GEOCHEMISTRY OF MN AND FE IN LAKE-SEDIMENTS IN RELATION TO LAKE ACIDITY

Concentrations of easily reducible Mn (ER Mn, Mn oxides), easily reducible + reducible Fe (ERR Fe, Fe oxides), and alkaline extracted Mn and Fe (org Mn and Fe, organically bound) were determined in sediments of three circumneutral (CN) and four acidic lakes. Up to 80% of total sediment Fe (total = ERR + org Fe) was removed as ERR Fe in the CN vs. acidic lakes. Acidic lakes contained twice the total Fe compared to CN lakes, which was distributed equally between the ERR Fe and org Fe sediment fractions. Mn was recovered as ER Mn from all seven lakes. Two of four acidic lakes contained little ER Mn (due to the pH-dependent decrease in particulate Mn precipitation). However, two acidic lakes contained surficial concentrations that were equivalent to CN lakes. ER Mn in sediments of acidic lakes is attributed to sediment diagenesis. Differences in ERR Fe and org Fe in sediments of acidic vs. CN lakes may be due to the pH-induced precipitation of Fe humic complexes.