. The carbon isotope composition of the shells ( δ 13C shell) of both species differs from the in situ δ 13C DIC found at the calcification depths of the specimens. The observed offset between the δ 13C shell and the ambient δ 13C 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 Δ δ 13C shell–DIC and Δ δ 18O shell–w: large shells of G. bulloides (250–355μm) are 0.33‰ ( δ 13C) and 0.23‰ ( δ 18O) higher compared to smaller ones (150–250 μm). For G. ruber, this is 0.39‰ ( δ 13C) and 0.17‰ ( δ 18O). Our field study shows that the δ 13C shell decreases as a result of lower δ 13C DIC values in upwelled waters, while the effects of the carbonate system and/or temperature act in an opposite direction and increase the δ 13C shell as a result lower [CO 3 2−] (or pH) values and/or lower temperature. The Δ δ 13C shell–DIC [CO 3 2−] slopes from our field data are close to those reported literature from laboratory culture experiments. Since seawater carbonate chemistry affects the δ 13C shell in an opposite sense, and often with a larger magnitude, than the change related to productivity (i.e. δ 13C DIC), higher δ 13C shell values may be expected during periods of upwelling.