The evolution of lithium isotope signatures in fluids draining actively weathering hillslopes

Jon K. Golla*, Marie L. Kuessner, Michael J. Henehan, Julien Bouchez, Daniella M. Rempe, Jennifer L. Druhan

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

25 Citations (Scopus)

Abstract

The stable isotopes of lithium (Li) serve as a robust proxy of silicate weathering. The fate and transport of these isotopes in the dissolved load of major rivers have been characterized to infer changes in both contemporary weathering regimes and paleo-conditions. In this contribution, we deconvolve this integrated signal into the individual processes that fractionate Li at the inception of silicate weathering by directly measuring Li isotope ratios of waters (δ7Li) transiting through a rapidly eroding first-order hillslope. We use these data to develop a multicomponent reactive transport framework, which shows that net dissolution of weathered material generates light δ7Li signatures (as low as −9.2‰) in the shallow portion of the vadose zone. An increase in δ7Li deeper into the vadose zone (as much as +18‰) reflects an increasing contribution of secondary mineral formation. Below the water table, congruent weathering occurs and imparts elevated cation concentrations and bedrock δ7Li. Silicate weathering continues within the saturated zone as groundwater travels downslope (δ7Li = +13 to + 24‰) to the stream. The stream signatures (δ7Li = +28 to +29‰) reflect the terminus of this network of silicate weathering reactions and the relative magnitude of each contributing process (e.g., transitions in secondary mineral formation, dissolution of weathered material). We show that fluid progressing through the weathering profile of this first-order hillslope is distinguished by a sequence of characteristic Li isotope signatures, which can be reproduced in a forward, process-based model framework. This model development offers an improved quantitative basis for the use of metal(loid) stable isotopes in disentangling catchment-scale chemical weathering fluxes.

Original languageEnglish
Article number116988
JournalEarth and Planetary Science Letters
Volume567
Early online date18 May 2021
DOIs
Publication statusPublished - 1 Aug 2021

Bibliographical note

Funding Information:
This research was partially funded by the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007-2013/ under REA agreement [ 608069 ] (ITN “IsoNose”), the IPGP multidisciplinary program PARI, and the Region île-de-France SESAME Grant No. 12015908 . J.K.G. is supported by the National Science Foundation (NSF) Graduate Research Fellowship Program. M.L.K. was supported by a joint OZCAR (French CZO Network) - CZEN (Critical Zone Exploration Network) fellowship. J.K.G. and J.L.D. acknowledge funding support from NSF - EAR-2047318 . The Eel River Critical Zone Observatory is supported by NSF - EAR-1331940 . M.J.H. and M.L.K. thank Jutta Schlegel and Josefine Buhk for assistance in the laboratory. We thank Gunnar Reith and Will Speiser for help with sample collection. We are grateful to Friedhelm von Blanckenburg for providing helpful feedback on an earlier version of the manuscript. We thank editor Louis Derry, Philip Pogge von Strandmann, and an anonymous reviewer for their constructive comments.

Publisher Copyright:
© 2021 The Author(s)

Keywords

  • Critical Zone
  • lithium isotopes
  • reactive transport
  • silicate weathering

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