Climate and vegetation of the Selandian-Thanetian Paleocene Sagwon Section, Northern Alaska (Palaeolatitude 85°N)

Spicer, R. A.; Daly, R. J.; Jolley, D. W.; Herman, A. B.; Ahlberg, A. and Moiseeva, M. (2009). Climate and vegetation of the Selandian-Thanetian Paleocene Sagwon Section, Northern Alaska (Palaeolatitude 85°N). In: Climatic and Biotic Evenets of the Paleogene, 12-15 Jan 2009, Wellington, New Zealand.


The Paleogene of the high paleo-Arctic provides valuable insights into the vegetation dynamics of one of the most climatically sensitive regions on Earth for past, present and future environmental change scenarios.
At Sagwon Bluffs, (69°23’N, 148°43’W) Northern Alaska (Fig. 1), combined sedimentological and palynological data (Fig. 2) has identified six depositional sequences within the Prince Creek Formation, Sagwon Member. The base of each sequence is marked by fluvial channel or crevasse-splay sediments, followed by emergent floodplain facies and subsequent deposition of peat mires. These mires gave way to lake sediments with the continued rise in water table and in turn replaced by floodplain water bodies, which in the oldest and youngest sequence contained brackish water algae.
Among the ninety-seven in situ palynomorph taxa recorded are the fungal spore Pesavis tagluensis, and the juglandaceous pollen Caryapollenites imparalis/inelegans indicating an age younger than Danian but older than Upper Paleocene. Taxa characteristic of the PETM are not recorded, suggesting that a Late Paleocene Selandian–Early Thanetian age (ca 61 Ma to 57 Ma; Gradstein et al. 2004) is appropriate. Paleogeographic reconstructions position the area at 85°N at that time (Smith et al. 1981; Ziegler et al. 1983).
We have used Correspondence Analysis (Hill 1979) to derive ecological groups representing successional “communities” in the floodplain sediments (e.g. Fig. 3). In some sections this includes a lacustrine group containing Azolla and other freshwater aquatics. These reflect different substrate stabilities and drainage regimes on the Sagwon floodplain. Similar analysis of the seven coal beds shows some originated as lacustrine accumulations of drifted wood and were later subjected to gleysol formation. Autocthonous coals showed a succession from polypodiaceous fern dominance through mid seam Betulaceae, Myricaceae and Fagaceae rich assemblages, to Taxodiaceae-Nyssaceae dominance. This succession is disrupted in some coals by the abundant charcoalified debris indicating that regular wildfires disturbed these mire communities.
Leaf, fruit and seed megafossil assemblages characterise more localised communities. These confirm a deciduous taxodiaceous conifer-dominated forest with a rare xeromorphic conifer component (Fokienopsis catenulata and cf. Mesocyparis) that we interpret as evergreens. This mixed coniferous forest supported a diversity of deciduous angiosperm understorey and riparian margin taxa (>25 morphotypes) derived in part from earlier Late Cretaceous, more southerly, first occurrences in N.E Russia (e.g. Fig. 4).
Well-developed tree rings (Fig. 5) are, on average, wider than those of the Maastrichtian of northern Alaska (Spicer and Parrish 1990a) (in some cases 5–10mm wide) with only small amounts of latewood (sensu Denne 1989; Mork 1928) indicative of benign summer growth conditions and a pronounced polar light regime. Overall the composition of the vegetation, coupled with the tree ring characteristics and the abundance and thickness of siliciclastic-free coals suggests high precipitation and a cool temperate thermal regime with mean annual temperatures (MATs) between 3–13°C (Wolfe 1979). However that MAT was likely to have been higher than the 5°C ± 2°C estimated for the Maastrichtian of the region (Spicer and Parrish 1990b) because the Sagwon flora is more diverse and the tree rings generally wider. The presence of some evergreen conifer components suggests winter temperatures were cold enough to depress metabolic rates sufficient to make retention of small xeromorphic leaves an energy-efficient competitive strategy for overwintering. There is no wood anatomical or sedimentological evidence for hard winter freezes. At such high paleolatitudes this is indicative of a weak polar high-pressure system compared to today.

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