Austral and Subtropical Gyre Radiolaria – latest Jurassic to Early Cretaceous Leg 123, Site 765, Argo Abyssal Plain revisited: Southern Hemisphere paleobiogeography and global climate change

The aim of this report is to 1) to formally describe Austral and Subtropical Gyre Radiolaria recovered from ODP Hole 123-765C), 2) to compare them with published records of Southern Hemisphere “non-Tethyan” assemblages and 3) to discuss radiolarian paleobiogeography of the Southern Hemisphere and its implications for global climate change during the Jurassic–Cretaceous transition. The Tithonian to Aptian/early Albian radiolarian record recovered from Hole 765C, Cores -62R to -36R in the Argo Abyssal Plain (AAP) is unique in its density of well-preserved samples and in its faunal contents. Radiolaria recovered from claystones yielded the low diversity, ecologically tolerant “Crypto-Archaeo” Assemblage, (chiefly cryptocephalic and cryptothoracic nassellarians and Archeodictyomitra spp.) interpreted herein as originated in the Subtropical Gyre (STG). In contrast, assemblages extracted from radiolarite layers, interpreted as pelagic turbidites derived from the deeper Australian margin, are dominated by Austral taxa. Neotethyan taxa are very rare to absent before the late Hauterivian/Barremian, when they gradually gain in diversity and abundance. Described Austral and STG taxa include 10 families, of which Fusitanellidae n. fam. and Windaliinae n. subfam. are new. Of 18 genera 7 are new (Nodosphaera, Praewindalia, Pachycingula, Archaeotanella, Morchella, Fusitanella, Argofusus) and of 55 species 30 new ones are formally described and 14 new ones are left in open nomenclature. The southern hemisphere Late Jurassic to Early Cretaceous radiolarian biogeography is defined from low- to high latitude: 1. the Neotethyan (NT) and 2. the Central Panthalassan (CP) realms, 3. Eastern Boundary Current (EBC) realm, 4. the Subtropical Gyre (STG) and the Austral (A) circum south-polar realm. Radiolarian biogeography and plate tectonic models support a scenario of palaeoceanographic and global climatic change during the Jurassic–Cretaceous transition related to progressive Pangea break-up with the following consequences: 1. an increased heat transfer to the Southern hemisphere which caused cooling of Neotethyan regions during the Late Tithonian dry event. 2. A northward shift of the southern summer Intertropical Conve nience Zone reduced the Neotethyan monsoon area and allowed the establishment of a southern Neotethyan subtropical gyre documented by the “Crypo-Archaeo” Assemblage. 3. The south-polar West Wind Drift may have forced a circum Antarctic-Australian cold current through the epicontinental rift between India and Antarctica-Australia since the Berriasian (140 my), transporting Austral Radiolaria into the AAP where they accumulated in radiolarite layers.