https://thehill.com/policy/energy-environment/4945872-arkansas-19-million-tons-lithium-battery-power/amp/
Researchers said in a recent article that Arkansas may have 19 million tons of lithium, which is used in rechargeable batteries for important products like phones and electric cars.
The researchers said in their article released last month in the journal Science Advances they had “calculated that there are 5.1 to 19 million tons of lithium in Smackover Formation brines in southern Arkansas,” making up “35 to 136% of the current US lithium resource estimate.”
Actual paper can be found here: https://www.science.org/doi/pdf/10.1126/sciadv.adp8149
All of this assumes the technology works to extract lithium from brine. This may be why the researchers are looking at "by 2040."