Bomb Radiocarbon in Tree Rings from Northern New South Wales, Australia: Implications for Dendrochronology, Atmospheric Transport, and Air-Sea Exchange of CO2

Abstract

We have analyzed by radiocarbon 27 consecutive single rings, starting from AD 1952, of a preliminarily cross-dated section (DFR 021) of Pinus radiata, which grew in Armidale, northern New South Wales, Australia. The bomb 14C results suggested the possibility of 2 false rings, and, consequently, 2 misidentified rings in the preliminary count for this section. This possibility was supported by a better ring-width correlation between the revised DFR 021 count and other Pinus radiata chronologies in the study region. This indicated that bomb 14C is a useful tool to complement the standard techniques of dendrochronology in tree species where annual rings are not always clearly defined. Our accelerator mass spectrometry (AMS) 14C results for Armidale Pinus radiata, on a corrected timescale, can be compared with previously published atmospheric and oceanic 14C data. The data show interesting features of atmospheric circulation and the regional air-sea exchange of CO2 for the bomb period. On average, the difference between Delta-14C values for Armidale (30 degrees S) and those for Tasmania (42 degrees S) was negligible, implying a small latitudinal 14C gradient in the Southern Hemisphere. However, small offsets between Armidale and Tasmania were observed for some periods. The variation of these offsets suggests some slight changes in the relative contributions of the 2 excess 14C sources (the northern troposphere and southern stratosphere) to the southern troposphere. In the decay of bomb 14C, atmospheric 14C reached a global equilibrium at the end of the 1960s and decreased exponentially, halving every 16 years. The time for air-sea exchange of CO2 for southern Pacific mid-latitudes was found to be about 7.5 yr, which was equivalent to a CO2 flux from the atmosphere to the oceans of 21.5 moles m-2 y-1 for the 1970s.