Late Pleistocene megalake system in the Yinchuan-Hetao rifting basins, upper reaches of the Yellow River

Abstract

Lacustrine paleoenvironment reconstructions integrating geomorphic and sedimentary archives yield critical insights into hydrologic variability across basins. The Yinchuan Basin (YCB) and Hetao Basin (HTB), large active rifting basins along the upper reaches of the Yellow River, are connected by the Wuhai broad valley (WBV). While the HTB was once occupied by "Megalake Hetao" (HTML) during the period from ∼100 ka to 60–50 ka, the coeval lacustrine histories of the YCB and WBV remain unclear. This study combines field investigations, optically stimulated luminescence (OSL) dating, and grain-size distribution (GSD) analyses to reconstruct the late Pleistocene paleoenvironmental conditions in the YCB and WBV. Our results reveal that lacustrine deposits in the WBV and YCB record two distinct phases of high lake levels during Marine Isotope Stage (MIS) 5–4. Rising lake levels in the YCB, HTB, and WBV coalesced to form a unified water body, designated as Megalake Yinchuan-Hetao. The first lake rise developed between ∼100 ka and 73 ka (MIS 5), facilitated by sustained low subsidence rates of basins coupled with elevated precipitation. An ephemeral lake-level drop at ∼73 ka exhibited temporal correlation with regional paleoseismic activity, potentially triggering dam breaching and lake-level drop. The second lake rise, spanning ∼73–67 ka (MIS 4), manifested under cooler climatic conditions, characterized by elevated sediment influx and contracted accommodation space of basins associated with persistent tectonic quiescence, ultimately culminating in lake spillover and drainage of lakes. These findings enhance our understanding of the broader hydrology and the evolutionary paleogeography of the upper reaches of the Yellow River, providing a case study on the tectonic and climatic factors that influenced paleolake formation within rifting basins in modern arid settings.