NSFGEO-NERC: Sulfur Cycling at Subduction Zones

This is a project that is jointly funded by the National Science Foundation's Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Agreement. This Agreement allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award, each Agency funds the proportion of the budget and the investigators associated with its own country.

Sulfur is a critical element in Earth's environment. When volcanoes erupt, sulfur is ejected into the atmosphere with cooling effects. Sulfur is also the transport agent and host of economic metals such as lead and zinc. The cluster of factors that control how much sulfur is available to volcanoes and metal deposits is still a major question in science, one that this project seeks to answer. One possible source of sulfur includes the sediments on the seafloor, which are subducted into the Earth to the depths where magmas form. This project is testing this hypothesis by measuring for the first time the sulfur forms and fluxes that feed volcanic systems at different subduction zones. A team of scientists from the US (Columbia University), the UK (Oxford University) and Italy (University of Palermo) will partner with the Ocean Drilling Program at Texas A&M University and the NERC Ion Microprobe Facility at the University of Edinburgh to make novel measurements of sulfur and its isotopes in sediments and volcanic material. Most of the project budget will support a Columbia Ph.D. student, a Columbia undergraduate, and an Oxford postdoc. The PI Team of Mather, Aiuppa and Plank will incubate special sessions at international meetings and a special journal issue focused on Deep Sulfur. This project, aimed at the very origins of sulfur in magmatic systems, may lead to novel connections between the sulfur supply to ore deposits and volcanic emissions at convergent margins.

This international project targets major unknowns in the sulfur cycle at subduction zones. The US-NSF focus of this project will fill a key knowledge gap in terms of S inputs to the mantle at subduction zones. It will involve extensive analysis of sedimentary sections at the Tonga, Marianas, Aleutians, Alaska and Central America trenches, chosen to represent end-member oceanic environments for sulfur deposition and diagenesis and extreme isotopic variations. Ocean Drilling Programs cores will be analyzed by XRF core scanning, a strategic approach to quantify heterogeneously disseminated pyrite and barite, major hosts of sulfur in sediment. Core scanning results will guide discrete sampling for bulk sulfur and sulfur isotope analyses at the University of Palermo by coupled Elemental Analyzer-Mass Spectrometry with a focus on the sulfide- vs. sulfate-dominated regimes that may occur in a single sedimentary section. The outcome will be the first comprehensive estimates (with uncertainties) for the fluxes and isotopic compositions of S into end-member trenches and improved global estimates. The UK-NERC part of this project will take a novel approach to understanding volcanic arc S outputs. It will measure for the first time the sulfur isotopic composition in undegassed olivine-hosted arc melt inclusions. Planned work will include well-studied melt inclusions suites from the same subducting systems as the sediment targets above. This will ensure close collaboration between the US, UK and Italian parts of this project, and allow for the first-time direct tracing of sulfur isotopes from sediment input to arc output. This project will not only provide the first comprehensive sediment input sulfur fluxes and arc isotopic compositions for any subduction zone, but also test the competing hypotheses that the concentrations and isotopic compositions of arc volcanic sulfur reflect a) the subducted inputs or b) the fractionation of sulfur species and isotopes in the subduction zone.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.