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

Abstract

On the basis of thin-section sedimentology, 137Cs and 210Pb 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 units of silt and clay. Depending on the location in the lake, the varves frequently contain one or more subannual rhythmites and inclusions of coarse sand and silt grains. Given the unstratified nature of the lake, the rhythmites are interpreted as products of sediment inflow events derived from rainfall, snowmelt, or mass wasting processes. In the most proximal site, these rhythmites may reflect insolation-driven diurnal variations in sediment transport. Isolated coarse grains in the varves are interpreted as eolian sediments washed off the lake ice cover. The lake is currently isothermal, and persistent ice cover and cold inflow prevent the formation of thermal stratification. The high accumulation rate is a critical factor in varve formation and it is probable that increased sediment yield during the past 500 years 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-elevation deglacial deposits have acted as an important fine-grained sediment source throughout the Holocene. These sediments moved progressively downstream through a series of basins by successive degradation and aggradation controlled 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 boundary conditions on sediment yield throughout the Holocene, and also indicate the difficulty of interpreting low-frequency yield variations as the direct consequence of changing climate in similar varve records.