Shadow Bay is a 4.3 km², 69 m deep basin situated in the Ahklun Mountains of southwest Alaska. Climate in the region is dynamic, marked by the interplay between maritime and polar continental regimes. The relative importance of these systems influences regional weather, particularly during the winter months when air temperature can fluctuate dramatically over diurnal time scales. The dynamic nature of climate in the region coupled with the annually-laminated (varved) nature of the lake sediments make Shadow Bay a prime location to investigate watershed response to climate variability. This project combines contemporary process studies of sediment traps with sedimentary records to further our understanding of hydroclimate variability in the region where no other annual-resolution records are available.

Stratigraphic analyses of lake cores collected in 2006-7 suggest that the Shadow Bay varve record is roughly 295 years long. The annual timescale of these sedimentary features is substantiated by independent ¹³⁷Cs and ^239+240Pu dating profiles that show the 1963 interval of peak radioisotope fallout to be located approximately 44 sedimentary units below the surface. The inferred varves are complex, with annual units commonly containing multiple sub-annual sedimentary structures. It appears that the composition of individual varves can be linked to spring snowmelt, rainfall, winter warming events, and avalanche activity. In addition to varve thickness and grain size measurements, a detailed classification system has been used to group varves into types based on their sedimentological characteristics (Fig. 1). This type of detailed sedimentary analysis provides an additional tool with which to evaluate relationships between regional hydroclimate and basin sedimentation.

Sediment traps deployed with lake temperature sensors and a weather station in 2006-7 allow evaluation of the linkages to be made between depositional processes and local weather conditions. Changes in grain size in the traps can be linked to the summer melt period, fall/winter storm activity and lake overturn, and clearly indicate that fall storm activity and winter warming events influence sedimentation in Shadow Bay (Fig. 2). Analysis of biogenic silica (BSi), indicative of the sedimentary abundance of diatoms, is underway and will be used to further link sub-annual sediment delivery phases to variations in lake productivity. Preliminary results show peak BSi concentration associated with maximum water temperatures, although nutrient supply may limit productivity in late spring/early summer.

These results represent the initial stages of a longer term effort to evaluate the paleoclimate of this region of southwestern Alaska and to contribute to larger syntheses of climate variability. Ongoing sediment trapping, lake instrumentation and catchment weather monitoring are expected to further increase our interpretive power of this complex sedimentary record.