Southern Hemisphere atmospheric radiocarbon during the past 60 years: filling up existing gaps and validating long-lasting tree species for further analysis

GUACIARA M. SANTOS; HADAD, M.; ROIG JUÑENT, FIDEL ALEJANDRO; ANDREUHAYLES, L

Mendoza

Conferencia; Third American Dendrochronology Conference; 2016

Tree Ring Society

Advances on radiocarbon (14C) dating allow very precise measurements, and consequently a better understanding of the C cycle and the human past. Nevertheless, because 14C ages are not true calendar ages, researchers rely on the precise 14C activity of biological materials that build up annual layers toreconstruct the 14C timescale. Continues efforts to optimize it have produced calibrations curves back to 50 kyrs BP. However, several gaps exist. While it is well established that atmospheric 14C signatures vary between the North and South Hemispheres (the inter-hemispheric offset [1]; there is little definitive data on 14C variation within the hemispheres (the intra-hemisphericoffsets, which define the zonal division of the present 14C atmospheric time-scale) [2-3]. Furthermore, the Southern Hemisphere (SH) 14C calibration dataset is presently based on data produced from samples from eastern SH sites within anarrow mid-latitude basd (New Zealand, Tasmania and South Africa). Consequently, the intra-hemispheric boundary between SH zone 1-2 and -3 on South America was established based on the putative position of the Inter Tropical Convergence Zone ITCZ; as no 14C data was available at the time of the SH 14C timescalecalibration effort [3]. Here, we use `bomb-pulse 14C', produced by aboveground nuclear weapon testing in the 1950s and 1960s to confirm the calendar year in which the C was fixed in tree-rings. The `bomb-pulse' approach can be used to establish the timing of the tree ring formation from recently fixedphotosynthetic products within ~1 year throughout the last 60 years, and therefore assure that the tree rings were not misidentified, that the chemical pretreatment can produce samples for meaningful 14C investigations, and most importantly that the tree species analyzed have annual growth layers. Ourefforts had produced two subtropical datasets from the Araucariaceae family e.g. Araucaria angustifolia from Brazil [4], and A. araucana from Argentina [5]; and a short tree-ring/14C cross-dating set for a tropical tree species (Pseudolmedia rigida) from Bolivia [6]. Those will be discussed. [1] McCormac et al.1998, Geophysical Research Letters 25, 1321[2] Hua et al. 1999, IAWA Journal 203; 261[3] Hua et al. 2013, Radiocarbon 55(4; 2059[4] Santos et al. 2015, Quaternary Geochronology 25, 96[5]Hadad et al. 2015, Quaternary Geochronology 30, 42[6] Andreu-Hayle & Santos et al.; 2015 Radiocarbon 57(1; 1)