in Jurassic and Cretaceous Stratigraphy

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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Evidences of Widespread Wildfires in Laurasia and Gondwana During the Middle Jurassic 376 L. Marynowski 1 , M. Zatoń 1 1. University of Silesia, Faculty of Earth Sciences, Będzińska 60 Street, PL-41-200 Sosnowiec, Poland (E-mail: leszek.marynowski@us.edu.pl) Wildfires play a key role in Recent and ancient terrestrial ecosystem modeling but their geological history after higher plants colonization is still not well-recorded. We used the charcoals (Fig.1) and pyrolytic polycyclic aromatic hydrocarbons (PAHs) co-occurrence as proxies of Middle Jurassic wildfires on both the Laurasia and Gondwana continents. The Laurasian evidence comes from the Lower Callovian terrigenous deposits of the eastern part of the Central European Basin. We have detected the charcoal frag- ments co-occurring with unsubstituted polycyclic aromatic hydrocarbons in the Lower Callovian, Papilė Formation of Lithuania, indicating that forest fires took place at this time. The sum of concentrations of the pyrolytic polycyclic aromatic hydrocarbons, including fluoranthene, pyrene, benzofluoranthenes, benzo[e] pyrene, benzo[ghi]perylene and coronene (Finkelstein et al., 2005) ranged from 4.66 and 6.57 μg/g TOC for charcoal-bearing clay and sand samples, respectively, while for the rest of the samples, the obtained values are below 0.5 μg/g TOC. Concentrations of PAHs are significantly lower than those from the Hettangian samples of Sołtyków and similar to those from the Podole and Lower Gromadzice localities in central Poland (Marynowski and Simoneit, 2009). Besides of pyrogenic PAHs, the charcoal-bearing horizons are characterized by the occurrence of oxygen-containing aromatic compounds. The most common are benzo[b] naphthofurans, dinaphthofurans, and benzobisben- zofurans. As far as we know, this is the first evidence of wildfires during Callovian, as based on multi-proxy documentation (Marynowski and Zatoń, 2010). The Gondwanan record of wildfires comes from the Neuquén province in Argentina, where the Upper Aalenian/Lower Bajocian to Bathonian terrigenous sequences are characterized by the occurrence of charcoal and PAHs, a distribution of which suggests their pyrolytic origin. This is the first multi-proxy evidence of Jurassic wildfires from the Gondwana continent and generally one of a very few records of wildfires in Gondwana during the Mesozoic. Fig.1 Scanning electron photomicrographs of: (a) relatively well preserved small charcoal fragment from the Lower Callovian clays of the Karpenai exposure, Lituania. (b) tangential section of excellently preserved charcoal from the Upper Aalenian-Lower Bajocian of the Rincón del Águila exposure, Argentina. Temperature interpretations, derived from iner- tinite reflectance data, show that in all cases the Middle Jurassic charcoals formed at low temperature surface fires that, in the case of Aalenian/Bajocian wildfires, sporadically reached the higher temperature crown fires. The occurrence of charcoals in the investigated deposits of the both continents confirms the recent results that the atmospheric oxygen level reached at least 15% during the Middle Jurassic times (Belcher and McElwain 2008; Berner, 2009). Acknowledgements: This work has been supported by the MNISW grant: N N307 2379 33 (for LM). Key words: Wildfires; Charcoal; PAHs; Middle Jurassic References: Belcher C.M., McElwain J.C. Limits for combustion in low O2 redefine paleoatmospheric predictions for
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 the Mesozoic. Science, 2008, 321: 1197-1200. Berner R.A. Phanerozoic atmospheric oxygen: New results using the GEOCARBSULF model. Ameri- can Journal of Science, 2009, 309: 603-606. Finklestein D.B., Pratt L.M., Curtin T.M., et al. Wildfires and seasonal aridity recorded in Late Cretaceous strata from south-eastern Arizona, USA. Sedimentology, 2005, 52: 587-599. Marynowski L., Simoneit B.R.T. Widespread Late Triassic to Early Jurassic wildfire records from Poland: Evidence from charcoal and pyrolytic polycyclic aromatic hydrocarbons. Palaios, 2009, 24: 785-798. Marynowski L., Zatoń M. Organic matter from the Callovian (Middle Jurassic) deposits of Lithuania: Compositions, sources and depositional environ- ments. Applied Geochemistry, 2010 (in press). 377
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