A novel approach for construction of radiocarbon-based chronologies for speleothems

Abstract

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Radiocarbon Dating of a Speleothem Record of Paleoclimate for Angkor, Cambodia

“…For the 2nd option with bomb 14 C onset at 5.9 mm depth, the speleothem growth rate for the post-bomb is 0.118 mm/yr (5.9 mm/50 yr), which is similar to that for the previous section of PC1 (5.9–34.4 mm depth) of 0.087–0.102 mm/yr based on the two dating methods (Hua et al 2012a; Lechleitner et al 2016a; see discussion later). While the growth rate for the 3rd option with bomb 14 C onset at 8.9 mm depth is 0.178 mm/yr (8.9 mm/50 yr) for the post-bomb, which is much higher than that for the previous section of PC1 (8.9–34.4 mm depth) of 0.092–0.127 mm/yr derived from the two dating methods (data are not shown).…”

“…This together with the likely short soil/karst residence time of cave drip water from the literature, which is less than 1 yr (Johnson et al 2006; Noronha et al 2015) to no more than 3–4 yr at maximum (e.g., Kluge et al 2010), suggests that 14 C increases at 1.7–9.7 mm depth are not related to DCF changes. In addition, the onset of bomb 14 C at 1.3 mm depth requires a large reduction in the growth rate for the top 1.3 mm of PC1 (1.3 mm/50 yr=0.026 mm/yr) compared to that for the previous section of 1.3–34.4 mm depth of 0.049–0.068 mm/yr derived from the two dating approaches (Hua et al 2012a; Lechleitner et al 2016a) mentioned above. This substantial growth rate change is unlikely because there are no obvious changes in the texture of PC1 in the top 34.4 mm.…”

“…A chronology of PC1 was built based on a series of DCF-corrected ages using the Bayesian OxCal P_sequence deposition model (Bronk Ramsey 2008). The second approach (Lechleitner et al 2016a) reconstructed the chronology by calculating the best-fit growth rate between a sequence of PC1 14 C data and a terrestrial 14 C calibration curve, and anchoring the estimated growth rate to a point of known age. This method does not depend on any prior knowledge or estimate of the DCF.…”

“…Age-depth modeled results are shown in Figures 5a–b with a model agreement index (A model ) of 154%, much higher than the accepted level of 60% (Bronk Ramsey 2008).

Figure 5 Age-depth models for (a) the top 51 mm of PC1 based on Hua et al (2012a)’s approach and (b) the top ~47 mm of PC1 derived from Hua et al (2012a)’s approach (black lines) and Lechleitner et al (2016a)’s approach (gray lines). For both age-depth models, 1σ age ranges are shown.

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“…Temporal variations in DCF have been reported by several researchers (e.g., Genty et al 2001; Griffiths et al 2012; Noronha et al 2014), but if DCF variability in a speleothem is properly constrained, a reliable 14 C-based chronology is achievable (Genty et al 1999; Hua 2009; Hua et al 2012a). In an alternative approach, age-depth modeling with 14 C-dated stalagmites is possible without prior knowledge of DCF, but requires the assumption of a quasi-constant growth rate (Lechleitner et al 2016a). In this paper, we use 14 C to generate a chronology for the top 51 mm of PC1.…”