The occurrence and significance of lead isotopes in pelagic sediments

Abstract

Leads representing a major portion of the continental surface of the earth have been sampled in pelagic sediments and isotopically analysed. About two-thirds of the lead in all Pleistocene pelagic sediments have been chemically precipitated from lead dissolved in sea water. The remaining one-third has been transported as a constituent of solid particles. The rate of chemical deposition of lead tends to be uniform everywhere, while the rate of mechanical deposition of lead varies considerably so that the .abundance ratio of these two types of occurrences is different in various pelagic deposits. Dissolved lead in the oceans originates from the chemical denudation of continents, and different drainage regions contribute leads of different isotopic composition to the oceans. The travel time of the dissolved lead which is chemically precipitated is short compared to the mixing time of different large water masses so that lead of a uniform isotopic composition is not simultaneously deposited everywhere on the ocean floors. This phenomenon prevents the use of lead as a chronometrie index in lead-rich, uranium-poor marine sediments for intervals shorter than 100 million years. The present rate of soluble lead denudation, free of industrial contamination, is seven times greater than the average rate of chemical precipitation for the Pleistocene. Lead is transported from the continents to pelagic regions in dispro-portionately greater amounts than uranium and thorium, so that the isotopic evolution of lead in surface materials of the continents is measurably affected, and the lead denuded from some regions of extensively reworked sediments contains abnormally large proportions of Pb 206. The gross features of the isotopic variations of leads denuded from continental surfaces can be accounted for by postulating: 1. (1) that these leads have evolved within the continental crust above the Mohorovicic discontinuity; 2. (2) that the crust consists of two phases, an exterior phase of high U/Pb and an interior phase of low U/Pb; 3. (3) that the exterior phase contains lead isotopically homogeneous with the rest of the crust at the time the exterior phase was formed; and 4. (4) that isotopic variations arise from different periods of aging within the exterior phase., The mean isotopic composition of lead to-day within the continental crust is obtained from the intersection of the marine lead isotopic regression line and the primary isochron of zero age for the earth. The values arer Pb 206Pb 204 = 18.5 8, Pb 207/P.b 204 = 15.7 7, Pb 208/Pb 204 = 38.8 7.