The Environmental Water of the Middle Eocene Arctic: Evidence from Δd, Δ18o and Δ13c Within Speciﬁc Compounds

Palaeogeography, Palaeoclimatology, Palaeoecology 271 (2009) 96–103 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo The environmental water of the middle Eocene Arctic: Evidence from δD, δ18O and δ13C within specific compounds A. Hope Jahren a,⁎, Monica C. Byrne b, Heather V. Graham c, Leonel S.L. Sternberg d, Roger E. Summons b a Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 6 96822 USA b Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 USA c Department of Geosciences, Pennsylvania State University, University Park, PA 16802 USA d Department of Biology, University of Miami, Coral Gables, FL 33124 USA article info abstract Article history: The extensive vertical exposure (N150 m) of terrestrial sediments on Axel Heiberg Island, which contain Received 21 February 2008 thick fossiliferous lignites, presents an exceptional opportunity to follow the establishment and re- Received in revised form 12 September 2008 establishment of Arctic Metasequoia forests during the middle Eocene. We compared δD values in n-alkanes Accepted 16 September 2008 of chain length 23, 25, 27 and 29 with δ18O values in phenylglucosazone (P-G) derived from α-cellulose; we also analyzed %-abundance of ferns, gymnosperms and angiosperms using pollen and spores isolated from each lignite. Our results showed that forest composition was altered upon uplift, as gymnosperms became more abundant within the relatively well-drained upland sediments. This was also reflected in the small (1‰), but significant, increase in the δ13C value of TOM from lowland to upland environments. However, neither the δD values of n-alkanes nor the δ18O in P-G were statistically different in the upland sediments, as compared to the lowland sediments; from this we inferred that the oxygen isotope signature of environmental water available to the forests for plant growth was relatively uniform throughout the time of the fossil forests. The δD value of environmental water implied by both n-alkanes and P-G ranged from −168 to −131% and was considerably enriched compared to all environmental water samples available from the modern Arctic region (b−180%). In addition to indicating a warmer Eocene Arctic, subject to meteoric transport patterns different from today's, these results argue against the presence of an Eocene polar ice cap. © 2008 Elsevier B.V. All rights reserved. 1. Introduction throughout the section. We interpret the results we see in terms of the contrasting ecosystems of meanderplain (lowland) forests, compared Evidence from plant fossils and paleogeographic studies indicates with alluvial fan (upland) forests, and the larger hydrological patterns that a lush deciduous conifer forest existed well above the Arctic Circle of the Arctic middle Eocene. during the middle Eocene (~45 Ma; Fig. 1; Basinger, 1986; Ricketts and McIntyre, 1986). The Buchanan Lake Formation, which crops out 2. Field site extensively on Axel Heiberg Island, contains more than 30 lignite layers comprised of exceptionally preserved fossil wood, forest litter The sediment of the Buchanan Lake Formation has been placed and amber, many of which are meters in thickness and are within the middle Eocene, based on the recognition of brontothere horizontally continuous for kilometers (Jahren, 2007). With few fossils associated with the upper strata (Eberle and Storer, 1999). exceptions, prior analyses of Axel Heiberg sediments come from the Unfortunately, a refined estimate of the time represented by the 150+ m area around the “K–L–M” lignites, a ~5 m thick section within the of vertical terrestrial deposits is extremely difficult, because of the large ~150 m sequence of Buchanan Lake sediments (e.g., Williams et al., uncertainties associated with terrestrial sedimentation rates. The 2003). Here we report the results of novel compound-specific analyses sequence represents sediment accumulation within a narrow inter- across the full sedimentary record of the Arctic middle Eocene, as montane basin (Fig. 2). The principal controls on sedimentation were recovered from Axel Heiberg Island. We present the first detailed and differential uplift and elevation of source terrane along tectonic strike complete stratigraphic section described for the Buchanan Lake leading to aggradation on an alluvial plain (Ricketts, 1991); because the Formation and include the first data detailing the isotopic composition basin is small and tectonically-controlled, very high rates of sedimenta- of specific compounds with successive establishment of fossil forests tion may apply (Blatt et al., 1980). For these reasons, most researchers have concluded that the sediments of the Buchanan Lake Formation represent geologically rapid cycles of forest establishment, coarser ⁎ Corresponding author. sediment deposition, and forest re-establishment. Tarnocai et al. (1991) E-mail address: [email protected] (A.H. Jahren). examined silica content within paleosols near the top of the section, and 0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2008.09.016 A.H. Jahren et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 271 (2009) 96–103 97 Fig. 1. Lambert azimuth map projections showing the location of the fossil forest site at present (A; 79°55′N) and during the time when the trees were alive (A′; ~45 Ma; 78.6±1.6°N). Drawn using www.odsn.de/odsn. concluded a soil development time of 103 to 104 yr, assuming that the Formation. Given the above, we estimate that the 150+ vertical meters of silica resulted from a concentration of plant phytolith material. Based on sediment on Axel Heiberg Island constitute the remains of at least thirty the supposition of 50 paleosols, Ricketts (1991) ventured an estimate of distinct episodes of Arctic forestation that were established and re- 0.5 million years as the total accumulation rate of the Buchanan Lake established on thousand-to-ten-thousand year cycles. Fig. 2. Stratigraphic column of sedimentary units within the Buchanan Lake Formation of Axel Heiberg Island, Nunavut, Canada. The strata are middle Eocene in age (Eberle and Storer, 1999) and include a rotating sequence of claystones, shales, mudstones, sandstones and lignites, ranging from a few cm to N10 m in thickness. The sequence shows two distinct sedimentary regimes associated with the Ancestral Princess Margaret Range: the lower portion of the column (below A′; b60 m) resulted from terrestrial deposition within a lowland meanderplain, the higher (above layer A′; 60 m and above) portions of the column correspond to relatively upland deposition within an alluvial fan (after Ricketts, 1991). 98 A.H. Jahren et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 271 (2009) 96–103 The depositional environment interpreted from sediment facies lipids, we found that n-alkanes derived from acetate precursors were has historically been separated into two regimes (Ricketts, 1991): the the optimal hydrogen isotopic carrier substances for comparisons coals (0–60 m) and the “prime” layers (60+ m; always labeled with a across the entire sample set. N-alkanes are also the compound class “prime” symbol). The “prime” layers (A′ to K′) are dominated by least susceptible to hydrogen isotopic exchange over geological sandstones, often exhibiting crossbedding, with thick lignites inter- timescales (Sessions et al., 2004). spersed. Because of the large grain size associated with the deposits, The appearance of the lignites sampled ranged from dry, loose, the “prime” portions of the section represent forests established light brown ashy material (e.g., H layer) to dark brown, moist, planar within the alluvial fan, or relatively upland, sedimentary environment. fragments (e.g., F′ layer). Samples were pulverized in a shatterbox for The coals, rich with fining-upward mudstones and claystones 1–2 min; ~12 g of each powdered sample was then packed into cells of interbedded with lignites, represent forests established within the a Dionex Accelerated Solvent Extractor 200. Because the samples were meanderplain, or relatively lowland, sedimentary environment organic-rich, each sample was extracted three times with a mixture of (Fig. 2). Because these changes in facies are continuous over several dichloromethane and methanol (1:1) at 150 °C and 1500 psi. The exposed kilometers, we believe the cause of change from lowland to resulting total lipid extract was separated using column chromato- upland regime was controlled by tectonic uplift of the basin, in graphy on silica gel by elution with hexane (saturated hydrocarbons), agreement with previous structural characterizations of the area hexane-dichloromethane (1:1, aromatic hydrocarbons) and dichlor- (Ricketts, 1994). omethane-methanol (1:1, polar compounds). Following further The extensive vertical exposure and exquisite preservation of the separation by an additional column chromatography step using silica lignitic material within the Buchanan Lake Formation presents us with gel impregnated with 5% AgNO3, fractions enriched in n-alkanes an excellent opportunity to look at trends in isotopic signatures within (hexane eluate) were quantified using gas chromatography with flame a successively re-establishing forests, changing from a floodplain ionization detection (GC-FID). Each ‘alkane’ fraction was concentrated (lowland) environment to an upland environment, as we move from in an appropriate volume of hexane,

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