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Radiocarbon Calibration

Radiocarbon dating is an important tool for studying the natural variability of the global climate system. High-resolution paleoclimate records show that many large, abrupt changes occurred during the last deglaciation, and there is increasing evidence that some of these may have been global in scope. Radiocarbon dating is necessary for determining the relative timing of these changes and their propagation through the climate system, but its utility is limited by the fact that the radiocarbon ‘clock’ runs at different speeds depending on the atmospheric 14C inventory. Studies have shown that atmospheric 14C concentration (D14C) has varied significantly through time, as a result of changes in 14C production rate (a function of variability in the Earth’s geomagnetic field strength and solar activity), and redistribution of 14C between reservoirs, (primarily a function of oceanic thermohaline circulation). Because of D14C changes, the radiocarbon time scale departs from true calendar ages in a nonlinear fashion by as much as 3000 years.

The highest resolution radiocarbon calibration data set, based on tree-ring chronologies from German oaks and pines, currently begins around 12 calendar kyr BP, just prior to the abrupt termination of the Younger Dryas cold period, a dramatic climatic oscillation lasting from about 13 to 11.7 cal kyr BP.

Tree rings thus do not provide calibration during several of the other large and abrupt climate changes of the last deglaciation. Previous attempts to extend radiocarbon calibration prior to the interval covered by tree-rings have used paired 14C-U/Th dates on corals as well as annually laminated sediment (varve) chronologies with 14C-dated macrofossils from European lakes and the Baltic Sea. Each of these data sets agrees with the tree-ring calibration for the period later than about 12 cal kyr BP. Prior to 12 cal kyr BP, however, the coral results disagree with the longer chronologies based on varves. The lack of agreement between coral and European varve data sets, together with the discontinuous or low-resolution nature of these time series, introduces substantial uncertainty to 14C calibration prior to 12 kyr BP.

A new 14C calibration data set has been constructed from varved marine sediments of the Cariaco Basin that spans most of the last deglaciation. Several independent lines of evidence demonstrate the accuracy of both Cariaco Basin calendar (varve) and 14C chronologies. These data extend an accurate, continuous radiocarbon calibration an additional 3000 years before tree-rings to the Glacial/Bølling event boundary and resolve short-lived changes in 14C during the early Younger Dryas and Bølling/Allerød periods that are not resolved by standard calibration data sets based on corals.