Microfossil evidence for changing salinity patterns in the bay of Bengal over the last 20 000 years

Abstract

The Monsson climate characterizing the northern Indian Ocean produces salinity gradients that are large compared with those in other open-ocean areas. These gradients are substantially independent of thermal and other ecologically important gradients. This situation makes it possible not only to isolate the effect of salinity on sea-bed distribution patterns of planktonic foraminifera, but also to investigate past salinity changes along a north-south transect of piston cores in the Bay of Bengal. Analysis of 152 surface sediment samples reveals that at least three factors other than salinity are important in controlling foraminiferal distribution: sea-bed dissolution, surface productivity, and surface temperature. Of these, dissolution is the most important. Therefore, in order to reconstruct past salinity patterns in the Bay of Bengal, faunal indices need to be developed that isolate the effect of salinity. We have developed two independent approaches. The first uses ternary diagrams of the three dominant dissolution-resistant species ( Globoquadrina dutertrei, Globorotalia menardii, Pulleniatina obliquiloculata). High values of G. dutertrei are associated with low surface salinities. The second approach uses dissolution-buffered salinity transfer functions based on the total foraminiferal fauna in the 152 surface samples from the northern Indian Ocean. Applied to downcore samples, these methods show the same basic historical pattern of salinity variation, which is in turn consistent with independent inferences based on preliminary oxygen-isotope data (Duplessy, 1980). Historical changes in the Bay of Bengal salinity are reconstructed along a north-south transect of seven cores at ∼90°E. The stratigraphy of the transect is controlled by oxygen isotope profiles that are similar to the generalized patterns observed in other areas over the past 20 000 yr. Salinity gradients are reconstructed for three time intervals: (1) the last glacial maximum (LGM), (2) the mid-termination (MT), and (3) the Holocene. During the LGM interval, salinity was highger in the Bay of Bengal, particularly in the north, thus reducing the north-south salinity gradient. During the MT, the middle of the transition from glacial to interglacial global climatic regimes, northern Bay of Bengal surface salinity was considerably reduced, while salinity to the south was generally similar to today. Thus, the salinity gradient between 10°N and 15°N was considerably steeper than today. In contrast, the Holocene gradient was approximately the same as the modern gradient, with absolute values along the transect being slightly but uniformly higher. These changes in the pattern of salinity are explained in terms of changes in the local patterns of precipitation and river run-off. During the LGM the salinity pattern probably results from decreased precipitation and decreased run-off by the Ganges-Brahmaputra River system. During the MT, decreased salinity in the northern Bay of Bengal is caused by increased precipitation and run-off rather than by the influx of glacial meltwater. The steeper MT salinity gradient in the central Bay of Bengal suggests increased eastward advection of equatorial surface waters by a Southwest Monsoon Current somewhat more vigorous than today. These patterns can be correlated with faunal evidence from the western Arabian Sea that suggest decreased upwelling during the LGM and increased upwelling during the MT. This coupling of events may be associated with changes in the pattern or intensity of circulation during the Southwest Monsoon: decreased intensity at the LGM (∼ 18 000 B.P.), and increased intensity at the MT (∼ 10 500 to 12 500 B.P.).