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.
Supplementary material to "Successional patterns of (trace) metals and microorganisms in the Rainbow hydrothermal vent plume at the Mid-Atlantic Ridge"