 |
| Titel |
Paleoclimatic changes at the Pliensbachian-Toarcian transition recorded by δD of n-alkanes and δ15Norg in a continental section of central Asia |
| VerfasserIn |
Romain Tramoy, Johann Schnyder, Thanh Thui Nguyen Tu, Johan Yans, Jacob Jeremy, Mathieu Sebilo, Sylvie Derenne, François Baudin |
| Konferenz |
EGU General Assembly 2015
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250111584
|
| Publikation (Nr.) |
 EGU/EGU2015-11718.pdf |
|
|
|
|
|
| Zusammenfassung |
| Major paleoenvironmental changes have been recognized at the Pliensbachian-Toarcian
transition. Cooler conditions are suggested during the Late Pliensbachian before a drastic
increase of temperatures in 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-sandstones most probably deposited in a
fluvial/lacustrine environment with nearby swampy areas. Rock-Eval pyrolysis
indicates a type-III organic matter, i.e.Âmainly derived from terrestrial plants. A
multi-isotope approach based on bulk organic nitrogen isotopes (δ15Norg) and hydrogen
isotopic composition (δD) of n-alkanes was developed to document paleoclimatic
changes in the area. In the literature, δ15Norg measured on modern or Quaternary
plants has been positively correlated with temperature and negatively correlated
with precipitations. According to these observations, δ15Norg measured on lignites
and clayey layers has been recently used to support humid/dry cycles around the
Paleocene-Eocene transition. In Taskomirsai, δ15Norg values ranged from 0.5o to 4.5o
with low values in lignite beds interpreted as humid periods and high values in
clayey layers interpreted as drier periods. The δD values of n-alkanes (C17 to C35)
ranged from -248oÂto -151o. Two groups of n-alkanes could be distinguished: an
aquatic group (C17 to C23) and a terrestrial group (C25 to C35). In the aquatic
group, low δD values in lignites (-219±17; n=10) suggest wetter and/or cooler
climate during their deposition, whereas high values in clayey layers (-179±13;
n=6) suggest a drier and/or warmer climate. Low δD 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 the Late
Pliensbachian. In contrast, drier/warmer conditions (high δD values) took over during
the Pliensbachian-Toarcian transition. Evapotranspiration was estimated from the
isotopic difference between δD values of the C23 and the C27 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 environment during most of the year
leading to D-enrichment of this pool. In conclusion, the δD and the δ15Norg are in
agreement, even though both proxies suggest different environment in some parts of the
section. This apparent discrepancy points to different integration spatial scale of those
two proxies: δD being under regional influence (precipitation regimes, air mass
temperatures) and δ15Norg being very sensible to local environmental parameters. |
|
|
|
|
|
|