An expanded Cretaceous−Tertiary transition in a pelagic setting of the Southern Alps (central-western Tethys)

The central part of the Piave River valley in the Venetian pre-Alps of NE Italy exposes an expanded and continuous marine sediment succession that encompasses the Paleocene series and the Paleocene to Eocene transition. The Paleocene through lower most Eocene succession is >100 m thick and was deposited at middle to lower bathyal depths in a hemipelagic, near-continental setting in the central western Tethys. In the Forada section, the Paleocene succession of limestone-marlm couplets is sharply interrupted by an ~3.30-m-thick unit of clays and marls (clay marlunit). The very base of this unit represents the biostratigraphic Paleocene-Eocene boundary, and the entire unit coincides with the main carbon isotopeexcursion of the Paleocene-Eocene thermal maximum event. Concentrations of hematite and biogenic carbonate, δ13C measurements, and abundance of radiolarians, all oscillate in a cyclical fashion and are interpreted to represent precession cycles. The main excursion interval spans fi ve complete cycles, that is, 105 ± 10 k.y. The overlying carbonisotope recovery interval, which is composed of six distinct limestone-marl couplets, is interpreted to represent six precessional cycles with a duration of 126 ± 12 k.y. The entire carbon isotope excursion interval in Forada has a total duration of ~231 ± 22 k.y., which is 5%–10% longer than previous estimates derived from open ocean sites (210–220 k.y.). Geochemical proxies for redox conditions indicate oxygenated conditions before, during, and after the carbon isotope excursion event. The Forada section exhibits a nonstepped sharp decrease in δ13C (−2.35‰) at the base of the clay marl unit. The hemipelagic, near-continental depositional setting of Forada and the sharply elevated sedimentation rates throughout the clay marl unit argue for continuous rather than interrupted deposition and show that the initial nonstepped carbon isotope shift was not caused by a hiatus. A single sample at the base of the unit lacks biogenic carbonate. Preservation of carbonate thereafter improves progressively up-section in the clay marl unit, which is consistent with a prodigiously abrupt and rapid acidifi cation of the oceans followed by a slower, successive deepening of the carbonate compensation depth. Increased sedimentation rates through the clay marl unit (approximately the main interval of the carbon isotope excursion) are consistent with an intensifi ed hydrological cycle driven by supergreenhouse conditions and enhanced weathering and transport of terrigenous material to this near-continental, hemipelagic environment in the central western Tethys. The sharp transition in lithology from the clay marl unit to the overlying limestonemarl couplets in the recovery interval and the coincident shift toward heavier δ13C values suggest that the silicate pump and continental weathering, the cause of the enhanced terrigenous fl ux to Forada, stopped abruptly. This implies that the source of the light CO2 ceased to be added to the ocean-atmosphere system at the top of the clay marl unit