Two large undocumented volcanic eruptions in 1698/9 and 1809 have been previously identified in ice cores as sulfate peaks, and in tree-ring latewood density data as low-density rings. These eruptions can also be recognized using tree-ring width data, and can be identified in the tree-ring record as narrow rings for several consecutive years. The first part of this study uses tree-ring width data from the Gulf of Alaska to provide further evidence that these eruptions occurred. The two eruptions also provide a natural experiment for investigating the effects large volcanic eruptions have on North Pacific climate, including the Pacific Decadal Oscillation (PDO), which is considered a primary driver of North Pacific climate. A recent modeling study suggests that strong volcanic eruptions (SVEs) can have significant impacts on Gulf of Alaska climate, including forcing the PDO into a negative phase. To gain further insight into this possible forcing, the second part of this study presents a PDO reconstruction that was created using tree-ring data from the Gulf of Alaska. The reconstruction is then analyzed at the times of the 1698/9 and 1809 eruptions. Our analyses find that while SVEs in 1698/9 and 1809 correlate with negative phases of the PDO, both eruptions occurred when the PDO had already entered a negative phase. Therefore, it appears that SVEs may intensify negative shifts of the PDO, but that the PDO is also driven by other factors including El Niño Southern Oscillation (ENSO), atmospheric circulation, ocean circulation and solar variability.


Pollock, Meagen

Second Advisor

Wiles, Greg






pacific decadal oscillation (pdo), tree-rings, strong volcanic eruptions (sves), volcanically forced cooling

Publication Date


Degree Granted

Bachelor of Arts

Document Type

Senior Independent Study Thesis Exemplar

Included in

Paleontology Commons



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