Iron behavior in a northern estuary : Large pools of non-sulfidized Fe(II)associated with organic matter

The estuaries of the Northern Baltic Sea (Gulf of Bothnia) receive an abundance of diagenetically reactivecatchment-derived Fe, which is to a large degree complexed with organic matter or present as Fe (hydr-)oxides.However, our understanding of sedimentary Fe diagenesis in these estuaries is limited. To address this limitation,the present study examines Fe geochemistry in a 3.5-m-thick estuarine benthic mud layer and three samples ofsuspended particulate matter of a catchment on the eastern Gulf of Bothnia. The age–depth model of the mud,constructed on the basis of sedimentary features as well as 137Cs and aquatic plant 14C determinations, revealeda high average rate of sedimentation (5 cm · yr−1) for the upper mud unit (0–182.5 cm, corresponding to1973–2011), in response to intensive land-use (ditching) in the catchment since the 1960s and 1970s. The intensiveland-use has resulted in a strong increase in the Fe accumulation rates, but has not caused a recognizableimpact on the diagenetic processes of Fe including features such as degree of sulfidization and solid-phasepartitioning. Iron X-ray absorption spectroscopy (XAS) indicated that in the suspended particulate matter,large proportions (47–58%) of Fe occur as Fe(III)-organic complexes and 2-line ferrihydrite. In the mud, the formeris completely reduced, and reactive Fe (defined via extraction with 1MHCl) was high throughout (52–68%,median=61%) and strongly dominated by Fe(II). This reactive Fe(II) pool was sulfidized to only a limited extent(degree of reactive sulfidization = 11–26%, median = 17%). This phenomenon is attributed to the brackishwaterconditions (i.e. low in sulfate) and the abundant input of reactive Fe(III) from the catchment, leading toa surplus of dissolved Fe2+ over dissolved sulfide in the sediment. The low availability of dissolved sulfide, incombinationwith the high average sedimentation rate, limits the formation of intermediate reduced sulfur compoundsat the water–sediment interface, the