Evidence for short term atmospheric ¹⁴C variations about 4,000 yr BP

Abstract

IT is well established from radiocarbon measurement of tree rings of known age that the natural ¹⁴C level in the Earth's atmosphere has undergone a gradual change of more than 10% over the past 7,000 yr (see, for example, ref. 1). During this time there have probably also been periods of irregular fluctuation spanning a few hundred years (the so-called Suess ‘wiggles’²). The existence, in addition to these effects, of substantial shorter term (∼ 10–20 yr) variations possibly correlated with solar activity is less certain. Farmer and Baxter (ref. 3) found ¹⁴C variations of up to 3% in 10 yr (equivalent to an age error of ±120 yr) over a consecutive sequence of 37 single tree rings taken from mid-nineteenth century wood grown in the Forest of Dean, England; but comparative measurements they made of twentieth century New Zealand wood were inconclusive. Twentieth century wood from Trondheim, Norway, showed a possible 0.6% variation⁴, and twentieth century North American wood showed about 0.3 % (ref. 5).