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



From: Ben Creisler
bcreisler@gmail.com


A news release associated with the article:


http://www.uibk.ac.at/ipoint/news/2013/bernsteine-liefern-neue-erkenntnisse-ueber-erdatmosphaere.html.en

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.