Award: OCE-1756558

Hydrothermal systems and the seafloor mineral deposits they create are increasingly recognized as having both economic and ecosystem services value. Until recently, most scientific research was focused on actively venting hydrothermal systems. Seafloor sites of hydrothermal venting have a life span and eventually the hot water stops flowing. The seafloor mineral deposits left behind after venting ceases are referred to as hydrothermally inactive. The importance of inactive seafloor mineral deposits in a global biodiversity and biogeochemistry context is not known. For example, it is currently unclear whether these deposits represent a distinct deep-sea ecosystem. To investigate this question, hydrothermally active vents and inactive deposits were sampled over three oceanographic expeditions to the East Pacific Rise 9-10oN mid-ocean ridge spreading center. Subsamples for mineralogy, description of microbial biofilms, and macrofauna were collected during the collaborative field work to allow multiple lines of evidence to be used together in describing the chemistry and biology of inactive seafloor mineral deposits. Mineralogy and chemical and morphological characteristics of microbial biofilms were described using several complementary methods, including imaging by scanning electron microscopy (SEM), and synchrotron microprobe X-ray fluorescence (micro-XRF) mapping, X-ray diffraction (micro-XRD) spectra, and iron X-ray absorption near edge structure (micro-XANES) spectroscopy. Overall, our data are consistent with the idea that hydrothermally inactive seafloor mineral deposits support mineral-attached microbial biofilms that get substrates from the minerals. These biofilms appear to be locations of grazing by deep-sea animals such as limpets (Neolepetopsis densata). This study provides evidence for describing inactive seafloor mineral deposits as a distinct ecosystem; an ecosystem formed initially by hydrothermal venting but ecologically unique after hydrothermal venting ceases. These dark, cold deep-sea environments should be investigated further in advance of deep-sea resource extraction such as mining. Figure Caption Within-rock location schematic (top row, left to right) Representative mineral specimen (left) and schematic (right) showing location where sub-samples were taken for four zones: inner, middle, boundary, and rind. Within-rock location (middle row, left to right) Bar charts display the frequency of mineral types observed for seafloor sulfide deposits from locations with active, proximal, and inactive hydrothermal venting. Petrographic thin sections were made from cross sections of seafloor sulfide deposits in up to four zones: inner, middle, boundary, and rind. Synchrotron X-ray microprobe analysis, X-ray diffraction (XRD) and iron X-ray absorption near edge structure (Fe XANES) spectroscopy, were conducted for each zone. Note that a rind was only observed in inactive samples. Hash pattern indicates minerals identified by Fe XANES and solid colors with no pattern indicate minerals identified by XRD. Bar chart legend: Fe(III) minerals group is primarily Fe(III) (oxyhydr)oxides, the Fe(II,III) minerals are mixed valence (oxyhydr)oxides, the sulfide minerals are primarily Fe sulfides, the non-sulfide Fe(II) is only detected by Fe XANES in the rind and represents an adsorbed Fe(II) component, the silicate minerals group is composed of rock-forming silicate minerals, and the other group (gray color) is primarily composed of anhydrite, metal chlorides, and non-Fe sulfides. Data show the changing proportion of minerals from inner to rind zone in seafloor sulfide deposits. Rock (bottom row, left to right) Representative images of seafloor sulfide deposits from locations with active, proximal, and inactive hydrothermal venting. The orange color on the exterior of the inactive specimen corresponds to the rind zone, while the dark gray color corresponds to boundary, middle, and inner zones. Last Modified: 01/20/2024 Submitted by: BrandyMToner