Transformation of Silicon in a Coastal Sandy Beach Ecosystem: Insights from Stable Silicon Isotopes in Ground- and Porewaters

Abstract:

Wave and tide dominated sandy beach ecosystems have a major effect on coastal carbon, nutrient, and trace element cycling. The coarse-grained sediments are characterized by a high permeability, which permits advective porewater transport. Due to the fast porewater flow in the sandy sediment, organic matter remineralization products (i.e. nutrients and trace metals) are only slightly enriched in the porewater and rapidly discharged to the coastal zone where they can potentially stimulate primary production.

Here we present stable silicon isotope (δ³⁰Si) data from groundwaters and porewaters from Spiekeroog Island, a barrier island in the southern North Sea, Germany. Groundwater was sampled in the freshwater lens down to 70 m depth in the center of the island. The beach porewaters were taken along a transect from the dune belt to the low water line at depths between 0.5 and 5.2 m. This transect represents the mixing zone of fresh and saline groundwater with seawater, whereby the terrestrial driven groundwater flow provides a potential source of terrestrially derived Si isotope compositions resulting in a modification of the marine-dominated porewater composition.

The groundwater is characterized by high Si concentrations of up to 750 µM and rather low δ³⁰Si signatures between +0.6‰ and +1.2‰, which decrease with depth (and age) within the freshwater lens. The porewaters have much lower Si concentrations around 50-60 µM and higher and more variable δ³⁰Si between +1.0‰ and +2.5‰. The lower δ³⁰Si values are found closer to the dunes and at greater depths in the beach sediments. These signatures possibly reflect the outflow of the fresh groundwater to the coastal zone. The higher values represent the influence of seawater, which delivers water with a high δ³⁰Si signature previously enriched by phytoplankton growth.