Investigations of potential offsets between the 14C-calibration curve and tree ring chronologies from Sweden
The investigations of a potential offsets between the 14C-calibration curve and tree ring chronologies is part of the PhD-project Dendroclimatic analysis of pine wood from Swedish peat bogs - towards a precise Holocene humidity record that aims to reconstruct precipitation and groundwater variations with high temporal precision since the last deglaciation. The project is mainly based on dendrochronological analysis of growth variability of Scots pine (Pinus sylvestris) buried in Swedish peat bogs.
Tree rings have proven to be a valuable source in reconstructions of the climate of the past. Trees growing at, or close to, their limits of distribution normally produce annual rings that provide climate information. Climate change can cause hydrological variations and groundwater table fluctuations that can be seen in e.g. peat bogs and the occurrence of wetness sensitive trees. Periods of relatively warm and dry conditions can allow tree establishment on peat bogs. Such bog trees can become buried in the peat and preserved for thousands of years. So far, chronologies covering more than 2 500 years have been developed from circa 300 trees collected at Viss mosse, Hällarydsmossen and Åbuamossen.
The wood material has been given an absolute age by statistical and visual correlation tests against German tree ring chronologies. Several samples from each chronology have also been radiocarbon dated. By knowing the relative age between different radiocarbon-dated annual rings it is possible to precisely connect the 14C ages to the calibration curve and to give the chronologies an accurate 14C-age. However, a 145 years bias between the absolute dendrochronological age and the 14C-dated age was detected in the chronology from Hällarydsmossen (covering the period 4839-3728 BC) and approximately 110 years offset was seen in the material from Åbuamossen (2077-1108 BC).
To find an offset between 14C- and dendroages was unexpected as the 14C-calibration curve is assumed to be suitable to calibrate 14C ages from trees irrespective of their origin. The discovered offsets will be studied by measuring high resolution 14C sequences over selected intervals. This will allow us to (1) check the absolute dating by 14C-wiggle match dating, (2) study the stability of the age offset through time and (3) investigate the implications for 14C dating in general. To investigate this, 14C-contents will be measured around periods with characteristic patterns in the 14C-calibration curve.
Uptake of old carbon leaking from the raised bogs might be a possible process behind the age deviation, but further studies on both living and buried bog trees, bog dynamics together with carbon cycling in bog environments is needed to explain the processes behind these possible 14C-anomalies. To understand the mechanisms behind the 14C-offset is important as it will improve the accuracy of the 14C-dating method in bog environments. It will also increase our understanding of bog dynamics together with the complex carbon cycling in bog environments, which is a significant factor for increased knowledge of CH4 and CO2-fluctations and mechanisms behind climate change