THREE ENVIRONMENTAL CHANGES IN BACTRIA AND SOGDIANA (CENTRAL ASIA, AFGHANISTAN, AND UZBEKISTAN) FROM THE NEOLITHIC TO THE LATE BRONZE AGE: Interaction with human occupation

INTRODUCTION

The Holocene climate change involving a general aridification and the timeline of its fluctuations are well established in Central Asia (Staubwasser and Weiss 2006; Chen et al. 2008). The Early Holocene climate was dry, when it changed to wetter or less dry from the Early to Mid-Holocene and finally moderately wet in the Late Holocene period (Mayewski et al. 2004; Chen et al. 2008). These rapid climatic changes or “RCC events” have been identified in the northern hemisphere in many fossil sequences (Bond et al. 1997; Mayewski et al. 2004). They led to significant environmental modifications and had a socioeconomic impact on ancient societies (Weiss and Bradley 2001; Weiss 2016), especially during the Neolithic and Bronze Age. However, while the correlation between abrupt climatic changes and collapse of civilizations is a fact for some authors (e.g., Weiss et al. 1993; Possehl 1997; Cullen et al. 2000; DeMenocal 2001; Madelia and Fuller 2006; Kaniewski et al. 2013; Welc and Marks 2014), it is less obvious for others (e.g., Coombes and Barber 2005; Knapp and Manning 2016). During the last few years, the relation between climate change and societal evolutions and the causal links between paleoclimates, environments, and culture changes have gained a new interest (Butzer 2012; Tang et al. 2013; Kuzucuoglu and Tsirtsoni 2016).

In southwestern arid Central Asia, the strong dependance of societies on water resources is well illustrated by the intertwined spatial distribution of ancient settlements and paleochannels as well as by the importance of runoffs for their subsistance, mainly provided by irrigated agriculture (Lewis 1966; Andrianov 1985, 2016; Francfort and Lecomte 2002). In this context, two phenomena should be considered that had a possible impact on the modifications in the settlement pattern: a decrease in the river flows and a change in the river courses. In order to render the spatio-temporal river course modifications, our research has focused on two deltaic floodplains, the Balkh River in northern Afghanistan and the Zeravshan River in Uzbekistan (Figure 3.1). What is the impact of aridification on the annual discharge of these rivers? What are the other factors controlling the changes of river courses?

The avulsion process is one of them, diverting flows from previous channels and leading to the abandonment of all or part of a meander belt (Allen 1965, 1978). It is a natural process in response to a variety of factors, such as topography, floodplain slopes, water table level, vegetation cover, and runoff energy. Some of these factors, exacerbated in the frame of a climatic change, result in an increase of the sedimentation rate, a change in the water discharge peak, and the degradation of the vegetation cover to varying extents according to the specific features of each river catchment (Mohrig et al. 2000). Avulsion is also an answer to exterior disruptions like local tectonics promoting a rise of the flow base level and obstructions of the drainage valley (Tòrnqvist 1994; Jones and Schumm 1999; Korup 2004; Stouthamer and Berendsen 2007; Strom and Abdrakhmatov 2009; Wang et al. 2015). In Central Asia, tectonics have been demonstrated as a major agent of fluvial instability during the Quaternary (Létolle and Mainguet 2004), and more light can be shed on other phenomena induced by seismicity such as dammed lakes linked to rockslides in the upper part of mountainous fluvial valleys or outburst floods (Strom 2013).

Our research’ has focused on the lower deltaic part of the Balkh River near the Aqcha oasis (Afghanistan) and on that of the Zeravshan River, near the adjacent Bukhara and Qaraqòl oases (Uzbekistan). Both are situated on the desertic Turan plain. Considering the sparse water resources and the sensitive environments of arid southwestern Central Asia, fluvial metamorphose is a step in the hydro-climatic and environmental degradation induced by aridification and tectonic activity. We argue that the geographical allocation of the successive paleochannels as well as their dating and the characterization of their flow dynamic through time make it possible to settle back the successive human occupations in their climatic context since they depended, for a significant part, on water availability. This, therefore, should enable us to understand their co-evolution. In order to explore and reproduce the river changes during the Holocene, we applied a geoarchaeological approach based upon the diagnostic role of fluvial geomorphology, optically stimulated luminescence (OSL) dating, sedimentology studies, and their correlation with archaeological data. These two case studies show that channel avulsions are paleoenvironmental clues that reflect the evolution of human occupation in regions later known as Bactria and Sogdiana.