Accretion rates of freshwater manganese deposits

Abstract

Freshwater ferromanganese concretions have been assigned growth rates ranging from 0.1 to 3,000 mm/103 yr (refs 1 and 2); however, most of these are simply educated guesses or are based on uncertain assumptions of the mechanism of their formation. Many of these guesses are based on the assumption that if nodules do not grow upward more rapidly than sediments accumulating nearby they will be buried. But in the deep sea there are many examples of manganese nodules with growth rates a factor of 103 lower than the adjacent sediment deposition rates3-5. So far no systematic studies to date freshwater nodules radiometrically have been reported. We have therefore studied the growth rates of these deposits by radiometric dating techniques which, coupled with a knowledge of their chemical composition, could provide information regarding elemental deposition rates and their geochemistry in lacustrine environments. We felt that 228Ra (half-life 6.7 yr) and 210Pb (half-life 22.3 yr) would be ideal for radiometrically measuring the rates of accretion of these deposits if the growth rates are in the range of 100 to 1,000 mm/103 yr.