Date of Completion

1-23-2017

Embargo Period

1-23-2020

Advisors

Dr. Lisa Park Boush and Dr. Will B. Ouimet

Field of Study

Geological Sciences

Degree

Master of Science

Open Access

Open Access

Abstract

Leaf wax biomarkers are increasingly becoming a fundamental aspect of paleoclimate research. The strength of the relationship between their molecular and isotopic properties and environmental conditions is critical to their effectiveness in reconstructing past environments. To address the current lack of data pertaining to arid and semi-arid environments, soil and modern plant samples (grasses, trees, forbs, and shrubs) from sites in the western United States were analyzed to assess relationships between normal alkane molecular distribution, δD and δ13C and seasonal/annual climatic conditions in semi-arid ecosystems. The data show a stronger relationship between δD of leaf waxes (δDwax) in soils and δD of mean annual precipitation (δDMAP) than the δD of plants. δDwax of grasses and soils correlate with latitude, mean annual vapor pressure deficit (VPD), mean annual temperature (MAT), and annual relative humidity (RH). No relationships were observed between δ13Cwax or the molecular distribution of the waxes and any of the climate variables tested. There is an offset between the δD and δ13C of these waxes and the δD and δ13C of the environment in which they were created. The apparent fractionation (εwax/atm) between δDwax and δDMAP for grasses and soils correlated with latitude as well as mean annual RH. There was no relationship found for the carbon discrimination between δ13Cwax and δ13Catm (Δleafwax/atm) and the climatic variables tested.

To further investigate the dominant environmental drivers controlling the composition of leaf wax biomarkers, data from the Western United States was then combined with published data from studies throughout the United States. These combined data show distinct distributions of εwax/atm from soils in different climates. The εwax/atm of the plants display correlations to mean annual VPD. The data also show an increase in the Average Chain Length (ACL) of the n-alkanes in soils with increased temperature and aridity. In contrast to previous studies, the offset between carbon isotope composition of bulk leaf tissue and leaf wax biomarkers varied in the soils. This suggests that the fractionation between the carbon isotope composition of bulk leaf tissue and leaf wax could be due to the differences in environmental or vegetation type. The data presented here suggest careful consideration of ecosystem type is critical to reconstructing past environmental conditions using leaf wax biomarkers.

Major Advisor

Dr. Michael T Hren

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