KNOWLEDGE of sedimentation rates in marine and lake deposits is of fundamental importance in understanding aquatic and sedimentary geochemical processes which may have been measurably altered since deposition times. A chronological method based on 210Pb (half life 21.3 yr) is useful in sedimentary environments in which sedimentation rates have been constant and sedimentation strata undisturbed by physical or biological mixing processes1–6. Studies in Lake Suwa and Osaka Bay have revealed, however, an anomaly in the 210Pb concentration in the lower level of the sediments. It is proposed here that the principal controls on 210Pb scavenging from water are both soil particles and hydrous oxides of manganese.