Anwar Alizai

Fluvial landscapes of the Harappan civilization

The collapse of the Bronze Age Harappan, one of the earliest urban civilizations, remains an enigma. Urbanism flourished in the western region of the Indo-Gangetic Plain for approximately 600 y, but since approximately 3,900 y ago, the total settled area and settlement sizes declined, many sites were abandoned, and a significant shift in site numbers and density towards the east is recorded. We report morphologic and chronologic evidence indicating that fluvial landscapes in Harappan territory became remarkably stable during the late Holocene as aridification intensified in the region after approximately 5,000 BP. Upstream on the alluvial plain, the large Himalayan rivers in Punjab stopped incising, while downstream, sedimentation slowed on the distinctive mega-fluvial ridge, which the Indus built in Sindh. This fluvial quiescence suggests a gradual decrease in flood intensity that probably stimulated intensive agriculture initially and encouraged urbanization around 4,500 BP. However, further decline in monsoon precipitation led to conditions adverse to both inundation- and rain-based farming. Contrary to earlier assumptions that a large glacier-fed Himalayan river, identified by some with the mythical Sarasvati, watered the Harappan heartland on the interfluve between the Indus and Ganges basins, we show that only monsoonal-fed rivers were active there during the Holocene. As the monsoon weakened, monsoonal rivers gradually dried or became seasonal, affecting habitability along their courses. Hydroclimatic stress increased the vulnerability of agricultural production supporting Harappan urbanism, leading to settlement downsizing, diversification of crops, and a drastic increase in settlements in the moister monsoon regions of the upper Punjab, Haryana, and Uttar Pradesh.

U-Pb zircon dating evidence for a Pleistocene Sarasvati River and capture of the Yamuna River

The Harappan Culture, one of the oldest known urban civilizations, thrived on the northwest edge of the Thar Desert (India and Pakistan) between 3200 and 1900 BCE. Its demise has been linked to rapid weakening of the summer monsoon at this time, yet reorganization of rivers may also have played a role. We sampled subsurface channel sand bodies predating ca. 4.0 ka and used U-Pb dating of zircon sand grains to constrain their provenance through comparison with the established character of modern river sands. Samples from close to archaeological sites to the north of the desert show little affinity with the Ghaggar-Hakra, the presumed source of the channels. Instead, we see at least two groups of sediments, showing similarities both to the Beas River in the west and to the Yamuna and Sutlej Rivers in the east. The channels were active until after 4.5 ka and were covered by dunes before 1.4 ka, although loss of the Yamuna from the Indus likely occurred as early as 49 ka and no later than 10 ka. Capture of the Yamuna to the east and the Sutlej to the north rerouted water away from the area of the Harappan centers, but this change significantly predated their final collapse.

Monsoon control over erosion patterns in the Western Himalaya: possible feed-back into the tectonic evolution

Abstract The Indus Delta is constructed of sediment eroded from the western Himalaya and since 20 ka has been subjected to strong variations in monsoon intensity. Provenance changes rapidly at 12–8 ka, although bulk and heavy mineral content remains relatively unchanged. Bulk sediment analyses shows more negative ϵNd and higher 87Sr/86Sr values, peaking around 8–9 ka. Apatite fission track ages and biotite Ar–Ar ages show younger grains ages at 8–9 ka compared to at the Last Glacial Maximum (LGM). At the same time δ13C climbs from –23 to –20‰, suggestive of a shift from terrestrial to more marine organic carbon as Early Holocene sea level rose. U–Pb zircon ages suggest enhanced erosion of the Lesser Himalaya and a relative reduction in erosion from the Transhimalaya and Karakoram since the LGM. The shift in erosion to the south correlates with those regions now affected by the heaviest summer monsoon rains. The focused erosion along the southern edge of Tibet required by current tectonic models for the Greater Himalaya would be impossible to achieve without a strong summer monsoon. Our work supports the idea that although long-term monsoon strengthening is caused by uplift of the Tibetan Plateau, monsoon-driven erosion controls Himalayan tectonic evolution. Supplementary material: A table of the population breakdown for zircons in sands and the predicted Nd isotope composition of sediments based on the zircons compared to the measured whole rock value is available at http://www.geolsoc.org.uk/SUP18412