Palaeoenvironment and ecology of the middle Cretaceous Grebenka flora of northeastern Asia

The Grebenka flora, from the main exposure of the Albian-Cenomanian Krivorechenskaya Formation in northeastern Russia, represents a range of plant communities from pioneer to mature forest that grew close to the mid-Cretaceous North Pole (> 72degreesN). The diversity of this flora is dominated by angiosperms followed by conifers, ferns and other plant groups. The age is constrained by Ar-40/Ar-39 analyses of associated volcaniclastics (similar to 96.5 Ma), coupled with biostratigraphic correlation of the plant-bearing non-marine beds with marine units of the Krivorechenskaya Formation and the overlying Dugovskaya Formation. Limited palaeosol development and pronounced episodic floodplain aggradation indicate that the 100-m-thick plant-bearing volcaniclastic floodplain succession was deposited rapidly, resulting in excellent trapping and preservation of the plant communities, but dilution of the palynoflora. Analysis of the megaflora (> 100 foliage taxa, plus woods and fructifications) provides a 'snapshot' of the mid-Cretaceous climate, and offers reliable quantitative climatic signals of conditions near the mid-Cretaceous North Pole. Multivariate analysis of leaf physiognomy (Climate Leaf Analysis Multivariate Program) on the whole flora suggests that the plants experienced a mean annual temperature of 13.0 +/- 1.8degreesC and a cold month mean temperature of 5.5 +/- 3.3degreesC. However, analyses of individual florules yield slightly different results that help constrain the uncertainties inherent in such an approach. These and other foliar physiognomic data are compared across the Arctic.