Constraints on surface temperature 3.4 billion years ago based on triple oxygen isotopes of cherts from the Barberton Greenstone Belt, South Africa, and the problem of sample selection

Reconstructing Past Elevations From Triple Oxygen Isotopes of Lacustrine Chert: Application to the Eocene Nevadaplano, Elko Basin, Nevada, United States

“…Specifically, we measured the negative voltage to the left of the mass 33 peak and calibrated this baseline correction (additional or missing voltage) against the mass 32 O 2 intensity every session (2 weeks to 1 month). All of our reported sample and standard analyses are relative to the mean published highprecision L1/UNM_Q (δ 18 O = 18.070; 17 O = −0.076) values reported in Wostbrock et al (2020) on the VSMOW2-SLAP2 scale adjusted for a new calibration of SCO, UWG-2, and NBS 28 (Sharp and Wostbrock, 2020), which are also measured regularly in our laboratory (Chamberlain et al, 2020;Lowe et al, 2020). Both δ 18 O and 17 O values are measured here relative to L1/UNM_Q.…”

“…We performed triple oxygen isotope measurements using a Thermo Scientific TM 253 Plus 10kV Isotope Ratio Mass Spectrometer (IRMS) on chert samples at Stanford University with O 2 as the analyte. We generated O 2 gas from silicates using the laser fluorination method after Sharp (1990), on the setup described in Chamberlain et al (2020) (see also Sharp et al, 2016;Wostbrock et al, 2018Wostbrock et al, , 2020Lowe et al, 2020;Kukla et al, 2021). This setup is similar to recent studies from other labs producing triple oxygen isotope measurements of chert and silica (Pack and Herwartz, 2014;Levin et al, 2014;Liljestrand et al, 2020;Sengupta et al, 2020;Zakharov et al, 2021).…”

A CO 2 greenhouse efficiently warmed the early Earth and decreased seawater 18 O/ 16 O before the onset of plate tectonics

“…2). This is presently acknowledged as a strong argument against low δ 18 O sw as an explanation for the low δ 18 O of Precambrian chemical sediments (24)(25)(26)(27)31). Most of the Δ' 17 O chert data do not fall on the black equilibrium curve in Fig.…”

“…Alteration lowers both δ 18 O and Δ' 17 O, hence most triple oxygen isotope data seem to suggest postdepositional alteration as the most viable hypothesis for the long-term shift in δ 18 O (24)(25)(26)(27). Due to samplesize requirements, the least altered cherts measured for Δ' 17 O chert comprise somewhat lower δ 18 O compared to the most pristine in situ δ 18 O analyses (27,31). Nevertheless, triple oxygen isotope compositions of well-preserved cherts from the Barberton Greenstone Belt, South Africa fall close to the equilibrium line consistent with hot Archean seawater (31).…”

“…Due to samplesize requirements, the least altered cherts measured for Δ' 17 O chert comprise somewhat lower δ 18 O compared to the most pristine in situ δ 18 O analyses (27,31). Nevertheless, triple oxygen isotope compositions of well-preserved cherts from the Barberton Greenstone Belt, South Africa fall close to the equilibrium line consistent with hot Archean seawater (31). By combining data from the two techniques, Lowe et al (31) conclude that ".…”