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The eruption of Mount Samalas in Indonesia ~1257 CE has been argued to be one of the largest eruptions of the last two millennia. It released a monumental amount of volcanic aerosols, and reduced incoming solar energy by more than 30 W/m² (Sigl et al., 2015). Large volcanic eruptions can cause short term (generally 5 years for tropical eruptions like Samalas) regional or global climate shifts, which include changes to monsoon rainfall (Ridley et al, 2015; Sigl et al., 2015). In order to investigate the impact of this eruption on the Indian summer monsoon in Nepal, I analyzed at ultra-high resolution the carbon and oxygen isotopes of a fast growing, precisely dated aragonite stalagmite from Siddha cave (28.26689°, 83.96851°, 820m), located in the Pokhara valley in central Nepal, leading up to and through the period spanned by the Mount Samalas eruption. Each micro milled sample was ~40 µm wide and the area sampled was ~1 cm , for a total of 261 analyses. Stalagmites are composed of calcium carbonate, with the oxygen coming primarily from precipitation dripping into the cave. Studies of oxygen isotopes in precipitation, both near Siddha cave and in Kathmandu (130 km to the southeast), reveal that the amount effects in oxygen isotopes are weak in this region. The amount effect, a relationship saying that the precipitation amount has a negative relationship with oxygen isotope values, is very weak in this region. Therefore, I relied on carbon isotopes as a proxy for site-specific rainfall. Carbon isotopes in stalagmite carbonate originate from the soil and bedrock around, and as the drip water infiltrates the earth, it begins to dissolve CO2. This process continues until the drip water is supersaturated and the pressure differential forces calcite (or aragonite) to precipitate out of the drip water and crystalize. The carbon isotopes define sinusoids that appear to represent annual cycles of rainfall associated with the summer monsoon and the winter dry season. While some outliers exist, the total number of season cycles (18-21) is within error of the number of years of growth as determined by U/Th dating (1 cm = 26 ± 8 years). To investigate the impact of the eruption on the regional climate, we detrended the carbon isotope data and then calculated anomaly values in the wet and dry season relative to the mean of those values. The most prominent feature of the time series is two large positive isotope anomalies, separated by a moderate negative isotope anomaly. I interpret these to reflect disruptions to both the wet and dry season precipitation cycles from aerosol forcing from Mount Samalas. If correct, then this data reveals, somewhat surprisingly, an anomalously wet monsoon season in the first year after the eruption and only 1 year of reduced summer monsoon rainfall following the eruption before a return to pre-eruption summer monsoon rainfall activity. References Ridley, H. E. et al. (2015). Aerosol forcing of the position of the intertropical convergence zone since ad 1550. Nature Geoscience, 8(3), 195–200. Sigl, M. et al. (2015). Timing and climate forcing of volcanic eruptions for the past 2,500 years. Nature, 523(7562), 543–549. 

The 1257 CE eruption of Mt. Samalas in Indonesia is argued to be the largest of the last two millennia in terms of global volcanic aerosol forcing, with a reduction in insolation of more than 30 W/m2 (Sigl et al., 2015, Nature, 523). Large volcanic eruptions are tied to short-term climatic shifts, including changes to monsoon rainfall (Ridley et al., 2015, Nature Geoscience, 8). In order to investigate the impact of this eruption on the Indian summer monsoon in Nepal, we analyzed at ultra-high resolution the carbon and oxygen isotopes of a fast-growing, precisely-dated aragonite stalagmite from Siddha cave in the Pokhara Valley of central Nepal (28.0˚ N, 84.1˚ E; ~850 m.a.s.l.). We micromilled the stalagmite in ~40 µm-wide traverses during the interval through the Mt. Samalas eruption (a total of 261 analyses). Studies near Siddha cave and in Kathmandu, 130 km to the southeast, reveal that amount effects of oxygen isotopes in precipitation in this region are weak, and so we rely on carbon isotopes as a proxy for rainfall. Carbon isotopes define sinusoids that appear to represent annual cycles of rainfall associated with the summer monsoon and winter dry season. The average magnitude of these cycles is ~0.3 to 0.6‰. While some ambiguities exist, the number of seasonal cycles (18-21) is within error of the years of growth for this interval as determined by U/Th dating (26±8 years). To investigate the impact of the eruption on regional hydroclimate, we detrended the carbon isotope data and then calculated anomalies in the wet and dry seasons relative to the mean of those values. The most prominent feature of the time series is two large positive isotope anomalies separated by a moderate negative isotope anomaly. We interpret these to reflect disruptions to both the monsoon and dry season precipitation regimes by aerosol forcing from Mt. Samalas. If true, then these results reveal somewhat surprising an anomalously wet monsoon season in the first year after the eruption and that seasonal sinusoids return to their pre-eruption pattern after only two years following the eruption. In order to better understand these results, we investigate this interval using the Last Millennium Ensemble, a state-of-the-art suite of climate model simulations conducted by the National Center for Atmospheric Research with the Community Earth System Model.