GALLITELLIA – A PROXY FOR PALAEO-MONSOONAL UPWELLING ON THE WESTERN COAST OF INDIA

This study investigates a Quaternary marine sequence exposed at Ambheta (N 21°40'49"; E 72°35'42") in the Narmada estuary of the Gulf of Cambay, western India, to infer monsoon-induced upwelling from indicator foraminifera. Summer monsoon winds cause coastal upwelling in the Arabian Sea and as a consequence, indicators of upwelling have become accepted proxies of palaeo-monsoon. Deeply incised vertical cliffs along the banks of rivers flanking the Gulf of Cambay expose Late Pleistocene and Holocene sediments that provide sections to study upwelling signatures and thereby the intensity of palaeo-monsoon.

The molluscan shells at the top and middle of the exposed Quaternary marine deposit at Ambheta are dated by the ESR method that indicates age of ~30 ka and ~90 ka respectively. Extrapolating these dates, the basal clay thus likely represents the deposit of Last Interglacial sea level highstand (~ 125 ka). The benthic foraminiferal assemblage of the sequence comprises of Ammonia, Florilus, Quinqueloculina, Bolivina, Brizalina, Cibicides and Spiroloculina. The associated planktonic foraminifera, Globigerina, Globigerinoides and Gallitellia, occur in varying abundance in the sequence. The peak occurrences of Gallitellia are observed at the top of unit C (~ 50 ka) and the lowermost unit A (~ 125 ka). Similar peak abundances of G. bulloides (an established indicator of monsoonal upwelling) are recorded at identical age intervals ~50 ka and ~ 125 ka in deep-sea sediment cores from the Arabian Sea. The textural and architectural studies of continental records in the estuary have indicated high monsoon intensity during 30-50 ka, 70-100 ka and 125 ka.

Gallitellia is a surface dweller and characteristically occurs in unstable environments, partially enclosed basins with high river run-off and upwelling currents. Its common occurrence in the Pleistocene marine clays of Ambheta suggest significant run-off and monsoonal induced upwelling during the ~ 125 ka and ~ 50 ka periods that is also corroborated by another biological proxy, G. bulloides in the Arabian Sea and the fluvial records in the Gulf of Cambay. Gallitellia is thus most likely a useful proxy of intense palaeo-monsoonal activity, particularly in restricted marine depositional settings.