Wind erosion in the Qaidam basin, central Asia: Implications for tectonics, paleoclimate, and the source of the Loess Plateau

Liquid water and ice are the dominant agents of erosion and sediment transport in most actively growing mountain belts. An exception is in the western Qaidam basin along the northeastern margin of the Tibetan Plateau, where wind and windblown sand have sculpted enormous yardang fields in actively folding sedimentary strata. Here, we present observations suggesting that since the late Pliocene, wind episodically (during glacial and stadial periods) removed strata from the western Qaidam basin at high rates (>0.12–1.1 mm/yr) and may have accelerated rates of tectonic folding. Severe wind erosion likely occurred during glacial and stadial periods when central Asia was drier and the main axis of the polar jet stream was located ~10° closer to the equator (over the Qaidam basin), as predicted by global climate models. Reconstructed wind patterns, the estimated volume of Qaidam basin material removed by wind, and numerical models of dust transport all support the hypothesis that the Qaidam basin was a major source of dust to the Loess Plateau. INTRODUCTION The 700-km-long, up to 300-km-wide Qaidam basin (Figs. 1 and 2A) encompasses one of the highest (~2800 m elevation) and driest (<50 mm/yr precipitation in its western part) deserts on Earth, and it is actively shortening NE-SW in response to the ongoing collision between India and Asia (Tapponnier et al., 2001). A common inference, likely because of its low relief (<300 m), intermontane and internally drained setting, and thick accumulations of late Cenozoic sediment, is that the Qaidam basin is actively filling with sediment. However, roughly one-third of the modern Qaidam basin floor (~3.88 × 10 km) actually exposes folded sedimentary strata (Fig. 2A) exhumed since the Pliocene due to uplift and wind erosion. Severe wind erosion is demonstrated in the western Qaidam basin by the presence of extensive fields of mega-yardangs (Figs. 2A–2C and 3A) (Goudie, 2007), which are ridges (and parallel troughs/ pans) carved in cohesive material by strong, unidirectional winds and saltating particles carried by the wind, with spacing and relief on the order of hundreds of meters and tens of meters (locally up to ~50 m), respectively. In most places, windcarved substrata consist of relatively friable, Plio-Quaternary lacustrine deposits. Yardang ridges form parallel to and taper in the direction of prevailing winds. The geometries of yardangs, and dune forms in ergs where present, reveal the prevailing wind pattern during their development (Fig. 2A). Northwesterly winds entered the Qaidam basin through topographic lows in the Altyn Tagh Range along its northwestern margin (Fig. 2A; Halimov and Fezer, 1989). Moving from west to east in the basin, the wind directions become more westerly, paralleling the trends of the >5000-m-high basin-bounding mountain ranges (Fig. 2A). Located downwind of the Qaidam basin is one of the most voluminous and best-exposed accumulations of Neogene-Quaternary loess on Earth—the Loess Plateau (e.g., Kukla, 1987; Porter, 2007) (Fig. 1). The alternating loess/paleosol stratigraphy of the Loess Plateau provides one of the richest terrestrial records of climate change since the Pliocene. Loess accumulation occurred primarily during glacial and stadial periods, when climatic conditions were drier in central Asia because of a weakened East Asian summer monsoon (e.g., An, 2000; Porter, 2007). Knowledge of the source regions for the loess, and how they have varied through time, is critical for assessing numerical models of atmospheric circulation during the Pliocene and Quaternary and the provenance of the nutrient-rich dust transported by high-level westerly winds into the Pacific Ocean, which may have increased marine productivity and contributed to the drawdown of atmospheric carbon dioxide during glacial periods (Rea, 1994; Bopp et al., 2003; Mahowald et al., 2006). In addition, Chinese loess input into the ocean has strongly altered seawater chemistry (e.g., Jacobson, 2004). Although it is widely argued that the bulk of Loess Plateau deposits was sourced from the Gobi and adjacent sand deserts (e.g., Sun, 2002; Sun et al., 2008), observations of spatially extensive fields of yardangs and the prevailing northwesterly to westerly wind pattern in the Qaidam basin suggest that its importance as a Loess Plateau source may presently be underappreciated. HISTORY OF QAIDAM WIND EROSION The wind erosion that produced the modern Qaidam yardangs must be younger than late Pleistocene because lacustrine strata of this age are widely exposed in the wind-eroded part of the basin (Pan et al., 2004). However, it does not appear that GSA Today, v. 21, no. 4/5, doi: 10.1130/GSATG99A.1 *[email protected] **Now at Institut für Geowissenschaften, Universität Potsdam, 14476 Potsdam, Germany