DISCORDANCY BETWEEN THE SEDIMENT ACCUMULATION RATES OBTAINED USING EXCESS PB-210, CS-137 AND PU-239,240: FACTORS AND PROCESSES

It is about 5 decades since the excess Pb-210 method was first employed to date sediment cores. Event markers such as Pu-239,240 and Cs-137 have been used over four decades for chronological applications. The distinct differences on the sources and their geochemical behavior of these nuclides pose continuous challenge in their effective utility as chronometers. The variable atmospheric deposition of Pb-210 due to varying amounts of precipitation, additional production of Pb-210 at the bottom of sediment cores from submarine groundwater discharge, physical and/or biological sediment mixing, and variable mass accumulation rates in a core lead to more challenges in obtaining reliable Pb-210 chronology. At present, Cs-137 peak have decreased to about 1/3 of the original concentration level due to radioactive decay. Diffusion of Cs-137 through porewater in certain environment has made the Cs-137 marker less useful. Furthermore, the event markers (Cs-137 and Pu) generally gives the age of one horizon and highly variable sedimentation rates since 1963 could make the chronology less reliable.

Based on our extensive sediment dating work from freshwater and saltwater systems, we present results that will discuss the following: i) How do the vertical profiles of excess Pb-210 are affected by submarine groundwater discharge in estuarine and freshwater systems which affects the chronology of sediment cores; ii) How do the variable inputs of Pb-210 due to varying amounts of precipitation affect the sediment chronology in freshwater and salt water systems; iii) What are the possible factors that cause the discordancy between the mass accumulation rates obtained using Cs-137 and Pb-210; iv) How do the physical/biological mixing rates affect the sediment accumulation rates obtained using Pb-210 and Cs-137; and v)What does the Pb-210 chorology tell us compared to that of Cs-137 and Pu? Why they are different in several cases? This talk will give an overview on problems associated with each of the isotope and provide possible strategies to effectively use these as chronological tracers.