Carbon isotopic composition of fossil leaves from the early Cretaceous sediments of Western India

Abstract

Stable carbon isotope analysis of fossil leaves from the Bhuj Formation, western India was carried out to infer the prevailing environmental conditions. Compression fossil leaves such as Pachypteris indica, Otozamite kachchhensis, Brachyphyllum royii and Dictyozamites sp. were recovered from three sedimentary successions of the Bhuj Formation, Early Cretaceous in age. A chronology was established based on faunal assemblage and palyno-stratigraphy and further constrained by carbon isotope stratigraphy. The three sampling sites were the Karawadi river bank near Dharesi; the Chawad river bank near Mathal; and the Pur river section near Trambau village in Gujarat. The Dharesi sample was also analyzed to investigate intra-leaf δ ¹³C variability. The mean δ ¹³C of the leaf was −24.6 ± 0.4%. which implied negligible systematic change along the leaf axis. The Mathal sample was fragmented in nature and showed considerable variation in carbon isotopic composition. The Trambau sample considered to be the oldest, dating to the middle of Aptian (ca. 116 Ma), shows the most depleted value in δ ¹³C among all of them. The overall δ ¹³C trend ranging from mid Aptian (ca. 116 Ma) to early Albian (ca. 110 Ma) shows a progressive increase in δ ¹³C from −26.8 to −20.5%.. Based on these measurements the carbon isotopic composition of atmospheric carbon dioxide of the Aptian-Albian period is estimated to be between −7.4 and −1.7%.. The ratio of the partial pressure of carbon dioxide in leaf to that of the ambient atmosphere calculated based on a model is estimated to be similar to that of the modern plants. This indicates that the Early-Cretaceous plants adapted to the prevailing high carbon dioxide regime by increasing their photosynthetic uptake.