Development of the Antarctic Circumpolar Current (ACC) during the Cenozoic is controversial in terms of timing and its role in major climate transitions. Some propose that the development of the ACC was instrumental in the continental scale glaciation of Antarctica and climate cooling at the Eocene/Oligocene boundary. Here we present climate model results that show that a coherent ACC was not possible during the Oligocene due to Australasian paleogeography, despite deep water connections through the Drake Passage and Tasman Gateway and the initiation of Antarctic glaciation. The simulations of ocean currents compare well to paleoenvironmental records relating to the physical oceanography of the Oligocene and provide a framework for understanding apparently contradictory dating of the initiation of the ACC. We conclude that the northward motion of the Australasian land masses and the reconfiguration of the Tasman Seaway and Drake Passage are necessary preconditions for the formation of a strong, coherent ACC.

Debate continues about the nature of the Cretaceous–Paleogene (K–Pg) mass extinction event. An abrupt crisis triggered by a bolide impact contrasts with ideas of a more gradual extinction involving flood volcanism or climatic changes. Evidence from high latitudes has also been used to suggest that the severity of the extinction decreased from low latitudes towards the poles. Here we present a record of the K–Pg extinction based on extensive assemblages of marine macrofossils (primarily new data from benthic molluscs) from a highly expanded Cretaceous–Paleogene succession: the López de Bertodano Formation of Seymour Island, Antarctica. We show that the extinction was rapid and severe in Antarctica, with no significant biotic decline during the latest Cretaceous, contrary to previous studies. These data are consistent with a catastrophic driver for the extinction, such as bolide impact, rather than a significant contribution from Deccan Traps volcanism during the late Maastrichtian.

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8The Late Cretaceous is generally considered to have been a time of greenhouse climate, 9 with no direct geological evidence for glaciation. We present, indirect evidence from the 10

The extensive Late Cretaceous – Early Paleogene sedimentary succession of Seymour Island, N.E. Antarctic Peninsula offers an unparalleled opportunity to examine the evolutionary origins of a modern polar marine fauna. Some 38 modern Southern Ocean molluscan genera (26 gastropods and 12 bivalves), representing approximately 18% of the total modern benthic molluscan fauna, can now be traced back through at least part of this sequence. As noted elsewhere in the world, the balance of the molluscan fauna changes sharply across the Cretaceous – Paleogene (K/Pg) boundary, with gastropods subsequently becoming more diverse than bivalves. A major reason for this is a significant radiation of the Neogastropoda, which today forms one of the most diverse clades in the sea. Buccinoidea is the dominant neogastropod superfamily in both the Paleocene Sobral Formation (SF) (56% of neogastropod genera) and Early - Middle Eocene La Meseta Formation (LMF) (47%), with the Conoidea (25%) being prominent for the first time in the latter. This radiation of Neogastropoda is linked to a significant pulse of global warming that reached at least 65°S, and terminates abruptly in the upper LMF in an extinction event that most likely heralds the onset of global cooling. It is also possible that the marked Early Paleogene expansion of neogastropods in Antarctica is in part due to a global increase in rates of origination following the K/Pg mass extinction event. The radiation of this and other clades at ∼65°S indicates that Antarctica was not necessarily an evolutionary refugium, or sink, in the Early – Middle Eocene. Evolutionary source – sink dynamics may have been significantly different between the Paleogene greenhouse and Neogene icehouse worlds.

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8One of the most expanded records to contain the final fortunes of ammonoid cephalopods is within 9 the López de Bertodano Formation of Seymour Island, James Ross Basin, Antarctica. Located at~65º 10 South now, and during the Cretaceous, this sequence is the highest southern latitude onshore outcrop 11 containing the Cretaceous-Paleogene (K-Pg) transition. We present comprehensive new 12 biostratigraphic range data for 14 ammonite and one nautiloid species based on the collection of >700 13 macrofossils from high-resolution sampling of parallel sedimentary sections, dated Maastrichtian to 14 earliest Danian in age, across southern Seymour Island. We find evidence for only a single, abrupt 15 pulse of cephalopod extinction at the end of the Cretaceous when the final seven ammonite species 16 disappeared, consistent with most evidence globally. In the lead up to the K-Pg extinction in the 17 James Ross Basin, starting during the Campanian, ammonite diversity decreased overall, but the 18 number of endemic taxa belonging to the family Kossmaticeratidae actually increased. This pattern 19 continued into the Maastrichtian and may be facies controlled, linked to changes in sea level and 20 seawater temperature. During the early Maastrichtian, ammonite diversity dropped significantly with 21 only two species recorded from the basal López de Bertodano Formation on Seymour Island. The 22 subsequent diversification of endemic taxa and reappearance of long-ranging, widespread species into 23 the basin resulted in an increase in ammonite diversity and abundance during the mid- Maastrichtian. 24 This was coincident with an apparent period of warming temperatures and sea level rise interpreted 25 from palynology and sedimentology, perhaps reflecting a high latitude expression of the Maastrichtian Event. Late Maastrichtian diversity levels remained stable despite reported climatic and 27 environmental variation. Ammonite diversity patterns during the Maastrichtian parallel those of 28 microfossil species such as nannofossil and planktonic foraminifera, suggesting that dynamic climatic 29 and environmental changes affected many planktonic and nektonic organisms during the latest 30Cretaceous. However, we suggest that these perturbations had a minimal effect on overall diversity 31 prior to the catastrophic extinction event at the K-Pg boundary. 32 33 3 Key words: Cretaceous; palaeoenvironments; ammonite; extinction; Antarctica; diversity. 34 35 1.Introduction 36The final demise of the ammonoid cephalopods at the end of the Cretaceous is a key 37 component of the ongoing debate about the nature of the Cretaceous -Paleogene (K-Pg) boundary 38 mass extinction event of 66 Ma (Gallagher, 1991;Ward et al., 1991;Marshall and Ward, 1996). 39Whether this event was caused by the devastating impact of an extra-terrestrial object (Alvarez et al., 40 1980; Schulte et al., 2010; Renne et al., 2013), or was drawn out through the final few million years of 41 the Cretaceous, with other factors such as climate and sea level cha...