Fossil charcoal from the Upper Triassic Karamay Formation in the Junggar Basin, NW China, and its geological implications

Abstract

Fossil charcoal can provide important evidence for understanding the ancient forest fires and predicting changes in vegetation, paleoclimate and paleoenvironment throughout geological history. In this research, a systematic study was conducted on fossil charcoal from the Upper Triassic Karamay Formation in the Junggar Basin, NW China. The charred wood is secondary xylem, pycnoxylic and homoxyleous, composed of tracheids and parenchymatous rays. Based on their anatomical features, the charcoal fossils are identified as Protophyllocladoxylon sp. The presence of well-preserved macroscopic charcoal fossils, along with carbonized leaves, suggests that these fossils were formed as a result of local surface fires. Based on the analysis of the mean random reflectance (Romean) values of charcoal from the Karamay Formation, the estimated temperature range for the production of the charcoal is between 407.49 °C and 369.17 °C. This temperature range aligns with the typical temperatures associated with surface fires, indicating that the fires in this region during the Late Triassic were of low intensity and occurred under relatively low temperatures. The presence of fossil charcoal in Junggar Basin suggests that the paleoatmospheric oxygen level in northwest China during Late Triassic was above 13% or 15%, which is the minimum required for sustained burning of natural wildfires. This finding is in accordance with the paleoatmospheric oxygen concentration models on the basis of variations in inertinite content and Phanerozoic wildfire history combined with the phosphorus cycle. The abundant presence of fossil charcoal herein suggests a humid and warm climate with intermittent droughts in the Late Triassic Junggar Basin.