High-resolution chemostratigraphy of the late Aptian-early Albian oceanic anoxic event (0AE 1b) from the Poggio le Guaine section (Umbria–Marche Basin, central Italy)
Résumé
An integrated study of multiple geochemical proxies (δ13Ccarb, δ13Corg, 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 b1%, 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 fromthe PLG section are characterized by the absence of correlative
shifts in δ13Ccarb and δ13Corg. The increase in the δ13Corg values in these levels is explained by an increase in the
relative contribution of 13C enriched marine planktonic archaeal biomass, while the concomitant negative
excursions recorded in the δ13Ccarb could reflect a major contribution of isotopically light terrestrial carbonate
ions from increased continental runoff during documented more humid conditions.