The Plenus Cold Event (PCE) temporarily interrupted the supergreenhouse conditions of the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2); it is coeval with the occurrence of Boreal benthic macroinvertebrates and of the nektonic belemnite Praeactinocamax plenus (de Blainville, 1827) (after which the event is named) in Europe, and to the re-oxygenation of bottom waters in the Northern Hemisphere. However, its effects on the sea-surface circulation are unknown and evidence for changes in the biogeography of planktic organisms are limited to the equatorward migration of dinoflagellate cysts grouped in the Cyclonephelium compactum-membraniphorum morphological plexus. The present study provides new planktic foraminiferal quantitative data from two complete OAE 2 records in the Anglo-Paris (Eastbourne, south-east England) and Vocontian (Clot Chevalier, south-east France) basins that show a transition from an oligo-mesotrophic Tethyan assemblage (rotaliporids and whiteinellids) at the onset of OAE 2 (prior to the PCE) to a cold and meso- to eutrophic assemblage (praeglobotruncanids, dicarinellids and muricohedbergellids) during the PCE. The cold-water assemblage shows strong affinities with the coeval fauna of the Norwegian Sea and yields the Boreal endemic species Muricohedbergella kyphoma (Hasegawa, 1999) and Praeglobotruncana plenusiensis Falzoni and Petrizzo (2020). Planktic foraminifera are passively transported by currents, thus changes in the assemblages have been interpreted to reflect the transition from a dominant influence of warm, saline and thermally stratified waters carried by the proto-Gulf Stream prior to the PCE to cold and low-saline Boreal waters originating in the Norwegian Sea during the PCE. We suggest that such changes were forced by the equatorward shift of the proto-Arctic Front (i.e., the boundary between warm saline Tethyan-Atlantic and cold low-saline Boreal waters) from offshore Norway to southern England. In this southerly position, the proto-Arctic Front likely represented an oceanographic barrier that limited the influence of the proto-Gulf Stream in the Anglo-Paris Basin, and favoured the inflow of Boreal waters from the north to the European epicontinental basins leading to ~20° equatorward expansion of the Boreal marine bioprovince. The sea-surface cooling and equatorward expansion of Boreal planktic assemblages during the PCE are of the same order of magnitude of those reconstructed between some glacial and interglacial intervals of the Plio-Pleistocene. Despite obvious differences between Cretaceous and Plio-Pleistocene palaeogeography and climate dynamics, the present study reviews the extent of environmental changes occurring in this interval and provides evidence of a profound re-organisation of sea-surface circulation patterns during a cold snap of the Cretaceous supergreenhouse (Falzoni and Petrizzo 2022).
Evidence for changes in sea-surface circulation patterns and ~20° equatorward expansion of the Boreal bioprovince during a cold snap of Oceanic Anoxic Event 2 (Late Cretaceous)
Francesca Falzoni;
2022
Abstract
The Plenus Cold Event (PCE) temporarily interrupted the supergreenhouse conditions of the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2); it is coeval with the occurrence of Boreal benthic macroinvertebrates and of the nektonic belemnite Praeactinocamax plenus (de Blainville, 1827) (after which the event is named) in Europe, and to the re-oxygenation of bottom waters in the Northern Hemisphere. However, its effects on the sea-surface circulation are unknown and evidence for changes in the biogeography of planktic organisms are limited to the equatorward migration of dinoflagellate cysts grouped in the Cyclonephelium compactum-membraniphorum morphological plexus. The present study provides new planktic foraminiferal quantitative data from two complete OAE 2 records in the Anglo-Paris (Eastbourne, south-east England) and Vocontian (Clot Chevalier, south-east France) basins that show a transition from an oligo-mesotrophic Tethyan assemblage (rotaliporids and whiteinellids) at the onset of OAE 2 (prior to the PCE) to a cold and meso- to eutrophic assemblage (praeglobotruncanids, dicarinellids and muricohedbergellids) during the PCE. The cold-water assemblage shows strong affinities with the coeval fauna of the Norwegian Sea and yields the Boreal endemic species Muricohedbergella kyphoma (Hasegawa, 1999) and Praeglobotruncana plenusiensis Falzoni and Petrizzo (2020). Planktic foraminifera are passively transported by currents, thus changes in the assemblages have been interpreted to reflect the transition from a dominant influence of warm, saline and thermally stratified waters carried by the proto-Gulf Stream prior to the PCE to cold and low-saline Boreal waters originating in the Norwegian Sea during the PCE. We suggest that such changes were forced by the equatorward shift of the proto-Arctic Front (i.e., the boundary between warm saline Tethyan-Atlantic and cold low-saline Boreal waters) from offshore Norway to southern England. In this southerly position, the proto-Arctic Front likely represented an oceanographic barrier that limited the influence of the proto-Gulf Stream in the Anglo-Paris Basin, and favoured the inflow of Boreal waters from the north to the European epicontinental basins leading to ~20° equatorward expansion of the Boreal marine bioprovince. The sea-surface cooling and equatorward expansion of Boreal planktic assemblages during the PCE are of the same order of magnitude of those reconstructed between some glacial and interglacial intervals of the Plio-Pleistocene. Despite obvious differences between Cretaceous and Plio-Pleistocene palaeogeography and climate dynamics, the present study reviews the extent of environmental changes occurring in this interval and provides evidence of a profound re-organisation of sea-surface circulation patterns during a cold snap of the Cretaceous supergreenhouse (Falzoni and Petrizzo 2022).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
