USING SEDIMENT CHRONOLOGIES TO ASSESS RECOVERY OF ANTHOROPOGENICALLY DISTURBED LAKES: MAKING CORRECT INTERPRETATIONS

Sediment cores are extremely useful tools for assessing recovery of anthropogenically disturbed lakes, since lakes act as chemical “tape recorders” of deposition over time. However, much of the information contained in lake sediments can be lost or misinterpreted if the approach to assess recovery is target specific (single chemical). For this study, a multi-elemental approach was used to assess recovery from historic anthropogenic inputs of mercury (Hg) into a Michigan lake, (Upper Peninsula) Michigan, which resulted in elevated concentrations of Hg in the sediments. Sediment cores were analyzed for 1) heavy metals in the sediments, 2) heavy metals in the porewater, 3) Hg isotopes and 4) 210Pb and 137Cs.

Cores were taken from this lake during the summer of 2000 using an MC-400 Lake/Shelf Multi-corer. One sub-core was sectioned for total metal content, which was extracted via nitric acid, microwave digestion; another core was “squeezed” to extract porewaters. Both the digestive fluids and porewaters were analyzed for twenty-two metals including mercury, using hexapole inductively coupled mass spectrometry and atomic absorption spectroscopy. The final sub-core was sectioned for 210Pb and 137Cs analysis, yielding sediment ages, porosity and sedimentation rates.

Results show constant Hg loadings for the last 10 years, which previously has been interpreted as recent sediments mixing with Hg contaminated sediments at depth (bioturbation). This suggests that Hg concentrations at the surface would be lower if mixing was not occurring, but concentration patterns vary during the same time frame for other elements (e.g. Sr, V, Zn,), suggesting bioturbation is in not the cause for constant Hg loadings. Also, isotope ratios of Hg change within the cores, this change has been interpreted to be a shift in source in other lakes (e.g., increasing atmospheric contribution), but this phenomenon was due to instrumental effects, which caused ratios to vary depending on concentration and not natural processes. Thus, although Hg loadings to the lake have decreased since the mid 80’s, concentrations are still high and indicate continued Hg input from the watershed, an interpretation not made from a target specific approach. Furthermore, apparent isotopic shifts in Hg must be viewed with caution when assessing recovery of lakes.