87Sr/86Sr record from the Lower Cretaceous pelagic Maiolica limestone (Central Apennines, Italy) and its offset from the global seawater reference curve
J. Belza, W. Alvarez, F. Vanhaecke, P. Claeys, "87Sr/86Sr record from the Lower Cretaceous pelagic Maiolica limestone (Central Apennines, Italy) and its offset from the global seawater reference curve", 250 Million Years of Earth History in Central Italy: Celebrating 25 Years of the Geological Observatory of Coldigioco, Christian Koeberl, David M. Bice
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This study provides a composite record of 87Sr/86Sr, δ18O, and δ13C for three sections in the Tethyan Lower Cretaceous Maiolica formation, a pelagic limestone from the Umbria-Marche Apennines of Italy, carefully tied to a magnetostratigraphically and biostratigraphically calibrated time scale. Although the 87Sr/86Sr record accurately follows the trend of the global marine 87Sr/86Sr reference curve, individual Sr isotope ratio values are relatively high for their inferred stratigraphic position, with all 87Sr/86Sr ratios yielding a fairly uniform +0.00007 to +0.0001 discrepancy. This offset likely results from incorporation of excess 87Sr through isotopic reequilibration with interstitial pore waters during progressive lithification of the calcareous ooze. Although the process occurs principally through dissolution-reprecipitation, buffering the contemporaneous seawater Sr isotopic signature, diffusive communication with the overlying water column and porous sediments will compete with the dissolution-precipitation process, homogenizing pore-fluid concentrations and isotope ratios throughout the sediment column. Because the secular trend in 87Sr/86Sr throughout the Maiolica time frame is one of constant increase before rebounding to lower 87Sr/86Sr ratios in the Barremian, the ratios of the Maiolica carbonates are systematically displaced from that of the seawater in which they were deposited toward more radiogenic (higher 87Sr/86Sr) values. In addition, the carbon and oxygen isotope record of the Maiolica limestone allows identification of the mid-Valanginian Weissert event, characterized by a positive excursion in the δ13C and the δ18O records. Furthermore, the Weissert event correlates with a positive spike (+0.0001) in 87Sr/86Sr. Both the Sr and O isotopic peak signals predate the maximum peak in the δ13C excursion. This is likely a diagenetic artifact and may support the hypothesis of diffusive communication during lithification of the calcareous ooze.
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250 Million Years of Earth History in Central Italy: Celebrating 25 Years of the Geological Observatory of Coldigioco
Central Italy has been a cradle of geology for centuries. For more than 100 years, studies at the Umbria and Marche Apennines have led to new ideas and a better understanding of the past, such as the Cretaceous–Paleogene (K-Pg) boundary event, or the events across the Eocene–Oligocene transition from a greenhouse to an icehouse world. The Umbria-Marche Apennines are entirely made of marine sedimentary rocks, representing a continuous record of the geotectonic evolution of an epeiric sea from the Early Triassic to the Pleistocene. The book includes reviews and original research works accomplished with the support of the Geological Observatory of Coldigioco, an independent research and educational center, which was founded in an abandoned medieval hamlet near Apiro in 1992.