Concentration of diatom frustules in Lake Michigan sediment cores1

The concentration of diatom frustules was determined in 14 sediment cores that were dated by 210Pb and 137Cs techniques. The concentration in the 0–0.5‐cm sediment layer varied from 6.31 × 105 to 3.71 × 107 frustules g dry wt−1. The annual deposition of frustules to the sediment ranged from 7.00 × 103 to 5.20 × 105 frustules cm−2 yr−1, <5% of the estimated annual diatom production based on average annual diatom standing crop, turnover rate, and depth of the euphotic zone. The vertical distribution of frustules in every core showed a logarithmic decrease of at least two orders of magnitude in a depth equivalent to 80–100 years B.P. Scanning electron photomicrographs of frustules from the sediment show evidence of dissolution. Dissolved silica concentrations in deep and interstitial water support the conclusion that diatom frustules are dissolving in both the water column and sediment. The permanent sediment is probably not a major sink for particulate biogenic silica in Lake Michigan.