A Comparison of Lake Sediments and Ombrotrophic Peat Deposits as Long-Term Monitors of Atmospheric Pollution

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Abstract
Ombrotrophic bogs receive all of their water and nutrients from the atmosphere. As such, these systems are potentially well suited to record the chronology and magnitude of atmospheric deposition of chemically nonmobile elements. Lakes receive focused material from their watersheds as well as directly from the atmosphere and thus may misestimate the atmospheric flux. Cores of profundal lake sediment and ombrotrophic peat from both a hummock and hollow were dated by Pb-210, utilizing the constant rate of supply (CRS) model, and analyzed for 13 major and trace elements. All cores, except the hollow, have yielded a pattern of increasing concentration of lead (Pb), zinc (Zn), and vanadium (V) since the mid-1800s. Net accumulation rates (NAR) over this period range from 0.1 to 5.0, 0.2 to 3.5, and 0.04 to 1.15 μg/cm2/year for Pb, Zn, and V, respectively, in the lakes. In the hollows, the values are 0.2 to 2.1, 0.1 to 0.86, and <0.01 to 0.11; in the hummocks, they are 0.2 to 4.1, 0.2 to 3.0, and <0.01 to 0.42. Actual atmospheric deposition rates are probably between the rates from the two sites in the bog. The possible overestimation of flux to lake sediment can be compensated for terrigenous input by normalization of trace metal flux to titanium dioxide (TiO2) flux. The adjusted ranges of atmospheric fluxes are 0 to 4.5, 0 to 2.4, and 0 to 0.4 μg/cm2/year for Pb, Zn, and V, respectively, in the lakes. The TiO2 normalization for peat chemistry is not a valid procedure, because the TiO2 content of peat is largely determined by atmospheric inputs that are erratic and that are large in relation to the background content. The large mineral TiO2 content of sediments results in a negligible atmospheric TiO2 signal in lake sediment.