Radium and radium-daughter nuclides in carbonates: a brief overview of strategies for determining chronologies

摘要

Radium isotopes have been used extensively to trace the movement of groundwater as well as oceanic water masses, but these radionuclides (and their daughters) are also useful chronometers for the determination of the time scales of other Earth and environmental processes. The purpose of this overview is to present the application of Ra and Ra daughters in the dating of carbonates. We show that the choice of dating method (decay of excess radionuclide or ingrowth of daughter) depends strongly on the parent/daughter activity ratios in the water in which the carbonate was precipitated. Thus freshly precipitated carbonates uniformly show excesses of ~(226)Ra relative to its parent ~(230)Th, and ~(226)Ra decay can provide ages of carbonates over Holocene time scales. In contrast, carbonates are precipitated in waters of greatly varying ~(210)Pb/~(226)Ra. Corals, deep-sea hydrothermal vent clams and the shelled cephalopod Nautilus live in waters with significant dissolved ~(210)Pb and all show excesses of ~(210)Pb in their carbonate. Bivalve molluscs from nearshore and coastal waters, and carbonates deposited from groundwater environments (e.g. travertines) in which ~(210)Pb is efficiently scavenged from solution, show deficiencies of ~(210)Pb relative to ~(226)Ra. In contrast, fish otoliths strongly discriminate against ~(210)Pb regardless of the environment in which the fish lives. Deficiencies of ~(228)Th relative to ~(228)Ra are common in all carbonates. Useful time ranges for the ~(210)Pb/~(226)Ra and ~(228)Th/~(228)Ra chronometers are ～ 100 y and ～ 10 y, respectively.