Source-sink recharge process constraints chemical and boron isotope evolution on the multi-decadal and monthly timescales in typical boron-rich lake basins on the Qinghai-Tibet Plateau

Da Qaidam Lake (DQL) and Xiao Qaidam Lake (XQL), located in close proximity with similar geological and hydrological settings, represent quintessential lakes harboring enriched boron (B) deposits on the northern Qinghai-Tibet Plateau (QTP). However, a significant disparity in both B concentrations and isotope values between these two lakes has emerged, showcasing distinct evolutionary trends in their B isotopes over the past three decades. The underlying mechanisms and evolutionary processes driving these variations remain ambiguous. This study elucidates the hydrochemical characteristics and delta 11B values for brines, geothermal springs, and river waters around the DQL-XQL, concurrently monitoring hydrogeochemical parameters in XQL and Tataleng River at a monthly temporal scale. Our findings corroborate that: (1) The delta 11B values of XQL waters have markedly increased from 0.52%o to 2.81%o, while those of DQL waters have decreased from 6.17%o to 4.16%o, exhibiting diametrically opposed isotopic trends in the face of warming-wetting climate change on the QTP. (2) Spatially, B concentration and delta 11B values in the low salinity region of DQL are predominantly influenced by the redissolution of B-rich sediments, witnessing inconspicuous changes in the high salinity region. Temporally, the delta 11B values of DQL waters experienced considerable fluctuations, ranging from 2.52%o to 10.81%o in the early 1990s, attributed to isotopic fractionation (solid-liquid separation) resulting from borate mining activities. After more than 30 years of recovery, its B isotope values have stabilized in recent years. (3) On a monthly time scale, delta 11B values of XQL and Tataleng River waters exhibited pronounced seasonal variations regulated by water residence time and the formation of secondary minerals, manifesting as high values in the dry season and low values in the wet season. Rainy or humid climatic conditions encourage an increase in weathering flux in B-rich areas, leading to a higher transport of resources into the terminal salt lake. Over a multi-decadal time scale, alterations in the hydrological budget and mean weathering flux induced by a warming-wetting climate may modify the proportion of contributions from various sources, thereby regulating the delta 11B values of XQL and Laguo Co. (4) When employing B isotopes to investigate the salt lakes evolution and paleo-environmental changes, it is imperative to consider the influence of alterations in water chemistry and isotope driven by changes in source-sink processes resulting from climate changes or tectonic movements.