Radiolarian response to the Jurassic-Cretaceous transition: Pangea breakup and the end of the Tethyan mega-monsoon

Unlike other system boundaries, the Jurassic-Cretaceous transition (J-K) is gradual, without a mass extinction event accompanied by major excursions of stable isotope curves. Significantly, continuous sedimentation of radiolarian-rich siliceous sediments occurred during the transition throughout the Neotethys (NT) and Panthalassa, which represented >80% of the J-K world ocean.

We studied radiolarian assemblages across the J-K in two distant areas: 1.) The Central Atlantic and Western Tethys and 2.) The Argo Abyssal Plain (AAP., ODP Site 765, off NW-Australia. J-K paleolatitude 45° S). Both areas share a similar geodynamic context of Pangaea breakup creating young ocean basins during the Middle-Late Jurassic. The studied sections reveal different radiolarian faunal evolutions but both respond to major palaeoceanographic and paleoclimatic changes across the J-K.

1.) In Western Tethys the change from radiolarite to pelagic cherty limestone is interpreted as the result of cooler, dryer climates during the Tithonian-Berriasian Dry Event.  Less continental weathering, hence, less nutrient input created more oligotrophic surface waters resulting in a major radiolarian turnover in the NT-realm.  UAZ13 (Baumgartner et al. 1995), straddling the J-K, is defined by 39 FO's at its base and 9 LO's at its top. Oligotrophic conditions also triggered the rise and explosion of calcareous nannoplankton and calpionellids. UAZ 13 correlates with the Crassicollaria Zone, the FO’s of hyaline calpionellids, and Nannoconus.

2.) In the AAP the first Tethyan radiolarians appear in Core 61 containing the J-K. Ranges of typical Tethyan species such as Podocapsa amphitreptera or Acaeniotyle umbilicata begin earlier in NT. Their later appearance in the AAP is the result of its opening towards NT. Tethyan taxa are rare before the Valanginian/Hauterivian and become common by the Aptian-Albian.

Several latest Jurassic-Early Cretaceous lineages of Austral Radiolaria (Nodosphaera, Praewindalia, Windalia, Archaeotanella, Morchella, Fusitanella, Argofusus, Baumgartner et al. 2023) are recorded in the AAP cores, but were also observed in S-Patagonia, Antarctic Peninsula, Wedell Sea (ODP Site 69), Kolbano Mega Sequence of Rote and Timor, originally deposited on the NW-Australian margin, and in Southern Tibet, interpreted as the NT distal continental margin of Greater India. The rapid evolution of Austral Radiolaria is the biotic response to the development during J-K of a an early circum-S-polar current system.

Radiolarian biogeography and plate tectonic models support a scenario of palaeoceanographic and global climatic change during the J-K related to progressive Pangea break-up with 3 consequences: 1). Increased heat transfer to the Southern hemisphere causing cooling of NT regions during the J-K Dry Event. 2.) Northward shift of the southern summer ITCZ reducing the NT monsoon area and establishment of a southern NT subtropical gyre. 3.) The south-polar West Wind Drift forced a circum-Antarctic-Australian cold current through the epicontinental rift between India and Antarctica since the J-K, transporting Austral Radiolaria into the AAP.

Baumgartner, P.O. et al. (Eds.). 1995. Middle Jurassic to Lower Cretaceous Radiolaria of Tethys. Mém. de Géol. Lausanne 23.

Baumgartner, P., Li, X., Matsuoka, A., Vérard, C. 2023. Austral and Subtropical Gyre Radiolaria – latest Jurassic to Early Cretaceous Leg 123, Site 765, Argo Abyssal Plain revisited. Micropaleontology. 69. 555-633. 10.47894/mpal.69.6.01.