Oxygen and hydrogen isotopes in fossil insect chitin as paleoenvironmental indicators.

摘要

Isotopic analyses of the minute quantities of chitin typically available from a fossil locality is facilitated by a new method of extracting δ 18O and δ2H values from a single organic sample. This involves the pyrolysis of materials in quartz-encapsulated nickel tubes. When heated to 1050°C hydrogen diffuses through the nickel and is held in the surrounding evacuated quartz envelope. This is transferred to a mass spectrometer for analysis, then CO₂ is extracted from the nickel tube on a vacuum-separation line and analyzed for δ18O.; Compensation for exchangeable hydrogen is achieved through equilibration with water of known δ2H at 0°C. In this procedure, purified chitin or cellulose is soaked in NaOH solution at about 0°C to open up the structure and make the maximum number of hydrogen atoms available for exchange. This method allows compensation for the influence of non-conservative, oxygen-bonded hydrogen in measured δ2H values.; The utility of chitin δ18O and δ2H isotopic analyses as hydrologic and environmental indicators is demonstrated. Using modern sites from across Canada (14 sites for δ18O) and across North America (46 sites for δ2H), links are established between chitin isotopic content and δ18O and δ2H of environmental water, relative humidity and temperature. Chitin isotopes are also compared to cellulose δ 18O and δ2H for sites where tree-feeding insects and food material were collected. These correlations are incorporated into a model with predictive capabilities which allows the determination of environmental water isotopic composition and relative humidity from chitin δ 18O and δ2H values. This is based on the Craig and Gordon model, describing steady-state evaporation from a terminal reservoir, and relates the fractionation factor between chitin and environmental water (18α_{chitin-environmental water} and 2α_{chitin-environmental water}) to humidity and net biochemical (α_n), equilibrium (α_e) and kinetic (α_k) fractionation factors. Evaporative enrichment of leaf water is related to α_e and α_k, which are approximated by fixed values controlled by temperature in the case of α _e and wind speed and leaf morphology in the case of α_k. The α_n is related to the biological processes which occur during synthesis of chitin and chitin precursors. For oxygen α _n is fixed, whereas α_n for hydrogen is temperature dependent, possibly because the seasonal activity of insects in cooler climates means they are preferentially exposed to summer precipitation, resulting in relative enrichment, in an effect analogous to snowmelt bypass in lakes. (Abstract shortened by UMI.)