ULTRA-HIGH RESOLUTION STABLE ISOTOPE ANALYSIS OF A NEPAL STALAGMITE REVEALS HYDROCLIMATE ANOMALIES ASSOCIATED WITH THE 1257 CE MT. SAMALAS ERUPTION

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.