High-resolution chemostratigraphy of the late Aptian-early Albian oceanic anoxic event (OAE 1b) from the Poggio le Guaine section (Umbria-Marche Basin, central Italy)

An integrated study of multiple geochemical proxies (delta C-13(carb), delta C-13(org), TOC, HI, CaCO3, trace elements/Al ratios) of the late Aptian-early Albian oceanic anoxic event (OAE) 1b has been performed on the pelagic sedimentary sequence of Poggio le Guaine (Umbria-Marche Basin, central Italy). A comparison of the newly collected stable isotope carbon curve with the records from the Vocontian Basin (SE France), DSDP Site 545 and Hole 1049C provided a reliable and precise identification of the four main prominent black shale levels (113/Jacob, Kilian, Urbino/Paquier and Leenhardt) that characterize the OAE 1b. The studied record shows an increase in the marine organic carbon accumulation rate, in particular in the 113/Jacob and Urbino/Paquier levels. In the other black shales, TOC values are <1%, with evidence of degraded marine organic matter. Completely anoxic conditions were never established during the sediment deposition, although evidence of oxygen depletion at the bottom of the basin is clearly documented by the distribution pattern of redox-sensitive trace metals. The results suggest an increase in organic carbon burial rates during the OAE 1b due to the effect of enhanced surface productivity, as supported by a major increase in Ba/Al, and reduced bottom water ventilation. Notably, the Kilian and Urbino/Paquier levels from the PLG section are characterized by the absence of correlative shifts in delta C-13(carb) and delta C-13(org). The increase in the delta C-13(org) values in these levels is explained by an increase in the relative contribution of C-13 enriched marine planktonic archaeal biomass, while the concomitant negative excursions recorded in the delta C-13(carb), could reflect a major contribution of isotopically light terrestrial carbonate ions from increased continental runoff during documented more humid conditions. (C) 2015 Elsevier B.V. All rights reserved.