Carbon Isotopes as Tracers of Organic and Inorganic Carbon in Baltic Sea Sediments

Distributions of Δ14CTOC studied in bottom sediments collected during 2011–2016 in the Curonian Lagoon and in the open Baltic Sea indicated wide variations of Δ14CTOC values. Laboratory experiments on differential carbon utilization by Pseudomonas putida isolated from bottom sediments were carried out for better understanding of impacts of different sources on Δ14CTOC variations. Preferential glucose uptake (up to 80%) as a carbon source and a rather low (2–10%) inorganic carbon incorporation was found in media with diesel fuel. Pseudomonas putida a specific biomarker analyzed in biomass cultivated on the media with different carbon sources has been used to characterize microbial communities responsible for degradation of organic substances in bottom sediments. Large 14C depletions observed in sediments collected in the Gotland Deep of the Baltic Sea may indicate leakage from dumped chemical weapons.

Queer times and Chemical Weapons, Suspended in the Gotland Deep

The video is also available at https://vimeo.com/220422987.

Fathoming chemical weapons in the Gotland Deep

At the end of World War II, tens of thousands of tons of chemical warfare agents – mostly mustard gas – were dumped in the Gotland Deep – a deep basin in the middle of the otherwise shallow Baltic Sea. Decades later, these weapons are being reactivated – both literally (perhaps on the faces of dead seals, and in fishermen’s nets) and also in our imaginations. In this story that recounts the beginning of our research into this situation, militarization meets with environmental concern: the past floats into the present, where humans and non-humans are equally implicated, where the sea itself conditions the kinds of questions we can ask, and answers we might get, and where terms like ‘threat’ and ‘risk’ remain undecided. After spending time on Gotland Island – the closest terrestrial site to these weapons dumps – we ask what kinds of research methods might be adequate to these tangled, underwater tales that we find so difficult to fathom.

Technical note: GODESS – a profiling mooring in the Gotland Basin

This note describes a profiling mooring with an interdisciplinary suite of sensors taking profiles between 180 and 30 m depth. It consists of an underwater winch, moored below 180 m depth, and a profiling instrumentation platform. In its described setup it can take about 200 profiles at pre-programmed times or intervals with one set of batteries. This allows for studies over an extended period of time (e.g. two daily profiles over a time of 3 months). The Gotland Deep Environmental Sampling Station (GODESS) in the Eastern Gotland Basin of the Baltic Sea is aimed at investigations of redoxcline dynamics. The described system can be readily adapted to other research foci by changing the profiling instrumentation platform and its payload.

Technical note: GODESS – A profiling mooring in the Gotland Basin

This note describes a profiling mooring with an interdisciplinary suite of sensors taking profiles between 180 m and 30 m depth. It consists of an underwater winch, moored below 180 m depth and a profiling instrumentation platform. In its described setup it can take about 200 profiles at pre-programmed times or intervals with one set of batteries. This allows studies over an extended period of time (e.g. two daily profiles over a time of three months). The Gotland Deep Environmental Sampling Station (GODESS) in the Eastern Gotland Basin of the Baltic Sea is aimed at investigations of redoxcline dynamics. The described system can be readily adapted to other research foci by changing the profiling instrumentation platform and its payload.

Are recent changes in sediment manganese sequestration in the euxinic basins of the Baltic Sea linked to the expansion of hypoxia?

Expanding hypoxia in the Baltic Sea over the past century has led to the development of anoxic and sulfidic (euxinic) deep basins that are only periodically ventilated by inflows of oxygenated waters from the North Sea. In this study, we investigate the potential consequences of the expanding hypoxia for manganese (Mn) burial in the Baltic Sea using a combination of pore water and sediment analyses of dated sediment cores from eight locations. Diffusive fluxes of dissolved Mn from sediments to overlying waters at oxic, hypoxic and euxinic sites are consistent with an active release of Mn from these areas. Although the present-day fluxes are significant (ranging up to ca. 240 μmol m−2 d−1), comparison to published water column data suggests that the current benthic release of Mn is small when compared to the large pool of Mn already present in the hypoxic and anoxic water column. Our results highlight two modes of Mn carbonate formation in sediments of the deep basins. In the Gotland Deep area, Mn carbonates likely form from Mn oxides that are precipitated from the water column directly following North Sea inflows. In the Landsort Deep, in contrast, Mn carbonate and Mn sulfide layers appear to form independently of inflow events, and are possibly related to the much larger and continuous input of Mn oxides linked to sediment focusing. Whereas Mn-enriched sediments continue to accumulate in the Landsort Deep, this does not hold for the Gotland Deep area. Here, a recent increase in euxinia, as evident from measured bottom water sulfide concentrations and elevated sediment molybdenum (Mo), coincides with a decline in sediment Mn content. Sediment analyses also reveal that recent inflows of oxygenated water (since ca. 1995) are no longer consistently recorded as Mn carbonate layers. Our data suggest that eutrophication has not only led to a recent rise in sulfate reduction rates but also to a decline in reactive Fe input to these basins. We hypothesize that these factors have jointly led to higher sulfide availability near the sediment–water interface after inflow events. As a consequence, the Mn oxides may be reductively dissolved more rapidly than in the past and Mn carbonates may no longer form. Using a simple diagenetic model for Mn dynamics in the surface sediment, we demonstrate that an enhancement of the rate of reduction of Mn oxides is consistent with such a scenario. Our results have important implications for the use of Mn carbonate enrichments as a redox proxy in marine systems.