Seasonal variability in silicate weathering signatures recorded by Li isotopes in cave drip-waters. (1st November 2021)

• Record Type: Journal Article
• Authors: Wilson, David J.; Pogge von Strandmann, Philip A.E.; White, Jo; Tarbuck, Gary; Marca, Alina D.; Atkinson, Tim C.; Hopley, Philip J.
• Is Part Of: Geochimica et cosmochimica acta. Volume 312(2021)
• Page Start: 194
• Page End: 216
• Publication Date: 2021-11-01
• Subjects: Chemical weathering -- Carbon cycle -- Climate -- Seasonality -- Geochemistry -- Lithium isotopes -- Porewater -- Caves; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2021.07.006
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Silicate weathering is a critical process in Earth's carbon cycle, but the fundamental controls on weathering are poorly understood and its response to future climate change is uncertain. In particular, the potential for changes in seasonality or extreme weather events to control silicate weathering rates or mechanisms has been little studied. Here, we use lithium (Li) isotope measurements in bimonthly sampled drip-waters from two caves in the Yorkshire Dales (U.K.) to assess the response of silicate weathering processes to changes in temperature and hydrology over seasonal timescales. While the caves are contained in limestone bedrock, the drip-water Li isotope signal predominantly reflects silicate weathering of the overlying soils that are dominated by glacial till. Drip-water Li isotope compositions record spatial and temporal variability ranging from δ 7 Li values of +1 to +17‰, with a mean of +11‰. These values are significantly higher than local lithogenic inputs (δ 7 Li = −1 ± 1‰), consistent with isotope fractionation during secondary mineral formation. Comparison to temperature, precipitation, drip rates, and drip-water chemistry enables the controls on the Li isotope weathering signatures to be explored, revealing possible roles for both temperature and fluid residence time in setting the balance between primary rock dissolution and secondary mineral formation. Specifically, our Li isotope data are consistent with a scenario in which cooler temperatures and/or longer fluid residence times lead to enhanced secondary mineral formation relative to silicate dissolution. Overall, our results indicate the potential for Li isotope variability over short temporal and spatial scales, which is important to consider when interpreting past changes in weathering processes or fluxes from paleo-records. In addition, the seasonal changes in Li isotopes suggest that weathering processes may be sensitive to seasonality or to extreme weather events, rather than responding only to the mean climate state. With warmer temperatures and more intense rainfall events expected in future, both increased primary rock dissolution and enhanced weathering efficiency (due to reduced secondary mineral formation) could potentially contribute to increased carbon dioxide drawdown by silicate weathering.