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Amber as clue to atmospheric oxygen and carbon dioxide levels during Mesozoic

From: Ben Creisler

A new paper that may be of interest:

Ralf Tappert, Ryan C. McKellar, Alexander P. Wolfe, Michelle C.
Tappert, Jaime Ortega-Blanco & Karlis Muehlenbachs (2013)
Stable carbon isotopes of C3 plant resins and ambers record changes in
atmospheric oxygen since the Triassic.
Geochimica et Cosmochimica Acta 121: 240–262

Estimating the partial pressure of atmospheric oxygen (rhoO2) in the
geological past has been challenging because of the lack of reliable
proxies. Here we develop a technique to estimate paleo-rhoO2 using the
stable carbon isotope composition (delta13C) 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 delta13C signatures over geological
timescales. The distribution of delta13C 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 delta13C of around 8‰
(delta13C 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 delta13C of resins, we used mean delta13C 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 rhoCO2 nor
in the delta13C of atmospheric CO2 can account for the observed shifts
in View the MathML source. The fractionation of 13C in resin-producing
plants (delta13C), instead, is primarily influenced by atmospheric
rhoO2, with more fractionation occurring at higher rhoO2. The enriched
View the MathML source values suggest that atmospheric rhoO2 during
most of the Mesozoic and Cenozoic was considerably lower (rhoO2 =
10–20%) than today (rhoO2 = 21%). In addition, a correlation between
the View the MathML source and the marine delta18O record implies that
rhoO2, rhoCO2, and global temperatures were inversely linked, which
suggests that intervals of low rhoO2 were generally accompanied by
high rhoCO2 and elevated global temperatures. Intervals with the
lowest inferred rhoO2, 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 rhoO2 during the mid-Cretaceous and the early
Eocene greenhouse periods. After the cessation in LIP volcanism and
the decrease in oxidative weathering rates, atmospheric rhoO2 levels
continuously increased over tens of millions of years, whereas CO2
levels and temperatures continuously declined. These findings suggest
that atmospheric rhoO2 had a considerable impact on the evolution of
the climate on Earth, and that the delta13C 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.