Deciphering seasonal to decadal climate variability during the Oligocene: an integrated approach based on bivalve sclerochronology and palynology
Zusammenfassung der Projektergebnisse
The goal of this project was to identify how short-term climate dynamics changed during the Oligocene, the last time in Earth history during which a unipolar glaciation was followed by a nearly polar ice-free world. Since boundary conditions were similar to present day, the Oligocene can serve as a modern analogue for future climate change and help to constrain the mode and tempo of changes in seasonality and decadal variability. The main results are as follows. (1) We have demonstrated that bivalves of the Oligocene are excellently preserved and can be employed as powerful tools in ultra-high-resolution climate reconstructions in deeper time of Earth history. Studied species recorded the full seasonal amplitude over many consecutive years in their shells. Based on a multi-taxon approach (bivalves, sea cows, sharks) it was possible to estimate sea surface temperature with great reliability. Through the use of different bivalve species, we were able to estimate seasonal temperatures in different water depths. (2) Tank experiments demonstrated that different bivalve species have developed different strategies to cope with ocean acidification and warming. Na/Cashell of some species potentially functions as a proxy for ocean pH. (3) Physical and chemical sample pretreatment – which is often necessary when working with fossil shells – and contamination can significantly bias light stable isotope data. Although only a few of the tested substances and methods strongly affected the isotope signatures, even small changes can accumulate to levels that are inacceptable for high-resolution paleoclimatology. Pretreatment of fossil bivalve shells with an intended use for isotope analysis should be kept at an absolute minimum and contamination with adhesives etc. should be avoided. Specimens from museum collections may be of limited value for isotope studies, unless the history of sample treatment is known. (4) Surface temperatures in the Central European Seaway during the Lower Oligocene were, on average, ca. 1°C lower than previously assumed and seasonal extremes were more pronounced. Seasonality also fluctuated greatly on inter-annual time-scales. (5) During the Rupelian, average SST rose by 4°C, with warming being more pronounced during summer (5°C) than during winter (3°C). Subsequently, SST dropped by 4°C. (6) As revealed by a combined analysis of proxy data and numerical climate simulations, the observed Lower Oligocene warming trend at paleolatitude 45°N resulted from a pCO2 increase of merely 160 ppm. Accordingly, the climate sensitivity of Central Europe is much greater than the global average. The pCO2 increase during the Rupelian compares well with carbon dioxide levels predicted for the next centuries. (7) An NAO-like atmospheric circulation pattern already established near the Eocene/Oligocene boundary, i.e., 20 Ma earlier than previously assumed, and replaced the ENSO-dominated climate. This likely contributed to the pervasive climate deterioration near the Eocene–Oligocene boundary and possibly played a role in the late Paleogene faunal turnover. Future studies should conduct similar studies in other regions and other time intervals of Earth history during which major climate shifts occurred. A combined model-proxy data approach can be particularly powerful. It may also be worthwhile to further investigate how the trace and minor element content of the shells as well as heavy isotopes (e.g., Nd) can be put at work. Calibration studies with modern species are inevitably required for this purpose.
Projektbezogene Publikationen (Auswahl)
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2015. The bivalve Glycymeris planicostalis as a high-resolution paleoclimate archive for the Rupelian (early Oligocene) of Central Europe. Climate of the Past 11, 653-668
Walliser EO, Schöne BR, Tütken T, Zirkel J, Grimm KI and Pross J
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2016. Response of Central European SST to atmospheric pCO2 forcing during the Oligocene – A combined proxy data and numerical climate model approach. Palaeogeography, Palaeoclimatology, Palaeoecology 459, 552-569
Walliser EO, Lohmann G, Niezgodzki I, Tütken T and Schöne BR
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2017. Effects of sample pretreatment and contamination on bivalve shell and Carrara marble δ18O and δ13C signatures. Palaeogeography, Palaeoclimatology, Palaeoecology
chöne BR, Schmitt K and Maus M
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2017. Insights from sodium into the impacts of elevated pCO2 and temperature on bivalve shell formation. Journal of Marine Experimental Biology and Ecology 486, 148-154
Zhao L, Schöne BR, Mertz-Kraus R and Yang F
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Inter-annual climate variability in Europe during the Oligocene Icehouse. Palaeogeography, Palaeoclimatology, Palaeoecology
Volume 475, 1 June 2017, Pages 140-153
Walliser EO, Lohmann G, Niezgodzki I and Schöne BR
