Understanding precipitation changes over Greenland through the ice sheet history is a critical component of determining if increased precipitation from reduced Arctic sea ice stabilizes the retreat of the ice sheet during warm periods. Ice core data is one method of reconstructing past precipitation. Deuterium and Oxygen18 isotopes can be used in reconstructing historic variables like atmospheric temperature and water vapor transport. Several deep ice cores exist from the Greenland Ice Sheet that span the entire Holocene, and will provide the type of long-term record needed to examine precipitation records, and many more shallow cores provide data that can be used to validate a shorter-term record. However ice core data alone will not provide the kind of spatial or temporal coverage needed to determine ice sheet margins through time. In order to to expand this coverage, the Snow on Ice project will use the hydrogen isotopes of leaf waxes found in sediments from lakes around the margins of the Greenland Ice Sheet.
Leaf waxes are molecules composed mainly of hydrogen and carbon, produced by terrestrial and aquatic plants as a protective coating on their leaves. Terrestrial plants produce leaf waxes with long carbon chains, whereas aquatic plants produce leaf waxes with shorter carbon chains, allowing us to measure δ2H of aquatic and terrestrial leaf waxes separately. Leaf waxes are blown or abraded off leaves and are deposited and preserved in lake sediment. Leaf wax hydrogen isotopes (δ2Hwax) reflect plant source water δ2H: i.e., aquatic δ2Hwax reflects mean annual precipitation as recorded in lake water and terrestrial δ2Hwax reflects growing season precipitation as recorded in soil water. The difference between aquatic and terrestrial δ2Hwax tells us about winter snowfall, an important parameter for ice sheet mass balance.
Hydrocarbon chains are stable. Thousands of years after the leaf waxes are blown into the lake, their hydrocarbon chains remain unchanged and can be used as a proxy for paleoclimate. Back in the lab, we pump solvent through the mud to release the waxes, and we then purify and analyze the hydrogen isotopes of the leaf waxes.