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Dissolved organic carbon (DOC) constitutes the largest pool of reduced carbon in the global ocean, with important contributions from both recently formed and aged, biologically refractory DOC (RDOC). The mechanisms regulating RDOC transformation and removal remain uncertain though hydrothermal vents have been identified as sources and sinks. This study examines RDOC sinks in the deep Pacific Ocean, highlighting the role of submarine hydrothermal systems. Geochemical survey data from GO‐SHIP and GEOTRACES projects, alongside specific investigations of Pacific hydrothermal systems, suggest that particulate iron introduced by hydrothermal systems plays a key role in scavenging DOC and delivering it to the seafloor, leaving a deficit in the RDOC of the deep ocean. Dilution of the oceanic water column by hydrothermal fluids exhibiting low DOC concentrations likely plays a secondary role. 

One of the most exciting results from the GEOTRACES program’s zonal and meridional sections has been the recognition that hydrothermally sourced Fe may persist long enough to be upwelled along shoaling isopycnals and act as an essential micronutrient, stimulating primary productivity at high latitudes. In Aug-Sep 2023 our team used a combination of predictive plume dispersion modelling, real-time current meter data from the Ocean Networks Canada observatory, and in situ sensing and sampling from the AUV Sentry to guide biogeochemical sampling of dispersing hydrothermal plumes above the Juan de Fuca Ridge. A key motivation for this study was to investigate what sets the export flux of dissolved Fe and Mn away from ridge-axis venting. We specifically targeted hydrothermal vents in the NE Pacific for this study, at the far end of the thermohaline circulation, to maximize predicted Fe oxidation times within the dispersing plume and, hence, optimize our ability to reveal distinct processes that may contribute to regulating Fe flux as a function of time and distance down-plume. We also targeted an overlooked gap in the length-scale over which hydrothermal processes may regulate export fluxes, between the ≤1km range typical of submersible-based investigations and the ~100km spacing for GEOTRACES Section stations. Over 3 weeks on station we were able to use the Sentry AUV equipped with an in situ oxidation-reduction potential (ORP) sensor, an optical backscatter sensor (OBS) and two methane sensors (METS, SAGE) to track predicted plume dispersion trajectories and guide a telescopically-expanding program of water column sampling for dissolved, soluble, colloidal and particulate species of Fe, Mn and other metals, at <0.1, 0.25, 0.50, 1, 2, 5 and 10km down-plume from the High Rise and Main Endeavour vent-sites. We will present results from Sentry sensor data revealing length scales over which hydrothermal plume signatures attenuated, together with complementary TEI data, all set within the context of our dispersing plume model. Our approach will ultimately allow us to assign both effective distances down-plume from source, for each sample collected, and model dispersion ages. This will provide insights into both the processes active within a dispersing hydrothermal plume and the rates at which those processes occur. 

While recent efforts to catalogue Earth’s microbial diversity have focused upon surface and marine habitats, 12–20 % of Earth’s biomass is suggested to exist in the terrestrial deep subsurface, compared to ~1.8 % in the deep subseafloor. Metagenomic studies of the terrestrial deep subsurface have yielded a trove of divergent and functionally important microbiomes from a range of localities. However, a wider perspective of microbial diversity and its relationship to environmental conditions within the terrestrial deep subsurface is still required. Our meta-analysis reveals that terrestrial deep subsurface microbiota are dominated byBetaproteobacteria, GammaproteobacteriaandFirmicutes, probably as a function of the diverse metabolic strategies of these taxa. Evidence was also found for a common small consortium of prevalentBetaproteobacteriaandGammaproteobacteriaoperational taxonomic units across the localities. This implies a core terrestrial deep subsurface community, irrespective of aquifer lithology, depth and other variables, that may play an important role in colonizing and sustaining microbial habitats in the deep terrestrial subsurface. Anin silicocontamination-aware approach to analysing this dataset underscores the importance of downstream methods for assuring that robust conclusions can be reached from deep subsurface-derived sequencing data. Understanding the global panorama of microbial diversity and ecological dynamics in the deep terrestrial subsurface provides a first step towards understanding the role of microbes in global subsurface element and nutrient cycling.