Introduction. Chemical weathering is the transformation of rocks into soils, a process that not only consumes atmospheric CO₂ but also releases nutrients from rock minerals and makes them available for life, creating the critical zone, the habitable part of the planet. Forest ecosystem services are limited by water and nutrient stresses. While the water, carbon, and nitrogen cycles have been and continue to be the subject of numerous studies, not enough attention has been paid to essential mineral nutrients including Ca, Mg, K, and P, whose ultimate origin can only be rock weathering. Chemical weathering is controlled by a number of factors including climate, landscape position, parent material, precipitation rates of secondary minerals, ecosystem productivity, and residence time of water in the critical zone. In this study, we investigate, under the favorable conditions of a tropical rain forest, the relationships among critical zone architecture, landscape position, hydrological flow path and river water chemistry.

Study site. The Quiock stream site is a small monitored catchment (8 ha) in the island of Basse Terre, French West Indies, part of the ObsErA Observatory, OZCAR critical zone observatory network (https://www.ozcar-ri.org). The whole catchment is underlain by 1 Myr old volcanic rocks and the rainfalls exceed 3500 mm/yr. Located in the National Parc of Guadeloupe, the catchment is covered by a primary rain forest. The Quiock stream is characterized by a 50 m-wide knickzone located 200 m upstream to the catchment outlet, indicating a transient river profile where the upper reaches are non-equilibrated. Geophysical investigations have revealed that the weathered mantle or saprolite is deep, reaching 40 m (Pasquet et al. 2022).

Analytical tools. We measured different elemental and isotopic tracers along the Quiock river from the spring to the catchment outlet and in various compartments of the system (soil, rain, vegetation, rocks). We were particularly interested in Strontium (Sr) and Uranium (U) isotopes as tracers of bedrock weathering vs. atmospheric inputs for mineral nutrients to ecosystems.

Results. The study shows that the chemistry of the river changes along the 700 m length from spring to mouth, indicating the contribution of waters with different origins. Sr isotopes vary from ⁸⁷Sr/⁸⁶Sr = 0.709 in the headwaters to 0.7055 at mouth. U isotopes increase from (²³⁴U/²³⁸U) = 1.15 in the headwaters to 1.30 at mouth. Upstream of the knickzone, most of the nutrients measured in the river are originating from marine aerosols in rainwater. Below the knickzone, nutrients are enriched and display a clear bedrock origin, despite the thickness of the weathered zone.

Discussion. U and Sr isotopes in the river water define a mixing line between a seawater-like endmember and a volcanic rock endmember. This mixing line allows us to calculate at each sampling location how much of the Sr, U and the other major nutrients are released by rock weathering and how much are added to the ecosystem by the dissolution of atmospheric marine aerosols. These results were compared with water flow lines simulated by a steady-state groundwater numerical model developed for the Quiock catchment. Modflow was used to solve the groundwater flow equations and Modpath to determine the flow lines (Abhervé et al. 2023). The hydrological model clearly shows a strong vertical partitioning of water and nutrients between the weathered zone and the bedrock, controlled by a hydraulic conductivity that is 150 times higher in the saprolite. While only 4% of the water, characterized by long residence time, circulates through the unweathered bedrock,this small fraction interacts with fresh minerals and releases nutrients. The model shows that the spatial distribution of water and nutrient fluxes is controlled by the surface topography, in particular the knickzone. Indeed, the ability of groundwater flowpaths to cross the weathered zone-bedrock interface and to discharge into the river is mainly controlled by the change in the hydraulic gradient associated with the knickzone.

This study therefore shows that even in deep mantle zones, the shape of the landscape controls groundwater flow paths and creates hot spots of weathering and nutrient release that can benefit and sustain ecosystem productivity.

Jérôme Gaillardet