Monsoon aridification over Holocene South India & agricultural adaptation

[from the archaeobotanist] A new article, out this week in Geophysical Research Letters,"Holocene aridification of India", by, Ponton, Giosan, an others, presents important new, and quite high resolution, data on past monsoon dynamics and vegetation of peninsular India spanning the whole Holocene. This research, lead by researchers from the Woods Hole Oceanographic Institute, analyzed evidence from a Bay of Bengal sediment core, which captures discharges from the large Godavari river system. The core data comes from carbon isotopes of leaf waxes, reflecting the amount of arid-adapted/ savannah vegetation in the Godavari catchment, and oxygen isotopes from a marine microfossil that record salinity. This points to a general aridification trend over the course of the middle and late Holocene, supporting what we already would infer from pollen data in Rajasthan or monsoon proxies in the Arabian Sea, but this time providing more direct evidence from South India. My own involvement in this work came in the form of trying to think about how this might be correlated with archaeological evidence for settlement, agriculture and population in South India-- where the archaeological record suggests increasing sedentism, population and agriculture in response to, or despite, aridification, a contrast from the Indus region for example where the long-term trend of population depletion as aridification proceeded. This suggests long term cultural adapatation processes to aridification in peninsular Indian agricultural practices.

To quote from part of our conclusion: "The significant aridification recorded after ca. 4,000 years ago may have spurred the widespread adoption of sedentary agriculture in central and south India capable of providing surplus food in a less secure hydroclimate. Archaeological site numbers and the summed probability distributions of calibrated radiocarbon dates from archaeological sites, which serve as proxies of agricultural population, increase markedly after 4,000 BP in peninsular India [discussed in detail in the electronic supplementary text]...In contrast, the same process of drying elicited the opposite response in the already arid northwestern region of the subcontinent along the Indus River. From 3,900 to 3,200 years BP, the urban Harappan civilization entered a phase of protracted collapse. Late Harrapan rural settlements became instead more numerous in the rainier regions at the foothills of the Himalaya and in the Ganges watershed."  Most of the archaeological information is summarized in the electronic supplement, Section 4., and included an attempt to sum Neolithic/Chalcolithihc radiocarbon dates (as limited as they are) and to tally known site numbers through the Iron Age. 

This work complements recent sedimentary studies of the Indus river system, such as the Clift et al Geology paper, blogged earlier.

A widening range of textiles on Harappan trading ships

The archaeobotanist blog summarizes some recent reports on textile and fibres identifictions from Harappan contexts, including hard evidence for jute (published by Wright et al). Taken together with jute and sunn hemp finds from eastern Iran, recent evidence for Harappan silk (made from the native Assam silk moth), and older evidence for flax and cotton, we can regard the Harappan civilization as the most diversified textile producers of the Bronze Age world. They also made nets made out wild palm fibres.

Sourcing the 'lost Saraswati' river: new geological evidence

[from thearchaeobotanist]

Recently published on-line in Geology  is a paper which might not appear on the surface to be very archaeobotanical, but which is important for thinking about the past agriculture of the Indus valley. This is by Clift et al (2012) on "U-Pb zircon dating evidnece for a Pleistocene Sarasvati River and capture of the Yamuna River". This paper provides sources for the headwater sediments in the various rivers of the Indus system based on zircon finger-printed (geological source dating in the 1000s of millions of years). These dated source profiles in turn are stratified in the Pleistocene and Holocene river sequences which have been dated by OSL. These river systems include the now extinct Ghaggar-Hakra river, often equated with the 'lost Saraswati" of Indian epic. The paper shows that while the Ghaggar-Hakra used to be much larger in the Pleistocene, drawing on the headwaters that now feed the  Yamuna, tha Yamuna had begun to flow  east into the Ganges before the End of the Pleistocene, and therefore well before the start of Harappan urban societies. Throughout the Holocene, including the Harappan period this river was fed only by seasonal monsoon rain in the east. This rain-fed Ghaggar-Hakra was  active until after 4.5 ka and was then covered by dunes before 1.4 ka. What this means is that the Ghaggar-Hakra, unlike any of the major Indus tributaries, was not fed by snow melt, which begins in Spring and may be unpredictable, but was entirely reliant on swelling its banks from the summer monsoon. This means it would have been an ideal river for winter crop agriculture, along the lines of the Nile flood regime which is keyed to the Blue Nile's monsoon source, with sowing of wheat and barley in Oct.-Nov. as the monsoon flood began to recede to leave behind a rich floodplain. These could then be left to mature until harvests in March or April, without fear of early snowmelt floods ruining crops. It really should come as no surprise then that so many Harappan Bronze Age sites concentrated in this valley. Nevertheless as monsoons gradually weakened (already underway during the Harappan period) with the flood water source retreating eastwards, and the Thar desert expanding, the valley became gradually drier and eventually choked with desert sands. This, however happened in Iron Age or post-Iorn Age times, so thus there is no basis for correlating any catastrophic shift in the Ghaggar-Hakra with the end of the Harappan civilization-- a notion which has often appealed to archaeologists.

[edited for typos 9.2.2102 DF]