Radiocarbon dating is one of the most reliable and well-established methods for dating the past ~50,000 years. 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 has not been constant.
Here, we report the highprecision, high-resolution atmospheric 14C record from a Huon pine in Tasmania for improved radiocarbon calibration for the early Younger Dryas. Our study also allows for a critical evaluation of mechanisms of atmospheric 14C variations and abrupt climate change during the Last Deglaciation, ~20,000 – 11,600 calendar years before present
Radiocarbon calibration
Variations in atmospheric 14C concentrations are mainly due to variations 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.
Theresult is that radiocarbon an 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.
The internationally ratified calibration curve charcoals and macro-fossils) covers the past 26,000 years [1]. This curve is reliably based
on dendrochronologically dated for the period 12,400 – 0 calendar years before present (BP). For the remaining period 26,000 – 14,000 calendar years BP, the curve is derived fromindependently dated marine samples such as corals and foraminifera in varved marine sediments.
However, these archives may not be ideal for radiocarbon calibration because they are subject to uncertainties in varve counting and in the assumption of a constant marine reservoir age correction (a 14C age offset due to upwelling of old carbon from the ocean interior) applied for each marine sampling site, which may not be valid for intervals of rapid climate change during the Last Deglaciation (~20,000 – 11,600 years ago).
Tasmanian Huon-pine logs
We have studied four sub-fossil logs of Huon pine with clearly defined and measured annual tree rings which were excavated from alluvial sediments along Stanley River in north-western Tasmania, Australia, at 41°41’S, 145°18’E. These logs are quite well preserved and contain several hundreds of tree rings each (Figure 1). We performed radiocarbon measurements on sequential decadal samples from each of the logs.
A total of 137 samples were pre-treated to alphacellulose, converted to CO2 and then graphite for Accelerator Mass Spectrometry 14C analysisusing the ANTARES facility at ANSTO [2]. Final total analytical 14C measurement precision for most samples ranged from 0.3 to 0.4% (24 – 32 14C years) obtained after a 1-hour runtime. Based on ring-width information and radiocarbon results, we have successfully constructed a floating 617-ring Huon pine chronology, covering an age range from 10,350 to 10,760 14C years BP. New tree-ring 14C record and its implications
The 14C age range and patterns of the older part and middle portion of the Huon pine record are similar to those of the younger part of a 1382- ring Late Glacial Pine chronology from Europe [3], and to those of the older portion of extended European absolute chronologies [4], respectively.
This allowed us to link our Huon pine 14C record to the floating Late Glacial Pine record and anchor it to the absolute tree-ring timescale (calendar years), by 14C wiggle matching (Fig. 2). This resulted in a continuous and reliable atmospheric 14C record based on tree rings for the past 14,000 cal BP (Fig. 3) for improved radiocarbon calibration.
Large differences of up to 400 years between tree-ring radiocarbon ages and those of marine samples during the early Younger Dryas from 13,000 to 12,600 calendar years BP (Fig. 3) indicate that marine-derived 14C records do not faithfully represent atmospheric 14C during periods of abrupt climate change, due to possibly large changes in marine reservoir age which were not accounted for in these records.
In addition, the availability of the continuous and reliable tree-ring-based atmospheric 14C record for the past 14,000 calendar years BP allows for a critical evaluation of mechanisms of atmospheric 14C variations and abrupt climate change during the Last Deglaciation. In particular, by comparing the tree-ring 14C with marine derived 14C and modelled 14C based on ice-core 10Be fluxes, we conclude that changes in ocean circulation weremainly responsible for the onset of the Younger Dryas cold reversal, while a combination of changes in ocean circulation and 14C production rate were responsible for atmospheric 14C variations for the remainder of the Younger Dryas [4].
Our work, together with previous studies, indicates that investigations of atmospheric radiocarbon variations over time can deliver crucial information about climate change and changes in ocean circulation in the past. It demonstrates that marine-based 14C records, which can be affected by changes in ocean circulation during abrupt climate change, are not ideal for radiocarbon calibration.
Authors
Quan Hua, David Fink, Vladimir Levchenko, Andrew Smith and Fiona Bertuch