Impact of natural re-oxygenation on the sediment dynamics of manganese, iron and phosphorus in a euxinic Baltic Sea basin

Abstract

The Baltic Sea is characterized by the largest area of hypoxic (oxygen (O 2 ) < 2 mg L -1 ) bottom 2 -depleted waters largely depends on episodic Major Baltic Inflows from the adjacent North Sea. In 2014 and 2015, two such inflows led to a strong rise in O 2 and decline in phosphate (HPO 4 2-) in waters below 125 m depth in the Eastern Gotland Basin. This provided the opportunity to assess the impact of such re-oxygenation events on the cycles of manganese (Mn), iron (Fe) and phosphorus (P) in the sediment for the first time. We demonstrate that the re-oxygenation induced the activity of sulphur (S)-oxidising bacteria, known as Beggiatoaceae in the surface sediment where a thin oxic and suboxic layer developed. At the two deepest sites, strong enrichments of total Mn and to a lesser extent Fe oxides and P were observed in this surface layer. A combination of sequential sediment extractions and synchrotron-based X-ray spectroscopy revealed evidence for the abundant presence of P-bearing rhodochrosite and Mn(II)phosphates. In contrast to what is typically assumed, the formation of Fe oxides in the surface sediment was limited. We attribute this lack of Fe oxide formation to the high flux of reductants, such as sulphide, from deeper sediments which allows Fe(II) in the form of FeS to be preserved and restricts the penetration of O 2 into the sediment. We estimate that enhanced P sequestration in surface sediments accounts for only ~5% of water column HPO 4 2removal in the Eastern Gotland Basin linked to the recent inflows. The remaining HPO 4 2was transported to adjacent areas in the Baltic Sea. Our results highlight that the benthic O 2 demand arising from the accumulation of organic-rich sediments over several decades, the legacy of hypoxia, has major implications for the biogeochemical response of euxinic basins to re-oxygenation. In particular, P sequestration in the sediment in association with Fe oxides is limited. This implies that artificial ventilation projects that aim at removing water column HPO 4 2and thereby improving water quality in the Baltic Sea will likely not have the desired effect.

Abundance and Biogeochemical Impact of Cable Bacteria in Baltic Sea Sediments

“…Sediment samples from core sectioning were freeze-dried and subsequently ground in an argon-purged glovebox. Solid-phase Fe was fractioned into (1) labile Fe(III) oxides and Fe(II) (FeS + FeCO 3 ), (2) crystalline Fe minerals, (3) magnetite, and (4) pyrite (FeS 2 ), using a combination of two extraction methods (Poulton and Canfield; 35 Claff et al 36 ) as described by Hermans et al 21 Solid-phase S was separated into (1) acid volatile sulfur (AVS; FeS) and (2) chromium reducible sulfur (CRS; FeS 2 ), using the method after Burton et al 37 as modified by Kraal et al 38 FeS and FeS 2 were quantified by iodometric titrations. 39 Total solid-phase Mn was determined by ICP-OES, after dissolution using a mixture of HF and HNO 3 .…”

Removal of phosphorus and nitrogen in sediments of the eutrophic Stockholm archipelago, Baltic Sea

“…High-resolution (50 µm) depth profiles of dissolved O 2 were obtained from one core per site, using microelectrodes (Unisense A/S, Denmark), as described in Hermans et al (2019b). The diffusive uptake of O 2 was determined by numerical modeling with PROFILE (Berg et al, 1998) using the high-resolution O 2 measurements.…”

Reply to Reviewer 3

“…Reply: In the revised text, we will specifically limit our discussion to artificial reoxygenation of the Stockholm Archipelago. We note, however, that a similar legacy effect due to the upward flux of hydrogen sulfide has been reported previously for the Gotland Deep following the most recent Major Baltic Inflow by Hermans et al (2019). Instead of visual observations and conclusions on the presence or absence of Fe oxides based on sediment imagery, Hermans et al (2019) quantified the sediment content of Fe ox-…”