Temporal variation in the interhemispheric 14C offset

Contemporaneous tree‐ring dated wood, from trees in the northern and southern hemispheres, gives different 14 C 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‰ and −5.12±0.62‰ 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‰ (27.2±4.7 yrs). However, data after 1895 AD has a mean offset of 0.66±1.06‰ (−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 14 C component and when burned preferentially depletes the northern hemisphere atmosphere of 14 C.