| Summary: | Detailed studies of the mid-Cretaceous sedimentary strata in the Umbria–Marche Basin in Italy revealed that aside from severalwell-known organic-rich bblack shaleQ horizons that record OAEs, several varicolored, mainly reddish horizons indicate oxicconditions at the ocean bottom. Eight such horizons have been identified in Aptian–Cenomanian sequences in the Umbria–MarcheBasin. The dysoxic/oxic beds alternate regularly but not cyclically and seem to be the result of non-random processes. Theiroccurrence 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 theTicinella 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 AustrianAlps, 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 bychanges in bioproductivity, basin geometry, sedimentation rates, paleocirculation and/or production of bottom waters with highercontent of dissolved oxygen in response to changes in paleoclimate. We suggest that the periodic inflow of colder, moreoxygenated bottom waters was the probable cause of ORBs development, either as a result of changes in the ocean bottomtopography, or as a result of brief cool climate periods. However, reliable proxies for changes in deep ocean circulation are stilllacking. If ORBs were the result of the paleoclimate and, therefore, indirectly caused by changes in CO2 in atmosphere, theydocument the increased sensitivity and instability of the mid-Cretaceous climate. Thus, changes in ocean dynamics were on a scaleof several hundred thousand years to several million years, which has not been considered by most theories of CO2 cycling, ...
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