Regional Radiocarbon Effect Due to Thawing of Frozen Earth

Accelerator mass spectrometry (AMS) measurement of 25 single-year tree rings from AD 1861–1885 at ca. ±3.5‰ precision shows no evidence of an anomalous 11-yr cycle of 14 C near the Arctic Circle in the Mackenzie River area. However, the Δ 14 C measurements are lower on average by 2.7 ± 0.9 (ō)‰ relative to 14 C measurements on tree rings from the Pacific Northwest (Stuiver and Braziunas 1993). We attribute this depression of Δ 14 C to thawing of the ice and snow cover followed by melting of frozen earth that releases trapped 14 C-depleted CO 2 to the atmosphere during the short growing season from May through August. Correlation of Δ 14 C with May–August estimated temperatures yields a correlation index of r = 0.60. The reduction in Δ 14 C is dominated by seven years of anomalous depletion. These years are 1861, 1867–1869, 1879–1880 and 1883. The years 1867–1869 are coincident with a very strong ENSO event.