Biogeochemical processes in Baltic Sea sediments as a recorder of environmental change over the Holocene

The Baltic Sea has been subject to significant environmental change over the Holocene, driven by climatic variability, glacio-isostatic rebound and anthropogenic activities. During the Integrated Ocean Drilling Program (IODP) Expedition 347, “Baltic Sea Paleoenvironment”, unique, long sediment cores were retrieved, enabling the reconstruction of environmental conditions and the temporal evolution of key biogeochemical processes in the Baltic Sea over the Holocene. Here, we review major findings from geochemical and microbial analyses of well-dated sedimentary records from three of the IODP sites, combining literature data with newly generated phosphorus speciation data for one of the IODP sites (the Little Belt) and reanalysed metagenomic datasets. Sedimentary redox proxies (Mo, C org /P tot and Fe/Al) are used to illustrate that two past intervals of deoxygenation in the Holocene were particularly pronounced in the central part of the Baltic Sea. By combining our proxy records with recent water column data, we show that present-day anthropogenically-driven deoxygenation developed faster, covers a larger area and is more intense than in the past. Sedimentary records of the nutrient phosphorus point towards the widespread occurrence of vivianite, an authigenic iron(II) phosphate mineral, which can form diagenetically in and below the methanogenic brackish/marine sediments. While vivianite likely acted as a major phosphorus sink in the Baltic Sea in the past, this is no longer the case because of eutrophication. Methanogenesis is a key pathway of organic matter degradation in deep Baltic Sea sediments. While upward diffusing methane is oxidized with sulfate as an electron acceptor in near-surface sediments, oxidation of methane that diffuses downwards into freshwater sediments may be coupled to iron-oxide reduction. Metagenomic analyses reveal the adaptation of the sediment microbial community to present-day salinity and methane concentrations, illustrating the need to understand changes that occur long after deposition. Our findings underscore the dynamic interplay between climate, geology, and anthropogenic influence in shaping Baltic Sea biogeochemistry, offering a glimpse into how other coastal marine systems may respond to projected global change in the future. • Spatial differences in bottom water redox during past deoxygenation in the Baltic Sea. • Modern deoxygenation: faster onset, more widespread and intense than in the past. • Vivianite formation acted as a key phosphorus sink in past Baltic Sea sediments. • At present, vivianite primarily forms diagenetically in deep freshwater sediments. • Microbes in deep sediments may couple methane oxidation to iron-oxide reduction.