Paleoclimatic changes recorded by δD of n-alkanes and δ15Norg in a continental section of central Asia (Early Jurassic)
Résumé
Major paleoenvironmental changes have been documented during the Early Jurassic (e.g. Morard et al., 2003; Suan et al., 2008). Most studies were carried out on European marine sediments, with little information on the environmental conditions that prevailed in terrestrial ecosystems. Here we present results on a continental section from Taskomirsai (South Kazakhstan) showing a succession of sedimentary cycles made of lignites, clayey layers and silty-sandstones most probably deposited in a fluvial/lacustrine environment with nearby swampy areas. Rock-Eval pyrolysis indicates an immature Type-III organic matter. A multi-isotope approach based on bulk organic nitrogen isotopes (δ 15 Norg) and hydrogen isotopic composition (δD) of n-alkanes was developed to document paleoclimatic changes in the area. To the best of our knowledge, it is the first time that these proxies are combined for a paleoclimatic approach in the Early Jurassic. In the literature, compound-specific δD is usually used to reconstruct paleohydrological conditions (Sachse et al., 2012). In the same way, δ 15 Norg measured on modern or Quaternary plants has been positively correlated with temperature and negatively correlated with precipitations (e.g. Austin and Vitousek, 1998; Liu and Wang, 2008). Then, these concepts were successfully used to evidence humid/dry cycles around the Paleocene-Eocene transition (Storme et al., 2012). In Taskomirsai, δ 15 Norg values ranged from 0.5‰ to 4.5‰. The lowest values are found in lignite beds and interpreted as humid periods, whereas the highest ones are recorded in clayey layers and suggest drier periods. The δD values of n-alkanes (C17 to C35) ranged from-248‰ to-151‰. Two groups of n-alkanes were distinguished based on their chain length and their δD values: an aquatic group (C17 to C23;-198‰ in average) and a terrestrial one (C25 to C35;-183‰ in average). In the aquatic group, low δD values in lignites (-219 17‰; n=10) suggest humid and/or cool climate during their formation, whereas high values in clayey layers (-179 13‰; n=6) suggest a drier and/or warmer climate. Based on main trends in the n-alkanes δD values, two " climatic units " are proposed named Unit 1 and Unit 2 (Figure 1). Decreasing δD values in Unit 1, recorded in the aquatic pool, suggest a cooling/humid climate trend (Figure 1). In contrast, drier/warmer conditions, inferred from high δD values, took over in Unit 2. δD values also suggest paleoclimatic variations at higher frequency than the two main trends described above. They seem to be linked with the sedimentary cycles. This finding rules out an autocyclic control on the sedimentation, which would have been generated by local sedimentary processes (e.g. moving channels of rivers). Thus, an allocyclic control driven by climate is most likely.
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