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Objective 3 |
Objective 3: Reconstruct Eocene
Temperatures From 18O/16O and D/H Analyses. d 18O values will be determined for
cellulose isolated from Eocene Metasequoia tree rings following
pyrolysis with mercuric chloride and conversion of resulting O2
gas to CO2 for measurement on the mass spectrometer. d D values in the same samples will be determined in
cellulose nitrate purified from cellulose and combusted in excess O2
to produce water which is then reduced to H2 gas for measurement
on the mass spectrometer. Methods used for cellulose isolation and analysis
will take advantage of new variants (Sheu and Chiu 1995) of the original
method of Epstein et al. 1976, 1977). For example, recent advances
allow for cellulose-isolation batch processing of small wholewood samples
(Leavitt and Danzer 1993, Loader et al. 1997) and high resolution
isotopic analysis of single rings and single tissues (Loader et al.
1995). Established relationships between d 18O value of cellulose and d 18O value of site precipitation (Burk
and Stuiver 1981) and between d D value
of cellulose-nitrate and d D value of site precipitation
(Yapp and Epstein 1977) allow determination of paleoprecipitation isotopic
composition. Furthermore, the documented relationship between the isotopic
composition of precipitation and site temperature (Dansgaard 1964) provide a
means for paleotemperature estimation. The above methodology has been used
extensively to determine paleotemperatures in the Holocene (e.g., Yapp and
Epstein 1977; Feng and Epstein 1994) but lack of well-preserved fossils has
prevented application in deep time. For verification, we propose to run
similar analyses on extant Metasequoia from a range of climates to
determine if the isotopic signatures give reasonable estimates of present
temperatures. As indicated in Figure
6, some trees have large annual rings, and the cellular anatomy suggest
differences in cell morphology in the spring and fall light/dark season
compared to cells produced in the continuously light midsummer period (Fig. 7). d 18O values may
vary across each annual ring in a way that reflects temperature. Thus, we
will attempt to determine seasonal temperatures by subsampling annual rings
of modern and fossil trees to obtain isotopic data for different parts of the
growing season. Again, the unusual preservation of wood, the presence of very
wide annual rings, and modern Metasequoia growing in a variety of
climates presents a unique opportunity to extract and verify paleoclimate
information. |
