Catchment and lake controls over the formation of varves in monomictic Nicolay Lake, Cornwall Island, Nunavut

On the basis of thin-section sedimentology, Cs-137 and Pb-210 profiles, and the pronounced seasonality of runoff and sediment delivery, sediments from Nicolay Lake, Cornwall Island, Nunavut (77 46'N, 94 40'W) are interpreted as varves. In thin section, the laminae are conformable, normally graded un its of silt and clay. Depending on the location in the lake, the varves fre quently contain one or more subannual rhythmites and inclusions of coarse s and and silt grains. Given the unstratified nature of the lake, the rhythmi tes are interpreted as products of sediment inflow events derived from rain fall, snowmelt, or mass wasting processes. In the most proximal site, these rhythmites may reflect insolation-driven diurnal variations in sediment tr ansport. Isolated coarse grains in the varves are interpreted as eolian sed iments washed off the lake ice cover. The lake is currently isothermal, and persistent ice cover and cold inflow prevent the formation of thermal stra tification. The high accumulation rate is a critical factor in varve format ion and it is probable that increased sediment yield during the past 500 ye ars has led to the formation of varves, compared to the underlying massive mud that accumulated when deposition was focused inland of the lake during higher relative sea level. Evidence in the catchment indicates that high-el evation deglacial deposits have acted as an important fine-grained sediment source throughout the Holocene. These sediments moved progressively downst ream through a series of basins by successive degradation and aggradation c ontrolled by glacioisostatic emergence, hence, limiting the progression of this paraglacial sediment wave to areas upstream of the lake until the late Holocene. These results identify the importance of shifting catchment boun dary conditions on sediment yield throughout the Holocene, and also indicat e the difficulty of interpreting low-frequency yield variations as the dire ct consequence of changing climate in similar varve records.