Processes and environmental significance of the subglacial chemical deposits in Tianshan Mountains

摘要

On the bedrock surface of Glacier No.1 in the headwater of Urumqi River, Tianshan Mts., well layered and crystallized subglacial calcite precipitations were discovered. Based on observations and analysis of the surface form, sedimentary texture and structure, and chemical composition of the deposits, clues about the subglacial processes and environment are deduced. The radial-growth crustation texture of the deposits, which builds up in the saturated CaCO_3 solution, proves the existence of pressure melting water and water films under Glacier No.1; and their rhythmic beddings, dissolved planes and unconformable contacts show that the water films responsible for the formation of these structures were in a wide range of spatial as well as temporal variations. Though formed under continental glacier in non-limestone area, the deposits are quite similar to those formed under temperate glaciers in limestone areas, a fact that shows a similar process of chemical precipitation between the two. Hence the enrichment of calcium in the subglacial melting water and the process of precipitation have actually little to do with the bedrock lithology and the glacier types. The cemented detritus in the deposits are rich in Fe and Al while depleted in K, Na and Si; also the included clay mineral consists mainly of illite, which reveals some weak chemical weathering under the continental glacier. The subglacial CaCO_3 precipitates when plenty of Ca~(++) melt into the subglacial melting water on a comparatively enclosed ice-bedrock interface under a high CO_2 partial pressure, the forming of subglacial chemical deposits therefore offers unequivocal evidence for the ongoing of subglacial chemical reactions.