Contact metamorphism, halocarbons, and environmental crises of the past

Abstract

Environmental context. What caused the biggest known mass extinction on Earth ~252 million years ago? A possible killer mechanism was the release of specific gases into the atmosphere, which eventually led to destruction of the ozone layer. This is now supported by new laboratory experiments in which ozone-destructing gases were generated when heating rocks from East Siberia (Russia) – reconstructing what happened naturally in Siberia during explosive gas eruptions 252 million years ago. Abstract. What triggered the largest know mass extinction at the Permian–Triassic boundary 252 million years ago, when 95% of the species in the oceans disappeared? New geological data suggest that eruptions of carbon (CH4, CO2) and halocarbon (CH3Cl and CH3Br) gases from the vast sedimentary basins of east Siberia could have triggered a period with global warming (5°–10°C) and terrestrial mass extinction. The gases were generated during contact metamorphism of sedimentary rocks around 1200°C hot igneous intrusions. One of the suggested end-Permian extinction mechanisms is the extreme ultraviolet radiation (UV-B) caused by a prolonged destruction of stratospheric ozone induced by the emitted halocarbons. This hypothesis is supported by a new set of experiments, where natural rock salt samples from Siberia were heated to 275°C. Among the gases generated during heating are methyl chloride (CH3Cl) and methyl bromide (CH3Br). These findings open up new possibilities for investigating ancient environmental crises.

High-precision geochronology links the Ferrar large igneous province with early-Jurassic ocean anoxia and biotic crisis

“…The seemingly protracted emplacement/eruption of the Karoo portion of the LIP (>2 Ma) suggests a potential decoupling of this magmatism from the short-duration environmental change characterizing the early Toarcian. However, as previously stated, further high-precision geochronology on rocks from the Karoo province is critical to resolve the disparity in duration proposed by Svensen et al (2012) and Sell et al (2014) and to determine over what duration the maximum volume of Karoo magmas was emplaced/erupted.…”

“…This relatively large uncertainty permits overlap between the timing of magmatism in both provinces but obscures any diachroneity at the sub-million year level. Recently, U-Pb ID-TIMS zircon geochronology was used to constrain Karoo LIP sill emplacement to a much narrower interval, from 183.0 ± 0.5 to 182.3 ± 0.6 Ma, with a probabilistically determined duration of 0.47 Ma (Svensen et al, 2012). New U-Pb ID-TIMS work on zircon and baddeleyite by Sell et al (2014) indicate that Karoo intrusive magmatism began by 183.02 ± 0.07 Ma and progressed for ∼2 Ma.…”

“…If the single sample dated from the Karoo LIP is included, the duration of Karoo-Ferrar magmatism defined by our dataset increases to 816 ± 58 kyr. Svensen et al (2012) used U-Pb zircon TIMS geochronology to constrain the duration of intrusive magmatism in the southern portion of the Karoo province to ∼500 kyr, between 183.0 ± 0.5 Ma and 182.3 ± 0.6 Ma. Sell et al (2014) dated baddeleyite and zircon from three Karoo sills and one Karoo lava by U-Pb TIMS and suggested that magmatism is more protracted, lasting a minimum of 2 Ma.…”

“…Sell et al (2014) dated baddeleyite and zircon from three Karoo sills and one Karoo lava by U-Pb TIMS and suggested that magmatism is more protracted, lasting a minimum of 2 Ma. Further high-precision geochronology on rocks (both lavas and sills) from the Karoo province are needed resolve the current disparity in duration proposed by Svensen et al (2012) and Sell et al (2014). Even though the datasets of Svensen et al (2012) and Sell et al (2014) and those from the present work were generated using the U-Pb zircon method, to directly compare the datasets of Svensen et al (2012) with Sell et al (2014) and this study requires propagation of external errors to account for uncertainty in the isotopic composition of the tracer solution used, which was different in the Svensen et al (2012) work.…”

“…Even though the datasets of Svensen et al (2012) and Sell et al (2014) and those from the present work were generated using the U-Pb zircon method, to directly compare the datasets of Svensen et al (2012) with Sell et al (2014) and this study requires propagation of external errors to account for uncertainty in the isotopic composition of the tracer solution used, which was different in the Svensen et al (2012) work. With or without this additional uncertainty, emplacement dates for all 14 sills dated by Svensen et al (2012) overlap with the dates of all 21 rocks dated in the present study. All Ferrar rocks dated here fall within the range of Karoo rocks dated by Sell et al (2014).…”