Silicon isotope composition of dissolved silica in surface waters of the Elbe Estuary and its tidal marshes

Abstract

Land ocean silica fluxes pass estuaries. Recent data suggest that the isotopic composition of dissolved silica (DSi) is not altered during this transition. This could have major implication for the oceanic isotopic silicon cycle. To improve our knowledge about isotopic Si cycling in estuaries we investigate the silicon isotopic composition (δ30Si) of DSi of the Elbe Estuary and for the first time of tidal marsh areas. DSi concentrations in the tidal marshes were generally higher during seepage phase than during bulk phase. Negligible tidal variation in δ30Si (1.71 ± 0.08–1.87 ± 0.13 ‰) occurred in the freshwater marsh. In the brackish marsh δ30Si was higher during the seepage phase than during the bulk phase, with highest noted values being >2.78 ± 0.11 ‰. In the salt marsh, seepage water had lower δ30Si than bulk water over a total range of 1.81 ± 0.03–2.62 ± 0.02 ‰. In the estuary in October, DSi concentrations in the freshwater zone were diminished through removal by diatoms. The δ30Si of DSi increased from 1.43 ± 0.11 to 2.33 ± 0.08 ‰. In December, DSi concentrations increased along the estuary through lateral input from tributaries and tidal marshes. δ30Si values in the freshwater and brackish zones were lower than in October. The most notable changes in δ30Si occurred in the tidal freshwater zone of the estuary. This underscores that this zone modulates the delivery of reactive silica from land to sea. If true for other systems estuarine transformation would significantly contribute to the long term control of the silicon isotopic composition of the ocean.