Semester

Fall

Date of Graduation

2021

Document Type

Thesis

Degree Type

MA

College

Eberly College of Arts and Sciences

Department

Geology and Geography

Committee Chair

Amy Hessl

Committee Member

Shikha Sharma

Committee Member

Kevin Anchukaitis

Abstract

Few annually dated stable isotope records exist across Oceania. In mid- to high-latitude locations in South America, tree-ring stable isotopes provide information about past climate dynamics such as the Southern Annular Mode (SAM). The SAM drives latitudinal shifts in Southern Hemisphere westerly winds, influencing temperature and moisture delivery across the mid- to high-latitudes, including Tasmania. Combinations of paleoclimate proxies from across the Southern Ocean might provide insight into dynamic processes like the SAM that are difficult to measure with a single proxy. Measuring stable carbon and oxygen isotope ratios from tree rings in Tasmania could provide complementary data to contribute to existing reconstructions heavily reliant on tree ring width chronologies.

A team collected tree core samples of Athrotaxis selaginoides from a high elevation site in Tasmania, Australia. The site receives abundant rainfall throughout the year and is ~130 km from the Global Network of Isotopes in Precipitation (GNIP) site at Cape Grim. I crossdated our new samples against an existing tree-ring width chronology (439 BCE - 2011 CE) and analyzed the δ13C and δ18O from the individual rings of ten trees for the period 1960 - 2018. Using high resolution (0.25 degrees) Australian Bureau of Meteorology Scientific Information for Land Owners (BOM SILO) climate data and ECMWF ERA5 reanalysis data, I disentangled the effects of local climate and source region on the isotopic signatures recorded in the annual rings. In addition, I used HYSPLIT backward trajectory analysis to determine the source region of precipitation to Mount Read and whether the source region is influenced by the SAM.

Median δ18OTR (n=10) is correlated with temperature and vapor pressure deficit in the early growing season. In addition, spatial correlations reveal that median δ18OTR is positively correlated with temperature and negatively correlated with precipitation in the source region. However, measurements of δ18OTR exhibit high inter-tree variation, particularly between 1960 - 1990. Suess-corrected δ13CTR measurements are not significantly correlated with local climate measures, and have no significant relationship with δ18OTR, implying that the influence of local moisture on the isotopic signatures is weak. During positive states of the SAM, there is a mid-latitude influence in the source region of precipitation to Mount Read that is not present in negative states of the SAM. Our results indicate that combined δ18OTR and δ13CTR proxies may provide additional information about past moisture conditions during the growing season, potentially contributing to more robust reconstructions of the SAM; however, additional sampling in a different environment may be necessary to resolve inter-tree variation in δ18OTR.

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