Impact and significance of tephra deposition from Mount Mazama and Holocene climate variability in the Pacific Northwest USA.

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Abstract

The mid-Holocene climactic eruption of Mount Mazama in Oregon, USA (Volcanic Explosivity Index, VEI-7) was among the largest eruptions globally during the Holocene. Despite evidence for possible hemispheric climatic impacts, the age of the eruption is not well-constrained and little is known about the environmental impacts of distal tephra deposition with previous studies showing no clear consensus. Further, the eruption occurred during a time of global climatic warming, raising questions about the impacts of tephra deposition in the context of longer-term change. Thus the aim of this thesis is to investigate the terrestrial and aquatic impacts of distal tephra deposition from the climactic eruption of Mount Mazama approximately 7700 years ago, and to reconstruct Holocene environmental change in the Pacific Northwest of North America. The Mazama tephra forms an important isochronous marker horizon. A refined age of 7682-7584 cal. years BP (95.4% probability range) for the eruption was acquired through Bayesian statistical modelling of 81 previously published radiocarbon age estimations. Through high resolution palaeoecological and statistical analyses (stratigraphy, tephra geochemistry, radiocarbon dating, pollen, diatoms and ordination) the aquatic and terrestrial impacts of tephra deposition are assessed. Records were examined from the centre and fringe of Moss Lake, Washington to elucidate regional and local effects on vegetation and to determine whether the observed aquatic impacts were consistent throughout the lake, or whether the diatoms were responding to other factors, such as climate or catchment changes. Tephra deposition from the climactic eruption of Mount Mazama caused a statistically significant local terrestrial impact with changes to open habitat vegetation (Cyperaceae and Poaceae) and changes in aquatic macrophytes (Myriophyllum spicatum, Equisetum) and alga (Pediastrum), but there was no significant regional impact of distal tephra deposition. Statistical testing suggests the regional changes observed were climate-driven, evidenced by longer-term, underlying environmental change. Tephra deposition had a statistically significant impact on the aquatic system with decreases of epiphytic taxa (Fragilaria brevistriata and Staurosira venter) and increases of epipelic (Brachysira brebissonii) and tychoplanktonic taxa (Aulacoseira sp.) indicating a change in habitat and an increase of the Si:P ratio, lasting approximately 150 years. Variance partitioning demonstrated tephra to be a significant environmental variable; however, directional change exerted most influence and interactions between variables are evident. This study clearly demonstrates that there are complex interactions between drivers of change which is evidenced through time series analysis of the diatom Holocene record, revealing periodicities of approximately 2000 years, 1300 years, and 450 years attributed to solar variation and ocean-atmosphere interactions. Overall, tephra had a significant local effect on the environment, but no significant impact on the region independent of underlying environmental changes. More studies of similar nature are needed to evaluate the wider regional significance of the localised impacts shown at Moss Lake.

Keyword(s)

Diatoms; Holocene environmental change; Mazama; Ordination; Pollen; Tephra impact; Time series analysis

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