Transfer of organic Br and Cl from the Biosphere to the Atmosphere during the Cretaceous/Tertiary Impact: Implications for the stratospheric Ozone Layer

Abstract

Abstract.Following the Cretaceous/Tertiary (K/T) meteoritic impact some 65 Myr ago, large portions of aboveground terrestrial biomass were burned. As a result, large amounts of various trace gases were injected to the atmosphere, inducing a wide range of effects on climate and ecosystems. Here, it is commented on the previously unaccounted for emission to the atmosphere of methyl bromide (CH 3 Br) and methyl chloride (CH 3 Cl) from extensive biomass burning that followed the impact. Based on reported biomass burning emission rates of the above organohalogens relative to CO 2 , it is estimated that their emissions from global fires resulted in tropospheric mixing ratios of around 20-65.8 ppbv organic Cl and 110-390 pptv organic Br. The above calculated mixing ratios of organic chlorine and bromine are more than an order of magnitude greater than their present, anthropogenically perturbed level and, although the ocean ultimately might absorb them, we argue here that they could still remain in the atmosphere for many years, and a substantial fraction could be transported to the stratosphere, thus substantially affecting the ozone layer. This would have led to very serious increases in short wavelength UV radiation reaching the lowermost atmosphere.

A two‐dimensional atmospheric chemistry modeling investigation of Earth's Phanerozoic O3 and near‐surface ultraviolet radiation history

“…However, the authors suggest that large‐scale changes in the flux of UVB may have been ameliorated because of the impactor striking an anhydrite (CaSO 4 ) rich carbonate platform, consequently loading the atmosphere with a sulphate haze. Biomass burning [ Wolbach et al , 1988] would result in the generation of methyl bromine and methyl chloride [ Kourtidis , 2005] prolonging O 3 depletion. However, evidence for prolonged, widespread biomass burning at the K‐T boundary is contentious [ Belcher et al , 2003].…”

Changes in biologically-active ultraviolet radiation reaching the Earth’s surface

“…95,96 There have been speculations that damage to the ozone layer may have occurred 65 Myr ago, as a result of widespread combustion of biomass following a large meteoritic impact. 97 These authors estimate that the concentrations of organic chlorine and bromine may have been an order of magnitude greater than at present, resulting in ozone loss and increased UV radiation, which could have damaged life on Earth. However, there is no proof that this contributed to any biospheric extinctions.…”

Environmental effects of ozone depletion and its interactions with climate change: Progress report, 2005

“…There are still large uncertainties in the identification and quantification of significant sources and sinks of methyl bromide (and other simple organic halides). 46 New sources [47][48][49][50] for these compounds are still being discovered but uncertainties prevail. The phase-out of methyl bromide usage is important for the short-term recovery of stratospheric ozone concentrations.…”