Radiocarbon analysis confirms annual periodicity in Cedrela odorata tree rings from the equatorial Amazon

Abstract

A Cedrela odorata tree ring width chronology spanning from 1786 to 2016 was developed in the quasi-equatorial eastern Amazon Basin. Annual calendar dates were assigned using dendrochronological techniques at the Federal University of Lavras, Brazil. Due to its strategic location at the edge of the Equator (approximately 0°57′S), an independent confirmation of the annual periodicity of this century-long chronology would be of great value, allowing its future use for climate reconstruction and for filling gaps in upcoming atmospheric radiocarbon (14C) compilations. For reconstruction of atmospheric 14C, high reliability of the dendrochronological calendar dates is a requirement. Here, we used high-precision 14C bomb pulse dating (BPD) of selected C. odorata tree rings as a robust independent method to validate the dendrochronological dates. Eight calendar years from across the pre-to post-bomb period were tested through 14C analysis of α-cellulose extracts (19 targets in total were produced from those 8 calendar years). All dendrochronologically dated tree rings measured produced 14C values in perfect alignment with the Southern Hemisphere 14C bomb curve, further confirming the annual growth of this important record. Extraction of α-cellulose was attained by a recently implemented procedure at the Lamont-Doherty Earth Observatory (LDEO). The method was 14C assessed by high-precision measurements at the Keck Carbon Cycle Accelerator Mass Spectrometer (KCCAMS) at the University of California, Irvine (UCI) by measuring reference materials and unknown samples. High reproducibility of reference materials (within uncertainties) showed that the novel 150-funnel system and protocol developed at LDEO is reliable and can potentially expedite cellulose extractions for 14C analysis.