Twentieth century varve formation in urban Twin Lake, Golden Valley, MN: Evidence for deposition of multiple laminae per year

Abstract

Lakes in urban settings are subject to a suite of environmental perturbations from the surrounding cityscape, such as increased flux of nutrients, input of road salts, and erosion of shoreline from housing and recreation. High-resolution lamination studies of a small, deep lake in central Minnesota suggest that the lake experiences unusually high sedimentation rates as a result of urbanization in the basin. Twin Lake is located in Golden Valley, Minnesota, five miles from downtown Minneapolis. It is a small, deep lake with a surface area of 8.5 hectares and a maximum depth of 16.6 meters. Twin Lake is relatively isolated from urban development due to its steeply sloped shoreline and remains sheltered from wind-driven mixing by its steep banks and tree cover on the hills surrounding the lake; the lake is bordered on the east by Wirth Park with luxury housing developments on the northern and western shores.

In order to understand the impact of twentieth century urban sprawl and construction on the sediment lithology, a freeze core was collected for high-resolution color analysis, thin section preparation, and smear slide analysis. Lamina counting and lead-210 dating provide contrasting age models for Twin Lake, suggesting multiple laminae are deposited annually, with up to 1 cm per year of sediment deposition. Mass accumulation rates reach 0.4 g/cm2 yr (over 10 times the background rate) during the late 1970s and early 1980s, when historical water quality monitoring data indicate poor conditions. Such extraordinary sedimentation rates are due to high rates of authigenic calcite precipitation, diatom deposition, and wash-in of terrigenous material from the steep banks surrounding the lake. Diverse diatom assemblages and diagenetic iron phosphate minerals in the sediment indicate high levels of nutrient flux to the lake and support the hypothesis that Twin Lake has experienced unusually high sedimentation rates in the past 100 years.