Utilization of synchrotron radiation X-ray microscopy, micro-probe, and spectroscopy to characterize the carbon, sulfur, and iron speciation of particles from buoyant, deep-sea hydrothermal plumes in the Mid-Cayman Rise

Abstract

The purpose of this study was to investigate the geochemical inputs to rising hydrothermal vent plumes of the Mid-Cayman Rise. To assess processes that modulate hydrothermal fluxes to the deep ocean at the Mid-Cayman Rise, the speciation of Fe, S, and C was measured for particles and aggregates using: (1) microprobe S 1s and Fe 1s X-ray absorption near edge structure (XANES) spectroscopy, (2) microprobe X-ray fluorescence (XRF) chemical mapping, and (3) scanning transmission X-ray microscopy (STXM) based C 1s and Fe 2p XANES. The Mid-Cayman Rise is an ultraslow spreading center located in the Caribbean Sea. The Mid-Cayman Rise hosts two hydrothermal vents that produce geochemically diverse fluids: (1) Beebe, the deepest (5000 ± 50 meters) known high-temperature (398 oC) vent site with high iron and sulfur, and (2) Von Damm (2300 ± 50 meters) with fluids at 110 to 200 oC (Kinsey and German, 2013). My samples were collected with a newly developed instrument, called the SUPR (SUspended Particle Rosette) (Breier et al., 2014). The SUPR was developed for high-precision collection of deep-sea samples, and it also made it possible to collect samples for complementary research efforts. Carbon XANES of Von Damm fluids reveal the presence of biomolecules such as proteins, lipids, polysaccharides, and chitin in plume particles. Iron 2p imaging and XANES indicate that nanoparticulate Fe minerals are associated with particulate organic C (POC). Sulfur 1s XANES and chemical mapping data reveal the presence of sulfonate, sulfone, and ester sulfate, as well as elemental S, and indicate that microbial processes and chemical oxidation occur in the subsurface or in the near vent samples. In the Von Damm particles, a shift from strongly reduced to oxidized, including the appearance of intermediate S-bearing functionalities, suggests turbulent mixing of Von Damm fluids with seawater provide oxic and pH neutral conditions where chemical and biological interactions can occur. The Fe 1s XANES observations capture trends that suggest particles within the Von Damm and Beebe Vents could be sourced from chemical processes within the plume and physical entrainment processes from multiple sources.