Release of Iron and Manganese from Louisiana Continental Shelf Sediments and Impact on the Water Column

Iron (Fe) and manganese (Mn) are key micronutrients for
phytoplankton. Continental shelves are a major source of
both Fe and Mn to the water column. The mechanisms
controlling benthic release and its lateral transport on
continental shelves (“shuttling”) depend on a range of factors
such as bottom water oxygen concentrations, rates of organic
matter deposition and macrofaunal activity. The role of each
of these factors in controlling the shelf dynamics of Mn and
Fe is still incompletely understood.
In this study, we assess the benthic release of Fe and Mn
at six stations along a shelf-to-basin transect on the Louisiana
shelf in the Gulf of Mexico using water column, porewater
and sediment data of Fe and Mn combined with in-situ
benthic flux measurements. The two shallowest sites of our
transect are located in the seasonally hypoxic area on the
shelf. These sites are characterized by high benthic release of
Mn and limited release of Fe. Further offshore, no benthic Fe
and Mn release was observed due to limited deposition of
organic matter. Water column concentrations of total Fe are
an order of magnitude higher than those of total Mn, while
dissolved Mn dominates over dissolved Fe. We discuss the
controls on benthic release of Fe and Mn at the coastal sites
including the potential for mobilization of both elements from
the sediment later in the year and the impact on the water
column. We also compare the benthic release and forms of Fe
and Mn in the water column on the Louisiana shelf with those
in the Baltic Sea and on the Romanian shelf in the Black Sea
[1].
[1] Lenstra, W. K., et al., The shelf-to-basin iron shuttle in
the Black Sea revisited, Chemical Geology,
doi:10.1016/j.chemgeo.2018.10.024, 2018.