Deciphering seasonal to decadal climate variability during the Oligocene: an integrated approach based on bivalve sclerochronology and palynology
Zusammenfassung der Projektergebnisse
The last time in Earth history characterized by a unipolar glaciation followed by a significant reduction of polar ice sheets was during the Oligocene. Hence, the Oligocene world may serve as a fossil analog for the climatic boundary conditions of the future, which will be initially characterized by a loss of Northern Hemisphere ice sheets. The resulting climate shifts will include changes in seasonality as well as in the frequency and intensity of decadal climate oscillations. However, the mode and tempo of these changes are as yet poorly constrained. This one-year pilot study focused on potential biogenic archives from which such data can likely be reconstructed: shells of long-lived marine bivalves (Glycymeris spp., Arctica spp.) and teeth of marine vertebrates (sea cows, sharks) from the Mainz and Kassel Basins, part of the Central European Epicontinental Seaway. Selected results are as follows. (1) Despite vital effects, Sr/Ca ratios of both modern Glycymeris glycymeris and Arctica islandica shells reflect distinct seasonal oscillations and – after mathematical elimination of ontogenetic age-related effects – are negatively and significantly correlated with ambient water temperature. Sr/Ca shell values of these two genera can provide reliable and independent temperature estimates for fossil environments. (2) The δ18Oshell profiles of well-preserved Glycymeris obovata from the Mainz Basin (Rupelian) revealed distinct seasonal oscillations, with amplitudes of more than ~4‰ in early ontogenetic stages. Shells from the Kassel Basin (Chattian) show much lower seasonal amplitudes (~2‰). In both cases, however, the most positive δ18Oshell values occur near major growth lines, confirming that they formed during the cold season of the year like in modern Glycymeris from the North Sea and the North Atlantic. (3) To compute water temperatures from δ18Oshell values, δ18Owater values were reconstructed from δ18O values of the phosphate phase of sea cow teeth (enamel) using the equation by Tütken (2003). For the Chattian of the Kassel Basin, an average δ18Owater value of -1.1±0.2‰ was obtained, which is well in agreement with the smaller ice volume at that time. Taking this δ18Owater into account, δ18Oshell-derived temperatures during the Chattian ranged from 8.7°C to 20.7°±0.9°C. We have double-checked this result by computing water temperatures from Sr/Ca-shell values. These data (9° to 18.5±1°C) were well in agreement with Tδ18O. Furthermore, water temperatures were computed from δ18O values of shark teeth (enameloid) from the same stratigraphic horizon using the paleotemperature equation by Pucéat et al. (2010). Likewise, the results were largely in the range obtained from the bivalves: 9.2° to 21.8±2.4°C. The large δ18Oshell amplitudes observed in the bivalves from Rupelian of Mainz Basin, however, suggest significant seasonal changes in freshwater influx and precipitation. The δ18O water values reconstructed from sea cow teeth were more negative than expected for unipolar glaciation, and highly variable as well (-1.50±1.51‰). By using these δ18O water values, temperatures reconstructed from δ18Oshell would range from 8.9° to 26.3±6.5°C. According to Sr/Ca shell data, however, the temperature range was close to that observed in the Kassel Basin (8.4° to 20.1±1°C). (4) Daily shell growth of modern Glycymeris glycymeris exhibits a statistically significant positive linear correlation with temperature proving another independent means of estimating water temperatures of the past. (5) Annual increment width time-series (corrected for age-related growth trends) from a specimen from the Mainz Basin revealed coherent spectral power at frequencies corresponding to periods of about 3 and 5 to 6 years, i.e. the modern NAO frequency band. With the results obtained by this pilot study, it is now possible to gain a detailed understanding of seasonal to inter-annual climate dynamics during the Oligocene epoch.
Projektbezogene Publikationen (Auswahl)
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2011. Sr/Ca and Mg/Ca ratios of ontogenetically old, long-lived bivalve shells (Arctica islandica) and their function as paleotemperature proxies. Palaeogeography, Palaeoclimatology, Palaeoecology 302, 31-42.
Schöne BR, Zhang Z, Radermacher P, Thébault J, Jacob D, Nunn EV and Maurer A-F
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Ivany LC, Brey T, Huber M, Buick DP and Schöne BR2011. El Niño in the Eocene greenhouse recorded by fossil bivalves and wood from Antarctica. Geophysical Research Letters 38, L16709
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2012. Crystal fabrics and element impurities (Sr/Ca, Mg/Ca, and Ba/Ca) in shells of Arctica islandica – implications for paleoclimate reconstructions. Palaeogeography, Palaeoclimatology, Palaeoecology
Schöne BR, Radermacher P, Zhang Z and Jacob DE
