NSFGEO-NERC: Sulfur Cycling at Subduction Zones

Subduction zones are a key valve mediating global S processing and the climatic effects of arc volcanism, the economic potential of arc magmas, and the oxidation state of solid Earth reservoirs. Yet, the inputs, processing and recycling of S throughout the subduction system are still inaccurately known. This international project targets major unknowns in the sulfur cycle at subduction zones. The US-NSF focus of this project (PI Plank, LDEO) 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, in collaboration with Prof. Aiuppa and Vizzini. Pilot data collected in Palermo demonstrate the quality of the coupled Elemental Analyzer-Mass Spectrometry technique and the clear 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 (PI Mather, Oxford) 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. Pilot data collected at NERC Ion Microprobe Facility at Edinburgh demonstrate the viability of the technique, and yield positive delta(34)S in melt inclusions from Fuego volcano. 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 and UK parts of this project, and allow for the first-time direct tracing of sulfur isotopes from sediment input to arc output.

1. Mentoring and advising a graduate student at LDEO. This project would form the core of a Ph.D. thesis. Plank has a track record of successfully mentoring graduate students, including Katie Kelley (now tenured at URI), Jennifer Wade (now a Program Officer at NSF), Mindy Zimmer (now permanent staff at Los Alamos National Lab), Alex Lloyd (now an Environmental Science teacher at the Hun School in Princeton, NJ), and Dan Rasmussen (soon to begin a Postdoctoral Fellowship at the Smithsonian, DC).

2. Undergraduate Research Experience at LDEO. This project will also support the undergraduate senior thesis of a Columbia or Barnard student, who will carry out a well-defined sub-project through the auspices of the Lamont REU Summer Intern program. Senior theses within Columbia's Department of Earth and Environmental Sciences involve a year-long course during which each student is advised in the ways of writing and presentation by an instructor in the course, and in conducting original research by a mentor in the department. Plank has successfully mentored four students in this program who have gone onto PhD programs, the Air Force, and High School science teaching.

3. Broader Scientific Impact. Sulfur is not only an essential chemical component to environmental, ecological and subduction cycles, but also in both monitoring volcanic eruptions and understanding of ore deposit formation. Our work, aimed at the very origins of sulfur in arc systems, may lead to novel to connections between sulfur supply to magmatic ore deposits and volcanic systems at convergent margins. For example, what governs whether a primary arc magma has 1000 or 6000 ppm S? This project is aimed the fundamental answer to this question, whether it be the subducted input or the sulfur-participating reactions in each subduction zone.

4. Special Sessions and Journal Issue on Deep Sulfur. Carbon has enjoyed ten years in the spotlight with the Deep Carbon Observatory, and the benefit of community focus has been significant. Plank, Aiuppa and Mather have all been leaders in DCO efforts, including being a member of the Executive Committee, a founding member of DECADE (Deep Earth Carbon Degassing) and a chair of an upcoming Deep Carbon Science Gordon Conference, respectively. Over the course of this project, the PI team will work together to bring sulfur onto the stage, by nurturing special session proposals on Deep Sulfur at upcoming international meetings. Mather and Aiuppa are also internationally prominent journal editors (for Earth and Planetary Science Letters and Journal of Volcanology and Geothermal Research, respectively), and they will use their perches to incubate a special journal issue dedicated to Deep Sulfur.