Over the past several decades U–Th dating, also known as U-series disequilibrium dating, has been applied to geologic archives of past sea-level position such as fossil coral reefs, submerged speleothems, aragonite carbonate bank sediments, and fossil mollusks in beach deposits.This chapter emphasizes the processes and issues pertaining to the U-series dating of fossil corals, which is the most widespread application of U-series dating in studies of past sea-level position.Where the motion of the land can be constrained, measurements of past sea level markers allow local sea level changes to be determined.
Higher precision, and longer ranging, chronology can, however, be determined through uranium-thorium dating.
When the coral precipitates its skeleton the calcium carbonate incorporates small quantities of natural uranium from the surrounding seawater (typically 3 parts per million).
To place these markers in the past, corals also prove particularly well suited.
Their calcium carbonate skeleton offers up the potential for carbon dating (up to 30-40,000 years), because it is 12% carbon.
To recover corals that grew at times when sea level was lower then at present it is necessary to drill into the sea floor at depths of up to 140 meters below sea level.
Two IODP expeditions to Tahiti, French Polynesia, and the Great Barrier Reef, Australia recovered such material.
The important characteristic of common lead is that it contains no significant proportion of radiogenic lead accumulated since the time that the mineral or rock phase was formed.
Of the four isotopes of lead, two are formed from the uranium isotopes and one is formed from the thorium isotope; only lead-204 is not known to have any long-lived radioactive progenitor.
Corals provide a useful sea level marker because they are exclusively marine organisms, so provide an absolute constraint that sea level must not have been bellow the position of the coral when it was alive.
Additionally many corals have a limited water depth habitat, and therefore provide some constraint as to how much higher sea level could have been above the coral when it grew.
Uranium-series dating is a critical tool in quaternary geochronology, including paleoclimate work, archaeology and geomorphology.