IODP 302 ACEX: Dating 'Zebra': was the Lomonosov Ridge an Arctic Island in the Oligocene?
Authors: Brinkhuis Henk , Sangiorgi Francesca , Wubben Evi , O'Regan Matt 
Published: March 2022

&lt;p&gt;Some fifteen years ago the pioneering Arctic IODP Expedition 302 (ACEX) partly recovered Cenozoic sedimentary successions at the Lomonosov Ridge (LR) close to the North Pole. Among the few more completely cored intervals one was regarded to likely encompass the Paleogene-Neogene (P/N) transition. In effect, on board and follow up marine palynological studies indicated that the P/N transition is here likely marked by a major hiatus spanning some 25 Myr (from ~44 Ma, mid Eocene, to ~18 Ma, early mid Miocene). According to these earlier studies, the hiatus occurs at the boundary between the local lithological subunits 1/6 and 1/5. The latter is informally often referred to as the &amp;#8220;&lt;em&gt;Zebra unit&amp;#8221;&lt;/em&gt;, owing to its characteristic (cross bedded) distinctly black and white colored alternations of silty clays. Subsequent detailed geochemical studies of subunits 1/6 and 1/5 indeed support the inference of a major hiatus, as many proxies show a sharp change at the subunit boundary. This while for both subunits generally marginal marine, restricted conditions are reconstructed. However, successive Rhenium&amp;#8208;Osmium (Re&amp;#8208;Os) isochron ages and complementary Os&amp;#8208;isotope measurements from subunits 1/6 and 1/5 led to propose that the P/N transition is essentially completely represented, albeit extremely condensed. This data hence challenges the presence of a major hiatus and thus would depict a very different geological evolution of the LR.&lt;/p&gt;&lt;p&gt;The key to biostratigraphically dating the Zebra unit is the massive occurrence of representatives of a &amp;#8211; back then - unknown organic walled dinoflagellate cyst genus, later formally described as &lt;em&gt;Arcticacysta.&lt;/em&gt; Yet, because of its distinct morphology, akin to typical Neogene dinocyst taxa, and because of e.g., patterns in reworked marine elements besides (abundant) terrestrial palynological signals, it was postulated that the Zebra unit should be early Neogene, rather than mid Paleogene in age. This, and other aspects, albeit based on poorly constrained bioevents, led to the inference of the major hiatus.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;This remained the case until now. Here we introduce new findings from 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 &lt;em&gt;Arcticacysta&lt;/em&gt;, allowing first-ever calibration of the Arctic record.&lt;/p&gt;&lt;p&gt;Crucially, these findings now - rather precisely - 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. The reported Rhenium&amp;#8208;Osmium (Re&amp;#8208;Os) isochron ages are attributed to reworking of older organic matter into the early Neogene. An important corollary is that the central Lomonosov Ridge was likely subaerial or ultra-shallow marine in the Oligocene, leading to a totally new perspective of its Cenozoic history.&lt;/p&gt;