ANTHROPOGENIC INFLUENCE ON SEDIMENT COMPOSITION AND MAGNETIC PROPERTIES OF BROWNIE LAKE, MINNEAPOLIS, MINNESOTA

Brownie Lake is the smallest water body in the Minneapolis Chain of Lakes, a focal point of natural beauty in the heart of the city. For a century these lakes have been heavily managed for water quality and lake level. Connections via artificial channels, dredging and filling shallow areas, and even reshaping the shorelines have been done for recreational and aesthetic purposes. These manipulations have dramatically affected the limnological conditions in Brownie Lake, which experienced a rapid drop in lake level of 2 to 3 m in the 1910s, caused by landscape modifications related to its connection to Cedar Lake, and to the construction of a nearby railway. As a result, lake area decreased by 50%, and the water column became meromictic due to better sheltering from wind in its lower position in the basin. The lake level drop was followed by a short period of high sediment flux from the exposed littoral area, which can be seen in the sediment column as a layer containing abundant siliciclastic material, as well as shallow-water authigenic carbonates and benthic diatoms. This spike in sediment flux is superimposed on sedimentation rates that have increased by an order of magnitude, from the inception of land clearing for agriculture by European settlers until the mid-twentieth century. The combination of agricultural and urban runoff had the effect of increasing nutrient loads, which led to the progressive eutrophication of Brownie Lake. This promoted at least seasonal anoxia in the bottom waters even before the artificial lake lowering, as evidenced by the appearance of varves, decreasing sedimentary Fe/Mn ratios, and a shift in diatom communities. The one order of magnitude increase in sedimentation rates can be seen in the flux of ferrimagnetic coarse detrital and fine-grained interacting particles delivered to the sediment column. By comparison, the flux of non-interacting biogenic magnetite particles and chains produced intracellularly by magnetotactic bacteria only increased by a factor of two. However, this lower rate of increase may just be an effect of the oxic-anoxic boundary migrating from the sediment into the water column. This upward shift could have created the conditions for magnetite chains to collapse while still in the water column, thus reducing the chance for their burial in intact form.