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The role of biological cycling on the chemistry of rivers remains poorly understood. in an attempt to close this knowledge gap, here we examine the difference between the elemental supply to catchments through rock degradation and the corresponding elemental riverine export, for two non-nutrient elements lithium (Li) and sodium (Na) and two nutrients-like elements potassium (K) and barium (Ba), in 20 of the largest world river catchments. Overall, the riverine export of K and Ba are lower than their estimated release by catchment scale rock degradation, while the two fluxes match for Li and Na. Barium isotope constraints lending support to this observation, we take this difference between these two element groups as a suggestion of the influence of biological uptake of rock-derived nutrients on river chemistry. Nevertheless, the magnitude of riverine K depletion cannot be reconciled with a pervasive growth of the biota on continents, nor with an "occult" export of organic material that would go unnoticed by common sampling protocols. One plausible explanation for this conundrum could lie in the complex partitioning of elements amongst soil, biota, and dead organic matter. As a consequence, our study emphasises the need for further work aiming at deciphering the cycle of rock-derived nutrients in the Critical Zone. ISSN:2410-339X ISSN:2410-3403

Poster presentation

Poster presentation

Barium and its isotopes are receiving growing interest in their capacity to record the variation of the oceanic biological pump through geological timescales. Nonetheless, only little is known about the continental sources and processes that can drive the Ba flux and isotope composition to the ocean. Whereas the role of processes such as secondary phase formation, sorption and biological uptake on Ba isotopes has been recently investigated, the potential of various rock sources in producing a range of Ba isotope compositions remains poorly known. In particular, black shales often exhibit enrichment in Ba that might potentially be associated with various Ba isotope signatures. In this study, we report the isotope composition of Ba in rivers with different relative contribution of black shale rocks in their catchments, located in the Mackenzie Basin (Northwest Canada). Both the dissolved and solid loads of the Mackenzie tributaries are enriched in heavy Ba isotopes with respect to the continental crust, which is consistent with an additional source to the weathering of igneous and siliciclastic rocks. Although the dissolved Ba abundance is partially driven by weathering processes, the river dissolved Ba isotope composition rather reflects a binary mixing between 1) a “classical” siliciclastic source (with a Ba isotope composition close to that of the upper continental crust δ138Ba ∼ 0.15 ± 0.05‰) and 2) an isotopically heavier source especially present in the Mackenzie Mountains (with a δ138Ba ∼ 0.40‰). The positive relationships between dissolved δ138Ba and radiogenic osmium isotope ratios, as well as with sulphuric acid production, indicate that the heavy Ba source is tightly linked to the weathering of black shale. Although the exact reason for such heavy Ba isotope signatures remains elusive, this study emphasises the potential of black shale weathering in shifting the Ba isotope composition of the dissolved flux to the ocean towards heavier values., Chemical Geology, 594, ISSN:0009-2541, ISSN:1872-6836