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COCKBURN, JACLYN M. H.  Queen's University.
Lamoureux, Scott F.  Queen's University.

Annually laminated (varved) lake sediments serve as useful proxies for past climate variability. In this study, the subannual structures within varved sediments from Summit Lake (875 m a.s.l.) in the Coast Mountains along the northern British Columbia/Alaska border were used to investigate interannual variability specific to nival-glacial and rainfall runoff for the past 700 years. The frequency and thickness of rainfall-induced sedimentary events increased abruptly at 1675 AD suggesting that there was an increase in Pacific airmasses at this time. In addition to the rainfall increase in the late 17th century, there was a distinct decrease in event frequency in the later half of the 19th century and subsequent increase at the beginning of the 20th century. At an adjacent lake (Meadow Lake), continuous varve deposition began at 1667 AD, and we interpret this as an indication of a glacial advance in the watershed caused by increased autumn precipitation. The subannual autumn rainfall record from Summit Lake in combination with the onset of continuous varve formation in Meadow Lake suggests that there was a major ocean-atmosphere shift over the North Pacific Ocean in the last part of the 17th century.

The northern Pacific Ocean exhibits known multi-decadal variability in the 20th century and our results suggest the onset of behaviour resembling the current ocean-atmosphere regime occurred at 1675 AD and was weak or absent in the preceding 375 years. The waxing and waning of Pacific decadal variability identified in our record may explain why it has been difficult to consistently reconstruct PDO from proxy records prior to 1900 AD. However, comparison between the Summit Lake varve record and other proxy records indicates a coherent phasing of the ocean-atmosphere system. These results suggest that long term variability in the northeastern Pacific region may have been more complex than indicated by instrumental records and point to the need for more well-dated proxy records from a range of environments.

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