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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 explanation may be the diagenetic alteration of fragile ammonite shells as primary strontium isotope composition may be affected by the contribution of isoto- pically heavier strontium transported by diagenetic fluids derived from weathered land areas. This scenario seems, however, to be controversial as studied ammonite shells were find to be well-preserved based on careful screening of aragonite contents and the presser- vation of the microstructure. Key words: Strontium isotope stratigraphy; Middle Jurassic; Belemnites; Ammonites References: Bartolini A., Baumgartner P.O., Hunziker J. Middle and Late Jurassic carbon stable-isotope stratigraphy and radiolarite sedimentation of the Umbria-Marche Basin (Central Italy). Eclogae Geologiae Helvetiae, 1996, 89: 811-844. Brigaud B., Durlet C., Deconinck J.F., et al. Facies and climate/environmental changes recorded on a carbonate ramp: A sedimentological and geochemical approach on Middle Jurassic carbonates (Paris Basin, France). Sedimentary Geology, 2009, 222: 181-206. Callomon J.H., Dietl G. On the proposed basal boundary stratotype (GSSP) of the Middle Jurassic Callovian Stage. Georesearch Forum, 2000, 6: 41-54. Cecca F., Martin Garin B., Marchand D., et al. Paleo- climatic control of biogeography and sedimentary events in Tethyan and peri-Tethyan areas during the Oxfordian (Late Jurassic). Palaeogeography, Palaeoclimatogy Palaeoecology, 2005, 222: 10-32. Dromart G., Garcia J-P., Gaumet F., et al. Perturbation of the carbon cycle at the Middle/Late Jurassic transition: Geological and geochemical evidence. American Journal of Science, 2003, 303: 667-707. Jenkyns H.C., Jones C.E., Gröcke D.R., et al. Chemo- stratigraphy of the Jurassic System: Applications, limitations and implications for palaeoceano- graphy. Journal of the Geological Society, London, 342 2002, 159: 351-378. Jones C.E., Jenkyns H.C. Seawater strontium isotopes, oceanic anoxic events, and seafloor hydrothermal activity in the Jurassic and Cretaceous. American Journal of Science, 2001, 301: 112-149. Jones C.E., Jenkyns H.C., Coe A.L., Hesselbo S.P. Strontium isotopic variations in Jurassic and Cretaceous seawater. Geochimica et Cosmo- chimica Acta, 1994, 58: 3061-3074. Morettini E., Santantonio M., Bartolini A., et al. Carbon isotope stratigraphy and carbonate production during the Early-Middle Jurassic: Examples from the Umbria-Marche-Sabina Apennines (central Italy). Palaeogeography Palaeoclimatology Palaeoecology, 2002, 184: 251–273. Norris M.S., Hallam A. Facies variations across the Middle–Upper Jurassic boundary in Western Europe and the relationship to sea-level changes. Palaeogeography, Palaeoclimatology, Palaeo- ecology, 1995, 116: 189-245. Page K.N., Meléndez G., Hart M.B., et al. Integrated stratigraphical study of the candidate Oxfordian Global Stratotype Section and Point (GSSP) at Redcliff Point, Weymouth, Dorset, UK. Volumina Jurassica, 2009, 7: 101-111. Podlaha O.G., Chalimourda A., Albeverio S. Nonlinear system analysis of a 87 Sr/ 86 Sr time series for Phanerozoic seawater. Chemical Geology, 1999, 161: 241-252. Wierzbowski H., Joachimski M. Reconstruction of late Bajocian-Bathonian marine palaeoenvironments using carbon and oxygen isotope ratios of cal- careous fossils from the Polish Jura Chain (central Poland). Palaeogeography Palaeo- climatology, Palaeoecology, 2007, 254: 523-540. Wierzbowski H., Dembicz K., Praszkier T. Oxygen and carbon isotope composition of Callovian–Lower Oxfordian (Middle–Upper Jurassic) belemnite rostra from central Poland: A record of a Late Callovian global sea-level rise? Palaeogeography, Palaeoclimatology, Palaeoecology, 2009, 283: 182-194.
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Stepwise Atmospheric Carbon Isotope Excursion During the Early Jurassic Oceanic Anoxic Event Stephen P. Hesselbo 1 , Grzegorz Pieñkowski 2 1. Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK (E-mail: Stephen.hesselbo@earth.ox.ac.uk ) 2. Polish Geological Institute – National Research Institute, Rakowiecka 4, PL-00-975 Warszawa, Poland (E-mail: grzegorz.pienkowski@pgi.gov.pl ) During the Mesozoic (250-64 Myr ago) geo- logically short intervals of about 0.5 Myr were subject to particularly severe environmental changes, including high sea-surface temperature and very low oxygen content of marine water. These so-called Oceanic Anoxic Events, or OAEs, occurred simultaneously with profound disturbance to the carbon cycle. A giant Early Toarcian (~182 Myr ago) negative carbon-isotope excursion, possibly the largest such anomaly in whole Phanerozoic, has been described from marine materials, and high-resolution datasets in marine sections have shown that the shift to light carbon-isotope values occur as a series of stratigraphically abrupt steps towards lighter isotopic values. (Hermoso et al., 2009; Jenkyns et al., 2001; Kemp et al., 2005). However, what is still poorly known about this event is its manifestation in non-marine and marginal marine environments. Al- though the prominent negative carbon-isotope anomaly, has been described from terrestrial organic matter in fully marine deposits (Hesselbo et al., 2000; Hesselbo et al., 2007), the same excursion towards light isotopic values has until now been documented from only a single marginal marine site, on the island of Bornholm, Denmark (Hesselbo et al., 2000; Hesselbo et al., 2007; McElwain et al., 2005) and, furthermore, the stepwise character of the excursion has not hitherto be identified from non-marine materials. Fig.1 Parkoszowice core, showing Lower Toarcian Ciechocinek Formation (mudstones, heteroliths and subordinate sandstones) and major sedimentary and geochemical events Here we present new carbon-isotope data from terrestrial organic matter (phytoclast separates), col- lected from 5 fully cored boreholes (Fig. 1) and one outcrop through a Toarcian coastal and marginal marine sedimentary succession (Ciechocinek Forma- tion) in the Polish Basin, a setting where hinterland climate and sea-level change are particularly well recorded. A comprehensive sequence stratigraphy and inferred relative sea-level history has previously been developed for the Early Jurassic succession in the Polish Basin, based on sedimentological and palaeon- tological study of boreholes and outcrops (Pieñkowski, 2004). Early Toarcian strata are assigned to a single unconformity-bounded depositional sequence, which has been objectively subdivided into five para- sequences (Pieñkowski, 2004). The principal new dataset constitutes >400 carbon-isotope analyses of phytoclasts separated manually from palynological preparations. The organic content consists mainly of phytoclasts (wood fragments, cuticle) and spores and is thus made up of material that originates almost entirely in the terrestrial environment. All profiles analyzed show the same well-developed excursion towards exceptionally light isotopic values through the Early Toarcian. The results show that the shift to light carbon-isotope values in atmospheric carbon dioxide also occurred in a series of major steps that match those identified from marine materials, where they have previously been identified with 100 kyr eccentricity forcing of climate change. The new data are particularly 343
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