Contrasting variations of chemical weathering processes during the initial ablation period in two different glacial drainage systems, Tibetan Plateau

Subglacial drainage system plays a central role in regulating chemical weathering processes in glacier environments. However, the influence of glacial drainage systems’ seasonal evolution on chemical weathering processes and consequent carbon sink effect is still unclear. This study selected the Parlung No. 4 Glacier in the southeast Tibetan Plateau (TP) and Kuoqionggangri Glacier in the central TP as the study areas, representing the typical temperate glacier and polar glacier, respectively. Sampling campaigns were conducted during the initial ablation period of those two glaciers when glacial drainage systems may undergo significant transformation. River water samples were collected almost daily for glacial runoff and 5-day intervals for outlets of the catchments. Evidences from tracer tests and Ca^2+*/SiO₂^* show that glacial drainage system of the Kuoqionggangri Glacier is quite constant and it is dominated by a supraglacial pattern (^* represents ionic concentration after atmospheric inputs correction). Subglacial drainage system of the Parlung No.4 glacier, however, transferred from a supraglacial pattern during the early monsoon season to a channelized pattern after June 25^th. Cationic budget and major anionic sources discrimination (HCO₃^-) show that chemical weathering processes in the polar glacial catchment (Kuoqiongqu) has been displaying minor temporal change even though water discharge experienced a 7 times increment. Nevertheless, carbonate dissolution in the temperate glacial catchment (Rinongqu) was 24% decreased but sulfide oxidation 23% increased with the elapse of monsoon season. Its DIC (equivalent to HCO₃^-) sources from biological CO₂ are 10% higher in early (before May 21^st) and late monitoring periods (June 26^th to July 10^th) while the input proportion of atmospheric CO₂ shows an opposite temporal change with 12% higher proportion from May 22^nd to June 25^th. The seasonal change of net CO₂ consumption rate caused by chemical weathering (ФCO₂_net) in the Kuoqiongqu catchment is positive correlated with water discharge, indicating carbon sink effect in the polar glacial catchments of the central TP is mainly governed by water discharge. ФCO₂_net in the early subglacial channels reopen period is even slightly lower than that from April 2^nd to May 10^th when water discharge is more than 2.4 times lower because of dramatic increases in CO₂ release rate caused by sulfuric acid dissolve carbonate (ФCO₂_sul). This study highlights the evolution of glacial drainage system exerted crucial effects on carbon cycle by regulating chemical weathering processes.