11:45 AM - 12:00 PM
[BCG05-05] Pangaea breakup and global change of the Jurassic-Cretaceous transition – evidence from southern hemisphere Radiolaria
★Invited Papers
Keywords:Radiolaria. (Siliceous plankton, protists) , Jurassic-Cretaceous, Global climate Change, Pangaea breakup, plate tectonics, palaeo-oceanography
Polycystine Radiolaria are marine protists that bear a skeleton made of biogenic silica, preserved in bio-siliceous sediments from early Cambrian to the Recent. Ocean dwelling Radiolaria were one of the dominant plankton groups from Late Palaeozoic to Early Cretaceous, deposited as radiolarian chert or radiolarite on remnants of ancient ocean floor.
Taxon-quantitative studies in the modern oceans show that most taxa live in the photic zone, as they host algal symbionts. These studies also show characteristic faunal spectra of oceanic water masses. Hence, taxon-quantitative studies of well-preserved fossil radiolarian assemblages are key to resolving ancient ocean configurations and gateways, witnessing the plate tectonic evolution of the Earth.
More than 30 years ago we discovered a spectacular sequence of radiolarite layers (Argo Abyssal Plain, AAP, ODP Hole 123-765C, Baumgartner 1992) that revealed a rapid evolution of so far unknown radiolarian taxa, spanning the latest Jurassic-Early Cretaceous. Identical taxa have been recovered form onshore W-Australia, the Antarctic Peninsula, S-Patagonia, the Weddell Sea and New Zealand. These taxa also occur in far northwards travelled terranes, such as Timor and Southern Tibet. We have since described these taxa as Austral Radiolaria from well-preserved assemblages extracted from the AAP Lower Cretacsous radiolarite layers (Babumgartner et al. submitted) In the AAP Neotethyan (low palaeolatitude) taxa are very rare to absent before the late Hauterivian/Barremian, when they gradually gain in diversity and abundance. Many of the new Austral taxa originated during the Late Jurassic and rapidly evolved during the Early Cretaceous in a proto-circum- S-polar current system (Figure1). Their principal characteristics include thick-shelled, thickly ornamented, often spinose tests of spindle-shaped multicyrtid nassellarians.
In contrast, the bulk of the Lower Cretaceous cores of the AAP is claystone yielding the low diversity, ecologically tolerant “Crypto-Archaeo” Assemblage, (chiefly cryptocephalic and cryptothoracic nassellarians and Archeodictyomitra spp. Murchey 1984) interpreted by us as originated in the Subtropical Gyre (STG). The described Austral and STG taxa include 10 families, of which one family and one subfamily are new. Of 18 genera 7 are new and of 55 species 30 new ones were formally described and 14 new ones were left in open nomenclature.
The sudden, latest Jurassic appearance of Austral radiolarian taxa in the AAP calls for the opening of a deeply rifted oceanic gateway between future Greater India and East-Antarctica-Western Australia (Figure 1, Gordon 1979, Baumgartner 1992, 1993, Munasri and Sashida 2018).
Radiolarian biogeography and plate tectonic models support a scenario of palaeoceanographic and global climatic change during the Jurassic-Cretaceous transition, related to progressive Pangaea break-up. During the Late Jurassic an oceanic seaway opened between E- Africa and Madagascar-Greater India. Simultaneously, the Proto-Caribbean became connected with southern high latitudes via the S-American Quebrada Grande and Rocas Verdes back-arc basins. Finally, the epicontinental rift between Greater India and E-Antarctica-Australia connected the south polar regions with the AAP (Figure 1)
Some of the global consequences include: 1. An increased heat and moisture transfer to the Southern hemisphere caused cooling in Neotethyan regions during the Late Tithonian dry event. 2. A northward shift of the northern winter Intertropical Convenience Zone reduced the Neotethyan Mega-Monsoon area and allowed the establishment of a S-Neotethyan Subtropical Gyre, documented by the “Crypo-Archaeo” Assemblage. 3. A south-polar West Wind Drift may have forced the circum-S-polar cold current through the epicontinental rift between India and Antarctica-Australia since the Berriasian (140 my), transporting Austral Radiolaria into the AAP.
Taxon-quantitative studies in the modern oceans show that most taxa live in the photic zone, as they host algal symbionts. These studies also show characteristic faunal spectra of oceanic water masses. Hence, taxon-quantitative studies of well-preserved fossil radiolarian assemblages are key to resolving ancient ocean configurations and gateways, witnessing the plate tectonic evolution of the Earth.
More than 30 years ago we discovered a spectacular sequence of radiolarite layers (Argo Abyssal Plain, AAP, ODP Hole 123-765C, Baumgartner 1992) that revealed a rapid evolution of so far unknown radiolarian taxa, spanning the latest Jurassic-Early Cretaceous. Identical taxa have been recovered form onshore W-Australia, the Antarctic Peninsula, S-Patagonia, the Weddell Sea and New Zealand. These taxa also occur in far northwards travelled terranes, such as Timor and Southern Tibet. We have since described these taxa as Austral Radiolaria from well-preserved assemblages extracted from the AAP Lower Cretacsous radiolarite layers (Babumgartner et al. submitted) In the AAP Neotethyan (low palaeolatitude) taxa are very rare to absent before the late Hauterivian/Barremian, when they gradually gain in diversity and abundance. Many of the new Austral taxa originated during the Late Jurassic and rapidly evolved during the Early Cretaceous in a proto-circum- S-polar current system (Figure1). Their principal characteristics include thick-shelled, thickly ornamented, often spinose tests of spindle-shaped multicyrtid nassellarians.
In contrast, the bulk of the Lower Cretaceous cores of the AAP is claystone yielding the low diversity, ecologically tolerant “Crypto-Archaeo” Assemblage, (chiefly cryptocephalic and cryptothoracic nassellarians and Archeodictyomitra spp. Murchey 1984) interpreted by us as originated in the Subtropical Gyre (STG). The described Austral and STG taxa include 10 families, of which one family and one subfamily are new. Of 18 genera 7 are new and of 55 species 30 new ones were formally described and 14 new ones were left in open nomenclature.
The sudden, latest Jurassic appearance of Austral radiolarian taxa in the AAP calls for the opening of a deeply rifted oceanic gateway between future Greater India and East-Antarctica-Western Australia (Figure 1, Gordon 1979, Baumgartner 1992, 1993, Munasri and Sashida 2018).
Radiolarian biogeography and plate tectonic models support a scenario of palaeoceanographic and global climatic change during the Jurassic-Cretaceous transition, related to progressive Pangaea break-up. During the Late Jurassic an oceanic seaway opened between E- Africa and Madagascar-Greater India. Simultaneously, the Proto-Caribbean became connected with southern high latitudes via the S-American Quebrada Grande and Rocas Verdes back-arc basins. Finally, the epicontinental rift between Greater India and E-Antarctica-Australia connected the south polar regions with the AAP (Figure 1)
Some of the global consequences include: 1. An increased heat and moisture transfer to the Southern hemisphere caused cooling in Neotethyan regions during the Late Tithonian dry event. 2. A northward shift of the northern winter Intertropical Convenience Zone reduced the Neotethyan Mega-Monsoon area and allowed the establishment of a S-Neotethyan Subtropical Gyre, documented by the “Crypo-Archaeo” Assemblage. 3. A south-polar West Wind Drift may have forced the circum-S-polar cold current through the epicontinental rift between India and Antarctica-Australia since the Berriasian (140 my), transporting Austral Radiolaria into the AAP.