Long and short orbital forcing of Jurassic wildfires.
- 1WildFIRE Lab, University of Exeter, Earth System Sciences, Falmouth, United Kingdom of Great Britain – England, Scotland, Wales (t.p.hollaar@exeter.ac.uk)
- 2Camborne School of Mines and Environment and Sustainability Institute, University of Exeter, Penryn Campus, UK
- 3Université de Bourgogne/Franche-Comté, UMR 6282 CNRS, Biogéosciences, Dijon, France
- 4Department of Environment, Earth and Ecosystems, The Open University, Milton Keynes, UK
- 5Department of Geology, Trinity College Dublin, The University of Dublin, Ireland
Fire regimes are changing due to anthropogenic climatic drivers and fuel management challenges in all regions of Earth. However, the planet is also subject to natural background variability due to Earth’s orbital parameters (Milkankovitch cycles). To date no studies have assessed a sedimentary record that is sufficiently long or has a resolution that is high enough to assess both long eccentricity and precessional forcings on fire. Here we present a ~350,000 yr record of wildfire activity, using fossil charcoal from Jurassic sediments.
The studied interval is part of the astronomically constrained Upper Pliensbachian of the Mochras borehole, Cardigan Bay Basin. The site was located within the Laurasian Seaway, south of the Viking Corridor that linked the north-western Tethys Ocean to the Boreal Sea, at a palaeolatitude of ~35°N. Clear lithological couplets of carbonate-rich and TOC-enhanced beds are observed, which show an orbital control on deposition. High resolution macrocharcoal (>125 um) and microcharcoal (10-125 um) abundance data have been obtained at a ~2 ky resolution over the studied interval. Charcoal data are coupled to estimates of variations in the hydrological cycle using clay mineral analyses, along with palynofacies and elemental analyses, and lithological and biogeochemical signatures.
We show that fire activity was strongly increased during (1) a period of maximum eccentricity (405,000 yr cycle) and (2) inferred maximum in seasonal contrast due to precession (20,000 yr cycles). In these periods with a strong seasonality, charcoal abundance indicates enhanced wildfire activity. This is coupled to a more seasonal pattern of rainfall as indicated by the relative abundance of smectite versus kaolinite. We argue that the shift to a more seasonal climate would have led to the increase in dry-adapted conifer forests. Conifers have biochemical and morphological traits that make them particularly flammable whether dry or live.This climate induced change in vegetation contributed to increased wildfire activity in the seasonal dry periods.
Increase in wildfire activity on an orbital time scale indicates that currently wildfires should be suppressed as Earth is close to an eccentricity minimum, such that man may have counteracted a relatively fire limited period.
How to cite: Hollaar, T. P., Baker, S. B., Hesselbo, S. P., Deconinck, J.-F., Mander, L., Ruhl, M., and Belcher, C. M.: Long and short orbital forcing of Jurassic wildfires., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15594, https://doi.org/10.5194/egusphere-egu21-15594, 2021.
