2 added 1053 characters in body edited Feb 5 '18 at 10:09 plannapus 4,5681717 silver badges5353 bronze badges To answer the $$p_{CO_2}$$ aspect of the question, here is a graph modified from Kent & Muttoni, 2013 (2013): It is based on compilations from Royer (2010) and Beerling & Royer (2011), using a wide variety of proxies, but mostly fossil leaves stomata for the Cretaceous. It might be a bit outdated now but most articles out there do give values on the same range: ca. 400~500 ppm at the Cretaceous-Paleogene boundary and values fluctuating between 500 and 1500 ppm before that, with a maximum during the mid-Cretaceous. As for $$p_{O_2}$$, I couldn't find a recent compilation, but according to Poulsen et al. (2015), estimates for the Cenomanian (i. e. the high $$p_{CO_2}$$ period from the last graph) vary widely from 10% to (indeed as the other answer suggests) 32%. Here is the quote: We focused on simulations of the Cenomanian, a mid-Cretaceous stage characterized by high $$p_{CO_2}$$ [e.g., (12)] and the warmest conditions of the past ~100 million years (13). Paleo-$$p_{O_2}$$ estimates inferred from stable-isotope carbon compositions indicate that the atmospheric percentage of $$O_2$$ was as low as 10 to 11% during this interval (7, 9), with one biogeochemical model indicating levels as high as 32.5% (6). References 7 and 9 are to Falkowski et al. (2005) and Tappert et al. (2013), while reference 6 is to Bergman et al. (2004). To answer the $$p_{CO_2}$$ aspect of the question, here is a graph modified from Kent & Muttoni, 2013: It is based on compilations from Royer (2010) and Beerling & Royer (2011), using a wide variety of proxies, but mostly fossil leaves stomata for the Cretaceous. It might be a bit outdated now but most articles out there do give values on the same range: ca. 400~500 ppm at the Cretaceous-Paleogene boundary and values fluctuating between 500 and 1500 ppm before that, with a maximum during the mid-Cretaceous. To answer the $$p_{CO_2}$$ aspect of the question, here is a graph modified from Kent & Muttoni (2013): It is based on compilations from Royer (2010) and Beerling & Royer (2011), using a wide variety of proxies, but mostly fossil leaves stomata for the Cretaceous. It might be a bit outdated now but most articles out there do give values on the same range: ca. 400~500 ppm at the Cretaceous-Paleogene boundary and values fluctuating between 500 and 1500 ppm before that, with a maximum during the mid-Cretaceous. As for $$p_{O_2}$$, I couldn't find a recent compilation, but according to Poulsen et al. (2015), estimates for the Cenomanian (i. e. the high $$p_{CO_2}$$ period from the last graph) vary widely from 10% to (indeed as the other answer suggests) 32%. Here is the quote: We focused on simulations of the Cenomanian, a mid-Cretaceous stage characterized by high $$p_{CO_2}$$ [e.g., (12)] and the warmest conditions of the past ~100 million years (13). Paleo-$$p_{O_2}$$ estimates inferred from stable-isotope carbon compositions indicate that the atmospheric percentage of $$O_2$$ was as low as 10 to 11% during this interval (7, 9), with one biogeochemical model indicating levels as high as 32.5% (6). References 7 and 9 are to Falkowski et al. (2005) and Tappert et al. (2013), while reference 6 is to Bergman et al. (2004). 1 answered Feb 5 '18 at 9:40 plannapus 4,5681717 silver badges5353 bronze badges To answer the $$p_{CO_2}$$ aspect of the question, here is a graph modified from Kent & Muttoni, 2013: It is based on compilations from Royer (2010) and Beerling & Royer (2011), using a wide variety of proxies, but mostly fossil leaves stomata for the Cretaceous. It might be a bit outdated now but most articles out there do give values on the same range: ca. 400~500 ppm at the Cretaceous-Paleogene boundary and values fluctuating between 500 and 1500 ppm before that, with a maximum during the mid-Cretaceous.