Charcoal in the Late Jurassic (Kimmeridgian) of Western and Central Europe—palaeoclimatic and palaeoenvironmental significance

Abstract

Although fossil charcoal, as direct evidence of palaeo-wildfires, occurs in the fossil record at least since the Late Silurian, it is not equally distributed in sedimentary rocks from different ages. As the occurrence of wildfires is indeed not only controlled by climatic and environmental parameters, but also by the concentration of atmospheric oxygen, it has been argued by various authors that the fossil record of charcoal must also be influenced by (long-term) variations in atmospheric oxygen concentrations. Geochemical models have reconstructed low oxygen concentrations during almost the entire Jurassic, resulting, at least theoretically, in very low fire frequencies during this period. Here we describe new discoveries of fossil charcoal fragments from two Late Jurassic (Kimmeridgian) localities in Western (Boulonnais area in northern France) and Central Europe (Nusplingen Lithographic Limestone Fossillagerstatte in southwestern Germany). Combining our new data with currently available—but rather scarce—data on the occurrences of charcoal fragments during this particular interval of time demonstrates that all of these occurrences lie either within a Late Jurassic winter-wet climate belt, characterised by a marked seasonality, or within the assumedly drier part of a temperate climate belt, near the boundaries of the winter-wet climate belt. This is somewhat surprising as the preservation potential of charcoal is generally considered to be rather low under comparable climatic conditions, although charcoal production is usually high under seasonally dry climatic conditions. As almost all Kimmeridgian charcoals discovered to date come from marine deposits, it seems likely that taphonomic factors may have favoured the preservation of charcoal in such environments. Considering all data and interpretations, it seems possible that on a global scale fire frequencies were low during the Kimmeridgian due to relatively low atmospheric oxygen conditions during this period. Only in areas with a pronounced seasonality (i.e. under a winter-wet climate) could fires have occurred frequently enough to produce a certain amount of charcoal, and this charcoal has only been preserved under favourable conditions in marine sediments or in peat bogs with relatively high fire frequencies.