Extreme climates and present-day analogues : from the last millennia to the Holocene optimum (4)

Abstract

Paleoclimatic records of the Asian monsoon show a slow decline throughout the Holocene period, superimposed by abrupt, transient decreases lasting several centuries. For the event centered around 2200 BC (known as the " of 4.2 k BP "), it is possible to discern a geographical impact on a global scale, with a drop in precipitation for intertropical and subtropical latitudes and increases higher up the latitude.

The impact of the 4.2 k event has been studied for the area of Pakistan, where the Harappan civilization developed between 2600 and 1900 BC. Its vast territory stretched around the Indus River and its tributaries, as well as along the coast of the Arabian Sea, from the present-day border between Pakistan and Iran to India's Gujarat region.

This region borders the zone of influence of the summer monsoon, with rainfall concentrated in the north of the country, upstream of the Indus, in the foothills of the Himalayan range. In winter, the region also receives lower rainfall from the west. Bronze Age civilization, based on agriculture and livestock breeding, was therefore highly dependent on water resources. The major Harappean urban centers were characterized by elaborate hydraulic architecture, with baths and piping systems (e.g. Mohenjo-daro, Dholavira, Lothal), as well as large reservoirs sometimes linked to rivers (e.g. Lothal). Numerous archaeological sites are concentrated along the Ghaggar-Hakra endoreic river, north of the Thar desert, which was then a tributary of the Indus.

The Harappean civilization possessed a script that has yet to be deciphered, due to the scarcity and brevity of texts essentially limited to a few inscriptions on seals. Archaeologists are still debating the reasons for the emergence, prosperity and rapid decline of this brilliant civilization. In the absence of text, it is useful to turn to paleoenvironmental records, such as lake and marine sediments and stalagmites.

To the north of the Thar Desert, lakes that have now dried up were watered at the height of the Indus civilization (e.g. Karsandi and Kotla Dahar paleolakes). Analyses of the ^18O/16Oand D/H ratios of structural water in gypsum crystals preserved in their sediments enable us to pinpoint the wet conditions of the period and compare them with isotopic series of stalagmites from north-west India (e.g. Sahiya cave).

Marine sediments collected off the Indus delta recorded fluvial and aeolian inputs, giving us an even broader temporal perspective. Pollen analysis confirms the long drying trend since 3000 BC, with a particularly marked drop in precipitation between 2500 BC and 1500 BC. It is also possible to distinguish between contributions of mountain pollen transported by the river from the Himalayas and contributions of pollen from the arid steppes of Baluchistan transported by winter monsoon winds. Further isotopic, geochemical and micropalaeontological analyses of marine sediments are investigating the combined influences of summer and winter monsoons. Signals suggest a decrease in summer monsoon influence between 2200 BC and 1900 BC, and a relative increase in winter monsoon influence.

These major regional hydrological variations certainly had decisive impacts in explaining the development and collapse of the Harappan civilization. The archaeological data themselves provide evidence of climatic influence. In the Ghaggar-Hakra region, the urban centers dotted along the river were abandoned after 2000 BC, but the population of the rural sites migrated to northern India to benefit from water supplies that enabled them to survive.