Stable carbon isotopes of c3 plant resins and ambers record changes in atmospheric oxygen since the triassic:

Estimating the partial pressure of atmospheric oxygen (pO 2) in the geological past has been challenging because of the lack of reliable proxies. Here we develop a technique to estimate paleo-pO 2 using the stable carbon isotope composition (δ 13 C) of plant resins—including amber, copal, and resinite—from a wide range of localities and ages (Triassic to modern). Plant resins are particularly suitable as proxies because their highly cross-linked terpenoid structures allow the preservation of pristine δ 13 C signatures over geological.

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Outermost (valence) electrons

Coal i believe hope this