Palynology and organic-carbon isotope ratios across a terrestrial Palaeocene/Eocene boundary section in the Williston Basin, North Dakota, USA

The Palaeocene–Eocene Thermal Maximum (PETM) at 55 Ma marks the Palaeocene/Eocene (P/E) boundary and represents a discrete period of abrupt, transient global warming. There are few vegetation records from within the PETM and such an absence of data prevents modelling of the vegetation response to climate warming. Outcrops exposing the Sentinel Butte member (upper Fort Union Formation) and the Golden Valley Formation (Bear Den and lower Camels Butte members) within the Williston Basin of western North Dakota, USA are known to span the P/E boundary. Pollen and spore floras at the Farmers Butte locality (Stark County, North Dakota; 46.92° N 102.11° W) record changes in abundance of some reed, fern and understorey plants across the Sentinel Butte–Bear Den contact but no other composition changes occur until the arrival of Eocene immigrants Platycarya spp. (walnut/pecan family) and Intratriporopollenites instructus (linden/sterculia/cotton tree families) at the top of the Bear Den member, c. 11 m above the change in co-occurrence and relative abundance patterns of range-through taxa. The exact stratigraphic level at which these Eocene marker taxa first occur is unclear owing to the heavily weathered nature of Bear Den strata below the Alamo Bluff lignite. This pattern of stratigraphic change may be correlative to the well documented “floral gap” of PETM records in Wyoming. Though bulk δ13Corg ratios decrease by 2.4‰ across the Alamo Bluff lignite, degradation of organic carbon within the upper Bear Den member partially masks full expression of the carbon isotope excursion associated with the PETM. Hence, strata around the Alamo Bluff lignite may represent a new terrestrial record of the PETM. In agreement with terrestrial PETM records from other U.S. western interior localities, palynological data indicate no floral extinction and little composition change across the Palaeocene/Eocene boundary.