Amber Isotope Ratios Give Much Lower Atmospheric Oxygen Levels Since Triassic Than Other Proxies

Stable carbon isotopes of C3 plant resins and ambers record changes in atmospheric oxygen since the Triassic

Authors:

Tappert et al.

Abstract:

Estimating the partial pressure of atmospheric oxygen (pO2) in the geological past has been challenging because of the lack of reliable proxies. Here we develop a technique to estimate paleo-pO2 using the stable carbon isotope composition (δ13C) 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 δ13C signatures over geological timescales. The distribution of δ13C values of modern resins (n = 126) indicates that (a) resin-producing plant families generally have a similar fractionation behavior during resin biosynthesis, and (b) the fractionation observed in resins is similar to that of bulk plant matter. Resins exhibit a natural variability in δ13C of around 8‰ (δ13C range: −31‰ to −23‰, mean: −27‰), which is caused by local environmental and ecological factors (e.g., water availability, water composition, light exposure, temperature, nutrient availability). To minimize the effects of local conditions and to determine long-term changes in the δ13C of resins, we used mean δ13C values (View the MathML source) for each geological resin deposit. Fossil resins (n = 412) are generally enriched in 13C compared to their modern counterparts, with shifts in View the MathML source of up to 6‰. These isotopic shifts follow distinctive trends through time, which are unrelated to post-depositional processes including polymerization and diagenesis. The most enriched fossil resin samples, with a View the MathML source between −22‰ and −21‰, formed during the Triassic, the mid-Cretaceous, and the early Eocene. Experimental evidence and theoretical considerations suggest that neither change in pCO2 nor in the δ13C of atmospheric CO2 can account for the observed shifts in View the MathML source. The fractionation of 13C in resin-producing plants (Δ13C), instead, is primarily influenced by atmospheric pO2, with more fractionation occurring at higher pO2. The enriched View the MathML source values suggest that atmospheric pO2 during most of the Mesozoic and Cenozoic was considerably lower (pO2 = 10–20%) than today (pO2 = 21%). In addition, a correlation between the View the MathML source and the marine δ18O record implies that pO2, pCO2, and global temperatures were inversely linked, which suggests that intervals of low pO2 were generally accompanied by high pCO2 and elevated global temperatures. Intervals with the lowest inferred pO2, including the mid-Cretaceous and the early Eocene, were preceded by large-scale volcanism during the emplacement of large igneous provinces (LIPs). This suggests that the influx of mantle-derived volcanic CO2 triggered an initial phase of warming, which led to an increase in oxidative weathering, thereby further increasing greenhouse forcing. This process resulted in the rapid decline of atmospheric pO2 during the mid-Cretaceous and the early Eocene greenhouse periods. After the cessation in LIP volcanism and the decrease in oxidative weathering rates, atmospheric pO2 levels continuously increased over tens of millions of years, whereas CO2 levels and temperatures continuously declined. These findings suggest that atmospheric pO2 had a considerable impact on the evolution of the climate on Earth, and that the δ13C of fossil resins can be used as a novel tool to assess the changes of atmospheric compositions since the emergence of resin-producing plants in the Paleozoic.