Hydrological control of river and seawater lithium isotopes

Over the past decades, the “Uplift-climate” hypothesis has triggered a long-standing and fruitful debate about the role of tectonic uplift in causing the global Cenozoic climate cooling. Multiple lines of new evidence from Li isotopes across a range of timescales do not support the uplift hypothesis.
Hydrological control of river and seawater lithium isotopes

In 2012, Misra and Froelich published a ground-breaking paper that provided a new record of seawater lithium isotope (a tracer of the weathering of silicate rocks) evolution over the Cenozoic. Silicate weathering is thought to be a major process that consumes atmospheric CO2, and thus that moderates global climate over geological timescales. They found a large (9‰) rise in δ7Li from the Paleocene to the present-day, and their favored explanation were that tectonic uplift in mountain belts shifted the global weathering regime from “supply-limited” (weathering rates are limited by the low supply of fresh minerals by physical erosion) towards more “kinetic-limited” conditions (weathering rates are limited by the by the kinetics of mineral dissolution), suggesting higher and more climate-sensitive weathering rates. However, controversy has persisted, notably in the apparent difficulty to build a coherent and global framework for interpreting the behavior of δ7Li values of river waters around the world.

Our new study proposes that climate, not tectonic uplift is the main driver of the Cenozoic δ7Li rise. The new compilation of river δ7Li data from samples collected across seasonal changes in river flow, show a clear link between δ7Li and hydrology. When adding these new data to a global compilation of rivers (across latitudes and basin sizes), we find a coherent story: when the climate is dry, river δ7Li values are high, and when the climate is wet, river δ7Li values are low. We interpret this result as reflecting the dominant control of water residence time on river δ7Li values. 

Fig.1 River and seawater δ7Li across various timescales.

We then re-examine geological records of shifts in δ7Li (from the last glacial to >106 years) and find that the published δ7Li paleo-records can be explained by a similar mechanism – shifts in the fluid residence time linked to changes in continental hydrology and the water cycle. Overall, Our paper show, for the first time, that a hydrological control mechanism can satisfactorily explain all δ7Li records across various climatic transitions during the last ~445 million years (Fig.1). This leads to a provocative conclusion: the Cenozoic seawater δ7Li record reflects overall drying of the continental climate over millions of years, rather than control by tectonic uplift.

The findings were published online in Nature Communications on 11 June, 2022.

This new study will stimulate new research about the role of changing hydrology in affecting global weathering flux and carbon cycle. As the reconstruction of hydrological regimes of the past remains a major challenge, the strong link between hydrology and δ7Li provides a new research direction for reconstructing past hydrology. 

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