Dating ‘Zebra’: A subaerial to ultra-shallow marine Lomonosov Ridge at the end of the Oligocene?

Twenty years after the pioneering IODP Expedition 302 (ACEX) to the Lomonosov Ridge, central Arctic Ocean, a robust age model was still lacking. This was foremost due to the overall poor core recovery, and frequent lack of age diagnostic features including (micro)fossils, notably in the likely Neogene succession. Interestingly, one of the few intervals that were relatively well-recovered was interpreted to span the Paleogene-Neogene transition. On board and follow up marine palynological studies indicated that within this interval, a potential ∼26 Myr long hiatus separated the local top of the Paleogene (∼44 Ma) from the Neogene (∼18 Ma), occurring between local lithological subunits 1/6 and 1/5. The latter was informally also referred to as the ‘Zebra’ interval, owing to its characteristic black/white alternations in the colour of the sediments. This hiatus would possibly imply a subaerial or ultra-shallow marine Lomonosov Ridge during late Oligocene to early Miocene times. A Neogene, (late) Early Miocene age for the base of ‘Zebra’ subunit 1/5 was tentatively provided by the finding of a new, later formally described, organic walled dinoflagellate cyst (dinocyst) genus we named Arcticacysta, which included two species, A. backmanii and A. moraniae. Subsequent work challenged the presence of a hiatus and the age of the ‘Zebra’ subunit. Here we present recent palynological (dinocyst) findings from Miocene sediments retrieved from the Pennell Basin during IODP Expedition 374 (Ross Sea, Antarctica) that contain the first appearance at 17.7 Ma of specimens of the same dinocyst genus, Arcticacysta, and species, and a more continuous presence between 17.0 and 15.9 Ma. Providing that the (first) occurrence of this dinocyst genus is synchronous in both (polar) records, these findings confirm the assignment of the Arctic Ocean ‘Zebra’ interval to the (late) Early Miocene, adding evidence for a large hiatus characterizing the Paleogene/Neogene transition on the Lomonosov Ridge.