Indian winter and summer monsoon strength over the 4.2 ka BP event in foraminifer isotope records from the Indus River delta in the Arabian Sea

Abstract

Abstract. The plains of northwest South Asia receive rainfall during both the Indian\nsummer (June–September) and winter (December–March) monsoon. Researchers\nhave long attempted to deconstruct the influence of these precipitation\nregimes in paleoclimate records, in order to better understand regional\nclimatic drivers and their potential impact on human populations. The\nmid–late Holocene transition between 5.3 and 3.3\u2009ka is of particular\ninterest in this region because it spans the period of the Indus Civilization\nfrom its early development, through its urbanization, and onto eventual\ntransformation into a rural society. An oxygen isotope record of the\nsurface-dwelling planktonic foraminifer Globigerinoides ruber from\nthe northeast Arabian Sea provided evidence for an abrupt decrease in\nrainfall and reduction in Indus River discharge at 4.2\u2009ka, which the authors\nlinked to the decline in the urban phase of the Indus Civilization\n(Staubwasser et al., 2003). Given the importance of this study, we used the\nsame core (63KA) to measure the oxygen isotope profiles of two other\nforaminifer species at decadal resolution over the interval from 5.4 to\n3.0\u2009ka and to replicate a larger size fraction of G. ruber than\nmeasured previously. By selecting both thermocline-dwelling\n(Neogloboquadrina dutertrei) and shallow-dwelling\n(Globigerinoides sacculifer) species, we provide enhanced detail of\nthe climatic changes that occurred over this crucial time interval. We found\nevidence for a period of increased surface water mixing, which we suggest was\nrelated to a strengthened winter monsoon with a peak intensity over 200\xa0years\nfrom 4.5 to 4.3\u2009ka. The time of greatest change occurred at 4.1\u2009ka when\nboth the summer and winter monsoon weakened, resulting in a reduction in\nrainfall in the Indus region. The earliest phase of the urban Mature Harappan\nperiod coincided with the period of inferred stronger winter monsoon between\n4.5 and 4.3\u2009ka, whereas the end of the urbanized phase occurred some time\nafter the decrease in both the summer and winter monsoon strength by 4.1\u2009ka.\nOur findings provide evidence that the initial growth of large Indus urban\ncenters coincided with increased winter rainfall, whereas the contraction of\nurbanism and change in subsistence strategies followed a reduction in\nrainfall of both seasons.