Reconstructing the delta 18O of atmospheric water vapour via the CAM epiphyte Tillandsia usneoides: seasonal controls on delta 18O in the field and large-scale reconstruction of delta 18O_a.

摘要

Using both oxygen isotope ratios of leaf water ( delta 18O_L) and cellulose ( delta 18O_C) of Tillandsiausneoides in situ, this paper examined how short- and long-term responses to environmental variation and model parameterization affected the reconstruction of the atmospheric water vapour ( delta 18O_a). During sample-intensive field campaigns, predictions of delta 18O_L matched observations well using a non-steady-state model, but the model required data-rich parameterization. Predictions from the more easily parameterized maximum enrichment model ( delta 18O_L-M) matched observed delta 18O_L and observed delta 18O_a when leaf water turnover was less than 3.5d. Using the delta 18O_L-M model and weekly samples of delta 18O_L across two growing seasons in Florida, USA, reconstructed delta 18O_a was -12.6+or-0.3 per mil. This is compared with delta 18O_a of -12.4+or-0.2 per mil resolved from the growing-season-weighted delta 18O_C. Both of these values were similar to delta 18O_a in equilibrium with precipitation, -12.9 per mil. delta 18O_a was also reconstructed through a large-scale transect with delta 18O_L and the growing-season-integrated delta 18O_C across the southeastern United States. There was considerable large-scale variation, but there was regional, weather-induced coherence in delta 18O_a when using delta 18O_L. The reconstruction of delta 18O_a with delta 18O_C generally supported the assumption of delta 18O_a being in equilibrium with precipitation delta 18O ( delta 18O_ppt), but the pool of delta 18O_ppt with which delta 18O_a was in equilibrium - growing season versus annual delta 18O_ppt - changed with latitude.