50 years of firn evolution on Grigoriev Ice Cap, Tien Shan, Kyrgyzstan.

Grigoriev Ice Cap, located in the Tien Shan mountains of Kyrgyzstan, has a rich history of firn and ice core drilling starting as early as 1962. Until now, the time series ended with a core drilled in 2007. Here we extend the exceptional record and describe an 18 m firn core, drilled in February 2018 on the summit of Grigoriev Ice Cap, at 4600 m a.s.l. The core has been analyzed for firn stratigraphy, major ions, black carbon, water stable isotope ratios and total β -activity. We find that the core covers 46±3 years and overlaps by two to three decades with legacy cores. A good agreement is found in major ion concentrations for the overlapping period. Concentration in ions, susceptible to being washed out, is reduced since the early 2000s. This indicates the onset of meltwater runoff. Apart from runoff evidence, however, the firn appears remarkably unchanged. We find little change in net accumulation since the 1980s. Firn temperatures fluctuate, with 2018 temperatures being the highest on record 1.6 °C at 17 m depth). However, temperatures in 2023 are again similar to the early 2000s at 2.5 °C. We hypothesize (i) that firn temperatures are stabilized by the removal of latent heat through lateral meltwater runoff, and (ii) that mass loss by runoff is compensated by an increase in snowfall. While data from a nearby weather station support the latter hypothesis, thorough testing of both hypotheses will require surface mass balance and firn modelling. [ABSTRACT FROM AUTHOR]