Late Quaternary persistent impacts of permafrost on lake hydrology and landscape evolution on the NE Tibetan Plateau

Abstract

Permafrost on the Tibetan Plateau (TP) plays an important role on influencing regional to local hydrology of lake systems by thermokarst processes. Rare investigations and research of such process variations and how they affect persistently the water budget and landscape cratering in alpine lake basin through the Late Quaternary are reported. We used Lake Heihai on the north-eastern Tibetan Plateau as an example to reveal how landscape change/collapse under the regime of permafrost and thermokarst processes occurred since the Lateglacial. Five shallow seismic profiles in combination with lake core sediments from the lake centre as well as fossil lacustrine deposits in the catchment were obtained and analysed by multiple proxies. The results show that Lake Heihai did not exist as a terminal lake during the Lateglacial and the beginning of the early Holocene with widespread permafrost in the basin. Permafrost thaw and glacier melt of short duration contributed to lake hydrology and sediment accumulation. Thermokarst development started in the early Holocene with climate warming and increased water storage in the shallow basin, finally enabling the onset of a thermokarst lake with subsidence of lake deposits. Thermokarst thaw contributed reworked materials to the lake during the formation of a growing talik layer beneath the lake. The damming of the previous outflow in the east by glacio-fluvial accumulation initiated a closed system, combined with the accelerated thermokarst subsidence, which promoted a rapid lake level rise during the middle Holocene, mainly supplied by glacier melt. This period was characterized by the withdrawal of permafrost to higher position in the catchment. Lake Heihai became open again after around 5.3 ka and initiated a significant shrinkage culminating at around 4 cal ka BP and lasted until 2.4 cal ka BP. Permafrost returned into the exposed middle Holocene lacustrine sediments which were then subject to strong seasonal thaw-induced erosion and back transport into the lake, while water depth increased to the present level with returned thermokarst processes at the lake margin. Recent occurrences and disappearances of permafrost phenomena near shore, such as lithalsa/pingos, might have existed in the past as well. Our research sheds new light on the long-lasting impacts of permafrost on the hydrological cycle and integrated landscape evolution in a lake basin on the Tibetan Plateau.