Decoupling δ13C response to palaeoflora cycles and climatic variation in coal: A case study from the Late Permian Bowen Basin, Queensland, Australia

摘要

The Late Permian coal measures of the Bowen Basin, Australia express both environmental and climatic changes that occurred prior to the Permian Triassic (P-T) boundary. In order to decouple the influence of environmental factors (salinity, pH, base level and temperature) from depositional and climatic factors (atmospheric CO_2) in organic δ~(13)C, a high resolution study was performed on 24 coal seams (total 24.6m) in the Late Permian stratigraphy in the northern Bowen Basin. The Late Permian stratigraphy of the Bowen Basin records a transition from deltaic and lacustrine conditions within the Tinowan Formation and Black Alley Shale Formation, to fluvial deposition in the Kaloola and Bandanna Formations. Intermittent volcanism is recorded by tuff layers during periods of peat accumulation. Variations of coal lithotypes were recorded and formed the basis of sampling for petrography and isotope analysis. Coal samples were etched to expose cellular anatomy, and systematically identified to recognise palaeoflora assemblages. When observed within seam, δ~(13)C of the coal varied cyclically (~(13)C enriched-depleted-enriched) as a response to environmental changes expressed in palaeoflora communities. The total range of δ~(13)C was -26.6‰ to -21.9‰. The overall trend of δ~(13)C progresses to increasing ~(13)C enrichment, corresponding with dull lithotypes (rich in inertinite) which indicate fluctuations in base level. The ~(13)C enrichment peaks at -22.5‰ within the Kaloola Member and shifting rapidly toward a depletion (maximum -26.6‰) of ~(13)C in the upper Bandanna Formation, prior to the P-T boundary. These changes are expressed in palaeoflora communities where ecosystems shifted from dominant Glossopteris flora, to climax community flora including Palaeosmunda, Cycadales and Ginkgo, suited to temperate, early Mesozoic climates. The results of this study represent an insight into the effects of environmental variables on ~(13)C uptake of plants. The identification of flora within coal gives an insight into palaeowetland evolution, and can be partnered with classic petrographical techniques for integrated analysis in coals. Both the geochemistry and the anatomical aspects of coal represent an important tool for future palaeowetland research.