Abteilung Oberflächengewässer

Lakes emit substantial amounts of carbon dioxide (CO₂) into the atmosphere, but why they do remains debated. The long-standing vision of lakes as solely respirators of the organic matter leaking from the soils has been challenged by evidence that inorganic carbon produced by weathering of the catchment bedrock could also support lake CO₂ emissions. How inorganic carbon inputs ultimately generate lake CO₂ outgassing remains a blind spot. We develop and introduce a calcite module in a coupled one-dimensional physical-biogeochemical model that we use to simulate the carbon cycle of the large Lake Geneva over the past 40 years. We mechanistically demonstrate how the so-far neglected process of calcite precipitation boosts net CO₂ emissions at the annual scale. Far from being anecdotal, we show that calcite precipitation could explain CO₂ outgassing across various lakes globally, including some of the largest lakes in the world.

Methane (CH₄) accumulation in the well-oxygenated lake epilimnion enhances the diffusive atmospheric CH₄ emission. Both lateral transport and in situ oxic methane production (OMP) have been suggested as potential sources. While the latter has been recently supported by increasing evidence, quantifying the exact contribution of OMP to atmospheric emissions remains challenging. Based on a large high-resolution field data set collected during 2019-2020 in the deep stratified Lake Stechlin and on three-dimensional hydrodynamic modeling, we improved existing CH₄ budgets by resolving each component of the mass balance model at a seasonal scale and therefore better constrained the residual OMP. All terms in our model showed a large temporal variability at scales from intraday to seasonal, and the modeled OMP was most sensitive to the surface CH₄ flux estimates. Future efforts are needed to reduce the uncertainties in estimating OMP rates using the mass balance approach by increasing the frequency of atmospheric CH₄ flux measurements.

Over the last decades, the chromium (Cr) stable isotope system (referred to as δ⁵³Cr) has emerged as a proxy to reconstruct past oxygenation changes in Earth’s atmosphere and oceans. Although Cr is a promising paleoproxy, uncertainties remain as to the modern marine Cr cycle, and limited data are yet available in large swaths of the ocean, including the Atlantic Ocean. Here we present dissolved seawater Cr concentrations ([Cr]) and δ⁵³Cr along a meridional transect from the North to the South Atlantic (AMT 29). Chromium concentrations range from of 2.51 to 3.96 nmol kg⁻¹ (n = 68) and δ⁵³Cr values range from +0.86 ± 0.04 ‰ (2SEM) to +1.20 ± 0.02 ‰ (2SEM) (n = 68). In contrast to data from other ocean basins [Cr] and δ⁵³Cr show only a weak correlation (δ⁵³Cr vs. Ln([Cr]) R² = 0.17), inconsistent with a closed-system Rayleigh distillation model. These results can mainly be explained by horizontal advection and water mass mixing, which our data demonstrate are the dominant processes controlling [Cr] and δ⁵³Cr distributions throughout much of the Atlantic, while the impact of in situ biogeochemical cycling is comparatively minor. There is, indeed no clear impact of biological productivity nor of dysoxic environments in the (sub)tropical Atlantic on the cycling of Cr along the transect. This is likely explained by insufficiently depleted oxygen concentrations and relatively low biological productivity, resulting in these processes being of secondary importance relative to water mass mixing in shaping the distribution of Cr in the low- to mid-latitude Atlantic Ocean.