Earth's first glaciation at 2.9 Ga revealed by triple oxygen isotopes

We here report the lowest (∼3 ‰ VSMOW) δ¹⁸O values for any weathering-related sedimentary rock in Earth's history, from shales and diamictites of the Mesoarchaean Pongola Supergroup of South Africa. This volcano-sedimentary succession was deposited in a shallow epeiric sea on continental crust of the Kaapvaal Craton and includes the record of the Earth's oldest surface glaciation. Oxygen isotope data of shales of the Mozaan Group indicate gradual climatic cooling of the surface environments that culminated in glacial conditions at ∼2.90 Ga. Mathematical inversion of measured Δ'¹⁷O and δ¹⁸O values results in δ¹⁸O values around -20 ‰ for weathering waters, suggesting cold climate conditions. These observations suggest continental weathering of the Kaapval Craton involving low δ¹⁸O meteoric waters, possibly in a near-polar position.