Nuclear techniques are providing valuable insights into the mechanisms of abrupt climate change, particularly for one of the most important and controversial periods during the last deglaciation, the Younger Dryas (YD) cold reversal, which occurred around 11,600-13,000 years ago.
By comparing the Carbon 14 (14C) signature in tree rings from sub-fossil Tasmanian Huon pine with atmospheric 14C derived from marine samples, researchers from ANSTO's Institute for Environmental Research (IER), in collaboration with investigators from the University of Queensland and universities and research institutes in Germany and Switzerland, were able to conclude that changes in ocean circulation were mainly responsible for the rapid onset of the YD, while a combination of changes in ocean circulation and 14C production rate were responsible for atmospheric 14C variations for the remainder of this period.
Radiocarbon dating is one of the most reliable and well-established methods for dating the past ~50,000 years. The radiocarbon age of a sample is determined by measuring its 14C concentration and by assuming a constant level of atmospheric 14C through time. However, not long after the establishment of the radiocarbon dating method in the late 1940s, it was recognised that the 14C concentration of the atmosphere in the past had not been constant.
These variations are due to changes in the rate of radiocarbon production in the atmosphere, caused by changes in the Earth's magnetic field, variability in solar activity and changes in the carbon cycle. As a result, radiocarbon and calendar ages are not identical, and the radiocarbon ages have to be converted to calendar ages using a calibration curve, which describes the atmospheric 14C concentration in the past measured in precisely and independently-dated materials.
ANSTO researchers, Drs. Quan Hua, David Fink, Vladimir Levchenko, Andrew Smith and Fiona Bertuch, took radiocarbon samples from four sub-fossil logs of Huon pine excavated from alluvial sediments along Stanley River in north-western Tasmania and analysed sequential decadal samples using ANSTO's ANTARES facility.
By linking their Huon pine 14C record to the floating Late Glacial Pine record and anchoring it to the absolute tree-ring timescale (calendar years), the team was able to provide a continuous and reliable atmospheric 14C record based on tree rings for the past 14,000 calendar years for improved radiocarbon calibration.
In the study, published in the December 2009 issue of the prestigious journal Quaternary Science Review, the team found large differences of up to 400 years between tree-ring radiocarbon ages and those derived from marine samples during the early YD from 13,000 to 12,600 years ago. This indicates that marine-derived 14C records do not faithfully represent atmospheric 14C during periods of abrupt climate change, highlighting the fact that changes in ocean circulation were mainly responsible for the onset of the YD.