Fossil charcoal from the Middle Jurassic of the Ordos Basin, China and its paleoatmospheric implications

The Yan'an Formation of the Ordos Basin is a sequence of four members, consisting of siliciclastic sediments deposited in alluvial, lacustrine and mire settings during the Middle Jurassic. Samples collected from Members Two and Four contain abundant blackened plant material identified through standard analytical techniques as fusain (fossil charcoal). The occurrence of fusain in fluvial sandstones at multiple horizons in the outcrops, combined with the previously reported high concentration of inertinite in the coals of Member One, indicates that paleowildfire was a common occurrence in the Ordos Basin during Yan'an deposition. Sedimentary evidence from Yan'an outcrops suggests that the paleoclimate was seasonal during deposition of Members Two through Four, which may have contributed to the wildfire frequency. The presence of fusain in the Yan'an Formation indicates that atmospheric oxygen levels were clearly above the minimum required for sustained combustion during the Middle Jurassic. This conclusion contradicts previous geochemical models for paleoatmospheric composition, but supports more recent studies.