Sediment Diatom and Metal Stratigraphy from Rocky Mountain Lakes with Special Reference to Atmospheric Deposition

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Abstract
We addressed the hypothesis that current levels of acidity in precipitation falling on Rocky Mountain National Park, USA, are greater than historical levels. Sediment cores from four subalpine lakes were sectioned and dated with 210Pb. The stratigraphic records of metals were used to detect trends in the deposition of Pb, Cu, Zn, and V because these metals are known to increase in areas receiving air masses polluted with fossil-fuel emissions. Diatom assemblages over time were used as indicators of change in lake water chemistry. Reconstructions of pH from four lakes were prepared using Index B, Index α, and predictive equations derived from an additional 19 lake surface sediments. The metal stratigraphy, diatom stratigraphy, and inferred pH profiles of the four study lakes indicate no historical influence on pH attributable to atmospheric deposition. Common to all four cored lakes, however, is a persistent increase in the deposition rate of Pb starting in the period between 1855 and 1905. The most likely source of the Pb is airborne particules from mining activity in Colorado which began in the mid-1800s. Recent high levels of Pb within the sediments may be due in part to emissions related to automotive leaded gasoline combustion.