Transfer of Iron from Continental Shelves to Anoxic Basins: A Comparison of the Black Sea and Baltic Sea

Iron (Fe) is a key micronutrient for phytoplankton.
Continental shelf sediments act as a source of Fe to offshore
marine waters. Here, we compare the release of Fe from shelf
sediments and its transfer to adjacent anoxic basins (“Fe
shuttling”) in the Black Sea and Baltic Sea using water
column, porewater and sediment analyses of Fe combined
with in-situ benthic flux measurements. We demonstrate that
strong release of Fe from shelf sediments occurs in both
basins. In the Black Sea, we only observe sediment Fe release
from coastal sediments overlain by oxic waters that receive a
relatively high input of organic matter and that are
characterized by high macrofaunal activity. In the Baltic Sea,
in contrast, the highest sediment Fe release is observed at
sites where bottom waters are low in oxygen but do not
contain sulfide.
Results of sequential extractions and synchrotron-based
X-ray spectroscopy of the suspended matter on the Black Sea
shelf highlight that poorly crystalline Fe-oxides and clays
account for most of the Fe that is transported laterally over
the shelf within the nepheloid layer. The forms of Fe in the
suspended matter and surface sediment on the Black Sea shelf
are very similar, consistent with transport of particulate Fe
through deposition and resuspension of particles in an
oxygenated environment. In the Baltic Sea, lateral transport
of Fe takes place in both dissolved and particulate form, with
the former only being important at sites with low bottom
water concentrations of both oxygen and sulfide. Our results
highlight that benthic Fe release from continental shelves is a
complex process controlled by bottom water oxygen
concentrations, macrofaunal activity and the supply of
organic matter and Fe-oxides. We will discuss the
consequences of our findings for the lateral transport of Fe to
the anoxic basins in the Black Sea and Baltic Sea.