Paleoclimatic changes at the Pliensbachian-Toarcian transition recorded byδD of n- alkanes and δ 15 N org in a continental section of central Asia

Abstract : Major paleoenvironmental changes have been recognized at the Pliensbachian-Toarcian transition. Cooler conditions are suggested during the Late Pliensbachian before a drastic increase of temperaturesin the Early Toarcian. Most studies were realized 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 (Kazakhstan, Central Asia) showing a succession of sedimentary cycles made of lignites, clayey layers and silty-sandstonesmost probably deposited in a fluvial/lacustrine environment with nearby swampy areas.Rock-Eval pyrolysis indicates a type-III organic matter, i.e. mainly derived from terrestrial plants. A multi-isotope approach based on bulk organic nitrogen isotopes (δ 15 N org) and hydrogen isotopic composition (δD)ofn-alkaneswas developed to document paleoclimatic changes in the area.In the literature, δ 15 N org measured on modern or Quaternary plants has been positively correlated with temperature and negatively correlated with precipitations. According to these observations, δ 15 N org measured on lignites and clayey layers has been recently used to support humid/dry cycles around the Paleocene-Eocene transition. In Taskomirsai, δ 15 N org values ranged from 0.5‰ to 4.5‰ with low values in lignite beds interpreted as humid periods and high values in clayey layers interpreted as drier periods. TheδD values ofn-alkanes (C 17 to C 35) ranged from-248‰ to-151‰.Two groups of n-alkanes could be distinguished: an aquatic group (C 17 to C 23) and a terrestrial group (C 25 to C 35).In the aquatic group, low δDvalues in lignitessuggest wetter and/or cooler climate during their deposition (-219±17; n=10), whereashigh values in clayey layers (-179±13; n=6) suggest a drier and/or warmer climate. LowδD values recorded in the aquatic pool prior to the Pliensbachian-Toarcian transition suggest a relatively cooler and wetter climate that could correspond to the global cooling recorded in theLate Pliensbachian. In contrast, drier/warmer conditions (highδD values) took overduring the Pliensbachian-Toarcian transition.Evapotranspiration was estimated from the isotopic difference betweenδD valuesof theC 23 and theC 27 n-alkanes. Maximal positive difference was recorded during the coolest/most humid interval (prior to the Pliensbachian-Toarcian), suggesting a contrasted seasonality with a warm/humid growing season. Difference close to zero was recorded in drier/warmer intervals pointing to preponderant evaporative conditions in the aquatic
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Submitted on : Monday, December 5, 2016 - 10:00:29 AM
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Romain Tramoy, Johann Schnyder, Thuy Nguyen Tu Thanh, Johan Yans, Jérémy Jacob, et al.. Paleoclimatic changes at the Pliensbachian-Toarcian transition recorded byδD of n- alkanes and δ 15 N org in a continental section of central Asia. European Geoscience Meeting 2015, Apr 2015, Vienne, Austria. ⟨hal-01401411⟩

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