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Transformation of silicon in a sandy beach ecosystem: insights from stable silicon isotopes from fresh and saline groundwaters

(Dr. Claudia Ehlert, Scientist)

Abstract  Dissolved silicon isotope compositions (δ30Si) have been analysed for the first time in groundwaters of beach sediments, which represent a subterranean estuary with fresh groundwater discharge from a freshwater reservoir and mixing with recirculated seawater. The fresh groundwater reservoir has high and variable dissolved silica concentrations between 136 and 736 μM, but homogeneous δ30Si of +1.0 ± 0.15‰. By contrast, the seawater is strongly depleted in dissolved silica with concentrations of 3 μM, and consequently characterised by high δ30Si of +3.0‰. The beach groundwaters are variably enriched in dissolved silica compared to seawater (23– 192 μM), and concentrations increase with depth at all sampling sites. The corresponding δ30Si values are highly variable (+0.3‰ to +2.2‰) and decrease with depth at each site. All groundwater δ30Si values are lower than seawater and most values are lower than dissolved δ30Si of freshwater discharge indicating a significant amount of lithogenic silica dissolution in beach sediments. In contrast to open North Sea sediments, diatom dissolution or formation of authigenic silica in beach sediments is very low (ca. 5 μmol Si g−1). Silica discharge from the beach to the coastal ocean is estimated as approximately 210 mol Si yr−1 per meter shoreline. Considering the extent of coastline this is, at least for the study area, a significant amount of the total Si budget and amounts to ca. 1% of river and 3.5% of backbarrier tidal flat area Si input.

Ehlert, C., Reckhardt, A., Greskowiak, J., Torres Liguori Pires, B., Böning, P., Paffrath, R., Brumsack, H.-J., Pahnke, K., 2016. Transformation of silicon in a sandy beach ecosystem: insights from stable silicon isotopes from fresh and saline groundwaters Chemical Geology 440, 207-218.

From Land to Sea: transport versus transformation of trace elements and nutrients in the Weser estuary

(Dr. Philipp Böning, Dr. Claudia Ehlert)

For many elements rivers are the main transport pathway from land into the ocean. However, on their way they have to pass through estuaries, a highly dynamic zone where fresh- and saltwater get in contact and get mixed. During this mixing many processes like dissolution, new formation or sedimentation of particles, or adsorption of dissolved elements onto particles occur. We study the Weser estuary in order to understand how these processes affect certain elements like silicon (Si) and thallium (Tl).

Silicon is a major element, which gets into solution during weathering of rocks on land and which is used in the ocean e.g. by diatoms as a nutrient. In estuaries we expect major biological and abiotic transformation processes. However, despite the importance of Si in global biogeochemical cycles it is still mostly unclear which effect different timescales (e.g., seasonality) or different zonal distribution within the estuary have.

Thallium, like cadmium, lead or mercury, it is a highly toxic trace element and pollutant. It can be released to the environment through anthropogenic processes like coal and ore combustion, cement manufacture and metal smelting. Despite its toxicity, very little is known so far about its distribution and behavior in the coastal marine environment, and we want to contribute to the basic understanding of Tl in estuaries.

Assessment of ground- and porewater-derived nutrient fluxes into the German North Sea – Is there a ‘Barrier Island Mass Effect’? (BIME)

We are involved in the collaborative project BIME at the ICBM with the subproject ‘SGD composition and constituent fluxes across the sediment-water interface’ with the measurement of Fe isotopes and REE on porewater and coastal seawater samples.

(Dr. Philipp Böning, Dr. Claudia Ehlert)

The Ecology of Molecules (EcoMol)

We are involved in the collaborative project EcoMol of the ICBM with the subproject ‘The fate of selected trace metals (Mo, Mn, Fe, REE) and their isotopes (Fe, Nd) in coastal waters 
of the Southern North Sea’

(Dr. Philipp Böning)

 

 

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