Biogeochemistry of Thoreau's Bog, Concord, Massachusetts

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Abstract
Thoreau's Bog in Concord, Massachusetts, is a floating—mat Sphagnum bog developed in a glacial kettle hole. Low shrub vegetation of the open mat is dominated by Chamaedaphne calyculata; trees include scattered Picea mariana and Larix laricina. Hydrological investigations show the bog to be ombrotrophic, with an annual water input of 1.45 m and an annual runoff of 0.24 m. Corresponding metal inputs are 88, 132, and 54 mg°m—2°yr—1 for K, Mg, and Pb, respectively. K and Mg are vertically distributed in the bog profile in accord with the inhomogeneous ion exchange chemistry of peat, while lead is distributed in accord with historical trends in atmospheric lead fallout. Isotopic dating using 210Pb is a valuable tool for determining net peat accumulation rate, which is 180 g°m—2°yr—1. Annual storage rates of K, Mg, and Pb in peat amount to 36, 54, and 46 mg°m—2°yr—1, respectively. Bog acidity (pH = 3.8) is maintained by organic acids at concentrations of 10—3 eq/L. The effect of cation exchange on bog acidity is modest, while the much larger contribution of "acid rain" is offset by alkalinity increases of the same magnitude resulting from sulfate reduction and nitrate uptake. These latter processes are, in effect, a strong buffer mechanism against acid rain. 210Pb dating and historical records suggest that the floating mat is relatively young, perhaps as few as 500 yr old. These data raise the possibility that the bog is not a relict of colder, early postglacial periods, but instead, may have developed under modern climatic conditions.