Objective 2

Objective 2. Reconstruct d ¹³C and possibly pCO₂ of the Eocene atmosphere.

Organic carbon in fossil wood, leaf and reproductive tissues, paleosol organic matter, and fossil roots will be prepared for stable isotope analysis via combustion in sealed tubes containing Cu, CuO, and Ag (Minagawa et al. 1984). Released CO₂ will be purified cryogenically, and collected for ¹³C/¹²C measurement on the mass spectrometer. We will infer the d ¹³C value of the mid Eocene atmosphere from the mean d ¹³C value of organic carbon from all site taxa using an empirical relationship between d ¹³C plant and d ¹³C atmosphere developed from 671 published d ¹³C plant measurements on 288 C3 plant species across a wide variety of environmental conditions (Arens et al., in review). This work shows that d ¹³C atmosphere = (d ¹³C plant + 18.92)/1.05 for the C3 vascular land plant tissue averaged over the contribution of several species. This relationship has shown dramatic changes in global carbon cycling in the Early Cretaceous when used to interpret d ¹³C measurements made on a terrestrial carbon sequence in Colombia (Jahren et al., in review); our results will identify the important carbon sources and sinks to the middle Eocene atmosphere, which can be recognized based on differences in the isotope composition of each carbon pool.

For individual plants, isotopic fractionation during carbon assimilation via the C3 photosynthetic pathway can be described by (Farquhar et al. 1982):

d ¹³C plant = d ¹³C atmosphere - a - (b - a)Ci /Ca [1]

Where d ¹³C plant is the isotopic composition of individual plant tissue derived from C3-photosynthetic carbon assimilation, d ¹³C atmosphere is the composition of the atmospheric CO₂; "a" is the isotopic discrimination dominated by a simple diffusivity comparison of d ¹³CO₂ vs. d ¹²CO₂ in air (Craig 1953) and does not depend on stomatal density or conductivity; "b" is the isotopic discrimination imparted during carboxylation, mainly through the initial carbon-fixation enzyme in C3 plants, RuBisCO; and Ci/Ca is the ratio of intercellular to atmospheric pCO₂ expressed in parts per million. The influence of Ci/Ca on d ¹³C values of plant tissue has been central to the application of [1] to carbon assimilation and water-use efficiency (WUE) studies. Theory predicts that when stomatal conductance is low relative to CO₂-fixation capacity, Ci is small and d ¹³C plant tends toward larger values. Both d ¹³C plant and Ci/Ca have been measured under a variety of controlled conditions; Farquhar and colleagues (1982) reported Ci/Ca in several species subjected to water-stress and reported a range in Ci/Ca value of 0.30-0.85. Therefore, if d ¹³C atmosphere is known (or can be determined via the above means), individual d ¹³C plant values can be inserted into [1] to solve for Ci/Ca in individual plants, thus giving an indication of individual water stress status. d ¹³C plant values have been used to indicate water-stress status in modern trees (Dupouey et al. 1993; Marshall and Zhang 1994) and other plants (Toft et al. 1989). We can determine if there are differences in modern Metasequoia growing in seasonally dry vs. continuously moist climates, and on dry vs. poorly drained sites as a means of verifying this approach. The exceptional preservation of Eocene wood from Axel Heiberg Island allows us to extend this technique to the fossil record.

Comparisons of d ¹³C values of paleosol organic matter and tissues of different taxa at the site will allow for estimation of relative contribution of different taxa to the overall productivity of the site, as averaged by soil forming processes. These results will be compared to independent estimates of taxa productivity gained from field measurements of annual wood production (see below).