This is a brief letter to the respected journal Nature.
In fact, here's the entire text, but see our comment, following:

Ancient atmospheric C02 pressures inferred from natural goethites

Crayton J. Yapp & Harald Poths

Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131, USA

THE role of changing atmospheric CO2 concentrations in controlling global temperature can be investigated by examining variations in both CO2 and climate preserved in the Earth's geological record. A model of the Earth's carbon cycle over the past 570 Myr suggests that, compared to its present value, the partial pressure of CO2 (P co2) may have been an order of magnitude higher in the early Palaeozoic, and about 4–6 times higher in the middle Mesozoic1,2. Cerling3 used carbon isotope ratios in soil carbonate minerals to constrain atmospheric P Co2m portions of the Mesozoic and Cenozoic eras. Studies of the common mineral goethite (alpha-FeOOH) have shown that it contains small quantities of a carbonate component (Fe(CO3)OH), the concentration and carbon isotope content of which preserves a record of ambient P Co2at the time of formation4–7. Here we present data for goethites from an ironstone in the Upper Ordovician Neda Formation (Wisconsin, USA)8, which suggest that 440 Myr ago atmospheric P Co2 was ~ 16 times higher than today. However, this enhanced level of atmospheric CO2 does not seem to have been accompanied by unusually warm temperatures in the tropics, and in fact may have been contemporaneous with high-latitude continental glaciation on Gondwanaland9,10.


Though a brief statement, the author backs it up with significant documentation, as indicated by the extensive reference notes. Access the link if you wish to check those out.

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