Cretaceous (Albian–Aptian) Conifer Wood from Northern Hemisphere High Latitudes: Forest Composition and Palaeoclimate ⁎ M
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Review of Palaeobotany and Palynology 143 (2007) 167–196 www.elsevier.com/locate/revpalbo Cretaceous (Albian–Aptian) conifer wood from Northern Hemisphere high latitudes: Forest composition and palaeoclimate ⁎ M. Harland a, , J.E. Francis a, S.J. Brentnall b, D.J. Beerling b a School of Earth and Environment, University of Leeds, LS2 9JT, UK b Department of Animal and Plant Sciences, University of Sheffield, S10 2TN, UK Received 19 January 2006; received in revised form 8 July 2006; accepted 16 July 2006 Available online 19 October 2006 Abstract Permineralised conifer wood is abundant in Cretaceous (Albian–Aptian) sediments in high northern latitudes (N60°). The wood provides evidence of conifer-dominated forests that extended across the northern polar regions during greenhouse periods when the climate was warmer than today. This study investigates the composition of the Cretaceous (Albian–Aptian) high latitude Northern Hemisphere conifer forests using wood from Spitsbergen, and Ellesmere and Axel Heiberg islands in the Canadian Arctic Archipelago. Identification of the fossil woods indicates that the conifers included Pityoxylon, Piceoxylon, Laricioxylon, Protopi- ceoxylon, Palaoepiceoxylon, Taxodioxylon, Juniperoxylon, Protocedroxylon, Araucariopitys, Xenoxylon, Cupressinoxylon and Taxaceoxylon. This study shows that Spitsbergen was dominated by Taxodioxylon (25%) and in the Canadian Arctic Pityoxylon (33%) was dominant. Climate analysis of the conifers indicates that the northern Cretaceous (Albian–Aptian) forests of Svalbard grew in moist cool upland areas with warmer temperate areas in the lowlands, probably with rivers and/or swampy areas present. The forests of the Canadian Arctic probably grew under slightly cooler conditions than on Svalbard, similar to northern Canada today. © 2006 Published by Elsevier B.V. Keywords: Cretaceous; fossil wood; Spitsbergen; Canadian Arctic; conifer; palaeoclimate 1. Introduction nuous sunlight (Creber and Chaloner, 1984; Chaloner and Creber, 1990; Vakhrameev, 1991; Spicer et al., 2002; Fossil evidence shows that during greenhouse periods Skelton, 2003). There are no modern analogues for these the climate was warm enough in the high northern forests in the Northern Hemisphere today, as the present latitudes (N60°) to allow forests to flourish under con- tree line does not extend far beyond 70°N, primarily ditions of elevated CO2, even under the unusual light determined by wind, temperature, moisture and land regime of long dark winters and summers with conti- availability (Krebs, 1972; Wilmking and Juday, 2005). Little work has been undertaken on the Cretaceous ⁎ Northern Hemisphere forests but a few reports suggest Corresponding author. Now at CASP, University of Cambridge, that conifers were a dominant element of the Northern West Building, 181a Huntingdon Road, Cambridge, CB3 0DH, UK. Fax: +44 1223 276606. Hemisphere forest communities (Arnold, 1953; Bannan E-mail address: [email protected] (M. Harland). and Fry, 1957; Spicer and Parrish, 1986; Spicer, 2003). 0034-6667/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.revpalbo.2006.07.005 168 M. Harland et al. / Review of Palaeobotany and Palynology 143 (2007) 167–196 For example, the floras of Albian to Cenomanian age ceous floras of Spitsbergen contained the conifers Ar- from the Grebenka River region of northeastern Russia aucarites and Podozamites (Harland, 1997). and from the North Slope of Alaska appear to have been Polar forests potentially exerted a significant effect very similar. These floras were highly diverse, contain- on climate dynamics at both regional and global scales, ing early successional communities dominated by due to feedback processes such as albedo, the land Equisetites and Birisia ferns (Spicer and Herman, surface heat budget, and hydrological and carbon cycles 2001; Spicer et al., 2002) with mature stands rich in (Foley et al., 1994; DeConto et al., 2000; Beringer et al., conifers of Araucarites, Pagiophyllum, Pityophylum, 2005). Recent studies have shown that polar vegetation Podozamites, Cephalotaxopsis and Sequoia, particular- may have been critical for maintaining high latitude ly by the early Cenomanian in drier areas (Parrish and warmth, through feedbacks such as evapotranspiration Spicer, 1988; Spicer and Parrish, 1990; Spicer and and carbon cycling, without concomitant warming of Herman, 2001; Spicer et al., 2002; Spicer, 2003). Al- the lower latitudes (Foley et al., 1994; Upchurch et al., though less diverse, the forests of Axel Heiberg and 1998; DeConto et al., 2000). Therefore it is important to Amund Ringness islands in the Canadian Arctic appear understand the distribution and diversity of these forests to have been dominated by the conifers Cedroxylon and to allow them to be accurately included within vegeta- Piceoxylon (Bannan and Fry, 1957). The Early Creta- tion models for coupling to climate models. Fig. 1. Geological setting of the Canadian Arctic Archipelago specimens. a) Present day map of Canada showing the location of Ellesmere and Axel Heiberg islands and the Sverdrup Basin in light grey (after Basinger, 1991). b) Enlarged map showing the location of the specimen collection areas (⁎). RR = Roll-rock valley, E = Eureka and BL = Buchanan Lake. c) Sedimentary log of Aptian–Albian Christopher Formation (compiled from Patchett et al., 2004; Hall et al., 2005). M. Harland et al. / Review of Palaeobotany and Palynology 143 (2007) 167–196 169 This paper focuses on Northern Hemisphere Creta- The fossil wood from the Canadian Arctic is pre- ceous (Albian–Aptian) fossil wood of Ellesmere and Axel served in the Sverdrup Basin. During the mid-Aptian rift Heiberg islands on the Canadian Arctic Archipelago and activity began (Ellesmere and Axel Heiberg islands; Spitsbergen, the main island of Svalbard. The biodiversity Patchett et al., 2004; Fig. 1a and b) and basaltic vol- and climatic interpretations are discussed. The results canism associated with the rifting led to the deposition have provided information about the polar forests to of thick, coarse, non-marine deposits throughout the provide a proxy for the verification of a new vegetation Sverdrup Basin (Fig. 1a). This was followed in the mid- model called the University of Sheffield Conifer Model Aptian and Albian by a marine transgression that (USCM, Brentnall et al., 2005). produced dark laminated mudstones of the Christopher Formation (Patchett et al., 2004; Fig. 1c) in which the 2. Geological background fossil wood was found. Some plate tectonic reconstructions for the early FossilwoodinboththeCanadianArcticand Mesozoic suggest that Svalbard and Ellesmere Island Svalbard was preserved within black mudstones that were close to each other, with Svalbard forming the represent deep marine conditions. The wood represents eastern extension to the Sverdrup Basin (Worsley et al., trees that lived on nearby land but were then washed as 1986). During the Early Cretaceous in the south of driftwood into adjacent marine basins. Wood in both Spitsbergen there was a gradual transition from pro- regions is preserved by calcite mineralization. delta shales through delta front to fluvial dominated Fig. 2. Geological setting of the Svalbard specimens. a) Present day map of Svalbard, the grey area is Jurassic/Cretaceous sediments (modified from Worsley et al., 1986). b) Enlarged map showing the location of the specimen collection areas (⁎) around Lundstromdalen and Storknausen Peak. c) Generalised section log for Lundstromdalen (J. Francis, unpublished data). 170 M. Harland et al. / Review of Palaeobotany and Palynology 143 (2007) 167–196 sequences (Worsley et al., 1986). Overlying these specimens within one morphogenus displayed differences sediments, and partially laterally equivalent, are marine in structure these were separated by using a morphogenera sandstones, siltstones and shales that suggest a relatively number (e.g. Type F1,TypeF2). open marine shelf setting that would have provided the In order for comparisons to be made between the carbonate for the formation of the concretions in which specimens used here and previous descriptions and to the fossil wood was found (Worsley et al., 1986). The aid identification, it was necessary to calculate the per- permineralised wood was collected from the southern centage of certain features present within the wood parts of the main island of Spitsbergen, from the structure. For example, bordered pits within a single Lundstromdalen and Storknausen areas (Fig. 2a and b). specimen may be uniseriate, biseriate or multiseriate and The fossil wood represents the remains of trees that the percentage of each type can aid in identifying the drifted from the land surface into a marine basin and specimen to morphogenus level. Therefore all quantita- then preserved within dark laminated mudstones of the tive data are given in Table 1. Aptian–Albian Carolinefjellet Formation (Fig. 2c). Although the fossil woods are Cretaceous in age, and thus direct modern affinities are unknown, palaeocli- 3. Material and methods mate is inferred from the fossil wood based on the climate tolerances of the living types, bearing in mind In total 23 fossil wood specimens were examined that the fossils may have had different ecological toler- from the Northern Hemisphere sites. Ten of these spe- ance in the Cretaceous. cimens are from the Canadian Arctic Archipelago (Ellesmere and Axel Heiberg islands). Thirteen speci- 4. Fossil wood descriptions and identification mens were examined from Spitsbergen.