HR: 10:35h
AN: PP22A-02    [Abstracts]
TI: Seasonal Variations in the Biochemical Fractionation of Hydrogen Isotopes by Spartina alterniflora.
AU: * Sessions, A L
EM: als@gps.caltech.edu
AF: Division of Geological & Planetary Sciences California Institute of Technology, 1200 E. California MC 100-23, Pasadena, CA 91125 United States
AB: Hydrogen isotope ratios (D/H) of lipids are being intensively explored as a paleoenvironmental proxy, particularly for continental regimes where organic preservation in lakes is generally high. Several studies have already shown good correlations between δD values of lake water and sedimentary (core-top) lipids, but the fractionations indicated by those correlations do not agree well between studies. Moreover, the data cannot be adequately described by a single biochemical fractionation. These difficulties suggest that the relationship between environmental water and plant lipid δD is controlled by multiple environmental and biochemical factors. Understanding these factors will lead to a more robust interpretation of D/H as a paleoclimate proxy. Here we examine seasonal changes in biochemical H-isotopic fractionation by the salt marsh grass Spartina alterniflora. Because S. alterniflora grows partially submerged in a tidal estuary, it has an unlimited and isotopically unvarying source of water for growth. Thus environmental influences on fractionation should be negligible, allowing us to examine seasonal changes in biochemical fractionations. C₂₇ and C₂₉ n-alkanes, β-sitosterol, phytol, and C₁₆ and C₁₈ fatty acids were extracted and analyzed from 35 samples of S. alterniflora harvested from the same location over a period of 18 months. All lipids except β-sitosterol exhibit statistically significant depletions of D during summer months relative to the rest of the year. The magnitude of the isotopic shift is up to 36‰ in the fatty acids (δD values from -130 to -166‰), 31‰ in n-alkanes (-161 to -192‰), and 24‰ in phytol (-252 to -276‰). The shift in D/H ratio is in the opposite direction from that expected due to increased evapotranspiration during the summer months. The largest D-depletions coincide with periods of maximal growth. The observed pattern is interpreted as resulting from increased use of stored carbohydrates as substrates for lipid biosynthesis during the spring and fall, whereas 'fresh' photosynthate is utilized during periods of maximal growth. Storage carbohydrates are strongly enriched in D as a result of isotopic exchange with water. Paleoclimate calibrations of D/H ratios in lipids from terrestrial plants will thus need to take the seasonality of lipid production and export into account, in addition to other environmental factors.
DE: 0424 Biosignatures and proxies
DE: 0438 Diel, seasonal, and annual cycles (4227)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0473 Paleoclimatology and paleoceanography (3344, 4900)
DE: 1055 Organic and biogenic geochemistry
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005