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 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.

Impact of Multiscale Variability on Last 6,000 Years Indian and West African Monsoon Rain

“…For each 500-year nonoverlapping windows, the seven colored squares show the changes in the respective influence of the first seven EOF modes for the two variability timescales refer to as SV (short term) for the 2-to 20-year variability and LV (long term) for the 50-to 500-year variability (see section 2.2 for analysis details). Random phasing between these two variability ranges alone can thus be involved to understand the major events that had profound impact in the tropics, such as the 4.2 ka BP event (Giesche et al, 2019;Staubwasser et al, 2003). The EOF numbers are plotted on the bars that have a size proportional to the load (relative unit) of each EOF mode for the respective monsoon indices.…”

“…higher values. Random phasing between these two variability ranges alone can thus be involved to understand the major events that had profound impact in the tropics, such as the 4.2 ka BP event (Giesche et al, 2019;Staubwasser et al, 2003).…”

Impacts of long term climate change during the collapse of the Akkadian Empire

“…Furthermore, this water shortage was recorded on the tomb inscription of Ankhtifi alongside details of starvation, famine, and civil war (Butzer, 2012). Meanwhile, the Indus Valley civilisation suffered from drought as a result of changes in both the Indian summer monsoon and winter rains from Western Disturbances (Giesche et al, 2019). This has been shown in pollen records and in sediment cores from Lonar Lake and the Indus delta.…”

“…This has been shown in pollen records and in sediment cores from Lonar Lake and the Indus delta. (Giesche et al, 2019;Prassad et al, 2014;Staubwasser and Weis, 2006;Staubwasser et al, 2003).…”

Mid–late Holocene event registered in organo-siliciclastic sediments of Lagoa Salgada carbonate system, southeast Brazil

“…Many studies involving OM have been done to characterize past and recent depositional environments (Megens et al, 2002;Pessenda et al, 2004;Salomons and Mook, 1981). The percentage of total organic carbon (% TOC) and the C : N ratio can also indicate the productivity and OM sources in paleoclimatic interpretations (Hartmann and Wünnemann, 2009). Thus, the objectives of this work were to evaluate the depositional processes related to sea level changes during the marine and lacustrine stage and to interpret the Holocene climatic changes during the last 5.8 kyr.…”