Sediments of sub-Antarctic islands have been proposed to
be important contributors to natural iron fertilization in the
Southern Ocean [1, 2]. This potential contribution depends
on biogeochemical processes within the sediment that may
result in an iron benthic flux, most likely related to the degradation
of organic matter (OM). Yet, the OM degradation
pathways vary strongly among different sedimentary settings.
We elucidate the role of environmental factors on the
prevailing biogeochemical pathways and reaction rates at
three contrasting sites of South Georgia, using comprehensive
solid-phase and pore-water analyses, as well as transportreaction
modelling. Samples were obtained along a transect
from a glacial fjord towards the shelf during cruise ANTXXIX/
4 of RV POLARSTERN in 2013.
Oxygen penetration depth at all sites is <1 cm. Sediments
recovered within the fjord are dominated by dissimilatory
iron reduction (DIR) and show very high dissolved Fe2+ concentrations
of up to 760 μM, while sulfide was not detected.
In addition, Fe reduction below the sulfate/methane transition
was observed. High input of reactive iron phases, possibly
enhanced by bioturbation and bubble ebullition, appear to
favour DIR as the dominant metabolic process for OM degradation
in the basin like fjord.
Shelf sediments outside the fjord are sulfidic throughout,
with H2S formed primarily by anaerobic oxidation of methane.
The conversion of Fe oxides into Fe sulfides significantly
alters the initial sediment composition along the shelf,
and impact the availability of iron to the water column.
OM is of marine origin at all three sites (C:N~7), indicating
that Fe oxide availability and reactivity rather than the
carbon source determine whether iron or sulfate reduction
dominantes.
[1] Moore & Braucher (2008) Biogeosciences 5, 631-656.
[2] Borrione et al., (2014) Biogeosciences 11, 1981–2001