A Technique for Disentangling Temporal Source and Sediment Variations from Radioactive Isotope Measurements with Depth

LIU JIANCHANG, and IAN LERCHE,* University of South Carolina, Columbia, SC, and JOLYNN CARROLL, University of Alaska, Fairbanks, AK

Radioactive isotope analyses are widely used in many aspects of geological studies--dominantly as a means of ascribing exact ages

to strata and to provide an assessment of sedimentation rates. However, because of the complexity of sediment transport, deposition, and accumulation, as well as of the effects of compaction, consolidation, and diagenesis, and because of the intrinsic variation of isotopic concentrations at deposition, it is often quite difficult to provide a quantitative "age profile," relating sediment depth to age using procedures developed to date.

This paper develops a quantitative procedure that permits the singular separation of effects due to source variations in time from those due to variations in sedimentation rate in observed profiles of any radioactive isotope concentration with subsurface depth.

The procedure guarantees to keep depth to time conversion positive, guarantees to provide a positive source variation with time, guarantees a least squares fit to measured isotope data with depth, and operates with any radioactive isotope.

Five excess activity (210)Pb profiles collected from Amazonian deltaic sediments are used to illustrate the general applicability and utility of the technique to diverse variations of profile patterns that are otherwise difficult to comprehend. The technique allow simultaneous determination of (1) sediment depth to time conversion; (2) intrinsic excess (210)Pb depositional variation with time (or depth); and (3) sediment accumulation rate variations with time (or depth), and such results are provided for each of the five cases. Sensitivity analyses provide an appreciation of the stability and parameter resolution obtainable, and illuminate the degree of uniqueness and accuracy of the disentanglement technique.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)