Abstract
The Baltic Sea is the largest brackish inland sea in the world with the salinity gradient
decrease from 27‰ in Danish Strain to < 5‰ in the Bothnia. Over the last 100 years, the
Baltic Sea has received a large amount of nutrients from surrounding watersheds (e.g
domestic waste, agriculture) that has increased the primary production, and eutrophication in
the water column. Thus, the seafloor of Baltic Sea has received large amounts of organic
matter that lead to the enhancement of gas bearing sediments (H2S, CH4, etc). In turn, the gas
release from sediments may impact ecosystems and climate. The biogeochemical and
geophysical controls representing the formation of shallow gas-bearing sediments were the
major goal of EU project - BONUS - Baltic Gas project. An important aspect was to study the
link between organic carbon delivery into the Baltic Sea floor and S, CH4 and C cycling as
function of salinity gradients and sediment accumulation rate.
Several sampling campaigns throughout the Baltic Sea took place in 2009 and 2010
from the Baltic Sea – North Sea transition zone to the Northern Baltic Sea (Aarhus Bay,
Merkenburg, Bornholm Basin, Gdansk Basin, Gotland Deep, Himmerfjärden estuary, Bothnia
Sea and Bothnia Bay) to test hypotheses covering the factors that impact biogeochemistry of
carbon and sulfur cycling in the Baltic Sea sediments. Overall, wet and solid geochemistry of
C, and S were used to delineate the distribution of C and S species in the sediments. The
radiotracer methods (14C, 35S) were also used to determine the turnover rate of C and S
cycling. The abundance of radionuclides of 210Pb, 137Cs were used to calculate the sediment
accumulation rate, whereas the natural stable isotopes of 34S, 32S were used as an indicator of
S cycling. Additionally, one dimensional modeling and calculations were also used to
estimate rates.
Several outcomes of my study could clarify the biogeochemical controls on C and S
cycling in the Baltic Sea. I examined: 1) The impact of sediment and organic matter fluxes on
methane and sulfur cycling in Himmerfjärden estuary sediments; 2) The role of sulfate
penetration depth on carbon preservation and sulfur burial in the sediments of in the Baltic
Sea; and 3) The impacts of reactive Fe reactivity on sulfurization of organic matter and
oxidative sulfur cycling in Gdansk Basin sediments.
Himmerfjärden (Swedish coast) represents a littoral of the central Baltic Sea. High
sediment accumulation rates (0.65-0.95 cm a-1) is resulted from high primary production and sediment delivery from the surrounding watershed. Likewise, the low concentration of sulfate
of the overlying water (4.3 - 4.8 mM) and rapid depletion in top 20 cm depth leads to an
increase of methane concentration (>2 mmol L-1) within in the top 20 cm sediment and a steep
gradient through the sulfate zone. Although the rate of bicarbonate methanogensis intergrated
over 1 m depth were low (0.96 -1.09), sulfate reduction rates in the upper 14 cm depth were
also low (1.46 -1.92 mmol m-2 a-1). Additionally, bioirrigation due to the invasive polychaete
Marenzelleria enhances flux of methane to the sediment-water interface (0.32 -0.78 mmol m-2
a-1). High sediment accumulation rates also limited organic matter exposure time in the sulfate
reduction and led to high rates of organic carbon and reduced sulfur preservation. These high
rates of sediment accumulation and organic carbon burial distinguish the littoral
Himmerfjärden sediments from the central Baltic Sea and typical continental margin
sediments. The Himmerfjärden littoral sediments, therefore, are distinguished from central
basin Baltic Sea sediments of the Baltic and typical continental shelf margin sediments.
The brackish conditions of the Baltic Sea (3-21 mM sulfate concentration at the
sediment –water interface) and high sediment accumulation rates (0.01 - 0.95 cm-1) tend to
limit sulfate penetration depletion (typically 0. 17 -1.47 mbsf). Due to high sediment
accumulation rates throughout the Baltic Sea (most > 0.1 g cm-1 a-1), anaerobic processes such
as microbially mediated sulfate reduction contribute significantly to Corg degradation in
organic rich sediments. We tested the idea that sulfate penetration depth impacts organic
carbon preservation (15-93%). A transport - reaction model was applied to fit dissolved
inorganic carbon and sulfate pore water profiles to estimate total organic carbon degradation
rates. Combined with organic carbon accumulation rates, we estimated the efficiency of
organic carbon preservation. We calculated high efficiencies of Corg preservation in the Baltic
Sea. Overall, we conclude that it is the exposure time of the Corg rich sediments to sulfate
(sulfate exposure time), that is a good predictor of Corg preservation.
In contrast, variations of local sulfate inputs from bottom seawater, groundwater
discharge and deeper ice lake sediments have established a unique environment in the Gdansk
sediments that potentially sustain both reductive and oxidative sulfur cycling. In addition to
sulfate profiles, the geochemistry of carbon and Fe was highly variable in Gdansk Basin
sediments. Reactive iron was shown to be the major possible factor to control sulfurization of
organic matter due to FeOOH reduction coupled sulfide oxidation and oxidative sulfur
cycling via enhancement of sulfide oxidation. These additional sulfide oxidation pathways
apparently play an important role in isotope fractionation of S cycling in the deeper sediments. In the Baltic Sea, low sulfate penetration, and high rate of sediment accumulation rates,
are the master variables that control the biogeochemistry of carbon and sulfur cycling; in
particular, they may significantly allow for high organic carbon preservation and eventual
methane release into the water column and in the Baltic Sea.
