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 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.
Highlights
The ∼ 4.2 ka BP event is considered to be a defining event of the mid–late Holocene transition period (Mayewski et al, 2004) and is marked by intense aridity in much of western Asia, which has been linked to cultural transitions in Mesopotamia, Egypt, and the Indus Civilization (Staubwasser and Weiss, 2006; Weiss, 2016)
This trend is observed with the SiZer analysis, which identifies a significant increase in δ18O anywhere from 4.9 to 4.2 ka depending on which smoothing window is selected (Fig. 4)
Using δ18O of the surface-dwelling foraminifera G. ruber, the original study inferred an abrupt reduction in Indus River discharge at ∼ 4.2 ka
Summary
The ∼ 4.2 ka BP event is considered to be a defining event of the mid–late Holocene transition period (Mayewski et al, 2004) and is marked by intense aridity in much of western Asia, which has been linked to cultural transitions in Mesopotamia, Egypt, and the Indus Civilization (Staubwasser and Weiss, 2006; Weiss, 2016). Defining the exact timing and extent of aridity at ∼ 4.2 ka remains an open question (Finné et al, 2011; Wanner et al, 2008). In this special issue devoted to the “4.2 ka BP event”, we provide new paleoclimate data from a marine core in the northern Arabian Sea over this critical time interval to bet-. Giesche et al.: The 4.2 ka BP event in foraminifer isotope records from the Indus River delta ter understand the changes that occurred in both winter and summer hydroclimate over the Indian subcontinent
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