Abstract
Temporal variations in the atmospheric concentration of radiocarbon sometimes
result in radiocarbon-based age-estimates of biogenic material that do not
agree with true calendar age. This problem is particularly severe beyond the
limit of the high-resolution radiocarbon calibration based on tree-ring data,
which stretches back only to, about 11.8 kyr
before present (BP), near the termination of the Younger Dryas cold
period. If a wide range of palaeoclimate records are to be exploited for better
understanding the rates and patterns of environmental change during the last
deglaciation, extending the well-calibrated radiocarbon timescale back further
in time is crucial. Several studies attempting such an extension, using uranium/thorium-dated
corals and laminae counts in varved sediments,
show conflicting results. Here we use radiocarbon data from varved sediments
in the Cariaco basin, in the southern Caribbean Sea, to construct an accurate
and continuous radiocarbon calibration for the period 9 to 14.5 kyr
BP, nearly 3,000 years beyond the tree-ring-based calibration. A simple
model compared to the calculated atmospheric radiocarbon concentration and
palaeoclimate data from the same sediment core suggests that North Atlantic
Deep Water formation shut down during the Younger Dryas period, but was gradually
replaced by an alternative mode of convection, possibly via the formation
of North Atlantic Intermediate Water.
