Little Ice Age wetting of interior Asian deserts and the rise of the Mongol Empire

https://doi.org/10.1016/j.quascirev.2015.10.033Get rights and content

Highlights

  • The Tarim Basin was wetter than today for most of the past 830 yrs.

  • Wet conditions correspond to Little Ice Age cooling.

  • Southward migration of rangelands facilitated spread of Mongol Empire.

  • Current warming could cause northward expansion of interior Asian deserts.

Abstract

The degree to which warming of the planet will alter Asia's water resources is an important question for food, energy, and economic security. Here we present geological evidence, underpinned by radiometric dating and dendrochronology, and bolstered by hydrological modeling, indicating that wetter-than-present conditions characterized the core of the inner Asian desert belt during the Little Ice Age, the last major Northern Hemispheric cold spell of the Holocene. These wetter conditions accompanied northern mid-latitude cooling, glacier expansion, a strengthened/southward-shifted boreal jet, and weakened south Asian monsoons. We suggest that southward migration of grasslands in response to these wetter conditions aided the spread of Mongol Empire steppe pastoralists across Asian drylands. Conversely, net drying over the 20th century has led to drought that is unprecedented for the past ∼830 years, and that could intensify with further heating of the Asian continent.

Introduction

Sub-millennial climate fluctuations and associated changes in moisture availability can have important effects on human civilizations (Zhang et al., 2008, Buckley et al., 2010, Cook et al., 2010, Pederson et al., 2014, Davi et al., 2015). Water-climate interactions are particularly important in Asia, Earth's largest and most populous continent, where hydroclimatic changes may have attended cultural shifts over the past two millennia (Zhang et al., 2008, Pederson et al., 2014). Although atmospheric temperatures over the center of the Asian continent are highly sensitive to radiative heating of the land surface (Solomon et al., 2007, McKinnon et al., 2013, McKinnon and Huybers, 2014), it is uncertain how the hydrological system will respond to future warming (Chiang and Friedman, 2012, Broecker and Putnam, 2013). Here we present geomorphological, biological, and historical evidence for hydroclimatic change during the past ∼830 yrs in the Tarim Basin of the inner Asian desert belt. Our reconstruction affords insight into relationships among climate change, water, and human culture in the heart of Asia, as well as how water resources might respond to future atmospheric warming.

The large (∼1,090,000 km2) and closed Tarim Basin of western China (42-36°N, 74-95°E) is well positioned for monitoring hydrologic responses to climatic change in the mid-latitude deserts of interior Asia (Fig. 1). The Tarim Basin is bordered by the high Tien Shan, Pamir, and Kunlun ranges along its northern, western, and southern edges (Fig. 1). It features the Taklamakan Desert, the second-largest shifting-sand desert on Earth, as well as the Lop Desert, a now-dry eastern lakebed. Major drainage systems of the Tarim Basin include the Tarim, Yarkand, Khotan, Keriya, Niya, Aktash, Endere, Cherchen, and Konqi Rivers, all of which now terminate in the Taklamakan Desert (Yang et al., 2006). The Tarim, Cherchen, and Konqi Rivers flow toward the Lop Desert, where in historical times there existed a large lake known as ‘Lop Nor’ (‘Nor’ is derived from the Mongolian word for ‘Lake’) (Li et al., 2008).

The surface elevation of the groundwater table in the Tarim Basin is modulated by rivers fed from spring/summer melt of winter snowpack in the high adjacent mountains, as well as by evaporation over the deserts (Yang et al., 2002, Chen et al., 2006b). Winter snow of the high mountain ranges alongside the Tarim Basin is nourished by orographic precipitation from strong westerly airflow, and is ablated by summer warmth. Thus temperature and atmospheric circulation are dominant controls on runoff (e.g., Aizen et al., 1995, Luce et al., 2014).

Landforms composed of waterlain sediments are widespread in the Taklamakan Desert (Yang et al., 2002, Yang et al., 2006), indicating higher-than-present groundwater surface elevations in the past. These landforms are wind-scoured platforms of laminated sand and silt overlain by shifting sand dunes meters to tens-of-meters in relief (Fig. 2, Fig. 3; see also site descriptions below). In the Lop Desert, a relict shoreline at 800 m above sea level (a.s.l.) marks a former highstand of Lop Nor (Fig. 1, Fig. 4) when it had an area of ∼19,800 km2 and a maximum depth of ∼18 m.

Stands of well-preserved subfossil Populus euphratica and Tamarix ramosissima phreatophyte trees are rooted in waterlain sediments preserved in the Taklamakan Desert. Both species are indicative of riparian forests and both require high groundwater surfaces for germination and sapling survival, as well as persistent access to groundwater for long-term growth (Gries et al., 2003). Subfossil Phragmites sp. [i.e., ‘common reed’ of Eurasian wetlands (Thevs et al., 2007)] were also found rooted in these waterlain sediments. At the Lop Desert, desiccated lakebed sediments contain bivalve and gastropod shells. Landforms in the now-dry Taklamakan Desert and paleolake shorelines in the Lop Desert, together with subfossil phreatophyte vegetation and faunal assemblages, constitute direct physical and biological evidence for higher-than-present groundwater surfaces.

We employed 14C dating to determine ages of subfossil wood, reeds, and lacustrine bivalves and gastropods. In addition, targeting subfossil P. euphratica and T. ramosissima specimens with visible rings, we combined 14C dating with dendrochronology to refine the timing of sapling recruitment during intervals when these trees were alive. Recruitment dates indicate times when the groundwater surface was persistently at or very close to the land surface, permitting germination and survival of sapling trees. We compiled recruitment dates based on tree-ring data collected in A.D. 1983 from living trees in regions in the northern Tarim Basin now unsuitable for sapling survival. Methods are reported in the Appendix. Whereas ages of recruitment and growth correspond to periods of sustained wetter-than-present conditions, and are thus climatically and hydrologically significant, we note that dates of tree death can relate to a number of non-climatic factors (such as disease, meandering stream channels, etc.). Therefore, in the interest of assessing implications for past hydroclimate, we discuss only chronological data that document tree recruitment and growth.

Section snippets

Geomorphology of study sites

We focused our study on five sites in the Tarim Basin (Fig. 1). Sites 1–4 in the Taklamakan Desert are associated with now-dry channels of river systems that emanated from the Kunlun and Tien Shan drainage basins. Sites 1 and 2 (Fig. 1) are two and three km east, respectively, of a dry distributary channel of the Khotan River (Fig. 1). Site 3 is ∼50 km north of where the dry northern end of the Aktash River channel is obscured by migrating sand dunes of the Taklamakan Desert. Site 4 is within a

Results

Forty-two 14C dates of organic remains collected from waterlain sediments at four Taklamakan Desert sites and one Lop Desert site range in age from A.D. 1180 to A.D. 1820 (Fig. 1, Fig. 5; Table A1). Thirty of these 14C dates are on P. euphratica wood samples from Taklamakan Desert sites. Ten of these thirty 14C dates are on wood samples from inner and outer tree rings of P. euphratica trees in which the intervening number of rings have been counted, thus improving dating uncertainties (Table A2

Historical Lop Nor and its desiccation

Our geomorphic and chronologic data indicate that the Tarim Basin was wetter than today beginning as early as A.D. 1180 and continuing until A.D. 1907. Historical accounts and maps reinforce this conclusion. The oldest physical description of Lop Nor (also referred to as Kara Khatun, Kara Koshun, and Kara Buran) comes from a Sogdian account called the Tarikh-i-Rashidi, written by Mirza Muhammad Haidar Dughlát between A.D. 1541–1544 (Dughlát, 1544). As described by Prejevalsky (1879; p. 26): “In

Hydrological modeling

We conducted hydrologic modeling to identify the climate conditions necessary to sustain a closed-basin Lop Nor at the documented level of 800 m a.s.l. We combined a snow mass-balance model (Birkel et al., 2012, Putnam et al., 2013), a watershed water-balance model (McCabe and Markstrom, 2007), and a lake-evaporation model (based on Priestley and Taylor, 1972) to simulate the overall steady-state hydrologic response of the lake under two derived climate scenarios: 1) the A.D. 1950s decade,

Tarim Basin hydroclimate and the Little Ice Age

The timing of the rise of the groundwater surface in the Tarim Basin, as provided by our reconstruction, suggests a wetter-than-present climate in the Tarim Basin and across much of the Asian mid-latitude desert belt. In the Tarim Basin, our chronology is compatible with, and augments, proxy reconstructions for local humidity, inferred from the δ13C of Tamarix sp. leaf matter (Liu et al., 2010) and lacustrine sediment proxies (Chen et al., 2006a) at sites in the northern Tarim Basin. Our record

Conclusions

  • 1)

    A 14C chronology of subfossil trees and reeds associated with waterlain sediments at four sites in the Taklamakan Desert, as well as mollusks associated with an elevated shoreline documenting the presence of the ∼19,800 km2 closed-basin lake Lop Nor, indicates that the Tarim Basin was continuously wetter than today at least as early as A.D. 1180 until the middle A.D. 1800s. This chronology is reinforced by 250 dendrochronologically constrained P. euphratica recruitment ages from living trees

Acknowledgments

We are grateful to the Comer Science and Education Foundation (CSEF), the Quesada Family Fund, and the Lamont Climate Center for support. Putnam received generous support from a Lamont-Doherty Earth Observatory Research Professorship, the Lenfest Foundation, and the CSEF. We thank Zhisheng An, Weijian Zhou, and Minlu Fu for generous support in China. John Chiang, David Battisti, and Tanzhuo Liu, provided helpful insights into the hydroclimate dynamics of Asia. We thank Chris Atwood for

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