In times of accelerated global change, integrated tools are urgently needed that allow process-based, quantitative assessment of how climatic drivers interact with land cover, soil water conditions and hydrogeology to control the inter-relationships between water fluxes and storage in the Critical Zone (CZ). These dynamic relationships determine the availability and quality of water resources during droughts at multiple spatial and temporal scales. This talk will present approaches to investigate the relationships between water fluxes through and storage in the CZ, together with the associated water ages and residence times. The focus will be on groundwater recharge, as groundwater levels have been dramatically affected by the recent extreme droughts in large parts of Central Europe. Multiple ecohydrological processes and transformations within the CZ are influencing groundwater recharge by partitioning incoming precipitation into green (i.e. evaporation and transpiration) and blue (i.e. groundwater recharge and streamflow generation) water fluxes.

Water stable isotopes help to constrain ecohydrological process understanding in the CZ. Incorporating isotopes into tracer-aided ecohydrological models allows water flux, storage and age dynamics to be quantified in both plot und catchment scale modelling. Using such novel analysis of the spatio-temporal interactions of water flux-storage-ages in the CZ improves understanding of the sensitivity and resilience of catchment functionality to hydroclimatic perturbations, and provides science-based evidence on which land management techniques have the potential to modify green water fluxes and to conserve soil water storage, to guide decision-making and build resilience to future droughts.

Streams are integrators over all biogeochemical reactions taking place in the Critical Zone. The resulting export of dissolved elements from a watershed is commonly used to infer chemical weathering fluxes. Yet, this approach rests on the assumption that the mass of a given element released from primary minerals that was not incorporated into secondary solids is quantitatively transferred into the stream in the dissolved form. The comparison of element-specific solute stream fluxes with chemical weathering fluxes determined in the residual solids shows that this is often not the case. An imbalance persists even after correcting for a bias potentially introduced by changes in water flow over these entirely different timescales.

To explore causes for imbalances between short-term and long-term weathering fluxes, described by their ratio in form of a “Dissolved Export Efficiency” (DEE), we sampled six Critical Zone water compartments for one year in the Conventwald (Black Forest, Germany). We found deficits in the dissolved load, which emerged in the deep saprolite. For Si, Al, and Fe the deficit can be attributed to an export pathway that includes a “hidden” Critical Zone compartment or pool of unsampled colloids that are exported preferentially during flushing events. In contrast, deficits found for nutritive elements (Ca, K, Mg, P) can be explained by deep nutrient uptake followed by nutrient retainment in re-growing forest biomass or export in form of biogenic particulates. Given the collective evidence for these imbalances the deep Critical Zone warrants attention towards a complete budget of element cycles.

Silicate weathering sequesters CO₂ from the atmosphere and stabilizes Earth’s climate over geologic timescales. In turn, weathering of accessory carbonate and sulfide minerals is a geologically relevant CO₂ source. Rock-uplift and -erosion is the primary mechanism by which fresh minerals are exposed to weathering at Earth’s surface. Therefore, the global inorganic carbon cycle is sensitive to mountain uplift and erosion. However, quantifying this sensitivity is complex, because existing data do not consider weathering of all relevant mineral phases, and because co-variation of multiple environmental factors obscures the role of erosion. Here, we analyze the sensitivity of silicate, carbonate, and sulfide weathering fluxes to erosion in solute chemistry from four small mountain streams that span well-defined erosion-rate gradients in relatively uniform metasedimentary lithologies. Across all datasets and 2-3 orders of magnitude of erosion rate, we find that silicate weathering fluxes are almost insensitive to erosion at rates >10^-2 mm yr^-1. In contrast, weathering fluxes from sulfide and carbonate minerals increase sub-linearly with erosion. By fitting a weathering model to these data, we show that the contrasting sensitivities of silicate, carbonate, and sulfide weathering produce a distinct CO₂-drawdown maximum at moderate erosion rates of ~0.1 mm/y. Below this maximum, mineral supply limits silicate weathering. Above the maximum, silicate weathering fluxes plateau and CO₂ emissions from coupled sulfide oxidation and carbonate weathering increasingly dominate the carbon budget. Overall, uplift of metasedimentary lithologies to moderate relief can substantially bolster Earth’s CO₂ sink whereas further uplift may decrease, or even reverse, CO₂ sequestration rates.

Ferricretes are hard iron layers forming in semi-arid to subtropical environments. We can observe them in i.e. Africa, Australia or Brazil. They are an important part of regional geomorphology, capping hills and protecting landscapes. Climatic dependance is very high. Ferricrete formation occurs under strongly seasonally contrasting climates with precipitation and dry cycles. During wet seasons, transport and accumulation of elements happens while during dry seasons, precipitation and hardening dominate. It is also known that ferricretes form in tectonically “quiet” environments, and approximately need 1 Myr to form meter thick layers.

There are two ferricrete formation hypotheses, the hydrological hypothesis and the laterite hypothesis.

Recently, we developed a numerical model for ferricrete formation based on the laterisation hypothesis. 33% of land surfaces are covered by laterites today. The thickest lateritic profiles evolved for millions of years and are found in the centre of tectonically inactive areas. Weathering is the main process responsible for laterisation, transforming bedrock into regolith. A typical lateritic profile is divided into multiple stages from the weathering front to the surface, starting with a coarse grained and then fine grained saprolite, a mottled zone and at the top, a ferricrete.

In our model, we assume that as the regolith ages, it undergoes a process of transformation that leads to hardening and compaction. Material is constantly eroded away from the regolith, thus making the model dependant on a constant material input for example through uplift. This is necessary to reach sufficient laterisation levels for ferricrete formation.

Land-ocean interactions in the coastal zone are of particular interest regarding the exchange of substances, like nutrients, carbon, sulfur, metals, and water. The rising sea level is and will enhance the pressure of salty solutions on previously fresh water ecosystems. Currently coastal areas in the North Eastern part of Germany under increasingly rewetted by the connection with the brackish Baltic Sea. We present results on the isotope biogeochemistry of a modern rewetted wetland, at the southern Baltic Sea, the Huetelmoor, that is under impact by event-type flooding by brackish seawater. These events lead to an enhancement of sulfate availability for microbial carbon transformations. Sediment cores on transects within the wetland were investigated for the pore water and soil composition, together with selected ground water wells and surface waters from the channeling system. Different fractions of the soils were analyzed for the elemental composition, mineral micro-textures, and the stable sulfur (and oxygen) isotope composition of different sulfur fractions to understand the water and biogeochemical carbon-sulfur-metal cycles and the geochemical signatures in authigenic mineral phases and organic matter. Adding sulfate creates space for mineral authigenesis and organic matter sulfurization. The soils are impacted by different intensities in sulfur cycling as reflected by isotope and textural signals. Further mechanistic investigations consider the role of DOS upon changing sulfur substrate availability. Results allow for a transfer of proxy information to other modern and past coastal organic-rich peatlands.

Acknowledgement for support by DFG Research Training Group BALTIC TRANSCOAST, ERASMUS, and DAAD