The Pliensbachian-Toarcian paleoclimate transition: New insights from organic geochemistry and C, H, N isotopes in a continental section from Central Asia

Abstract : The Taskomirsay section (South Kazakhstan) is a unique Pliensbachian-Toarcian sequence of lignites, clayey layers and silty-sandstones deposited in a fluvial/lacustrine environment with nearby swampy areas. This period, characterized by a drastic climate change, has been particularly studied in Western Tethyan marine environments, whereas very few studies focused on continental settings. Paleoflora analyses, associated with a multi-isotope approach, based on well-preserved Type-III bulk organic nitrogen isotopes (à15Norg) and hydrogen isotopic composition (à2H) of n-alkanes, were developed to document paleoclimatic changes in the area. Sporomorph associations and fossil woods revealed a globally warm- to cool-temperate climate – characterized by Xenoxylon, a conifer morphogenus biogeographically related to cool/humid settings – apart from slightly less humid and warmer conditions in the early Toarcian. Warmer conditions are supported by reconstructed Mean Annual Air Temperatures (MAATs), based on the first branched glycerol dialkyl glycerol tetraethers (brGDGTs) ever recorded in the Early Jurassic. Nevertheless, no drastic changes were recorded in the à15Norg values; its signal being attributed to tenuous equilibrium between water- and nutrient-availability via intense N-recycling. Based on n-alkane distributions, sources of organic matter were separated in two pools: (i) a purely terrestrial (n-C27) and (ii) an “aquatic” pool (n-C23) constituted of vegetation that thrived under almost permanent water supply. The n-alkane à 2H values (␣248 to ␣151‰) as well as their amounts and average chain lengths (ACL) are in agreement with cool-temperate conditions in the Pliensbachian and less humid/warmer conditions in the early Toarcian. The isotopic difference between à2H values of n-C27 and n-C23 (␣2Hter-aq) suggests enhanced seasonality during the Pliensbachian-Toarcian transition and low seasonality in the early Toarcian, in agreement with temperate climate-regime. Finally, contrasted response to paleoclimate changes between markers suggests different spatial integration of those proxies. The role of sea-level variations for à2H values might also resolve this contrasted response.
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Romain Tramoy, Johann Schnyder, T.T. Nguyen Tu, J. Yans, Jérémy Jacob, et al.. The Pliensbachian-Toarcian paleoclimate transition: New insights from organic geochemistry and C, H, N isotopes in a continental section from Central Asia. Palaeogeography, Palaeoclimatology, Palaeoecology, Elsevier, 2016, 461, pp.310 - 327. ⟨10.1016/j.palaeo.2016.08.020⟩. ⟨hal-01382763⟩

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