Palaeomonsoon and palaeoproductivity records of δ¹⁸O, δ¹³C and CaCO₃ variations in the northern Indian Ocean sedimentsC and CaCO₃ variations in the northern Indian Ocean sediments

Abstract

δ¹⁸O and δ¹³C of G. sacculifer have been measured in five cores from the northern Indian Ocean. In addition, high resolution analysis (1 to 2 cm) was performed on one core (SK-20-185) for both δ¹⁸O and δ¹³C in five species of planktonic foraminifera. CaCO₃ variation was measured in two cores. The results, presented here, show that • the summer monsoon was weaker during 18 ka and was stronger during 9 ka, relative to modern conditions; - • δ¹³C variations are consistent with independent evidence that shows that during the last glacial maximum (LGM; 18 ka) the upwelling was reduced while during 9 ka it was vigorous; • calculation of CaCO₃ flux shows that the LGM was characterized by low biogenic productivity in the Arabian Sea while during the Holocene productivity increased by ~65%, as a direct consequence of the changes in upwelling. Similar changes (of lesser magnitude) are also seen in the equatorial Indian Ocean. The amount of terrigenous input into the Arabian Sea doubled during LGM possibly due to the higher erosion rate along the west coast. • δ¹⁸O values indicate that the Arabian Sea was saltier by 1 to 2%o during LGM. The northern part was dominated by evaporation while in the equatorial part there was an increased precipitation.