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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 03 Sep 2018

Research article | 03 Sep 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Climate of the Past (CP).

Re-examining the 4.2 ka BP event in foraminifer isotope records from the Indus River delta in the Arabian Sea

Alena Giesche1, Michael Staubwasser2, Cameron A. Petrie3, and David A. Hodell1 Alena Giesche et al.
  • 1Godwin Laboratory for Palaeoclimate Research, Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, United Kingdom
  • 2Institute for Geology und Mineralogy, University of Cologne, Zülpicher Str. 49a, 50674 Cologne, Germany
  • 3Department of Archaeology, University of Cambridge, Cambridge, CB2 3DZ, United Kingdom

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 both 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 5.3–3.3kaBP is of particular interest in this region because it spans the period of the Indus Civilization from its early development to eventual transformation. The 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 4.2kaBP, which the authors linked to the decline of the urban phase of the Indus Civilization (Staubwasser et al., 2003). Given the importance of this study, we used the same core (63KA) to replicate the oxygen isotope profiles of a larger size fraction of G. ruber than measured previously and, in addition, we measured two other foraminifer species at decadal resolution over the interval from 5.4 to 3.0kaBP. 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 4.3kaBP. The time of greatest change occurred at 4.1kaBP when both the summer and winter monsoon weakened, resulting in a reduction in rainfall in the Indus region. The earliest phase of the Mature Harappan period coincided with the period of inferred stronger winter monsoon between 4.5–4.3kaBP, whereas the end of the urbanized phase followed the decrease in both the summer and winter monsoon strength by 4.1kaBP. Our findings provide evidence that the initial growth of large Indus urban centers was coincident with increased winter rainfall, whereas the contraction of urbanism and change in subsistence strategies followed a reduction in rainfall of both seasons.

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Short summary
A northeastern Arabian Sea foraminifer oxygen isotope record was used to reconstruct winter and summer monsoon strength from 5.4 to 3.0 ka BP. We found a 200-year period of strengthened winter monsoon (4.5–4.3 ka BP) that coincides with the beginnings of the Mature Harappan phase of the Indus Civilization, followed by weakened winter and summer monsoons by 4.1 ka BP. Aridity spanning both rainfall seasons at 4.1 ka BP may help to explain some of the observed archaeological shifts.
A northeastern Arabian Sea foraminifer oxygen isotope record was used to reconstruct winter and...