Arctic plant diversity in the Early Eocene greenhouse

Abstract

For the majority of the Early Caenozoic, a remarkable expanse of humid, mesothermal to temperate forests spread across Northern Polar regions that now contain specialized plant and animal communities adapted to life in extreme environments. Little is known on the taxonomic diversity of Arctic floras during greenhouse periods of the Caenozoic. We show for the first time that plant richness in the globally warm Early Eocene (approx. 55-52 Myr) in the Canadian High Arctic (76° N) is comparable with that approximately 3500 km further south at mid-latitudes in the US western interior (44-47° N). Arctic Eocene pollen floras are most comparable in richness with today's forests in the southeastern United States, some 5000 km further south of the Arctic. Nearly half of the Eocene, Arctic plant taxa are endemic and the richness of pollen floras implies significant patchiness to the vegetation type and clear regional richness of angiosperms. The reduced latitudinal diversity gradient in Early Eocene North American plant species demonstrates that extreme photoperiod in the Arctic did not limit taxonomic diversity of plants.

Figure 1.

Location map of Stenkul Fiord, Nunavat, Canada indicating study sections on the southwest of the fiord.

Figure 2.

Stratigraphic measured sections from Stenkul Fiord, Nunavat, Canada indicating position of pollen samples. Important vertebrate and plant megafossil horizons are indicated within sections. Coal bed numbers are adapted from Reidiger & Bustin [23].

Figure 3.

Species accumulation curves for Early Eocene pollen assemblages from Stenkul Fiord, the US western interior and the Late Holocene from modern vegetation types of eastern North America. The mixed forest is spread over a wider geographical area than the deciduous forest (electronic supplementary material, figure S1). Therefore, the apparently lower richness of the deciduous forest is an area/geographical artefact only.

Figure 4.

(a) Bray–Curtis similarity of Early Eocene pollen floras from different North American and European sites. Numbers on the cluster nodes indicate bootstrap support through 1000 random replicates. (b) Location of North American Early Eocene sites used in panel (a). SF, Stenkul Fiord; MB, MacKenzie Basin; NS, North Slope; BHB, Bighorn Basin; PRB, Powder River Basin; WB, Williston Basin; VA, Virginia; USG, US Gulf Coast. European sites are off the map. (c) Distance decay of Early Eocene pollen flora composition with increasing sampling distance using the Bray–Curtis metric. Dashed line at 6000 km indicates the breakpoint.