Post-doctoral Fellow Nicole Keller and associate scientist Alison Shaw, both of the Department of Geology and Geophysics at WHOI, pause during their hike to the Poas volcano in Costa Rica in 2008. Behind them, gases vent from fumaroles in the dome of the volcano. They went to the area to collect samples of gases, fluids, and rocks for their study of how sulfur cycles through volcanic systems. Sulfur is exchanged between the oceans and atmosphere, then removed from the oceans via sedimentation, microbial processes, and alteration of minerals in the oceanic crust. The oceanic sediments and crust are then delivered to the mantle by tectonic processes. Some of the sulfur is eventually released back to the Earth’s surface via volcanic activity. The goal of this project is to quantify how much of this sulfur is actually recycled.