INFLOW AND LAKE CONTROLS ON SHORT-TERM MASS ACCUMULATION AND PARTICLE SIZE: IMPLICATIONS FOR INTERPRETING VARVED LACUSTRINE SEDIMENTARY RECORDS

Physical processes were monitored in a varved lake in the Canadian High Arctic through three melt seasons and revealed that seasonal sediment deposition rates and patterns were highly dependent on short-lived inflow events driven by high suspended sediment concentrations that varied with changes to snow melt and runoff intensity. Furthermore, results illustrate that sediment accumulation rate varied spatially in accordance with suspended sediment discharge into the lake in a given year, indicative of the dependence on short-lived intense inflow conditions through the season. In addition, there is strong evidence that points to substantial decoupling between sediment deposition rates and mean grain size of the deposit. These results have important implications for paleoclimate interpretations of annually laminated sedimentary records from dynamic lake environments and suggest that grain size measures (e.g., mean, median, or mode) may not be representative proxies of river competence. As sediment is deposited through the season, the grain size fraction becomes diluted by proportionately finer grain accumulation and thus the mean grain size of a given sample may be biased towards the finer fraction and therefore could potentially underestimate the intensity of inflow processes. Grain size indices based on a measure of the coarser fraction rather than the bulk sediment (e.g., Francus, 1998) may be more appropriate to link contemporary processes (e.g., runoff intensity, flow competence, discharge magnitude) to sedimentary characteristics (e.g., varve thickness, down-core grain size variability).

Reference: Francus, P., 1998. An image-analysis technique to measure grain-size variation in thin sections of soft clastic sediments. Sedimentary Geology 121: 289-298.