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| Titel |
Benthic foraminiferal and isotopic patterns during the Early Eocene Climatic Optimum (Aktulagay section, Kazakhstan) |
| VerfasserIn |
Arne Deprez, Steven Tesseur, Peter Stassen, Simon D'Haenens, Etienne Steurbaut, Christopher King, Philippe Claeys, Robert P. Speijer |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250109711
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| Publikation (Nr.) |
 EGU/EGU2015-9647.pdf |
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| Zusammenfassung |
| The early Eocene is characterized by long-term global warming culminating in the Early
Eocene Climatic Optimum (EECO). During this time interval, the Peri-Tethys was connected
to the Arctic and Atlantic Oceans by north-south and east-west trending seaways. The
Aktulagay section in Kazakhstan provides an expanded record of the middle Ypresian
(NP11-13, ~54-50ÂMa; King et al., 2013), including the EECO. The marl sequence features
a series of sapropel beds, observed throughout the Peri-Tethys, indicative of basin-wide
episodic hypoxic events. In order to unravel paleoenvironmental changes, we carried out
quantitative faunal studies and stable isotopic (C, O) investigations on excellently preserved
foraminiferal assemblages.
The period from 54 to 52.5ÂMa (NP11 to lower NP12; Alashen Formation) is
characterized by a diverse assemblage of deep outer neritic (~200-250Âm) benthic
foraminifera, with common Pulsiphonina prima and Paralabamina lunata. The initially
(54ÂMa) well-ventilated oligo- to mesotrophic seafloor conditions gradually changed to
more eutrophic and oxygen-limited. These conditions were more permanent in the
sapropel-bearing unit at 52.5-52ÂMa (middle NP12; Aktulagay B1 unit). This observation is
based on the dominance of Anomalinoides acutus and Bulimina aksuatica and the lower
diversity. Also the upward migration of endobenthic species, as suggested by rising
δ13Cendobenthic, supports this interpretation. These low-oxygen conditions might have been
caused by a transgression, flooding lowlands. Benthic foraminiferal assemblages dominated
by Epistominella minuta at ~52-50ÂMa (top NP12-NP13; Aktulagay B2 unit) suggest an
oligotrophic environment, with transient pulses of phytodetritus. Dinoflagellate blooms and
Acarinina isotope values at ~50.5ÂMa indicate lower salinity (lower δ18O) and
higher productivity (higher δ13C), possibly due to riverine input. Large river plumes,
episodically reaching the area, in a monsoonal climate context, might explain this
basin development. Akhmetiev and Beniamovskii (2009) also found a transition
from a paratropical to a monsoonal (seasonal) climate, based on simultaneous land
vegetation changes. This change coincides with the peak of the EECO, as indicated by
its position close to the base of NP13 and rising δ13Cepibenthic values from 52 to
50ÂMa.
Although it is tempting to link the observed patterns to climate change, we cannot exclude
that changing paleogeography and variable connections between the Peri-Tethys and the
Tethys, Atlantic and the Arctic Oceans largely determined the long-term period
of dysoxia and anoxia during deposition of the sapropel beds at the Peri-Tethyan
seafloor.
Akhmetiev, M.A., Beniamovskii, V.N., 2009. Geologica Acta 7, 297-309.
King, C. et al., 2013. Stratigraphy 10, 171-209. |
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