Seafloor weathering and aging of SMS deposits
Rachel A Mills
National Oceanography Centre, Southampton, University of Southampton, SO14 3ZH,
UK.
Email: Rachel.Mills@soton.ac.uk, Tel: +44 2380592678
Seafloor massive sulfide deposits are thermodynamically unstable when exposed to
oxic seawater and the ultimate fate of SMS deposits depends on the processes and
rates of alteration. Widespread seawater recharge into active permeable sulfide
deposits generates lower temperature fluids which evolve within the sulfide mound as
mixing and reaction occurs. Drilling of the active TAG hydrothermal deposit has
provided insights into the consequences of subsurface seawater penetration, reaction
and mineralisation in a 5MT active seafloor deposit. Once active mineralisation
ceases, the ridge crest deposits are oxidised and eroded at the seafloor and are
gradually overlain with carbonate sediments. The significant buffering capacity of
seawater and the input of carbonate materials generates circumneutral conditions near
to the sulfide interface which is a fundamental difference from subaerial acid mine
drainage conditions. Oxic fluids penetrate into the upper regions of the sulfide deposit
and steep pH and redox gradients are generated. A wide variety of inorganic and
microbially mediated metal cycling and redox reactions occur at the sulfide-oxic
interface which can be assessed through sampling of slumped debris at the periphery
of sulfide deposits. We observe enrichments in a range of metals including Cu, Mn,
Ag, Au, U, As, Cd, Zn and Pb at the active redox interface and elevated prokaryote
numbers relative to oxic and anoxic sediments. We have demonstrated that
marinobacter sp enhance pyrite and chalcopyrite oxidation under circumneutral
conditions. The non-equilibrium mineral assemblages within the redox transition zone
provide insights into the wide range of environmental conditions observed at the
mineral scale which provide niches for a diverse microbial population. Complete
replacement of sulfide phases with Fe oxides and silicates occurs over time generating
ochreous sediment whose organic biomarkers and trace element content reflect the
weathering processes. We have a good understanding of weathering processes at
SMS deposits but much less knowledge of the rates of reaction and timescales
involved in SMS aging.