Redox and magma recharge controls on excess sulfur build-up at Mount Samalas, 1257 CE.

The 1257 CE eruption of Mount Samalas (Indonesia) caused the largest stratospheric sulfur injection of the Common Era, as recorded by Greenland and Antarctic ice cores, and led to strong climate perturbations. About 80% of the eruption's sulfur output likely came from vapor stored in the magma reservoir. However, the factors that controlled prodigious excess sulfur buildup at Samalas remain enigmatic. To address this problem, we analyzed sulfur and iron speciation, along with sulfur isotopes, in plagioclase-hosted melt inclusions and matrix glasses from 1257 pumice samples, and we modeled magma reservoir processes. Our results indicate that intermediate redox conditions, with sulfur as both sulfate and sulfide, and progressive magma recharge maximized pre-eruptive sulfur accumulation at Samalas. Redox-controlled sulfur speciation in the magma and magma reservoir growth via incremental recharge may thus modulate the sulfur emissions and climate impacts of large explosive volcanic eruptions.