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

Giesche, Alena and Staubwasser, Michael and Petrie, Cameron A. and Hodell, David A. (2018) 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. Climate of the past, 15. pp. 73-90. ISSN 1814-9340 DOI https://doi.org/10.5194/cp-15-73-2019

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Abstract

The plains of northwest South Asia receive rainfall during both the Indian summer (June–September) and winter (December–March) monsoon. Researchers have long attempted to deconstruct the influence of these precipitation regimes in paleoclimate records, in order to better understand regional climatic drivers and their potential impact on human populations. The mid–late Holocene transition between 5.3 and 3.3 ka is of particular interest in this region because it spans the period of the Indus Civilization from its early development, through its urbanization, and onto eventual transformation into a rural society. An oxygen isotope record of the surface-dwelling planktonic foraminifer Globigerinoides ruber from the northeast Arabian Sea provided evidence for an abrupt decrease in rainfall and reduction in Indus River discharge at 4.2 ka, which the authors linked to the decline in the urban phase of the Indus Civilization (Staubwasser et al., 2003). Given the importance of this study, we used the same core (63KA) to measure the oxygen isotope profiles of two other foraminifer species at decadal resolution over the interval from 5.4 to 3.0 ka and to replicate a larger size fraction of G. ruber than measured previously. By selecting both thermocline-dwelling (Neogloboquadrina dutertrei) and shallow-dwelling (Globigerinoides sacculifer) species, we provide enhanced detail of the climatic changes that occurred over this crucial time interval. We found evidence for a period of increased surface water mixing, which we suggest was related to a strengthened winter monsoon with a peak intensity over 200 years from 4.5 to 4.3 ka. The time of greatest change occurred at 4.1 ka when both the summer and winter monsoon weakened, resulting in a reduction in rainfall in the Indus region. The earliest phase of the urban Mature Harappan period coincided with the period of inferred stronger winter monsoon between 4.5 and 4.3 ka, whereas the end of the urbanized phase occurred some time after the decrease in both the summer and winter monsoon strength by 4.1 ka. Our findings provide evidence that the initial growth of large Indus urban centers coincided with increased winter rainfall, whereas the contraction of urbanism and change in subsistence strategies followed a reduction in rainfall of both seasons.

Item Type: Article
Uncontrolled Keywords: 2018AREP; IA74
Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions: 01 - Climate Change and Earth-Ocean Atmosphere Systems
07 - Gold Open Access
12 - PhD
Journal or Publication Title: Climate of the past
Volume: 15
Page Range: pp. 73-90
Identification Number: https://doi.org/10.5194/cp-15-73-2019
Depositing User: Sarah Humbert
Date Deposited: 07 Dec 2018 12:30
Last Modified: 22 Jan 2019 13:34
URI: http://eprints.esc.cam.ac.uk/id/eprint/4371

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