Using varve thickness records from three proglacial lakes in south-central Alaska as proxies for climate variations on multidecadal and interannual timescales

Glacial varves are powerful tools for environmental reconstructions thanks to their annual timeframe and capacity to record high-resolution climate variability. This study paves the way for future environmental investigations... [ view full abstract ]

Glacial varves are powerful tools for environmental reconstructions thanks to their annual timeframe and capacity to record high-resolution climate variability. This study paves the way for future environmental investigations in the proglacial Eklutna, Kenai and Skilak Lakes (south-central Alaska) by confirming the annual nature of laminations in a series of gravity cores. Radionuclide dating (210Pb/137Cs) has been conducted on top sections from Eklutna and Skilak. Both 210Pb age-depth models and AD 1963 137Cs-peaks show good agreement with lamination counts, permitting the term ‘varves’ to be used. Especially in Skilak Lake, this has been a topic of speculation in the past. Repeated varve counts have been conducted by independent observers, allowing their reproducibility to be assessed and chronology errors on age-depth models and varve thickness records to be estimated. Apart from radionuclide dating, marker beds are also able to provide absolute dates if they can be related to historically reported events (e.g. earthquakes, floods). Identification of erosive mass-transport deposits and turbidites triggered by the AD 1964 Prince William Sound (PWS) earthquake (Mw 9.2) is fairly secure, given their position close to the AD 1963 137Cs-peaks in Eklutna and Skilak cores. In the Kenai Lake records as well, a similar turbidite can be observed and attributed to the AD 1964 PWS earthquake, compensating for the absence of other absolute dates in the latter lake deposits. In order to assess the relationship between annual sedimentation rates (i.e. varve thicknesses) and climate parameters (temperature, rainfall and snowfall), average varve thickness records for each lake were constructed and calibrated with data from weather stations close to the studied lakes. Relationships between varve thicknesses and climate parameters were investigated on multidecadal (by visual comparison) as well as on interannual timescales (by calculation of linear correlation coefficients). Results show that annual cycles of sediment accumulation in Eklutna and Skilak Lake on the one hand, and in Kenai Lake on the other hand, respond differently to fluctuating climate parameters. This difference can be related to the degree of proximity and dominance of large glaciers in the lake catchments. Water and sediment supply to Eklutna and Skilak are strongly bound to climate influenced glacier dynamics, whereas Kenai possesses a rather complex catchment, in which glacier-free valleys are prominent. As a result, rainfall in the Kenai catchment plays a larger role in generating increased river discharges with elevated sediment entrainment capacities, and hence thicker varves.