title: Constraints on the Role of Marine Authigenic Clay Formation in Determining Seawater Lithium Isotope Composition
Creators: name: Läuchli, Charlotte
name: Gaviria‐Lugo, Nestor
name: Bernhardt, Anne
name: Wittmann, Hella
name: Sachse, Dirk
name: Mohtadi, Mahyar
name: Lückge, Andreas
name: Frings, Patrick J.
Publisher: name: Humboldt-Universität zu Berlin
Year: 2025
Subjects: subject: authigenic clays
subject: lithium isotopes
subject: deep marine sediments
subject: 550 Geowissenschaften

description: Silicate weathering reactions generate alkalinity that drives carbonate burial in the ocean, ultimately considered to balance solid‐earth CO2 degassing. Because silicate weathering occurs at a climate dependent rate, it provides a negative feedback on long‐term climate evolution that regulates Earth's temperature within a habitable window. Quantifying the effect of silicate weathering on CO2 sequestration is challenging, because an unknown fraction of the alkalinity is consumed by the formation of marine authigenic clays. This process – termed reverse weathering – counteracts the degassing‐weathering feedback loop and retains carbon in the ocean‐atmosphere system. Quantifying reverse weathering is challenging due to the difficulties in detecting and quantifying marine authigenic aluminosilicate clays (MAAC) in marine sediments and distinguishing them from byproducts of alkalinity generating marine (forward) silicate weathering reactions. Here, a comparison of the lithium contents and isotope3 ratios of the clay‐sized fraction of marine and fluvial sediments along Chile suggests that higher lithium contents and 7Li/6Li ratios of marine detrital sediments compared to their fluvial counterparts are diagnostic of the presence of MAACs. The increases in Li content and isotope ratios were used to derive a lithium isotope fractionation of ca. −30‰ associated with the formation of MAACs in sediments derived from continents. Using this new constraint on authigenic clay formation, we reevaluate the global lithium budget and derive a flux of lithium associated with MAAC formation in marine detrital sediments.
description: Plain Language Summary: The role of the silicate weathering feedback as a thermostat for Earth's climate by removing CO2 from the atmosphere is widely accepted. Nevertheless, there is some uncertainty about the form of the relationship between climate and CO2 sequestration via weathering – and hence the drivers of long‐term climate evolution – since other processes within the long‐term carbon cycle remain stubbornly difficult to quantify. One such process is reverse weathering, where clays form on the ocean floor and release CO2 otherwise sequestered by weathering. This process remains poorly constrained due to the difficulties in detecting and quantifying spatially heterogenous marine authigenic aluminosilicate clays as well as distinguishing their reverse weathering or marine (forward) weathering origins. Here, we demonstrate the potential of the lithium system to detect and quantify marine authigenic clay formation in marine sediments derived from the continents. Using the lithium content and the isotope ratios of marine sediments compared to fluvial sediments along Chile, we derive a new constraint on the lithium isotope fractionation associated with marine authigenic clay formation in detrital sediments. With this new fractionation, we reevaluate the ocean lithium isotope budget and quantify at a global scale the flux of lithium associated with authigenic clay formation.
description: Key Points: Detection of marine authigenic clay formation using the lithium composition of clay‐sized marine sediments derived from continents. Field‐based estimate of lithium isotope fractionation associated with marine authigenic clay formation in dominantly detrital sediments. Quantification of the Li sink associated with marine authigenic clay formation using an inversion of the ocean Li isotope budget.