A framework for interpreting Li isotopes in speleothems using records from the U.K., Siberia, and South Africa

Résumé fourni par la source

The lithium isotopic composition (𝛿7Li) of dripwater from which speleothems precipitate is hypothesised to be determined by local changes in weathering congruency in the epikarst: the ratio of primary mineral dissolution to secondary mineral formation. Therefore, speleothem 𝛿7Li records may provide an avenue to determine past changes in local terrestrial silicate weathering processes and/or intensity, a key feedback mechanism for removing atmospheric CO2 over millennial to million-year-timescales. However, these records are complex to interpret due to persistent uncertainty in the relative importance of hydrology and weathering congruency in controlling fluid 𝛿7Li values. By evaluating three overlapping speleothem 𝛿7Li records spanning 12.3 – 0.5 ka from Lancaster Hole, Yorkshire Dales, U.K., and comparing them with new and existing 𝛿13C, 𝛿18O, Mg/Ca, and Sr/Ca records from the same samples, we have developed a framework for interpreting 𝛿7Li in speleothems.Both hydrology and weathering congruency affect speleothem 𝛿7Li records from the Yorkshire Dales. From correlated and elevated 𝛿7Li, 𝛿13C, Mg/Ca, and Sr/Ca records we infer increased epikarst residence times, usually driven by decreased effective infiltration above the cave. This scenario is characterised by increased prior carbonate precipitation, decreased drip rates, and prolonged water-rock interaction times, supporting a hydrological control on Li isotope ratios. However, on millennial timescales our 𝛿7Li records do not replicate across speleothems, indicating that water residence times in the epikarst can be highly localised due to different flow path lengths. In addition, the hydrologically-controlled correlation between 𝛿7Li and 𝛿13C, Mg/Ca, and Sr/Ca records is not consistent for the entire records from the Yorkshire Dales. An excursion to low 𝛿7Li values coupled with elevated Mg/Ca, Sr/Ca, and 𝛿13C values is observed prior to 11 ka, immediately after the Younger Dryas. This might indicate that regional changes in weathering congruency, driven by decreased surface vegetation, increased supply of primary silicates, and high denudation rates following the Younger Dryas, can override the local hydrological control on speleothem 𝛿7Li values. We apply this framework to help interpret two new 𝛿7Li speleothem datasets: i) a series of Siberian speleothems spanning interglacials MIS 9 – 15, and ii) a flowstone record from Buffalo Cave, South Africa, spanning 1.5 – 1.7 Ma. We discuss these datasets and investigate how fluid residence times and weathering congruency fluctuate over interglacials in permafrost and savannah terrains.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.