Intellectual merit: Precipitation isotopic ratio is arguably the best proxy for quantitative continental paleoclimate reconstructions. Traditional archives, ice cores and speleothems, are spatially restricted. dD values of plant leaf waxes represent a potentially powerful and widely applicable proxy. However, there is major uncertainty in the hydrogen isotopic fractionation between leaf waxes and precipitation (åwax-p) under different climatic and environmental conditions. Several studies have attempted to obtain the mean åwax-p values from lake surface sediments, or even directly from plant leaves, but with only limited success. We propose to determine the dD values of leaf waxes from modern aerosol samples from contrasting ecosystems. Leaf waxes in aerosols integrate current production from higher plants, allowing us to accurately define the åwax-p values throughout the seasonal cycle under specific environmental settings. For the proposed EAGER project, we intend to study two climatically distinct sites (Arizona and Georgia in the US), in order to demonstrate the feasibility of the proposed approach. If successful, we intend to study a global assortment of sites across vegetation and climate gradients.
Broader impacts: The proposed work will generate the most accurate values of the apparent hydrogen isotopic fractionation (Ã¥wax-p) between modern plant leaf waxes and rainfall in the two highly contrasting climatic and environmental settings, and will pave the way for more comprehensive studies in the future. The results are fundamental for quantitative translation of leaf wax hydrogen isotopic data into paleoclimatic records. This study will promote multidisciplinary collaborations (organic/isotopic geochemistry and atmospheric science), and allow cross-disciplinary training of graduate and undergraduate students. The results have broad impacts in paleoclimatology, stable isotopes in earth science and biology, organic geochemistry.
Quantitative understanding of continential climate change is crucial for the prosperity of human society in the coming century. One of the major tools to improve our ability to predict future climate change is to obtain highly quantitative climate records in various continental locations for hundreds to 10s of thousands of years, so that we can accurately parameterize state-of-the-art climate models. An emerging powerful tool for quantifying past continental climate change is hydrogen isotopic ratios of terrestrial plant leaf waxes. In the past few years, many high profile papers have been published using leaf wax hydrogen isotopic values extracted from lake or ocean sediment cores. Despite of the initial success, we lack fundamental understanding of the hydrogen isotopic fractionation of leaf waxes relative to source water, which is the essential information for quantifying past climate change. Researchers have studied lake surface sediments from transects of lakes and from modern plants to obtain the best estimate of the isotopic fractionation of plant leaf waxes. However, while both methods provide extremely valuable information, there are a number of potential problems. The top section of lake surface sediments span an unknown amount of time (decades to hundreds of years depending on sedimentation rates of individual lakes). The fractionation values generated using such sediment transect samples are thus not fully comparable at different sites. Acquiring mean frationation values from an ecosystem by analyzing individual plants is an extremely challenging task, because different terrestrial plants can have very different hydrogen isotopic fractionation values. It is impossible to analyze all the plant species in an ecosystem and to determine the proportion of contribution to the total leaf wax output from individual plants, given leaf wax production can differ greatly in different plants. We demonstrate that continental aerosols are excellent for determining the ecosystem-level, time-resolved, integrated leaf wax hydrogen isotopic and molecular signatures. Our results from two drastically different subtropical ecosystems (i.e., desert Arizona vs. humid Atlanta) demonstrate hydrogen isotope fractionation between leaf waxes and annual mean precipitation does not differ significantly over the summer growth season. Our data indicate that influence of relative humidity and associated vegetation assemblage on leaf wax hydrogen isotope fractionation at ecosystem scales is relatively small. Our results represent an important step toward quantitative interpretation of leaf wax hydrogen isotopic records. We have also discovered new characteistics in biochemistry of leaf waxes. Leaf wax chain-length distribution data display systematic difference between the two ecosystems, with longer average chain-length for desert environment (Arizona) and shorter average chain-length for humid environment (Atlanta), supporting the use of leaf wax chain length for paleohydrology reconstructions. One graduate student, Li Gao, and two undergraduate students have received training in experimental procedures and/or project design and scientific writing as a result of this award. Under the partial support of this grant, 1 paper has been published, and three papers have been submitted for publication. 1. Gao L., Zheng M., Fraser M. and Huang Y. Comparable hydrogen isotopic fractionation of plant leaf waxes in arid and humid subtropical ecosystems', JGR, Biogeosciences, in review (submitted on June 15, 2012). 2. Gao L and Huang Y, Inverse gradients in leaf waxes dD and d13C values along grass blades: implications for leaf wax reproduction and plant physiology. Submitted to Oecologia on April 26, 2012, for publication. 3. Gao L, Tsai Y-J, Huang Y, Assessing the rate and timing of leaf wax regeneration in Fraxinus americana using stable hydrogen isotope labeling. Submitted to Rapid communications in Mass Spectrometry on May 10, 2012 for publication. 4. Gao L, Burnier A, Huang Y (2012) Quantifying instantaneous regeneration rates of plant leaf waxes using stable hydrogen isotope labeling. Rapid communications in Mass Spectrometry, 26, 115-122.
