EVOLUTION OF THE ATMOSPHERIC FLUX OF LEAD IN SOUTHERN MICHIGAN: INSIGHTS FROM A LAKE SEDIMENT CORE

The release of lead (Pb) and other toxic metal(loid)s into the atmosphere and their subsequent deposition into North American lakes have changed in response to anthropogenic activities over the last 200 years. Small headwater lakes located far from urban and industrial centers are ideal sites for studying long range pollution preserved in lake sediments. Lead concentrations driven by expanding use of coal and leaded gasoline generally increased over time, followed by decreasing concentrations after the phase-out of leaded gasoline began in 1975. This trend has been observed in lakes both big and small and isotopic studies have documented historical changes in the relative proportions of anthropogenic lead sources.

In this study, a 30 cm sediment core from the deepest part (10 m) of Matteson Lake, a 1.3 km² (320 acre) drainage lake in south central Michigan, USA, was collected in May 2023 for major and trace element concentration, and ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb isotope analyses using ICP-MS. Screening analyses of lead concentrations from 1 cm sections of the core stored in polyethylene Whirl-Pak bags were also measured with a Bruker Tracer 5i portable x-ray fluorescence (pXRF) device in mudrock mode. Curiously, Pb concentrations displayed an anomalous trend with negligible readings from the sediment surface to a depth of 27.5 cm in the core, where Pb concentrations increased to detectable levels. Although pXRF measurements of metal concentrations are known to be sensitive to moisture and organic matter, pXRF is generally able to detect Pb readily. Therefore, the screening concentrations were inferred to reflect the Pb profile.

If the pXRF measurements are confirmed by ICP-MS, the atypical trend may have resulted from homogenization of the upper portion of the core (bioturbation); however, we did not observe disrupted laminae. Alternatively, land use changes in the Matteson Lake watershed such as agricultural pivot irrigation and drainage tile systems installed over the past 50 years may have accelerated the rate of sedimentation since 1975. The sediment core will be dated using ²¹⁰Pb and ¹³⁷Cs activities measured with gamma spectrometry to determine sedimentation rates and reconstruct the historical atmospheric flux of Pb (and other toxic metal(loid)s) that reached the sediment-water interface of Matteson Lake.