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 Ca(2+), CO(3)(2-), H(+) and O(2) 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 O(2) at shallow depths while only 28% was decomposed aerobically at the deepest location. Fluxes were highest at 8 m depth (24.4 mM m(-2) d(-1)) and lowest at the deepest site of 85 m (7.1 mM m(-2) d(-1)). Dissolution of calcite from the sediment was also depth dependent. A total of 1.1 mM m(-2) d(-1) was found for the shallowest site, and values decreased with depth to 0.6 mM m(-2) d(-1) at the deepest site.