Regional Radiocarbon Effect Due to Thawing of Frozen Earth

Abstract

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 14C near the Arctic Circle in the Mackenzie River area. However, the Δ14C measurements are lower on average by 2.7 ± 0.9 (ō)‰ relative to 14C measurements on tree rings from the Pacific Northwest (Stuiver and Braziunas 1993). We attribute this depression of Δ14C to thawing of the ice and snow cover followed by melting of frozen earth that releases trapped 14C-depleted CO2 to the atmosphere during the short growing season from May through August. Correlation of Δ14C with May–August estimated temperatures yields a correlation index of r = 0.60. The reduction in Δ14C 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.