Radium and radium-daughter nuclides in carbonates: a brief overview of strategies for determining chronologies.
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 226Ra relative to its parent 230Th, and 226Ra decay can provide ages of carbonates over Holocene time scales. In contrast, carbonates are precipitated in waters of greatly varying 210Pb/226Ra. Corals, deep-sea hydrothermal vent clams and the shelled cephalopod Nautilus live in waters with significant dissolved 210Pb and all show excesses of 210Pb in their carbonate. Bivalve molluscs from nearshore and coastal waters, and carbonates deposited from groundwater environments (e.g. travertines) in which 210Pb is efficiently scavenged from solution, show deficiencies of 210Pb relative to 226Ra. In contrast, fish otoliths strongly discriminate against 210Pb regardless of the environment in which the fish lives. Deficiencies of 228Th relative to 228Ra are common in all carbonates. Useful time ranges for the 210Pb/226Ra and 228Th/228Ra chronometers are approximately 100 y and approximately 10 y, respectively.
Schmidt
Sabine
S
CNRS, UMR5805 EPOC, Avenue des Facultés, F-33405 Talence Cedex, France. s.schmidt@epoc.u-bordeaux1.fr
Cochran
J Kirk
JK
D000818:Animals; D037421:Anthozoa / Q000737:chemistry; D049872:Bivalvia / Q000737:chemistry; D002254:Carbonates / Q000737:chemistry*; D002910:Chronology as Topic; D004777:Environment; D005399:Fishes; D005843:Geography; D007856:Lead Radioisotopes / Q000032:analysis; D049850:Nautilus / Q000737:chemistry; D009792:Oceans and Seas; D010037:Otolithic Membrane / Q000737:chemistry; D011883:Radium / Q000032:analysis*; D013910:Thorium / Q000032:analysis; D014867:Water / Q000737:chemistry*; D014875:Water Pollutants, Radioactive / Q000032:analysis*
D002254:Carbonates; D007856:Lead Radioisotopes; D014875:Water Pollutants, Radioactive; D014867:Water; D013910:Thorium; D011883:Radium