GEOC 2 |
| Compound-specific hydrogen isotope ratios of leaf wax lipids hold great promise as paleohydrologic proxy because more arid conditions cause greater evaporative enrichment of D in leaf water. Leaf-wax compounds synthesized from leaf water record the degree of evaporative enrichment. However, some challenges remain in the calibration and application of this proxy. First, hydrogen isotope fractionation between leaf-wax n-alkanes and water added to the soil differs between C3 and C4 grasses (monocots) and between monocots and dicots. Observed variations are on the order of 20-40 per mil. Differences observed within the grasses can be accounted for by inter-veinal distance. Differences between monocots and dicots are still being explored, but are likely also be related to leaf anatomy and plant water budgets. Because hydrogen isotope fractionation varies with plant types, plant community change will affect isotopic records in the geologic past irrespective of climate. Therefore, any interpretation of leaf-wax hydrogen isotope records in terms of hydrologic condition must take into account the potential effects of changing plant communities. Second, to interpret hydrogen isotope ratios of leaf waxes quantitatively in terms of relative humidity requires that the source water (precipitation or soil water) isotope ratio be measured or inferred. Several potential methods of doing this will be discussed including short-chain n-alkanes, tooth enamel, and pedogenic clays. In addition, we introduce a new method that combines cellulose and n-alkane hydrogen isotope ratios to determine relative humidity independent of source water isotope values. |
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Modern Calibrations and Terrestrial Applications of Stable Isotope Paleo Proxies
9:00 AM-12:10 PM, Sunday, August 19, 2007 Boston Park Plaza -- St. James Rm, Oral
Division of Geochemistry |