HR: 10:50h
AN: PP22A-03 INVITED     [Abstracts]
TI: Patterns of Storage, Synthesis and Changing Light Levels Revealed by Carbon Isotope Microsampling within Eocene Metasequoia Tree Rings
AU: * Jahren, H
EM: jahren@jhu.edu
AF: Johns Hopkins University, Earth and Planetary Sciences, Baltimore, MD 21218 United States
AU: Sternberg, L
EM: leo@bio.miami.edu
AF: University of Miami, Department of Biology, Coral Gables, FL 33124 United States
AB: Fossil tree rings from Axel Heiberg Island were microanalyzed for δ¹³C value in order to assess patterns of tree growth and carbon storage within the Middle Eocene (~45 Ma) Arctic paleoenvironment. Wood from four Metasequoia-type individuals was subsampled for analysis: each individual fossil consisted of between 4 and 10 large (~1 cm thick) consecutive tree rings. One of the fossils displayed an obvious concentric pattern, allowing for the determination of the direction of growth with isotopic pattern. Each ring was divided into ~1 mm thick subsamples, resulting in 5-10 δ¹³C value determinations per period of ring growth (i.e., growing season). All rings revealed a distinct pattern that was characteristic across growing seasons and across individual fossils. Early in the season, δ¹³C was at its highest value but descended systematically and sharply to its lowest value at the end of the growing season. Total decrease ranged between 3 and 5 ‰ over the course of each growing season. Identical patterns were observed in the δ¹³C value of alpha-cellulose isolated from each subsample, indicating that the trends observed did not represent changing levels of secondary metabolites, but rather a seasonal adjustment in the bulk source of carbon used during biosynthesis. Our results are consistent with the following annual pattern of wood synthesis 1.) complete dependence on the mobilization of stored carbon compounds early in the growing season; 2.) systematically increasing use of actively-acquired photosynthate during the growing season; 3.) complete reliance on active photosynthate by the end of the growing season. An additional and significant source of ¹³C discrimination is declining light levels late in the growing season, and likely contributes to the extreme pattern of δ¹³C decrease seen across each ring. Our results mimic those seen from modern broadleaf deciduous trees (Helle & Schlesser 2004), but differ from those seen in modern conifers (Barbour et al 2002), underlining the important influence of dormancy, storage and changing light levels on carbon cycling within the Paleogene polar forests.

Helle & Schleser 2004: Plant, Cell and Environment 27:367-380.
Barbour, Walcroft & Farquhar 2002: Plant, Cell and Environment 25: 1483-1499.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 4853 Photosynthesis
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005