Paper No. 12
Presentation Time: 9:00 AM-6:30 PM
THE USE OF δ13C VALUES IN FOSSIL RESINS AS A PROXY FOR CHANGES IN ATMOSPHERIC PO2
Estimating the partial pressure of atmospheric oxygen (pO2) in the geological past has been challenging due to a lack of reliable proxies. Here we present a technique to estimate paleo-pO2 using the stable carbon isotope composition (δ13C) of modern and fossil plant resins. Resins exhibit a natural variability in δ13C of around 8‰, due to local environmental and ecological factors (e.g., water availability, water composition, light exposure, temperature, nutrient availability, etc.). Despite this variability, fossil resins are on average enriched in 13C by up to 6‰, compared to modern resins. Shifts in fossil resin isotopic composition follow distinctive trends through time, with the greatest enrichment taking place during the Triassic, mid-Cretaceous, and early Eocene. Experimental evidence and theoretical considerations suggest that neither changes in pCO2 nor in the δ13C of atmospheric CO2 can account for the observed shifts in resin δ13C. The fractionation of 13C in resin-producing plants, instead, is primarily influenced by atmospheric pO2, with more fractionation occurring at higher pO2. Enriched δ13C values suggest that atmospheric pO2 during most of the Mesozoic and Cenozoic was considerably lower (pO2 ≥10%) than today. Furthermore, a correlation between resin δ13C and the marine δ18O record implies that pO2, pCO2, and global temperatures were inversely linked (i.e., intervals of low pO2 were generally accompanied by high pCO2 and elevated global temperatures). Interestingly, intervals with the lowest inferred pO2 were preceded by large-scale volcanism. This suggests a relationship in which mantle-derived volcanic CO2 may have triggered an initial warming, leading to an increase in oxidative weathering, and ultimately, a decline of pO2 during greenhouse periods. Conversely, after volcanic output ceased, pO2 levels would have increased in response to reduced weathering rates, whereas CO2 levels and temperatures declined.