Re-examining 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 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 5.3–3.3 ka BP 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 4.2 ka BP, 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 3.0 ka BP. 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 BP. The time of greatest change occurred at 4.1 ka BP 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 4.5–4.3 ka BP, whereas the end of the urbanized phase followed the decrease in both the summer and winter monsoon strength by 4.1 ka BP. 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.

Neoglacial climate anomalies and the Harappan metamorphosis

“…The authors comment on our work and we provide a brief reply herein. Giesche et al (2018) used multi-species planktonic foraminifer δ 18 O and δ 13 C from a core close to our site to infer a history of the Indian winter monsoon between 4.5 and 3.0 ka BP that is different than what we propose. We suggest that the ancient DNA and % Globigerina falconensis proxies are better suited to reconstruct monsoon changes by providing the right balance between planktonic whole-ecosystem change and proxy specificity, respectively.…”