Influence of organic carbon decomposition on calcite dissolution in surficial sediments of a freshwater lake

A multi-sensor approach was used to determine high-resolution porewater gradients of Ca2+, CO32-, H+ and O2 in sediment cores along a depth transect from eutrophic Lake Sempach (Switzerland). We quantified the reproducibility of measurements and analyzed concentration profiles with a one-dimensional diffusion-reaction model to quantify benthic fluxes. Calculation of oxic respiration in the sediment showed that almost all settled organic carbon was degraded with O2 at shallow depths while only 28% was decomposed aerobically at the deepest location. Fluxes were highest at 8 m depth (24.4 mmol m^(-2) d^(-1) and lowest at the deepest site of 85 m (7.1 mmol m^(-2) d^(-1). Dissolution of calcite from the sediment was also depth dependent. A total of 1.1 mmol m^(-2) d^(-1) was found for the shallowest site, and values decreased with depth to 0.6 mmol m^(-2) d^(-1) at the deepest site.