Dunes and dunefields in the Bolivian Chaco as potential records of environmental change
Introduction
The lowlands of the Bolivian Chaco are situated along the southern margin of the Amazon basin, where the effects of late Quaternary latitudinal changes in vegetation and climate are clearly manifest in a wide range of relict landforms (Iriondo, 1993, Latrubesse et al., 2012, May, 2006, May, 2011, Werding, 1977a), alluvial stratigraphy (May et al., 2008a, May and Veit, 2009, May et al., 2008b, Servant et al., 1981), and pollen records (Burbridge et al., 2004, Mayle et al., 2000, Mourguiart and Ledru, 2003, Whitney et al., 2011). Further north – in the Amazon basin of inland Brazil and Venezuela – late Quaternary environmental changes have been inferred from studies of aeolian system, which are expressed in numerous forest covered palaeodunes and dunefields (Carneiro Filho et al., 2002, De Oliveira et al., 1999, Latrubesse and Nelson, 2001, Parolin and Stevaux, 2006, Tatumi et al., 2002, Tricart, 1974). Luminescence based chronologies for late Quaternary aeolian activity have also been published from dunefields and sand seas in the Argentinean Pampas (Kruck et al., 2011, Tripaldi and Forman, 2007, Tripaldi et al., 2011, Zárate and Tripaldi, 2012) and the Paraguayan Chaco (Kruck, 1996, Kruck et al., 2011). In the Bolivian Chaco, a number of studies have focused on problems related to recent aeolian activity (Derpsch, 1974, Herzog, 1914, Jordan, 1981, Jordan, 1982), whereas the only available information on type and controls of longer-term aeolian activity still relies on a very limited set of localized radiocarbon and luminescence ages (Latrubesse et al., 2012, Servant et al., 1981). Thus, much of the aeolian geomorphology in the extensive dunefields of eastern Bolivia remains to be explored (Iriondo, 1993, May, 2006), and has therefore been overlooked in hemispheric and continent-scale reviews on the Quaternary aeolian record (Muhs and Zárate, 2001, Munyikwa, 2005a). Despite the lack of detailed studies, the Bolivian Chaco has been suggested as an important source area for the widespread Quaternary loess cover across northern Argentina (Iriondo, 1990, Iriondo, 1997, Schellenberger and Veit, 2006, Zárate, 2003), and aeolian activity has generally been associated with dry climatic intervals in the Last Glacial Maximum (LGM) and/or the mid-Holocene (Clapperton, 1993, Latrubesse, 2003). Such generalized interpretation of the aeolian record as a proxy for regional to global-scale aridity (e.g. Sarnthein, 1978), however, has recently been debated (Chase, 2009, Kocurek and Lancaster, 1999, Munyikwa, 2005b, Thomas, 2013, Tsoar, 2005). As the development of sand dunes is mainly controlled by sediment supply, wind strength, and vegetation (Kocurek and Lancaster, 1999, Pye and Tsoar, 2009), a detailed knowledge of dune morphology and setting is required for a sound palaeoenvironmental – and even palaeoclimatic – interpretation of the aeolian record (Chase, 2009, Thomas, 201313). Consequently, this paper attempts the first comprehensive documentation of aeolian geomorphology in the Bolivian Chaco by mapping dunes, palaeodunes and dunefields from remote sensing data, and will thus provide the base to reconstructing mechanisms and controls of dune formation over late Quaternary timescales.
Section snippets
Study area
The evolution of the modern Andean foreland in eastern Bolivia is closely related to the Cenozoic uplift and deformation history of the Central Andes, and can be regarded as the consequence of crustal thickening, eastward propagation of the Andes and topographic loading (Allmendinger et al., 1997, Horton and DeCelles, 1997, Isacks, 1988). The Gran Chaco – a ∼850 000 km2 large area dominated by xerophytic deciduous forest (Riveros, 2004) – occupies much of the foreland basin in southeastern
Methods
To provide a comprehensive and detailed overview of aeolian landforms and processes in eastern Bolivia, data acquired on different spatial scales have been combined (Breed and Grow, 1979). (i) The analysis of regional-scale aeolian geomorphology in the Bolivian Chaco mainly relies on the visual interpretation (Hugenholtz et al., 2012, Rosenfeld, 1984, Verstappen, 1977) of freely available remote sensing data (source: US Geological Survey, http://earthexplorer.usgs.gov) such as Landsat imagery
Regional aeolian geomorphology
A variety of aeolian depositional landforms were mapped in the study area between approximately 17.5–20.5°S and 60–63°W (Fig. 2b). Depending on the spatial pattern and density of dunes, they were grouped into 10 dunefields, and then further classified according to the three main geomorphic settings (Fig. 2b) of the piedmont (A1 and A2), the foreland megafans (B1.1, B1.2, B1.3, B2, B3.1, and B3.2), and the erosional uplands of the Altos de Izozog in the eastern study area (C1 and C2).
The areas,
Discussion
The presented dataset is the first attempt to document, quantify and explain the information stored in the extensive dunes and dunefields of the Chaco in eastern Bolivia. It forms the base for the following discussion of three major issues related to the interpretation of the aeolian record: (i) what does the geomorphology of dunefields reveal about the long-term controls on their formation, (ii) what geomorphic and environmental information is stored in the spectrum of active and inactive
Conclusions
In conclusion, the detailed mapping of aeolian dunefields in eastern Bolivia has provided the first in-depth overview of the different types of aeolian landforms preserved in the Bolivian Chaco. The location and size of the dunefields generally show a dominant role of the fluvial system in the longer-term formation and preservation of the dunefields. The dominance of parabolic dunes in the Bolivian Chaco also indicates that, similar to today, vegetation and varying wind regimes were also
Acknowledgements
This research has been funded by SNF-project ‘Palaeoecology of Eastern Bolivia’ (200020-105228/1). Haim Tsoar is thanked for kindly providing Fig. 2a. The author would also like to thank Tim Cohen, Luke Gliganic, Nick Lancaster, Edgardo Latrubesse, Frank Preusser, Haim Tsoar, Heinz Veit, and Xiaoping Yang for fruitful discussions on aeolian geomorphology in the Chaco. The comments of two anonymous reviewers helped to improve the quality of this paper. Also, the logistical support of the Museo
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