Semester

Summer

Date of Graduation

2024

Document Type

Thesis

Degree Type

MS

College

Eberly College of Arts and Sciences

Department

Geology and Geography

Committee Chair

Amy Hessl

Committee Co-Chair

Charles Shobe

Committee Member

Charles Shobe

Committee Member

Laia Andreu-Hayles

Committee Member

Lankeswar Dey

Abstract

Cosmic rays and solar energetic particles (SEP) bombard the Earth’s geomagnetic field, posing a threat to satellites, space stations, and human space exploration. These rays and particles also produce radiocarbon (14C) in Earth’s atmosphere, thus records of past 14C concentration in the atmosphere may be indicative of past cosmic and solar activity. Rapid increases in the concentration of atmospheric radiocarbon 14C, (Miyake events) first identified in tree rings, are thought to be a result of solar eruptive activity triggering the release of solar energetic particles, though the precise nature of past events remains unresolved. The first Miyake event, (774/775 CE), showed a 12‰ increase in Δ14C in tree ring measurements, while the event in 993/994 CE exhibited a ~9‰ increase. These Miyake events were later confirmed by observed changes in cosmogenic isotopes 10Be and 36Cl from ice in Greenland and Antarctica Since the discovery of the 774 CE and 993 CE atmospheric 14C production events, additional studies have identified seven other strong excursions in the last 14,000 years, suggesting that past solar activity may have substantially exceeded what has been observed during the modern instrumental record. Developing a long and well-replicated history of spikes in atmospheric 14C concentration and reconstructing the magnitude of 14C production events may ultimately support forecasts of space weather. However, there are a range of uncertainties associated with tree rings as proxies for past 14C and by extension past 14C production events. The goal of my research is to isolate sources of uncertainty in annual 14C measurements between different tree species using Δ14C data and models of start date, duration, solar phase, and spike production of past extreme SEP events. I hypothesize that tree ring estimates of Δ14C during Miyake events differ based on tree species physiology, where deciduous species show a lagged effect in 14C measurements, with implications for estimates of timing, duration, and magnitude of past 14C production. To address this hypothesis, I 1) compared Δ14C in the annual rings of an evergreen conifer and a deciduous conifer growing at the same sites in Mongolia; and 2) used these Δ14C measurements alongside a network of Δ14C data from Northern Hemisphere sites in a Bayesian modeling framework to evaluate the impact of tree physiology on estimates of start date, duration, and spike production of past 14C production events. The accelerated mass spectrometer (AMS) results do not confirm our hypothesis that Δ14C differs between an evergreen conifer (Siberian pine) and a deciduous conifer (Siberian larch). The two species with different functional types growing at the same site yield similar Δ14C results for the 774 CE event. Results are more a variable for the 993 CE event, though modal parameter estimates of start date are similar. Results from the Northern Hemisphere comparison indicated that evergreen and deciduous conifers record an earlier rise in Δ14C relative to deciduous broadleaf species, likely due to lags in stored carbohydrates. Deciduous broadleaf species, consistent with their physiology, suggest later start dates and differing estimates of solar phase and spike production. Future work using deciduous broadleaf species should consider these differences and they should be factored into reconstructions of past solar activity at the annual scale.

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