Abstract
Abstract. Hydrothermal vent fields found at mid-ocean ridges emit hydrothermal fluids that disperse as neutrally buoyant plumes. From these fluids seafloor massive sulfides (SMS) deposits are formed, which are being explored as possible new mining sites for (trace) metals and rare earth elements (REEs). It has been suggested that during mining activities large amounts of suspended matter will appear in the water column due to excavation processes and discharge of mining waste from the surface vessel. Understanding how hydrothermal plumes can be characterised by means of geochemistry and microbiology as they spread away from their source and how they affect their surrounding environment may help in characterising the behaviour of the dilute distal part of chemically enriched mining plumes. This study on the extensive Rainbow hydrothermal plume, observed up to 25 km downstream from the vent site, enabled us to investigate how microbial communities and (trace) metal composition change in a natural plume with distance. The (trace) metal and REE content of suspended particulate matter (SPM) was determined using sector field inductively coupled plasma mass spectrometry (SF-ICP-MS) with high resolution (HR), and the microbial communities of the neutrally buoyant plume, above-plume, below-plume, and near-bottom water and sediment were characterised by using 16S rRNA amplicon sequencing methods. Both vertically in the water column and horizontally along the neutrally buoyant plume, geochemical and biological changes were evident, as the neutrally buoyant plume stood out by its enrichments in (trace) metals and REEs, as, for example, Fe, Cu, V, Mn and REEs were enriched by factors of up to ∼80, ∼90, ∼52, ∼2.5 and ∼40, respectively, compared to above-plume water samples taken at 1000 m water depth. The concentrations of these elements changed as the plume aged, shown by the decrease in element ∕ Fe molar ratios of chalcophile elements (Cu, Co, Zn), indicative of rapid removal from the hydrothermal plume or removal from the solid phase. Conversely, increasing REE ∕ Fe molar ratios imply uptake of REEs from the ambient seawater onto Fe-oxyhydroxides. This was also reflected in the background pelagic system, as Epsilonproteobacteria started to dominate and univariate microbial biodiversity declined with distance away from the Rainbow hydrothermal vent field. The Rainbow hydrothermal plume provides a geochemically enriched natural environment, which is a heterogeneous, dynamic habitat that is conducive to ecological changes in a short time span. This study of a hydrothermal plume provides a baseline study to characterise the natural plume before the interference of deep-sea mining.
Highlights
Hydrothermal vent fields found at mid-ocean ridges and back-arc basins are known for discharging fluids rich in potential microbial energy sources, such as H2, H2S, CH4, NH4 and Fe (Jannasch and Mottl, 1985; McCollom, 2000)
As seafloor massive sulfide (SMS) mining will concentrate on deposits around hydrothermal vents and not on active vents or chimneys due to technical risks associated with high temperatures (Gwyther et al, 2008), it is likely that the background and extinct vent communities will be impacted through habitat loss, mechanical destruction, noise, smothering and bioaccumulation of toxic substances (Levin et al, 2016)
Whilst mechanical understanding of microbial and geochemical interactions in the plume would have required a different experimental setup, which was beyond the scope of the TREASURE project, this paper aims to contribute to knowledge of geochemical and biological heterogeneity in the surroundings of an SMS site, induced by the presence of an active hydrothermal plume, which should be taken into account in environmental impact assessments of SMS mining
Summary
Hydrothermal vent fields found at mid-ocean ridges and back-arc basins are known for discharging fluids rich in potential microbial energy sources, such as H2, H2S, CH4, NH4 and Fe (Jannasch and Mottl, 1985; McCollom, 2000). Because of the steadily increasing demand for these metals and their geopolitical distribution on land, hydrothermal vent deposits are explored as new mining sites (Hoagland, 2010). Since such areas accommodate unique and vulnerable marine life, serious concerns exist about the environmental sustainability of seafloor massive sulfide (SMS) deposit mining (Boschen et al, 2013; Collins et al, 2013), especially with regard to the effects of the different plumes, which are generated during the excavation of ores and by the return flow of wastes in the vicinity of hydrothermal vents (Ramirez-Llodra et al, 2011; Vare et al, 2018). Knowledge about the background ecosystem and natural plume is sparse, as the vents and their proximal fauna have attracted most of the attention in, for example, microbiology (e.g. Han et al, 2018; Cerqueira et al, 2018)
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