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Clim. Past, 14, 1669–1686, 2018 https://doi.org/10.5194/cp-14-1669-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. 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 1Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, USA 2Department of Earth and Environmental Sciences, Paleontology & Geobiology, Ludwig-Maximilians-Universität München, Munich, Germany 3GeoBio-CenterLMU, Ludwig-Maximilians-Universität München, Munich, Germany 4Faculty of Science and Engineering, Curtin University, Perth, Australia 5Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI, USA 6Geology & Geophysics, Louisiana State University, USA 7Marine Chemistry & Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, USA 8Institute of Archaeology, University College London, London, UK Correspondence: Liviu Giosan ([email protected]) Received: 26 March 2018 – Discussion started: 4 April 2018 Revised: 17 October 2018 – Accepted: 18 October 2018 – Published: 13 November 2018 Abstract. Climate exerted constraints on the growth and de- helped trigger the metamorphosis of the urban Harappan civ- cline of past human societies but our knowledge of temporal ilization into a rural society through a push–pull migration and spatial climatic patterns is often too restricted to address from summer flood-deficient river valleys to the Himalayan causal connections. At a global scale, the inter-hemispheric piedmont plains with augmented winter rains. The decline thermal balance provides an emergent framework for under- in the winter monsoon between 3300 and 3000 years ago standing regional Holocene climate variability. As the ther- at the end of ENA could have played a role in the demise mal balance adjusted to gradual changes in the seasonality of the rural late Harappans during that time as the first Iron of insolation, the Intertropical Convergence Zone migrated Age culture established itself on the Ghaggar-Hakra inter- southward accompanied by a weakening of the Indian sum- fluve. Finally, we speculate that time-transgressive land cover mer monsoon. Superimposed on this trend, anomalies such as changes due to aridification of the tropics may have led to a the Little Ice Age point to asymmetric changes in the extra- generalized instability of the global climate during ENA at tropics of either hemisphere. Here we present a reconstruc- the transition from the warmer Holocene thermal maximum tion of the Indian winter monsoon in the Arabian Sea for to the cooler Neoglacial. the last 6000 years based on paleobiological records in sed- iments from the continental margin of Pakistan at two lev- els of ecological complexity: sedimentary ancient DNA re- flecting water column environmental states and planktonic 1 Introduction foraminifers sensitive to winter conditions. We show that strong winter monsoons between ca. 4500 and 3000 years The growth and decline of human societies can be affected ago occurred during a period characterized by a series of by climate (e.g., Butzer, 2012; deMenocal, 2001) but ad- weak interhemispheric temperature contrast intervals, which dressing causal connections is difficult, especially when no we identify as the early neoglacial anomalies (ENA). The written records exist. Human agency sometimes confounds strong winter monsoons during ENA were accompanied by such connections by acting to mitigate climate pressures or, changes in wind and precipitation patterns that are partic- on the contrary, increasing the brittleness of social systems ularly evident across the eastern Northern Hemisphere and in face of climate variability (Rosen, 2007). Moreover, our tropics. This coordinated climate reorganization may have knowledge of temporal and spatial climatic patterns remains too restricted, especially deeper in time, to fully address so- Published by Copernicus Publications on behalf of the European Geosciences Union. 1670 L. Giosan et al.: Neoglacial climate anomalies and the Harappan metamorphosis cial dynamics. Significant progress in addressing this prob- semi-arid landscape of the alluvial plain (Karim and Veizer, lem has been made especially for historical intervals (e.g., 2002). Carey, 2012; McMichael, 2012; Brooke, 2014; Izdebski et The climatic trigger for the urban Harappan collapse was al., 2016; d’Alpoim Guedes et al., 2016; Nelson et al., 2016; probably the decline of the summer monsoon (e.g., Dixit et Ljungqvist, 2017; Haldon et al., 2018) using theoretical re- al., 2014; Kathayat et al., 2017; MacDonald, 2011; Singh considerations, novel sources of data and sophisticated deep et al., 1971; Staubwasser et al., 2003; Stein, 1931) that led time modeling that could lead to better consilience between to less extensive and more erratic floods, making inunda- natural scientists, historians and archaeologists. The coa- tion agriculture less sustainable along the Indus and its trib- lescence of migration phenomena, profound cultural trans- utaries (Giosan et al., 2012) and may have led to bio-socio- formations and/or collapse of prehistorical societies regard- economic stress and disruptions (e.g., Meadow, 1991; Schug less of geographical and cultural boundaries during certain et al., 2013). Still, the remarkable longevity of the decentral- time periods characterized by climatic anomalies, events or ized rural phase until ca. 3200 years ago, in the face of persis- regime shifts suggests that large scale climate variability tent late Holocene aridity (Dixit et al., 2014; Fleitmann et al., may be involved (e.g., Donges et al., 2015 and references 2003; Ponton et al., 2012; Prasad and Enzel, 2006), remains therein). At the global scale, the interhemispheric thermal puzzling. Whether the Harappan metamorphosis was simply balance provides an emergent framework for understanding the result of habitat tracking toward regions where summer such major Holocene climate events (Boos and Korty, 2016; monsoon floods were still reliable or also reflected a signifi- Broecker and Putnam, 2013; McGee et al., 2014; Schneider cant increase in winter rain remains unknown (Giosan et al., et al., 2014). As this balance adjusted over the Holocene to 2012; Madella and Fuller, 2006; Petrie et al., 2017; Wright gradual changes in the seasonality of insolation (Berger and et al., 2008). To address this dilemma, we present a proxy Loutre, 1991), the Intertropical Convergence Zone (ITCZ) record for the Indian winter monsoon in the Arabian Sea and migrated southward (e.g., Arbuszewski et al., 2013; Haug et show that its variability was an expression of large scale cli- al., 2001) accompanied by a weakening of the Indian summer mate reorganization across the eastern Northern Hemisphere monsoon (e.g., Fleitmann et al., 2003; Ponton et al., 2012). and tropics affecting precipitation patterns across the Harap- Superimposed on this trend, centennial- to millennial-scale pan territory. Aided by an analysis of Harappan archaeologi- anomalies point to asymmetric changes in the extratropics cal site redistribution, we speculate that the Harappan reloca- of either hemisphere (Boos and Korty, 2016; Broccoli et al., tion after the collapse of its urban phase may have conformed 2006; Chiang and Bitz, 2005; Schneider et al., 2014). to a push–pull migration model. The most extensive but least understood among the early urban civilizations, the Harappan (Figs. 1 and 2; see Sup- plement for distribution of archaeological sites), collapsed 2 Background ca. 3900 years ago (e.g., Shaffer, 1992). At their peak, the Harappans spread over the alluvial plain of the Indus and its Under modern climatological conditions (Fig. 3), the sum- tributaries, encroaching onto the Sutlej–Yamuna or Ghaggar- mer monsoon delivers most of the precipitation to the for- Hakra (G-H) interfluve that separates the Indus and Ganges mer Harappan territory, but winter rains are also significant in drainage basins (Fig. 1; see more information on the Harap- quantity along the Himalayan piedmont (i.e., between 15 % pans in Appendix A). In the late Harappan phase that was and 30 % annually). Winter rain is brought in primarily by characterized by more regional artefact styles and trading extra-tropical cyclones embedded in the westerlies (Dimri networks, cities and settlements along the Indus and its trib- et al., 2015) and are known locally as western disturbances utaries declined while the number of rural sites increased on (WD). These cyclones distribute winter rains to a zonal swath the upper G-H interfluve (Gangal et al., 2010; Kenoyer, 1998; extending from the Mediterranean through Mesopotamia, the Mughal, 1997; Possehl, 2002; Wright, 2010). The agricul- Iranian Plateau and Balochistan, all and across to the west- tural Harappan economy showed a large degree of versatility ern Himalayas (Fig. 3). Stronger and more frequent WD rains by adapting to water availability (e.g., Fuller, 2011; Giosan et in northwestern India are associated with southern shifts of al., 2012; Madella and Fuller, 2006; Petrie et al., 2017; Weber the westerly jet in the upper troposphere (e.g., Dimri et al., et al., 2010; Wright et al., 2008). Two precipitation sources, 2015). Surface winter monsoon winds are generally directed the summer monsoon and winter westerlies (Fig. 1), provide towards the southwest but they blow preferentially