Recent hydroclimate dynamics in southwest Alaska: understanding multidecadal climate variability through sedimentary process studies and varve sedimentology.
Kaufman, Claire Allyn
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Sedimentological analyses of sediment traps and 289 years (1717-2006) of varved sediments from Shadow Bay (60.01°N, 159.18°W), Lake Chauekuktuli were compared to available hydrometeorological data in order to investigate basin response to hydroclimatic variability in southwest Alaska. On a subannual timescale, the characteristics of the coarse basal unit typical of clastic varves were controlled by spring snowmelt and runoff, while the fine-grained cap was shaped by fall and winter conditions. Coarse subannual laminations within the clay cap were associated with autumn storm activity and winter warming events brought about by the interplay between regional maritime and continental air masses. Biogenic silica (BSi) profiles show two annual peaks in siliceous algal deposition; the first occurred at the end of the spring freshet, the second in the late summer. On an annual timescale, varve thickness was dependent on total annual discharge (r2=0.75, n=43, p<0.0001), while maximum annual grain size was determined to be dependent on both maximum spring discharge (r2=0.63, n=43, p<0.0001) and total annual discharge (r2=0.61, n=43, p<0.0001). On interannual timescales, relationships between climate variables (temperature, precipitation, North Pacific (NP) and Pacific Decadal Oscillation (PDO) indices) and both regional discharge and varve thickness were insignificant. When the same data sets were analyzed on multidecadal timescales, regime shifts in varve thickness and total annual discharge coherent with shifts in NP and PDO indices were identified. Periods of increased varve thickness and total annual discharge were associated with warm PDO phases and a strengthened Aleutian Low. The 289-year varve record was used to reconstruct PDO dynamics prior to 1900. Multidecadal shifts were apparent throughout the 19th century, but were absent in the 18th century. This study provides insight into the linkages between regional climate, physical and biological deposition, and can be used to improve the interpretation of seasonal and annual-scale paleoclimate reconstructions for basins similar to Shadow Bay. This study also sheds light on the nature of multidecadal climate variability in the Pacific Northwest region prior to the instrumental record.