Temporal variation in the interhemispheric C-14 offset

Contemporaneous tree-ring dated wood, from trees in the northern and southern hemispheres, gives different C-14 dates. Previous studies [Vogel et al., 1986; 1993] using wood from South Africa and The Netherlands have shown depletion's of -4.56 +/- 0.85 parts per thousand and -5.12 +/- 0.62 parts per thousand respectively. This translates to age differences of 36 +/- 7 and 41 +/- 5 years (yrs) with the southern hemisphere giving the older dates. More recently, Stuiver and Braziunas [1998] have shown that an offset of 23 +/- 4 yrs exists between combined 19th century wood measurements from Tasmania and Chile in the southern hemisphere and the west coast of the U.S. (Washington) in the northern hemisphere. In this study measurements on contemporaneous decadal samples of oak from the British Isles and cedar from New Zealand over the period 1725 to 1885 AD show a depletion of -3.4 +/- 0.58 parts per thousand (27.2 +/- 4.7 yrs). However, data after 1895 AD has a mean offset of 0.66 +/- 1.06 parts per thousand (-5.3 +/- 8.5 yrs) with increased variance compared to 19th century data. This, we believe, is attributable to anthropogenic fossil fuel, which, due to its long residence time in the earth, has long since lost any C-14 component and when burned preferentially depletes the northern hemisphere atmosphere of C-14.