I read about deriving the oxygen content in the atmosphere in the Cretaceous by analyzing the air trapped in bubbles in amber from that period, but I have also found papers disputing the study's methods and showing that air isn't actually trapped in the bubbles because it's capable of diffusing through the amber (the amber isn't a perfect sealant), and thus the air found in that amber isn't necessarily as ancient as the amber itself.
My problem is is that almost all the relevant papers are from the late 80's; all those I've found are pro- or con-, I don't have the background to tell who is right, I haven't found a source that describes an actual consensus the field would have come to since then, and I see the claim about the original study showing what oxygen levels were in the Cretaceous cited in various places, but I find it hard to tell if it's because the objections were dismissed or if it's because it's a zombie claim.
What seems like the first paper to make the claim:
Gas bubbles in fossil amber as possible indicators of the major gas composition of ancient air
This source seems to be more recent and find the same conclusions but it's hard to tell (looks like at least one person is the same as the 1988 source):
Air Bubbles, Amber, and Dinosaurs - USGS
The main disagreement:
Is the air in amber ancient?
An unrelated recent paper that comes to a different conclusion on Cretaceous air content (based on amber as well but carbon isotope analysis thereof):
Stable carbon isotopes of C3 plant resins and ambers record changes in atmospheric oxygen since the Triassic
Does anyone know if a consensus was arrived at on this question, and/or if one side is clearly more right than the other?