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| Titel |
The record of Tethyan planktonic foraminifera at the early Paleogene hyperthermal events and Middle Eocene Climatic Optimum in northeastern Italy: are they comparable? |
| VerfasserIn |
Valeria Luciani, Luca Giusberti, Claudia Agnini, Eliana Fornaciari, Domenico Rio |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250039053
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| Zusammenfassung |
| The early Paleogene is one of the more climatically and evolutionary dynamic periods in the
Earth history that records a pronounced warming trend peaking in the Early Eocene, and a
successive composite transition towards the modern icehouse world. Ever increasingly
scientific attention is dedicated to definitely comprehend timing, nature and characters of the
complex, non-linear evolution of the Paleogene climate.
Several complete and expanded Paleogene successions (Forada, Possagno, Alano, Farra),
with a sound magneto-biochronostratigraphic and stable isotope record crop out in the
Venetian Southern Alps (Northeast Italy). Recent studies (Giusberti et. al., 2007; Luciani et
al., 2007; Agnini et al., 2008) and unpublished data document the presence in these section of
the main short-lived warming events (hyperthermals) of the Eocene (Paleocene-Eocene
Thermal Maximum, PETM, ca 55 Ma, Eocene Layer of Mysterious Origin (ELMO, ca 53,6
Ma), X-event (ca 52.5 Ma), of the Early Eocene Climatic Optimum (EECO, ca 50-52 Ma)
and of the Middle Eocene Climatic Optimum (MECO, ca 40 Ma; Zachos et al., 2001.
2008). All these events are typified by marked negative shifts in δ13C curves that
correspond to carbonate decrease related to rise of the carbonate compensation
depth in turn induced by large introduction in the ocean-atmosphere system of
CO2.
Common features to the warming events are pronounced and complex changes in
planktonic foraminiferal assemblages, indicating strong environmental perturbations that
perfectly parallel the variations of the stable isotope curves in all the examined
events. These strict correspondences indicate close cause-effect relationships between
changes in environmental conditions and modifications of the assemblages. Our
analysis shows that the most striking variations are recorded by the PETM and MECO
assemblages that reflect highly perturbed environments. The ELMO, X-event and EECO
exhibit planktic foraminiferal responses that are similar to, though less intense than,
those observed across the PETM and the MECO. In addition, sedimentological and
quantitative micropaleontological data from the hyperthermal events from the Venetian
Southern Alps essentially suggest as the main response to the pronounced warmth,
increased weathering and runoff as well as sea surface eutrophication. A pronounced
shift from relatively oligotrophic to eutrophic, opportunist planktonic foraminiferal
assemblages was observed at the MECO as well, thus showing analogies with the
hyperthermal events recorded in the same area. The taxa indicating eutrophic environmental
conditions are however different at the MECO from the Alano section; on the other
hand we can expect that the planktonic foraminiferal taxa indicating analogous
scenarios might be different in different Eocene time-intervals. Remarkably, the
PETM and MECO events record a significant occurrence of giant and malformed
foraminifera, evidence of transient alteration in the ocean chemistry, including possible pH
oscillations and increase in trace metal content. Our data suggests therefore that a major
threshold in the photic zone ocean chemistry has been passed only for those prominent
events.
In conclusion, from the biotic response to the hyperthermal events, to the EECO and
MECO we deduce that the most important effect of pronounced warming, that is the aspect
common to all these events, has been the eutrophication of surface waters, as a consequence
of modification in the hydrological cycle. The location adjacent to land masses of the studied
Tethyan setting evidently facilitated the terrigenous input that was apparently the main
responsible for the increase in nutrient availability during the cited Paleogene warming
events.
Finally, several lines of evidence indicate that PETM, EECO and MECO were linked to
permanent changes in planktonic foraminiferal evolution beside the transient, ecologically
controlled variations. Even though the true mechanisms forcing evolution of life on Earth are
still unexplained, our record of the major climatic Paleogene events suggests a close
interaction between global climate and biological evolution.
REFERENCES
Agnini et al., 2008. Rend. Soc. Geol. It. 4, 5-12.
Giusberti et., 2007; Geol. Soc. Am. Bull. 119, 391–412.
Luciani et al., 2007. Mar. Micopaleont. 64, 189-214.
Zachos et al., 2001. Science 292, 686–693.
Zachos et al 2008 Nature 451, 279–283. |
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