Anthropogenic Impacts on Landscape in River Basins of SE and East Asia

Abstract:

The settlement of Asia by humans has resulted in significant impacts on the landscape and river systems of the continent. The start of metal smelting has been shown to contaminate rivers with As, Zn and Cu, but earlier disturbances can be seen through increases in chemical weathering proxies. During the early and mid Holocene the degree of alteration of sediment deposited in the deltas and slopes offshore of the Pearl and Red Rivers tracks the intensity of the summer monsoon, as reconstructed from speleothems. However, in both drainages the Late Holocene has shown an increase in weathering intensity that does not have any links to changing climate. Indeed, monsoon has generally weakened since 8000 years ago. We suggest that the pulse of weathered materials that dates from 2500 and 1800 years ago respectively in the Pearl and Red Rivers is related to heavier settlement and the start of significant agriculture. Enhanced human activity (deforestation, cultivation, mining) since the end of the Chinese Han Dynasty (220 CE) affect the Pearl River. Human settlement of that basin increased up to 2.4 ka, involving increased cultural unity prior to the conquest of the delta by the Qin Dynasty, before 2.2 ka. Sugarcane cultivation became established in the region and was a major crop by the Song Dynasty (~1.0 ka). Intensified agriculture allowed older, more altered soil materials to be eroded into the rivers and transported rapid to the delta. Enhanced human activity (deforestation, cultivation, mining) is slightly younger in the Red River catchment and dates from the end of the Chinese Han Dynasty (220 CE). We conclude that in both systems human impact has overwhelmed the natural climatic controls on erosion long before the industrial revolution.