Permo-Pennsylvanian palaeotemperatures from Fe-Oxide and phyllosilicate δ18O values

Abstract

The oxygen isotope composition of fossil roots that have been permineralized by hematite are presented from eight different stratigraphic levels spanning the Upper Pennsylvanian and Lower Permian strata of north-central Texas. Hematite δ 18O values range from − 0.4% to 3.7%. The most negative δ 18O values occur in the upper Pennsylvanian strata, and there is a progressive trend toward more positive δ 18O values upward through the lower Permian strata. This stratigraphic pattern is similar in magnitude and style to δ 18O values reported for penecontemporaneous authigenic palaeosol phyllosilicates and calcites, suggesting that all three minerals record similar paragenetic histories that are probably attributed to temporal palaeoenvironmental changes across the Late Pennsylvanian and Early Permian landscapes. Palaeotemperature estimates based on paired δ 18O values between penecontemporaneous hematite and phyllosilicate samples suggest these minerals co-precipitated at relatively low temperatures that are consistent with a supergene origin in a low-latitude soil-forming environment. Hematite–phyllosilicate δ 18O pairs indicate (1) relatively low soil temperatures (∼ 24 ± 3 °C) during deposition of the upper Pennsylvanian strata followed by (2) a considerable rise in soil temperatures (∼ 35–37 ± 3 °C) during deposition of the lowermost Permian strata. Significantly, δD and δ 18O values of contemporaneous phyllosilicates provide single mineral palaeotemperature estimates that are analytically indistinguishable from temperature estimates based on hematite–phyllosilicate oxygen isotope pairs. The results between the two temperature-proxy methods suggest that the inferred large temperature change across the Upper Pennsylvanian–Lower Permian boundary might be taken seriously. If real, such a significant climate change would have undoubtedly had far-reaching ecological effects within this region of Pangaea. Notably, there are important lithological and palaeobotanical changes, such as disappearance of coal and coal swamp floras, across the Upper Pennsylvanian–Early Permian boundary of north-central Texas that may be consistent with major climatic change toward warmer conditions.