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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 04 Apr 2018

Research article | 04 Apr 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Climate of the Past (CP).

Neoglacial Climate Anomalies and the Harappan Metamorphosis

Liviu Giosan1, William D. Orsi2,3, Marco Coolen4, Cornelia Wuchter4, Ann G. Dunlea1, Kaustubh Thirumalai5, Samuel E. Munoz1, Peter D. Clift6, Jeffrey P. Donnelly1, Valier Galy7, and Dorian Q. Fuller8 Liviu Giosan et al.
  • 1Geology & Geophysics, Woods Hole Oceanographic Institution, MA, USA
  • 2Department of Earth and Environmental Sciences, Paleontology & Geobiology, Ludwig Maximilians-Universität München, 80333 Munich, Germany
  • 3GeoBio-CenterLMU, Ludwig-Maximilians-Universität München, 80333 Munich, Germany
  • 4Curtin University, Perth, Australia
  • 5Brown University, Providence, RI, USA
  • 6Geology & Geophysics, Louisiana State University, USA
  • 7Marine Chemistry & Geochemistry, Woods Hole Oceanographic Institution, MA, USA
  • 8Institute of Archaeology, University College London, London, UK

Abstract. Climate exerted constraints on the growth and decline of past human societies but our knowledge of temporal and spatial climatic patterns is often too restricted to address causal connections. At a global scale, the inter-hemispheric thermal balance provides an emergent framework for understanding regional Holocene climate variability. As the thermal balance adjusted to gradual changes in the seasonality of insolation, the Inter-Tropical Convergence Zone migrated southward accompanied by a weakening of the Indian summer monsoon. Superimposed on this trend, anomalies such as the Little Ice Age point to asymmetric changes in the extratropics of either hemisphere. Here we present a reconstruction of the Indian winter monsoon in the Arabian Sea for the last 6000 years based on paleobiological records in sediments from the continental margin of Pakistan at two levels of ecological complexity: sedimentary paleo-DNA reflecting water column environmental states and planktonic foraminifers sensitive to winter conditions. We show that strong winter monsoons between ca. 4,500 and 3,000 years ago occurred during an interval of weak interhemispheric temperature contrast, which we identify as the Early Neoglacial Anomaly (ENA), and were accompanied by changes in wind and precipitation patterns across the eastern Northern Hemisphere and Tropics. This coordinated climate reorganization may have helped trigger the metamorphosis of the urban Harappan civilization into a rural society through a push-pull migration from summer flood-deficient river valleys to the Himalayan piedmont plains with augmented winter rains. Finally, we speculate that time-transgressive landcover changes due to aridification of the Tropics may have led to a generalized instability of the global climate during ENA at the transition from the warmer Holocene Optimum to the cooler Neoglacial.

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Short summary
Climate reorganization during the Early Neoglacial Anomaly (ENA) may explain the Harappan Civilization metamorphosis from an urban, expansive culture to a rural, geographically-confined one. Landcover change is a candidate for causing this climate instability. During ENA agriculture along the flood-deficient floodplains of the Indus became too risky, which pushed people out. In the same time the Himalayan piedmont received augmented winter rain and steady summer precipitation, pulling people in.
Climate reorganization during the Early Neoglacial Anomaly (ENA) may explain the Harappan...