Mid-Cretaceous oceanic red beds in the Umbria–Marche Basin, central Italy: Constraints on paleoceanography and paleoclimate

Abstract

Detailed studies of the mid-Cretaceous sedimentary strata in the Umbria–Marche Basin in Italy revealed that aside from several well-known organic-rich “black shale” horizons that record OAEs, several varicolored, mainly reddish horizons indicate oxic conditions at the ocean bottom. Eight such horizons have been identified in Aptian–Cenomanian sequences in the Umbria–Marche Basin. The dysoxic/oxic beds alternate regularly but not cyclically and seem to be the result of non-random processes. Their occurrence is not a completely random process. The duration of deposition of these oceanic red beds (ORBs) varies from ∼0.13 my (ORB4) recorded in the Ticinella primula zone, to ∼4.54 my for ORB1, which spans the Globigerinelloides ferreolensis zone to the Ticinella bejaouaensis zone in the Piobbico core. Mid-Cretaceous ORBs are not a local phenomenon because they occur in the Tethyan deposits in the Southern and Austrian Alps, the Carpathians, the Northern Caucasus, the central North Atlantic, in northeastern England and in the western Himalayas. They provide evidence for periodic changes in redox conditions at the ocean bottom. Such changes could have been caused by changes in bioproductivity, basin geometry, sedimentation rates, paleocirculation and/or production of bottom waters with higher content of dissolved oxygen in response to changes in paleoclimate. We suggest that the periodic inflow of colder, more oxygenated bottom waters was the probable cause of ORBs development, either as a result of changes in the ocean bottom topography, or as a result of brief cool climate periods. However, reliable proxies for changes in deep ocean circulation are still lacking. If ORBs were the result of the paleoclimate and, therefore, indirectly caused by changes in CO 2 in atmosphere, they document the increased sensitivity and instability of the mid-Cretaceous climate. Thus, changes in ocean dynamics were on a scale of several hundred thousand years to several million years, which has not been considered by most theories of CO 2 cycling, mid-Cretaceous greenhouse paleoclimate and paleocean dynamics.