Rhythmic Shifts in Fire Activity in Earth's Geological Past were Driven by Orbital Cycles

Abstract

Fire is a key component of the Earth System and has shaped terrestrial ecosystems since the Late Silurian, shortly after the arrival of the first land plants. Present day anthropogenic warming has increased the fire weather season and the risk of extreme fire events in many regions of the world. The probability of fire will continue to rise with the future predicted warming. To understand long-term patterns and feedbacks in climate and fire activity we need to look at the geological record. Past periods of global warmth are all associated with enhanced wildfire, in a similar way to current global warming, and models exceeding the worst case scenario of 4 °C warming by the Intergovernmental Panel on Climate Change (IPPC). Fire activity has been reconstructed for some specific past events of global warmth accompanied by perturbations of the carbon cycle reflected in carbon-isotope records. However, it remains unknown how the fire regime and behaviour acted at times of no perturbation in the carbon-cycle in the geological past. Research on fire activity in the Quaternary (last 2.6 Myr) provides strong evidence that wildfires are driven by climate forcing on an orbital time scale. Fire activity has been shown to increase at times of enhanced seasonal contrast driven by precession and the amplitude modulator eccentricity in several regions. As in present day fire-prone areas, this creates a humid season that allows biomass to gather, followed by a dry season that lowers the fuel moisture levels to allow ignition and fire spread. These climate driven changes in fire activity are often linked to a change in fuel type, load and structure on an orbital time scale. In this thesis I present three reconstructions of wildfire activity from the Early Jurassic and examine the role of orbital forcing on these records: (i) one record spanning ~350,000 yr (350 kyr) at a time of no carbon-perturbation (background climate); (ii) one ~800,000 yr (800 kyr) record spanning the so-called Late Pliensbachian Event, and; (iii) a ~900,000 yr (900 kyr) record spanning the Sinemurian-Pliensbachian boundary. The first record serves to assess wildfire activity in geological time during a climatically relatively stable period, and the other two records examine if the wildfire regime is impacted by climatic cooling or warming, respectively. The study location is Mochras, Cardigan Bay Basin, NW Wales, UK. The Llanbedr (Mochras Farm) core constitutes an ideal archive for study of orbital forcing of wildfire, due to the well-established astrochronological framework and the high terrestrial organic content. Wildfire activity in the geological past is inferred from fossil charcoal abundance and two sets of charcoal counts are presented from a range of depositional environments and ages (Jurassic to Miocene) to assess the reproducibility of the fossil charcoal proxy. The reproducibility of charcoal counts between two researchers was significant in all depositional environments and in order to take small differences in charcoal counts into account an error bar of ~40 charcoal particles is suggested. A multi-proxy dataset, from the Late Pliensbachian, comprising charcoal counts, clay mineralogy, palynofacies (marine and terrestrial organic microfossils) and carbon mass spectrometry records, shows that fire activity in a ‘background’ climate during the Late Pliensbachian is strongly driven by ~20 kyr precession and modulated by 405 kyr eccentricity forcing. Also in the Early Jurassic, wildfire activity is greatest at times of high seasonal contrast in rainfall, where the rainy season allows biomass to build up and the subsequent dry season lowers the moisture status of the fuels and increases the ignitability. These climatically driven shifts in wildfire activity are potentially accelerated via orbital shifts in vegetation. Following, the Late Pliensbachian ‘background’ record is extended and spans two long eccentricity cycles. This record also covers the start of the Late Pliensbachian Event. Long eccentricity modulates changes in the hydrological cycle as inferred from clay mineralogy, grain-size inferred from elemental data (core-scan XRF), and microscopic charcoal abundance. The positive carbon-isotope excursion marking the onset of the Late Pliensbachian Event is associated with enhanced physical erosion relative to chemical weathering. Lastly, multi-proxy records of the Late Pliensbachian and the Sinemurian-Pliensbachian boundary are compared; both records comprising charcoal counts, clay mineralogy, palynofacies and mass spectrometry. I compare the fire record of the Late Pliensbachian associated with climatic cooling and the fire reconstruction from the Sinemurian-Pliensbachian boundary associated with climatic warming. Both fire records show a predominant control of the 100 kyr eccentricity cycle. Placing both fire records on the intermediate-productivity gradient indicates that both fire regimes were limited by moisture and not productivity (biomass). Thus, although these records likely represent different climatic backgrounds, fire activity was suppressed due to high fuel moisture levels and not low biomass abundance in the Mochras core. Overall, this thesis shows that fire activity in the Cardigan Bay region during the late Sinemurian and Pliensbachian was strongly driven by orbital forcing. Insolation driven changes in humidity at a precessional, short-, and long-eccentricity time scales led to fivefold increases and decreases in charcoal and inferred fire activity. Strong seasonality and intermediate levels of moisture and biomass productivity are linked to extremes in fire activity.