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



From: Ben Creisler
bcreisler@gmail.com


A free pdf of the paper can be downloaded from Research Gate.

This long link will work, or you can Google the full title of the
paper and get a live link that way.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&ved=0CD0QFjAD&url=http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F256655103_Stable_carbon_isotopes_of_C3_plant_resins_and_ambers_record_changes_in_atmospheric_oxygen_since_the_Triassic%2Ffile%2F60b7d52389cbaf41b9.pdf&ei=sOhrUryTOsrHigLh84Eo&usg=AFQjCNHB3bPa4HcMz1DN0EWckcSS-yiBVw&sig2=bNjMjs6WSApbi_8V1BX2RQ&bvm=bv.55123115,d.cGE


This paper contradicts recent studies that found high oxygen levels
during the Cretaceous of up to 30 percent based on charcoal from
fires.

Glasspool I. J. and Scott A. C. (2010) Phanerozoic concentrations of
atmospheric oxygen reconstructed from sedimentary charcoal. Nature
Geoscience 3: 627–630.


On Sat, Oct 26, 2013 at 8:22 AM, Ben Creisler <bcreisler@gmail.com> wrote:
> From: Ben Creisler
> bcreisler@gmail.com
>
>
> 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
> http://dx.doi.org/10.1016/j.gca.2013.07.011
> http://www.sciencedirect.com/science/article/pii/S0016703713003906
>
>
>
>
> 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.