IODP 302: Dating 'Zebra'; was the Lomonosov Ridge a central Arctic Ocean Island in the Oligocene?

Some fifteen years ago, the pioneering Arctic IODP Expedition 302 (ACEX) drilled, and partly recovered Cenozoic sedimentary successions at the Lomonosov Ridge (LR) close to the North Pole. Of the few intervals recovered, one was regarded to likely encompass the Paleogene-Neogene (P/N) transition. On board and follow up marine palynological (mainly dinoflagellate cyst) studies indicate that within this P/N section, a hiatus lasting ~ 25 Myr likely separates the top of the recovered Paleogene (dated ~44 Ma, mid Eocene) from the locally recovered base of the Neogene (likely dated ~18 Ma, mid Early Miocene).

 

The hiatus is represented by the boundary between local lithological subunits 1/6 and 1/5. Unit 1/5 is informally often referred to as the “Zebra unit”, owing to its characteristic (cross bedded) black/white colored alternations of silty clays. Palynological and elemental and organic geochemical studies of subunits 1/6 and 1/5 supported the inference of a major hiatus, as the proxies show a sharp change at the subunit boundary, although the reconstructed paleoenvironments of both subunits indicate marginal marine, restricted conditions. This aspect on its own already represents a challenge for geophysical models, which placed the LR at deeper waters at the P/N boundary. A key finding in the “Zebra unit” is a massive occurrence of representatives of a – back then - unknown dinoflagellate cyst genus, later formally described as Arcticacysta. Because of its morphology, akin to typical Neogene dinocyst taxa, it was postulated that the Zebra interval was early Neogene in age, confirming the existence of a major hiatus. However, successive Rhenium‐Osmium (Re‐Os) isochron ages and complementary Os‐isotope measurements from subunits 1/6 and 1/5 led to postulate that the P/N transition was in essence complete, albeit extremely condensed. This data hence challenged the presence of a major hiatus and depicted a very different geological evolution of the LR.

 

Here we introduce new findings from the lower to mid-Miocene sediments retrieved from the Pennell Basin during IODP Expedition 374 (Ross Sea, Antarctica) in 2018. These now constitute the second known record containing specimens of Arcticacysta. Importantly, these findings now confirm the initial age assignment of the Arctic “Zebra Unit” to the early Miocene and provide decisive evidence for a large hiatus characterizing the P/N transition on the central Lomonosov Ridge. An important corollary is that the central Lomonosov Ridge was likely subaerial or ultra-shallow marine by the end of Oligocene, leading to a totally new perspective of its Cenozoic history.