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

Robust chronologies are crucial for the correct interpretation of climate proxy records and for detailed reconstructions of palaeoclimate. Stalagmites have garnered strong interest as recorders of past climate in part due to their amenability to U-series dating. However, many stalagmites are not dateable using this technique due to low 238U and/or high detrital Th concentrations (e.g., many tropical cave systems (Adkins et al., 2013)), and occasionally these issues affect stalagmites across wide geographical regions (e.g., large parts of Australia (Green et al. 2013)) complicating the use of stalagmites in these areas. Radiocarbon (14C) offers an alternative method of dating stalagmites, but issues associated with the ‘dead carbon fraction’ (DCF) have historically hindered this approach. Here, a novel 14C-based method for dating stalagmites is presented and discussed. The technique calculates a best-fit growth rate between a time-series of stalagmite 14C data and known atmospheric 14C variability. The new method produces excellent results for stalagmites that satisfy four requirements: i) the absence of long-term secular variability in DCF (i.e., stalagmite DCF varies around a mean value with no long-term trend), ii) stalagmite growth rate does not vary significantly (the technique identifies stalagmites with substantial growth rate variability), iii) the stalagmite record is long enough that measurable 14C decay has occurred, and iv) one ‘anchor’ point exists where the calendar age is known. The model produces good results for a previously U–Th dated stalagmite from Heshang Cave, China, and is then applied to an undated stalagmite from southern Poland. The new method will not replace high-precision U–Th measurements, because the precision of the technique is difficult to quantify. However, it provides a means for dating certain stalagmites undateable by conventional U–Th methods and for refining coarse U–Th chronologies

Hydrological and climatological controls on radiocarbon concentrations in a tropical stalagmite

Precisely-dated stalagmites are increasingly important archives for the reconstruction of terrestrial paleoclimate at very high temporal resolution. In-depth understanding of local conditions at the cave site and of the processes driving stalagmite deposition is of paramount importance for interpreting proxy signals incorporated in stalagmite carbonate. Here we present a sub-decadally resolved dead carbon fraction (DCF) record for a stalagmite from Yok Balum Cave (southern Belize). The record is coupled to parallel stable carbon isotope (δ13C) and U/Ca measurements, as well as radiocarbon (14C) measurements from soils overlying the cave system. Using a karst carbon cycle model we disentangle the importance of soil and karst processes on stalagmite DCF incorporation, revealing a dominant host rock dissolution control on total DCF. Covariation between DCF, δ13C, and U/Ca indicates that karst processes are a common driver of all three parameters, suggesting possible use of δ13C and trace element ratios to independently quantify DCF variability. A statistically significant multi-decadal lag of variable length exists between DCF and reconstructed solar activity, suggesting that solar activity influenced regional precipitation in Mesoamerica over the past 1500 years, but that the relationship was non-static. Although the precise nature of the observed lag is unclear, solar-induced changes in North Atlantic oceanic and atmospheric dynamics may play a role