Jennifer C. McElwain

Changes in carbon dioxide during an oceanic anoxic event linked to intrusion into Gondwana coals

The marine sedimentary record exhibits evidence for episodes of enhanced organic carbon burial known as ‘oceanic anoxic events’ (OAEs). They are characterized by carbon-isotope excursions in marine and terrestrial reservoirs and mass extinction of marine faunas. Causal mechanisms for the enhancement of organic carbon burial during OAEs are still debated, but it is thought that such events should draw down significant quantities of atmospheric carbon dioxide. In the case of the Toarcian OAE (∼183 million years ago), a short-lived negative carbon-isotope excursion in oceanic and terrestrial reservoirs has been interpreted to indicate raised atmospheric carbon dioxide caused by oxidation of methane catastrophically released from either marine gas hydrates or magma-intruded organic-rich rocks. Here we test these two leading hypotheses for a negative carbon isotopic excursion marking the initiation of the Toarcian OAE using a high-resolution atmospheric carbon dioxide record obtained from fossil leaf stomatal frequency. We find that coincident with the negative carbon-isotope excursion carbon dioxide is first drawn down by 350 ± 100 p.p.m.v. and then abruptly elevated by 1,200 ± 400 p.p.m.v, and infer a global cooling and greenhouse warming of 2.5 ± 0.1 °C and 6.5 ± 1 °C, respectively. The pattern and magnitude of carbon dioxide change are difficult to reconcile with catastrophic input of isotopically light methane from hydrates as the cause of the negative isotopic signal. Our carbon dioxide record better supports a magma-intrusion hypothesis, and suggests that injection of isotopically light carbon from the release of thermogenic methane occurred owing to the intrusion of Gondwana coals by Toarcian-aged Karoo-Ferrar dolerites.

Macroecological responses of terrestrial vegetation to climatic and atmospheric change across the Triassic/Jurassic boundary in East Greenland

Abstract The magnitude and pace of terrestrial plant extinction and macroecological change associated with the Triassic/Jurassic (Tr/J) mass extinction boundary have not been quantified using paleoecological data. However, tracking the diversity and ecology of primary producers provides an ideal surrogate with which to explore patterns of ecosystem stability, collapse, and recovery and to explicitly test for gradual versus catastrophic causal mechanisms of extinction. We present an analysis of the vegetation dynamics in the Jameson Land Basin, East Greenland, spanning the Tr/J extinction event, from a census collected paleoecological data set of 4303 fossil leaf specimens, in an attempt to better constrain our understanding of the causes and consequences of the fourth greatest extinction event in earth history. Our analyses reveal (1) regional turnover of ecological dominants between Triassic and Jurassic plant communities, (2) marked structural changes in the vegetation as reflected by potential loss of a mid-canopy habit, and (3) decline in generic-level richness and evenness and change in ecological composition prior to the Tr/J boundary; all of these findings argue against a single catastrophic causal mechanism, such as a meteorite impact for Tr/J extinctions. We identify various key ecological and biological traits that increased extinction risk at the Tr/J boundary and corroborate predictions of meta-population theory or plant ecophysiological models. These include ecological rarity, complex reproductive biology, and large leaf size. Recovery in terms of generic-level richness was quite rapid following Tr/J extinctions; however, species-level turnover in earliest Jurassic plant communities remained an order of magnitude higher than observed for the Triassic. We hypothesize, on the basis of evidence for geographically extensive macrofossil and palynological turnover across the entire Jameson Land Basin, that the nature and magnitude of paleoecological changes recorded in this study reflect wider vegetation change across the whole region. How exactly these changes in dominance patterns of plant primary production affected the entire ecosystem remains an important avenue of future research.

Limits for Combustion in Low O2 Redefine Paleoatmospheric Predictions for the Mesozoic

Several studies have attempted to determine the lower limit of atmospheric oxygen under which combustion can occur; however, none have been conducted within a fully controlled and realistic atmospheric environment. We performed experimental burns (using pine wood, moss, matches, paper, and a candle) at 20°C in O2 concentrations ranging from 9 to 21% and at ambient and high CO2 (2000 parts per million) in a controlled environment room, which was equipped with a thermal imaging system and full atmospheric, temperature, and humidity control. Our data reveal that the lower O2 limit for combustion should be increased from 12 to 15%. These results, coupled with a record of Mesozoic paleowildfires, are incompatible with the prediction of prolonged intervals of low atmospheric O2 levels (10 to 12%) in the Mesozoic.

Carbon-cycle perturbation in the Middle Jurassic and Accompanying Changes in the Terrestrial Paleoenvironment

Carbon‐isotope analyses of fossil wood from the Middle Jurassic Ravenscar Group, Yorkshire, NE England, reveal a significant excursion toward light isotopic values (δ13C change of −3 to −4‰) at about the Aalenian‐Bajocian boundary (∼174 Ma). A positive carbon isotopic excursion is also shown for the middle Bajocian (∼170 Ma) but is less clearly defined. These isotopic patterns are very similar to the few published marine carbonate records available for this time, in particular one based on belemnites from the Hebrides basin, NW Scotland, and others from pelagic limestones in Italy. The similarity of the terrestrial and marine isotope curves is an indication that the observed isotopic signal is a global phenomenon. Through parts of the Ravenscar Group (the Scarborough Formation), supplementary data from bulk organic carbon and palynofacies analysis confirm that isotopic curves based on bulk analyses may be strongly influenced by the balance of terrestrial versus marine organic matter present in the samples. The negative isotope excursion at the Aalenian‐Bajocian boundary marks a change from charcoal to coal as the dominant preservational mode of the macroscopic wood fossils, which is interpreted here as a shift to a more continuously humid climate in the Early Bajocian. Upsection, charcoal once again becomes common, reflecting a return to more fire‐prone (presumably seasonally arid) environments in the middle Bajocian. Paradoxically, floral assemblages associated with the lithological unit in which the negative excursion occurs display characteristics that would normally be interpreted as adaptations to water stress brought about by relative aridity or salinity. Preliminary analyses of leaf stomatal densities show some evidence of raised pCO2 relative to background values at about the level of the negative excursion.

Stomatal frequency adjustment of four conifer species to historical changes in atmospheric CO2

The species-specific inverse relation between atmospheric CO(2) concentration and stomatal frequency for many woody angiosperm species is being used increasingly with fossil leaves to reconstruct past atmospheric CO(2) levels. To extend our limited knowledge of the responsiveness of conifer needles to CO(2) fluctuations, the stomatal frequency response of four native North American conifer species (Tsuga heterophylla, Picea glauca, Picea mariana, and Larix laricina) to a range of historical CO(2) mixing ratios (290 to 370 ppmV) was analyzed. Because of the specific mode of leaf development and the subsequent stomatal patterning in conifer needles, the stomatal index of these species was not affected by CO(2). In contrast, a new measure of stomatal frequency, based on the number of stomata per millimeter of needle length, decreased significantly with increasing CO(2). For Tsuga heterophylla, the stomatal frequency response to CO(2) changes in the last century is validated through assessment of the influence of other biological and environmental variables. Because of their sensitive response to CO(2), combined with a high preservation capacity, fossil needles of Tsuga heterophylla, Picea glauca, P. mariana, and Larix laricina have great potential for detecting and quantifying past atmospheric CO(2) fluctuations.

Baseline intrinsic flammability of Earth’s ecosystems estimated from paleoatmospheric oxygen over the past 350 million years

Atmospheric oxygen (O2) is estimated to have varied greatly throughout Earth’s history and has been capable of influencing wildfire activity wherever fuel and ignition sources were present. Fires consume huge quantities of biomass in all ecosystems and play an important role in biogeochemical cycles. This means that understanding the influence of O2 on past fire activity has far-reaching consequences for the evolution of life and Earth’s biodiversity over geological timescales. We have used a strong electrical ignition source to ignite smoldering fires, and we measured their self-sustaining propagation in atmospheres of different oxygen concentrations. These data have been used to build a model that we use to estimate the baseline intrinsic flammability of Earth’s ecosystems according to variations in O2 over the past 350 million years (Ma). Our aim is to highlight times in Earth’s history when fire has been capable of influencing the Earth system. We reveal that fire activity would be greatly suppressed below 18.5% O2, entirely switched off below 16% O2, and rapidly enhanced between 19–22% O2. We show that fire activity and, therefore, its influence on the Earth system would have been high during the Carboniferous (350–300 Ma) and Cretaceous (145–65 Ma) periods; intermediate in the Permian (299–251 Ma), Late Triassic (285–201 Ma), and Jurassic (201–145 Ma) periods; and surprisingly low to lacking in the Early–Middle Triassic period between 250–240 Ma. These baseline variations in Earth’s flammability must be factored into our understanding of past vegetation, biodiversity, evolution, and biogeochemical cycles.

Fossil plant relative abundances indicate sudden loss of late triassic biodiversity in East Greenland.

Extinction Distinction The Triassic-Jurassic extinction approximately 200 million years ago is one of the five major extinctions in Earths history. It has been primarily recognized through the loss of marine species, as well as the subsequent emergence of dinosaurs, but its pace, both on land and in sea, has been unclear. McElwain et al. (p. 1554) now provide evidence from the plant fossil record from rocks in East Greenland. The total number of taxa and the number of common taxa decreased across the extinction boundary. The decrease was fairly abrupt and seemed to coincide with a period with increased atmospheric CO2 levels. Plant fossils from East Greenland record an abrupt decrease in abundance as CO2 levels increased. The pace of Late Triassic (LT) biodiversity loss is uncertain, yet it could help to decipher causal mechanisms of mass extinction. We investigated relative abundance distributions (RADs) of six LT plant assemblages from the Kap Stewart Group, East Greenland, to determine the pace of collapse of LT primary productivity. RADs displayed not simply decreases in the number of taxa, but decreases in the number of common taxa. Likelihood tests rejected a hypothesis of continuously declining diversity. Instead, the RAD shift occurred over the upper two-to-four fossil plant assemblages and most likely over the last three (final 13 meters), coinciding with increased atmospheric carbon dioxide concentration and global warming. Thus, although the LT event did not induce mass extinction of plant families, it accompanied major and abrupt change in their ecology and diversity.

Mid-Cretaceous pCO2 based on stomata of the extinct conifer Pseudofrenelopsis (Cheirolepidiaceae)

Stomatal characteristics of an extinct Cretaceous conifer, Pseudofrenelopsis parceramosa (Fontaine) Watson, are used to reconstruct atmospheric carbon dioxide ( p CO 2 ) over a time previously inferred to exhibit major fluctuations in this greenhouse gas. Samples are from nonmarine to marine strata of the Wealden and Lower Greensand Groups of England and the Potomac Group of the eastern United States, of Hauterivian to Albian age (136–100 Ma). Atmospheric p CO 2 is estimated from the ratios between stomatal indices of fossil cuticles and those from four modern analogs (nearest living equivalent plants). Using this approach, and two calibration methods to explore ranges, results show relatively low and only slightly varying p CO 2 over the Hauterivian–Albian interval: a low of ∼560–960 ppm in the early Barremian and a high of ∼620–1200 ppm in the Albian. Data from the Barremian Wealden Group yield p CO 2 values indistinguishable from a soil-carbonate–based estimate from the same beds. The new p CO 2 estimates are compatible with sedimentological and oxygen-isotope evidence for relatively cool mid-Cretaceous climates.

Climate-independent paleoaltimetry using stomatal density in fossil leaves as a proxy for CO2 partial pressure

Existing methods for determining paleoelevation are primarily limited by (1) large errors (±450 m), (2) a reliance on incorrect assumptions that lapse rates in terrestrial temperature decrease with altitude in a globally predictable manner, and/or (3) are inherently climate dependent. Here I present a novel paleoelevation tool, based on a predictable, globally conserved decrease in CO 2 partial pressure ( p CO 2 ) with altitude, as indicated by increased stomatal frequency of plant leaves. The approach was validated using historical populations of black oak ( Quercus kelloggii ). These analyses demonstrate highly significant inverse relationships between stomatal frequency and p CO 2 ( r 2 > 0.73), independent of ecological or local climatic variability. As such, this is the first paleobotanical method to be globally applicable and independent of long-term Cenozoic climate change. Further, tests on modern leaves of known elevations indicate that species-specific application to the fossil record of Q. kelloggii (= Q. pseudolyrata ) will yield paleoelevation estimates within average errors of ∼±300 m, representing a significant improvement in accuracy over the majority of existing methods.

Ancient Atmospheres and the Evolution of Oxygen Sensing Via the Hypoxia-Inducible Factor in Metazoans

Metazoan diversification occurred during a time when atmospheric oxygen levels fluctuated between 15 and 30%. The hypoxia-inducible factor (HIF) is a primary regulator of the adaptive transcriptional response to hypoxia. Although the HIF pathway is highly conserved, its complexity increased during periods when atmospheric oxygen concentrations were increasing. Thus atmospheric oxygen levels may have provided a selection force on the development of cellular oxygen-sensing pathways.