Barry H. Lomax

An atmospheric pCO2 reconstruction across the Cretaceous-Tertiary boundary from leaf megafossils

The end-Cretaceous mass extinctions, 65 million years ago, profoundly influenced the course of biotic evolution. These extinctions coincided with a major extraterrestrial impact event and massive volcanism in India. Determining the relative importance of each event as a driver of environmental and biotic change across the Cretaceous-Tertiary boundary (KTB) crucially depends on constraining the mass of CO2 injected into the atmospheric carbon reservoir. Using the inverse relationship between atmospheric CO2 and the stomatal index of land plant leaves, we reconstruct Late Cretaceous-Early Tertiary atmospheric CO2 concentration (pCO2) levels with special emphasis on providing a pCO2 estimate directly above the KTB. Our record shows stable Late Cretaceous/Early Tertiary background pCO2 levels of 350–500 ppm by volume, but with a marked increase to at least 2,300 ppm by volume within 10,000 years of the KTB. Numerical simulations with a global biogeochemical carbon cycle model indicate that CO2 outgassing during the eruption of the Deccan Trap basalts fails to fully account for the inferred pCO2 increase. Instead, we calculate that the postboundary pCO2 rise is most consistent with the instantaneous transfer of ≈4,600 Gt C from the lithic to the atmospheric reservoir by a large extraterrestrial bolide impact. A resultant climatic forcing of +12 W⋅m−2 would have been sufficient to warm the Earths surface by ≈7.5°C, in the absence of counter forcing by sulfate aerosols. This finding reinforces previous evidence for major climatic warming after the KTB impact and implies that severe and abrupt global warming during the earliest Paleocene was an important factor in biotic extinction at the KTB.

The stability of the stratospheric ozone layer during the end-Permian eruption of the Siberian Traps

The discovery of mutated palynomorphs in end-Permian rocks led to the hypothesis that the eruption of the Siberian Traps through older organic-rich sediments synthesized and released massive quantities of organohalogens, which caused widespread O3 depletion and allowed increased terrestrial incidence of harmful ultraviolet-B radiation (UV-B, 280–315 nm; Visscher et al. 2004 Proc. Natl Acad. Sci. USA 101, 12 952–12 956). Here, we use an extended version of the Cambridge two-dimensional chemistry–transport model to evaluate quantitatively this possibility along with two other potential causes of O3 loss at this time: (i) direct effects of HCl release by the Siberian Traps and (ii) the indirect release of organohalogens from dispersed organic matter. According to our simulations, CH3Cl released from the heating of coals alone caused comparatively minor O3 depletion (5–20% maximum) because this mechanism fails to deliver sufficiently large amounts of Cl into the stratosphere. The unusual explosive nature of the Siberian Traps, combined with the direct release of large quantities of HCl, depleted the model O3 layer in the high northern latitudes by 33–55%, given a main eruptive phase of less than or equal to 200 kyr. Nevertheless, O3 depletion was most extensive when HCl release from the Siberian Traps was combined with massive CH3Cl release synthesized from a large reservoir of dispersed organic matter in Siberian rocks. This suite of model experiments produced column O3 depletion of 70–85% and 55–80% in the high northern and southern latitudes, respectively, given eruption durations of 100–200 kyr. On longer eruption time scales of 400–600 kyr, corresponding O3 depletion was 30–40% and 20–30%, respectively. Calculated year-round increases in total near-surface biologically effective (BE) UV-B radiation following these reductions in O3 layer range from 30–60 (kJ m−2 d−1)BE up to 50–100 (kJ m−2 d−1)BE. These ranges of daily UV-B doses appear sufficient to exert mutagenic effects on plants, especially if sustained over tens of thousands of years, unlike either rising temperatures or SO2 concentrations.

Metabolomic and physiological responses reveal multi-phasic acclimation of Arabidopsis thaliana to chronic UV radiation.

Biochemical changes in vivo and pathway interactions were investigated using integrated physiological and metabolic responses of Arabidopsis thaliana L. to ultraviolet (UV) radiation (280-400 nm) at 9.96 kJ m(-2) d(-1) over the entire life cycle from seed to seed (8 weeks). Columbia-0 (Col-0) and a UV-B sensitive accession (fah-1) showed significant (P < 0.001) reductions in leaf growth after 6 weeks. Col-0 recovered growth after 8 weeks, with recovery corresponding to a switch from production of phenylpropanoids to flavonoids. fah-1 failed to recover, indicating that sinapate production is an essential component of recovery. Epidermal features show that UV radiation caused significant (P < 0.001) increases in trichome density, which may act as a structural defence response. Stomatal indices showed a significant (P < 0.0001) reduction in Col-0 and a significant (P < 0.001) increase in fah-1. Epidermal cell density was significantly increased under UV radiation on the abaxial leaf surface, suggesting that that a fully functioning phenylpropanoid pathway is a requirement for cell expansion and leaf development. Despite wild-type acclimation, the costs of adaptation lead to reduced plant fitness by decreasing flower numbers and total seed biomass. A multi-phasic acclimation to UV radiation and the induction of specific metabolites link stress-induced biochemical responses to enhanced acclimation.

Rapid determination of spore chemistry using thermochemolysis gas chromatography-mass spectrometry and micro-Fourier transform infrared spectroscopy

Spore chemistry is at the centre of investigations aimed at producing a proxy record of harmful ultraviolet radiation (UV-B) through time. A biochemical proxy is essential owing to an absence of long-term (century or more) instrumental records. Spore cell material contains UV-B absorbing compounds that appear to be synthesised in variable amounts dependent on the ambient UV-B flux. To facilitate these investigations we have developed a rapid method for detecting variations in spore chemistry using combined thermochemolysis gas chromatography-mass spectrometry and micro-Fourier transform infrared spectroscopy. Our method was tested using spores obtained from five populations of the tropical lycopsid Lycopodium cernuum growing across an altitudinal gradient (650-1981 m a.s.l.) in S.E. Asia with the assumption that they experienced a range of UV-B radiation doses. Thermochemolysis and subsequent pyrolysis liberated UV-B pigments (ferulic and para-coumaric acid) from the spores. All of the aromatic compounds liberated from spores by thermochemolysis and pyrolysis were active in UV-B protection. The various functional groups associated with UV-B protecting pigments were rapidly detected by micro-FTIR and included the aromatic C[double bond, length as m-dash]C absorption band which was exclusive to the pigments. We show increases in micro-FTIR aromatic absorption (1510 cm(-1)) with altitude that may reflect a chemical response to higher UV-B flux. Our results indicate that rapid chemical analyses of historical spore samples could provide a record ideally suited to investigations of a proxy for stratospheric O3 layer variability and UV-B flux over historical (century to millennia) timescales.

Evidence for the recovery of terrestrial ecosystems ahead of marine primary production following a biotic crisis at the Cretaceous–Tertiary boundary

The fossil record demonstrates that mass extinction across the Cretaceous–Tertiary (K–T) boundary is more severe in the marine than the terrestrial realm. We hypothesize that terrestrial ecosystems were able to recover faster than their marine counterparts. To test this hypothesis, we measured sedimentary δ13C as a tracer for global carbon cycle changes and compared it with palaeovegetational changes reconstructed from palynomorphs and cuticles across the K–T boundary at Sugarite, New Mexico, USA. Different patterns of perturbation and timescales of recovery of isotopic and palaeobotanical records indicate that the δ13C excursion reflects the longer recovery time of marine versus terrestrial ecosystems.

Genome size as a predictor of guard cell length in Arabidopsis thaliana is independent of environmental conditions

The recent discovery of a strong positive relationship between angiosperm genome size and stomatal guard cell length (GCL) opens the possibility of using plant fossil guard cell size as a proxy for changes in angiosperm genome size over periods of environmental change. The responses of GCL to environmental stimuli are currently unknown and may obscure this predictive relationship. Here, we investigated the effects of environmental variables (atmospheric CO2, drought, relative humidity, irradiance, ultraviolet radiation and pathogen attack) on GCL in the model plant Arabidopsis thaliana to quantify environmentally induced variation. GCL responded to all variables tested, but the changes incurred did not significantly impinge on the predictive capability of the relationship.

Fire across the K–T boundary: initial results from the Sugarite Coal, New Mexico, USA

Abstract It has been hypothesized that major global fires occurred following a bolide impact at the Cretaceous–Tertiary boundary. Evidence for this has been mainly from ‘soot’ or black carbon at a number of marine boundary sites as well as the occurrence of inertinites (fusinite) in coals above the boundary. In addition, the occurrence of fossil charcoal (inertinites) in potential Tsunami deposits has been used to strengthen this idea. However, fire is known to have been widespread throughout the Cretaceous based on the distribution of fossil charcoal, and it is necessary to evaluate the claims for a global fire at the K–T boundary within the context of this more extensive record of ancient fires. The occurrence of the K–T boundary within the Sugarite coal sequence in the Raton Basin, New Mexico, offers the opportunity to assess the incidence of fire in local peat-forming vegetation, in the latest Cretaceous, across the Cretaceous–Tertiary boundary interval and in the earliest Tertiary. The distribution of fire products, i.e. fossil charcoal, is assessed using a combination of inertinite group macerals in polished blocks of coal and plant particles released by chemical maceration of coal. Inertinite group macerals (fusinite, semifusinite, inertodetrinite), which represent fossil charcoal, and particles of charcoal in maceration residues are abundant throughout the Sugarite coal sequence, both before, during and after the K–T boundary event. Samples from below the boundary yield inertinites that constitute more than 20% of the coal; three horizons are inertinite rich (>50%). Fire was obviously an important element of the terrestrial environment during the latest Cretaceous. High inertinite values and abundant charcoal particles characterize the latest Cretaceous part of the coal, the carbonaceous shale just below the boundary, the boundary interval itself and Tertiary coals and carbonaceous shales above the boundary. Charcoal in mineral-rich units of the latest Cretaceous, immediately below the boundary, in the boundary interval and in the Tertiary, is dominated by small inertodetrinite particles. These might have been wind blown from regional fires or they may reflect reworking during erosion, which led to sediment input into the mire. In either case the charcoal signature in these units is the same, irrespective of their position in the sequence. Our data, combined with that from two other sites in terrestrial sequences in North America, show that fires were an integral part of mire ecosystems through the latest Cretaceous and into the early Tertiary. Therefore, evidence for distinctive global wildfires at the Cretaceous–Tertiary boundary would have to be sought from careful consideration of other aspects of charcoal deposition, including flux rates for charcoal and soot.

UV-B absorbing pigments in spores: biochemical responses to shade in a high-latitude birch forest and implications for sporopollenin-based proxies of past environmental change

Current attempts to develop a proxy for Earths surface ultraviolet-B (UV-B) flux focus on the organic chemistry of pollen and spores because their constituent biopolymer, sporopollenin, contains UV-B absorbing pigments whose relative abundance may respond to the ambient UV-B flux. Fourier transform infrared (FTIR) microspectroscopy provides a useful tool for rapidly determining the pigment content of spores. In this paper, we use FTIR to detect a chemical response of spore wall UV-B absorbing pigments that correspond with levels of shade beneath the canopy of a high-latitude Swedish birch forest. A 27% reduction in UV-B flux beneath the canopy leads to a significant (p<0.05) 7.3% reduction in concentration of UV-B absorbing compounds in sporopollenin. The field data from this natural flux gradient in UV-B further support our earlier work on sporopollenin-based proxies derived from sedimentary records and herbaria collections.

The impact of oxidation on spore and pollen chemistry

Sporomorphs (pollen and spores) have an outer wall composed of sporopollenin. Sporopollenin chemistry contains both a signature of ambient ultraviolet-B flux and taxonomic information, but it is currently unknown how sensitive this is to standard palynological processing techniques. Oxidation in particular is known to cause physical degradation to sporomorphs, and it is expected that this should have a concordant impact on sporopollenin chemistry. Here, we test this by experimentally oxidizing Lycopodium (clubmoss) spores using two common oxidation techniques: acetolysis and nitric acid. We also carry out acetolysis on eight angiosperm (flowering plant) taxa to test the generality of our results. Using Fourier Transform infrared (FTIR) spectroscopy, we find that acetolysis removes labile, non-fossilizable components of sporomorphs, but has a limited impact upon the chemistry of sporopollenin under normal processing durations. Nitric acid is more aggressive and does break down sporopollenin and reorganize its chemical structure, but when limited to short treatments (i.e. ≤10 min) at room temperature sporomorphs still contain most of the original chemical signal. These findings suggest that when used carefully oxidation does not adversely affect sporopollenin chemistry, and that palaeoclimatic and taxonomic signatures contained within the sporomorph wall are recoverable from standard palynological preparations. Supplementary material: R code for all analyses, the complete dataset and additional figures are available at http://www.geolsoc.org.uk/SUP18811

Rapid (10-yr) recovery of terrestrial productivity in a simulation study of the terminal Cretaceous impact event

Abstract Investigations of short-term (up to 103 yr) environmental change across the Cretaceous–Tertiary boundary provide evidence for reduced temperatures, consistent with the injection of debris and sulphate aerosols into the upper atmosphere by a large impact event. Concomitant with this was a postulated massive addition of CO2 to the atmospheric carbon reservoir by impact vaporisation of the Chicxulub carbonate platform. Taken together, a high CO2 but low irradiance environment would have created unusual conditions for the operation of the terrestrial biosphere. Here, we have evaluated this environmental influence on terrestrial ecosystems using a process-based dynamic global vegetation model forced with post-impact global climates, derived by modification of the GENESIS atmospheric climate model simulation for the latest Cretaceous. Our results suggest that terrestrial primary productivity initially collapsed and then recovered to pre-impact levels within a decade. Global terrestrial carbon storage in vegetation biomass exhibited a similar collapse but complete recovery took place on a 60–80 yr timescale. The recovery of both terrestrial net primary productivity and vegetation biomass was largely mediated by the high CO2 concentration stimulating ecosystem photosynthetic productivity in the warm low latitudes. An apparently rapid recovery of terrestrial ecosystem function stands in marked contrast to the situation for the marine realm, where the organic carbon flux to the deep ocean was suppressed for up to 3 million years.