The paper by Ying Cui, Brian A. Schubert and A. Hope Jahren is the #3 most downloaded paper in Geology right now.
The ranking is here: https://pubs.geoscienceworld.org/geology
- and the paper is here (Open Access): https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G47681.1/586769/A-23-m-y-record-of-low-atmospheric-CO2