| 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 Earths 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. |