Climatic and limnologic setting of Elk Lake
Elk Lake is located on the Itasca moraine near the source of the Mississippi River in northwestern Minnesota. The basin is in calcareous glacial drift, and the lake water is a dilute solution of calcium and magnesium bicarbonate. Low-magnesian calcite formed by precipitation from the lake water has been a major component of the sediment throughout the lake’s history. The sediment also is laminated with alternating light and dark, millimeter-thick layers containing diatoms, organic matter, Fe(OH) 3 , and CaCO 3 . The sediment microstratigraphy has been preserved because the lake is unusually deep (maximum depth is 30 m) for its size (surface area is 1.01 km 2 ). Oxygen is present in low concentrations or absent in the deepest water during summer and winter. Water movements in the deepest part of the lake are insufficient some years for the complete aeration of the deepst water during spring and autumn circulation periods. Phytoplankton photosynthesis, which occurs mostly in the surficial 6 m of water, typically removes 0.5 g C m −2 day −1 from the epilimnion, which becomes strongly oversaturated with calcite during late spring and summer as the pH increases above the equilibrium pH (7.73) for calcite saturation. Most of the CaCO 3 that makes up the light-colored layers of sediments probably is formed during the late summer, when concentrations of calcium in the epilimnion decrease most rapidly. The silica and organic matter that form the darker sediment laminae are deposited earlier in the year, during a period extending from April to late June, when silica decreases fastest in the epilimnion.