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 reactive catchment-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 of suspended 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, revealed a high average rate of sedimentation (5 cm · yr^{− 1}) for the upper mud unit (0–182.5 cm, corresponding to 1973–2011), in response to intensive land-use (ditching) in the catchment since the 1960s and 1970s. The intensive land-use has resulted in a strong increase in the Fe accumulation rates, but has not caused a recognizable impact on the diagenetic processes of Fe including features such as degree of sulfidization and solid-phase partitioning. 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 former is completely reduced, and reactive Fe (defined via extraction with 1 M HCl) 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 brackish-water conditions (i.e. low in sulfate) and the abundant input of reactive Fe(III) from the catchment, leading to a surplus of dissolved Fe^{2 +} over dissolved sulfide in the sediment. The low availability of dissolved sulfide, in combination with the high average sedimentation rate, limits the formation of intermediate reduced sulfur compounds at the water–sediment interface, thereby retarding the conversion of FeS into pyrite (ratios of pyrite-S to AVS = 0.17–1.73, median = 0.37; degree of pyritization = 1–17%, median = 3%). Iron XAS, in combination with wavelet transform analysis, of representative sediment segments from the upper and lower mud units suggests that the non-sulfidized Fe(II) pool is dominantly complexed by organic matter, with the remaining Fe(II) occurring as mackinawite. This has implications for the understanding of early Fe diagenesis in settings with a high input of organic matter and relatively low supply of sulfate.