Seawater lithium isotopes (δ7Li) record changes over Earth history, including a ~9‰ increase during the Cenozoic, which is interpreted as the reflection of either a change in continental silicate weathering rate or weathering feedback strength, associated with tectonic uplift. However, mechanisms controlling the dissolved δ7Li remain debated.
Researchers from the Institute of Earth Environment of the Chinese Academy of Sciences and their collaborators from the U.K, France and Australia have found a clear link between δ7Li and hydrology based on data from river samples collected across seasonal changes in river flow.
The findings were published in Nature Communications on June 10.
When adding these new data to a global compilation of rivers (across latitudes and basin sizes), they found a consistent story: when the climate is dry, river δ7Li values are high, and when the climate is wet, river δ7Li values are low. They interpreted this result in emerging theme regarding continental weathering: that water residence time controlled river δ7Li values.
Then they re-examined geological records of shifts in δ7Li (from the last glacial to >106 years) and found that the dataset can be explained by a similar mechanism – shifts in the fluid residence time linked to changes in continental hydrology and the water cycle.
The researchers showed, for the first time, that a hydrological control mechanism can explain all δ7Li records across various climatic transitions during the last ~445 million years, and led to a provocative conclusion: the Cenozoic seawater δ7Li record reflected overall drying of the continental climate over millions of years, rather than control by tectonic uplift.
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