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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. 

Abstract We examined a Late Holocene sea-level stillstand using phreatic overgrowths on speleothems (POS) recovered from Medvjeđa Špilja [Bear Cave] (northern Adriatic Sea) from −1.28 ± 0.15 m below present mean sea level. Different mineralogical analyses were performed to characterize the POS and better understand the mechanisms of their formation. Results reveal that the fibrous overgrowth is formed of calcite and that both the supporting soda straw and the overgrowth have very similar trace element compositions. This suggests that the drip-water and groundwater pool from which the POS formed have similar chemical compositions. Four subsamples were dated by means of uranium-series. We found that ca. 2800 years ago, the relative sea level was stable for about 300 years at a depth of approximately −1.28 ± 0.15 m below the current mean sea level. This finding roughly corresponds with the end of a relatively stable sea-level period, between 3250 and 2800 cal yr BP, previously noted in the southern Adriatic. Our research confirms the presence of POS in the Adriatic region and establishes the Medvjeđa Špilja pool as a conducive environment for calcite POS formation, which encourages further investigations at this study site. 

Abstract We report exceptionally negative δ238U values for spring water (−2.5‰ to −0.8‰) and travertine calcite (−3.2‰ to −1.1‰) from an area where the Jemez lineament intersects the western margins of the Rio Grande rift, west-central New Mexico (southwestern United States). The highest anomalies come from the southern margins of the Valles Caldera and are related to upwelling CO2-charged spring water forming travertine mounds along joints and faults. The anomaly likely occurs due to CO2 lixiviation of uranium in a deep-seated reduced environment where 235U is preferentially leached along a long flow path through Precambrian granitic basement, resulting in spring water with exceptionally low δ238U values inherited by the calcite that precipitated near or at the surface at relatively low temperatures, i.e., ~40 °C (modern temperatures). The lowest δ238U values are preserved in settings where upwelling waters are least diluted by oxidized aquifer groundwaters. Given these low δ238U values in travertine are associated with and possibly indicators of upwelling CO2 related to tectonic and magmatic activity, studies such as ours may be used to identify this association far back in time. 

Stalagmites and climate models reveal ITCZ shifts drove concurrent changes in Australian tropical cyclone and monsoon rainfall. 

Modern global sea-level rise is anomalous relative to any natural variability over the past 4000 years. 

Abstract. Close coupling of Iberian hydroclimate and North Atlantic seasurface temperature (SST) during recent glacial periods has been identifiedthrough the analysis of marine sediment and pollen grains co-deposited on thePortuguese continental margin. While offering precisely correlatable records,these time series have lacked a directly dated, site-specific record ofcontinental Iberian climate spanning multiple glacial cycles as a point ofcomparison. Here we present a high-resolution, multi-proxy (growth dynamicsandδ¹³C, δ¹⁸O, and δ²³⁴Uvalues) composite stalagmite record of hydroclimate from two caves in westernPortugal across the majority of the last two glacial cycles (∼220ka).At orbital and millennial scales, stalagmite-based proxies for hydroclimateproxies covaried with SST, with elevated δ¹³C,δ¹⁸O, and δ²³⁴U values and/or growth hiatusesindicating reduced effective moisture coincident with periods of lowered SSTduring major ice-rafted debris events, in agreement with changes inpalynological reconstructions of continental climate. While in many cases thePortuguese stalagmite record can be scaled to SST, in some intervals themagnitudes of stalagmite isotopic shifts, and possibly hydroclimate, appearto have been somewhat decoupled from SST.