A stable isotope stratigraphy for the Axel Heiberg Fossil Forest and its application to Eocene climate

Abstract

The Eocene era was a warm, climatically dynamic transitional period between the Paleocene greenhouse world and the Oligocene icehouse world. This study details carbon and hydrogen isotopic and biomarker analyses of samples of lignite (bulk fossil leaves), wood, paleosol, and resinite from the Middle to Late Eocene age fossil forest stratigraphy on Axel Heiberg Island, Nunavut, Canada. Bulk carbon isotopes show a record of frequent, large fluctuations on the scale of the Paleocene-Eocene Thermal Maximum benthic carbon excursion of [approx.] 2.6%₀ (Zachos 1999). However, terrestrial flora are less sensitive to CO₂ fluctuations given their capacity to regulate stomatal intake and the comparatively easy diffusion of CO₂ in air. Resinites (-22.8 +/- 1.7%₀) are enriched relative to bulk lignite (-24.7 +/- 0.75%₀), and wood (-21.66 +/- 0.45%₀) is also enriched relative to bulk lignite. Both 1) a scenario of periodic methane hydrate pulses and 2) a scenario of fluctuating forest stand LAI (leaf area index) are not inconsistent with our data. Either mechanism could be responsible for large carbon isotope shifts. Higher plant input dominated the n-alkane signature. Compound-specific hydrogen isotopes in n-alkanes show a record of marked secular change, with isotopes becoming generally lighter over the time span of the stratigraphy, though punctuated by singular fluctuations as large as 32%₀.

(cont.) Polycyclic isoprenoid lipids (-266%₀ to -375%₀, mean 300%₀ +/- 38%₀) are characteristically depleted relative to n-alkanes (-238%₀ to -295%₀, mean -268%₀ +/- 10%₀). From the n-alkanes, we estimate that environmental water in the Eocene on Axel Heiberg Island was depleted -150%o +/- 24.8%o, which agrees with an estimate derived from cellulose, [delta]D-environmental = -133%₀ (Jahren 2003). (For comparison, modern precipitation at the site has a [delta]D value of [approx.] -213%₀, though precipitation should not be considered equivalent to environmental water.) This datum is consistent with a meridional weather patterns that may have carried moisture over continents towards high latitudes in the absence of a polar front, isotopically depleting precipitation to a greater extent than occurs today. However, seasonality cannot be discounted as a mechanism, given that colder temperatures would lead to colder condensation temperatures and thus, isotopically lighter precipitation.