Stable C and O isotopic study of the Campanian chalk from the Mielnik section (eastern Poland): Signals from bulk rock, belemnites, benthic foraminifera, nannofossils and microcrystalline cements - Université Pierre et Marie Curie Accéder directement au contenu
Article Dans Une Revue Palaeogeography, Palaeoclimatology, Palaeoecology Année : 2017

Stable C and O isotopic study of the Campanian chalk from the Mielnik section (eastern Poland): Signals from bulk rock, belemnites, benthic foraminifera, nannofossils and microcrystalline cements

Résumé

Exceptionally well preserved chalk from the Mielnik section in eastern Poland has been analyzed for Sr, C and O isotope composition recorded in bulk rock samples, belemnite rostra, hand-picked benthic foraminifera and separated fine fractions (incl. calcareous nannoplankton and calcite cements). The stratigraphic δ13C and δ18O trends are consistent with the global seawater variations during the Campanian and confirm strong climatic perturbation accompanying the global sea-level fall during the uppermost Campanian. Chalk samples contain disseminated early-diagenetic microcrystalline calcite cement having δ13C and δ18O values only slightly different (by < 1‰) than those of separated nannoplankton. This cementation did not alter significantly the original bulk C and O isotopic composition of bulk chalk, which supports the applicability of isotopic analyses of chalk samples, even those containing carbonate cements, for paleoceanographic and chemostratigraphic investigations. Significant differences in δ13C and δ18O values between different constituents measured from the same levels are discernible. They are related with thermal gradients in the water column and with the habitat and vital effects of biogenic components. Benthic foraminifera reflect strong vital effects in δ13C and δ18O with a tendency to lower values than the other sample types. The minimum strength of the disequilibrium fractionation effect for δ18O during foraminiferal shell secretion is estimated up to 2‰. Belemnites show offsets in δ13C values, which are difficult to estimate, but their δ18O values appear to have recorded composition of seawater. The large difference in δ18O values between belemnites and nannoplankton or bulk material suggests that belemnites were nectobenthic organisms living close to the seafloor. The 87Sr/86Sr values and biostratigraphic position of the belemnites correspond well to those in the Lägerdorf (McArthur et al., 1993), the reference section for the European, Upper Cretaceous epicontinental marine deposits, but are not in agreement with the Campanian stratigraphy defined in the Geological Time Scale 2012 (Gradstein et al., 2012), where positions of some biostratigraphic divisions are not well calibrated against the absolute age.
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Dates et versions

hal-01437960 , version 1 (17-01-2017)

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Maciej J. Bojanowski, Zofia Dubicka, Fabrice Minoletti, Danuta Olzewska-Nejbert, Michal Surowski. Stable C and O isotopic study of the Campanian chalk from the Mielnik section (eastern Poland): Signals from bulk rock, belemnites, benthic foraminifera, nannofossils and microcrystalline cements. Palaeogeography, Palaeoclimatology, Palaeoecology, 2017, 465 (Part A), pp.193-211. ⟨10.1016/j.palaeo.2016.10.032⟩. ⟨hal-01437960⟩
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