EFFECTS OF URBAN DEVELOPMENT AND ROAD SALT INPUT ON SEDIMENTATION RATES, HISTORICAL STRATIFICATION REGIME, AND VARVE FORMATION IN TANNERS LAKE, OAKDALE/WOODBURY, MN

Tanners Lake (in the eastern St. Paul, MN, metro area) is bordered by Interstate 94, a trailer park community, suburban housing, a public park and beach, and the 3M Corporation Center. Its surroundings and well-documented history make Tanners Lake ideal to study the effects of land use on lacustrine sedimentation. Salt used to de-ice roads since the 1960s have made lake bottom water more saline and have inhibited annual mixing and reoxygenation of the water column, allowing sedimentary laminations to be preserved. Land development including residential and commercial facilities, and a large increase in impervious area (parking lots and roads) has increased sedimentation rates. We studied a transect of cores from shallow to deep water, focusing on a freeze core collected from the deepest part of the lake, which represents the last ~170 years of the lake’s history. We used color analysis of a digital image of the freeze core to estimate sediment composition at a sub-millimeter scale, and loss-on-ignition (LOI) analysis for coarser quantitative compositional analysis. We used our LOI data to correlate our core with a previously-collected core that was dated using lead-210. Embedding slices of the core in epoxy allowed us to produce translucent thin sections which were inspected using both petrographic and scanning electron microscopes. The sediments show signs of water-column anoxia beginning in the lake around the year 1910. Given the area’s history, this likely occurred during the land-use transition from farmland to urban housing, although agriculture continued to be an important activity in the area and probably contributed considerable nutrients to the lake water. There are also signs of the construction of I-94 (and its predecessor State Highway 12) in the more recent sediment. The siliciclastic content of the sediment increases dramatically across the lake toward I-94 at its south end, and there is more organic matter away from I-94 in the shallow areas of the lake. Tanners Lake exhibits many sedimentary features which can be attributed to its surroundings and used to learn more about anthropogenic effects on urban lakes.