Analyses for 81 Kr and noble gases on groundwater from the deepest aquifer system of the Baltic Artesian Basin (BAB) were performed to determine groundwater ages and uncover the flow dynamics of the system on a timescale of several hundred thousand years. We find that the system is controlled by mixing of three distinct water masses: Interglacial or recent meteoric water (δ 18 O ≈ −10.4‰) with a poorly evolved chemical and noble gas signature, glacial meltwater (δ 18 O ≤ −18 ‰) with elevated noble gas concentrations, and an old, high-salinity brine component (δ 18 O ≥ −4.5‰, ≥ 90 g Cl − /L) with strongly depleted atmospheric noble gas concentrations. The 81 Kr measurements are interpreted within this mixing framework to estimate the age of the endmembers. Deconvoluted 81 Kr ages range from 300 ka to 1.3 Ma for interglacial or recent meteoric water and glacial meltwater. For the brine component, ages exceed the dating range of the ATTA-3 instrument of 1.3 Ma. The radiogenic noble gas components 4 He* and 40 Ar* are less conclusive butalso support an age of > 1 Ma for the brine. Based on the chemical and noble gas concentrations and the dating results, we conclude that the brine originates from evaporated seawater that has been modified by later water-rock interaction. As the obtained tracer ages cover several glacial cycles, we discuss the impact of the glacial cycles on flow patterns in the studied aquifer system. Virbulis et al., 2013), the latter of which estimated the hydraulic age of groundwater in the CAS to be on the order of several hundreds of ka to 1 Ma. In the light of such long residence times, it is crucial to consider the effect of repeated glacial cycles on the long-term evolution of groundwater composition and flow. Sampling the deeper parts of the CAS on a regional scale for chemistry, noble gases, and multiple dating tracers ( 81 Kr, 85 Kr, 39 Ar, 14 C, 4 He, 40 Ar) allows us to elucidate the evolution of the brine, mixing proportions of the different groundwater components, and the flow dynamics over the last 1 Ma.

The Tien Shan and Pamir mountains host over 28,000 glaciers providing essential water resources for increasing water demand in Central Asia. A disequilibrium between glaciers and climate affects meltwater release to Central Asian rivers, challenging the region's water availability. Previous research has neglected temporal variability. We present glacier mass balance estimates based on transient snowline and geodetic surveys with unprecedented spatiotemporal resolution from 1999/00 to 2017/18. Our results reveal spatiotemporal heterogeneity characterized by two mass balance clusters: (a) positive, low variability, and (b) negative, high variability. This translates into variable glacial meltwater release (≈1-16%) of annual river runoff for two watersheds. Our study reveals more complex climate forcingrunoff responses and importance of glacial meltwater variability for the region than suggested previously.Plain Language Summary Glaciers in Central Asia act as water towers for millions of people by storing and releasing water in response to climate. Monitoring glaciers is difficult due to their often very remote locations. Satellite remote sensing has emerged as a powerful method but a drawback is their (semi-)decadal resolution for glacier mass change surveys. We present a methodology, combining multiyear elevation change maps with frequent snowline observations to estimate mass changes and variability at annual scale, which allows us identifying so far unrecognized regions of contrasting trends for the Tien Shan and Pamir mountains. These "hot spots" reveal a far more complex climate-glacier interplay than previously known. The additional meltwater released from the retreating glaciers varies considerably and contributes to the river flow for warm dry years by twice as much as for cold wet years. Our findings will help to better understand the impact of climate change on Central Asian glaciers and their meltwater release. BARANDUN ET AL.

http://www.link.springer.com/article/10.1007%2Fs10040-013-0970-

Abstract. Glacier mass loss is among the clearest indicators of atmospheric warming. The observation of these changes is one of the major objectives of the international climate monitoring strategy developed by the Global Climate Observing System (GCOS). Long-term glacier mass balance measurements are furthermore the basis for calibrating and validating models simulating future runoff of glacierised catchments. This is essential for Central Asia, which is one of the driest continental regions of the Northern Hemisphere. In the highly populated regions, water shortage due to decreased glacierisation potentially leads to pronounced political instability, drastic ecological changes and endangered food security. As a consequence of the collapse of the former Soviet Union, however, many valuable glacier monitoring sites in the Tien Shan and Pamir Mountains were abandoned. In recent years, multinational actors have re-established a set of important in situ measuring sites to continue the invaluable long-term data series. This paper introduces the applied monitoring strategy for selected glaciers in the Kyrgyz and Uzbek Tien Shan and Pamir, highlights the existing and the new measurements on these glaciers, and presents an example for how the old and new data can be combined to establish multi-decadal mass balance time series. This is crucial for understanding the impact of climate change on glaciers in this region.

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