87Sr/86Sr record from the Lower Cretaceous pelagic Maiolica limestone (Central Apennines, Italy) and its offset from the global seawater reference curve

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 isotoper 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.

Jaar van publicatie:2019

Toegankelijkheid:Open