Thank you very much for your reply. It is very helpful to me.
However I have noticed that there are different considerations and studies on oxygen levels in the lower Cretaceous. Some of these studies indicate higher current levels (21%).For this reason I would like to know the most recent studies on this.
Here are a few more recently-published studies that bear on your area of interest:
Belcher, C. M., and J. C. McElwain. "Limits for combustion in low O2 redefine paleoatmospheric predictions for the Mesozoic." Science 321.5893 (2008): 1197-1200.
Glasspool, Ian J., and Andrew C. Scott. "Phanerozoic concentrations of atmospheric oxygen reconstructed from sedimentary charcoal." Nature Geoscience 3.9 (2010): 627-630.
Brown, Sarah AE, et al. "Cretaceous wildfires and their impact on the Earth system." Cretaceous Research 36 (2012): 162-190.
Berner’s most recent iteration of Phanerozoic Oxygen levels using GEOCARB is reported in Am Jour Sci:
Berner, Robert A. "Phanerozoic atmospheric oxygen: New results using the GEOCARBSULF model." American Journal of Science 309.7 (2009): 603-606.