Carbon-isotope, petrological and floral record in coals: implication for Bajocian (Middle Jurassic) climate change

Much of our existing knowledge of Middle Jurassic paleoclimate is based on well-dated marine isotopic records that show fluctuations between warm (greenhouse effected) and cool climates. In contrast, much less is known from contemporaneous terrestrial deposits that are often difficult to correlate stratigraphically with marine successions, and are typically considered as showing regional rather than global signals. In this paper we show that the carbon-isotopic, petrological and floral record through 20 m thick coals in the Northern Qaidam Basin (NW, China) represents a relatively comprehensive northern hemisphere Bajocian (Middle Jurassic) record for terrestrial climatic change. δ13Ccoal values for 88 samples from the Yuqia area in Northern Qaidam Basin range from–25.3‰ to –22.5‰, indicating that C3 plants were the main coal-forming vegetation in the region during the Middle Jurassic. This isotopic variation follows fluctuations in the composition of ferns and gymnosperms, where higher ratios correspond to more negative δ13C values. The similar carbon isotope values of gymnosperms and coal from the Yuqia area indicate that the coal comprises a high proportion of organic material derived from gymnosperm taxa, which is consistent with the very high abundance of diterpenoids in coal and especially pimarane and abietane that are produced primarily by gymnosperms. Results from the coal matrix provide an opportunity to record paleoclimate changes, showing several striking and regular coal-forming cycles with distinct long- and short-term variations. Trends of δ13Ccoal values changes coupled with the evolution of coal-forming plants may record a gradual increase in paleo-CO2 (pCO2) concentration, water-table level changes and the decrease in abundance of Classopollis conifer pollen through Bajocian. This result is in accordance with the published marine carbonate records for this time, with correlation enabled through matching carbon isotope curves from the terrestrial succession at Yuqia and marine records. The similarity of the terrestrial and marine geochemical and floral record is an indication that the observed paleoclimate signal is a global phenomenon. Furthermore, the high-frequency fluctuation of δ13Ccoal values, along with the coal petrologic variations may record short-term changes of environmental factors (e.g. temperature or humidity/precipitation), especially during intervals when palaeobotanical composition has not fluctuated intensely.