Wildfire activity enhanced during phases of maximum orbital eccentricity and precessional forcing in the Early Jurassic

Abstract

Fire regimes are changing due to both anthropogenic climatic drivers and vegetation management challenges, making it difficult to determine how climate alone might influence wildfire activity. Earth has been subject to natural-background climate variability throughout its past due to variations in Earth’s orbital parameters (Milkankovitch cycles), which provides an opportunity to assess climate-only driven variations in wildfire. Here we present a 350,000 yr long record of fossil charcoal from mid-latitude (~35°N) Jurassic sedimentary rocks. These results are coupled to estimates of variations in the hydrological cycle using clay mineral, palynofacies and elemental analyses, and lithological and biogeochemical signatures. We show that fire activity strongly increased during extreme seasonal contrast (monsoonal climate), which has been linked to maximal precessional forcing (boreal summer in perihelion) (21,000 yr cycles), and we hypothesize that long eccentricity modulation further enhances precession-forced fire activity.

Environmental changes during the onset of the Late Pliensbachian Event (Early Jurassic) in the Mochras Borehole, Cardigan Bay Basin, NW Wales

“…2). The δ 13 Corg displays a minor (~0.5 ‰) shift towards more positive values at ~944 mbs (as reported in Storm et al, 2020;Hollaar et al, 2021). At ~ 930 mbs an abrupt shift of ~1.8 ‰ (Fig.…”

“…2). Previous assessments of the palaeoenvironmental signature of these TOCenhanced and Ca-rich couplets indicate strongly that the different depositional modes are driven by orbital precession (Ruhl et al, 2016;Hinnov et al, 2018;Storm et al, 2020;Hollaar et al, 2021;Pieńkowski et al, 2021). Precession driven changes in monsoonal strength have been suggested to influence the deposition and preservation of TOC and carbonate in the Cardigan Bay Basin (Ruhl et al, 2016), although the impact may have been expressed, at least partially, by changes in strength of bottom currents in the seaway as a whole (Pieńkowski et al, 2021).…”

“…These data were then scaled up to the known quantity of the sample according the method of Belcher et al (2005). freshwater algae, marine palynomorphs, structured phytoclasts, unstructured phytoclasts, black debris, amorphous organic matter (AOM), and charcoal (further described in Hollaar et al, 2021). The palynofacies were quantified on a palynological slide using the optical light microscope (40x10 magnification) and counting a minimum of 300 particles per slide.…”

“…To assess the character of the observed fluctuations in charcoal abundance, whether changes in charcoal can be related to enhanced runoff from the land and/or organic preservation, or if the charcoal signifies changes in fire activity on land, the charcoal record was corrected for detrital influx. We adjust the charcoal particle abundances using the XRF elemental record, normalizing to the total terrigenous influx following Daniau et al (2013) and Hollaar et al (2021). The stratigraphic trends in the normalized microcharcoal for Eter/Ca, Si/Al, Ti/Al and Fe/Al remain the same (SI Fig.…”

Increased fire activity under high atmospheric oxygen concentrations is compatible with the presence of forests

“…Whether global forests could be totally eradicated by fire under high atmospheric oxygen concentrations also depends on factors such as resprouting and vegetative reproduction as well as seasonality. A study conducted by Hollaar et al 68 measuring smectite and kaolinite abundance from the Jurassic as proxies for seasonality and the hydrological cycle a strong correlation between lower humidity and greater fire activity. While these studies support our results, they leave open the question of whether moisture could sufficiently protect forest vegetation from fire even at very high concentrations of oxygen.…”