IntCal, SHCal, or a Mixed Curve? Choosing a¹⁴C Calibration Curve for Archaeological and Paleoenvironmental Records from Tropical South America

Abstract

Because the 14C calibration curves IntCal and SHCal are based on data from temperate latitudes, it remains unclear which curve is more suitable for archaeological and paleoenvironmental records from tropical South America. A review of climate dynamics reveals a significant influx of Northern Hemisphere air masses and moisture over a substantial part of the continent during the South American Summer Monsoon (SASM). Areas affected by the SASM receive unknown amounts of input from both hemispheres, where an argument could be made for either curve. Until localized tree-ring data can resolve this, we suggest using a mixed calibration curve, which accounts for inputs from both hemispheres, as a third calibration option. We present a calibration example from a crucial period of environmental and cultural change in the southern Lake Titicaca. Given our current lack of data on past ∆14C variation in South America, our calibrations and chronologies will likely change in the future. We hope this paper spurs new research into this topic and encourages researchers to make an informed and explicit choice of which curve to use, which is particularly relevant in research on past human–environmental relationships.

SHCal20 Southern Hemisphere Calibration, 0–55,000 Years cal BP

“…The use of either IntCal20 or SHCal20 curve for regions which are influenced by monsoon circulation, might be therefore inappropriate, and a mixed curve accounting for north-south air-mass mixing can be used for these regions. This approach is similar to that for tropical South America proposed by Marsh et al (2018).…”

The IntCal20 Northern Hemisphere Radiocarbon Age Calibration Curve (0–55 cal kBP)

“…In extreme situations, the ITCZ appears to have experienced a major southward migration across Amazonia during Heinrich stadials (Cheng et al 2013). Trees growing within the ITCZ are potentially subjected to air masses from different hemispheres at certain times of the year (Marsh et al 2018;Hogg et al 2020 in this issue). For example, Southern Hemisphere air masses in tropical and subtropical Brazil have been detected in 14 C measurements from trees growing in the 1960s (Lisi et al 2001).…”

New Evidence for a Mid- to Late-Holocene Change in the Marine Reservoir Effect Across the South Pacific Gyre

“…ΔR values for both 14 C and U/Th dated pairs (Table 1) have been calculated using the online tool found at http://calib.org/deltar/ (Reimer and Reimer 2017), which first calibrates the terrestrial 14 C age with the appropriate calibration curve and then reverse-calibrates discrete points of the resulting probability density function with the marine calibration curve (Marine13; Reimer et al 2013). Because the islands under consideration lie within the SPCZ, which merges with the ITCZ to the west (Figure 1) we have opted to use the Northern Hemisphere calibration curve (IntCal13; Reimer et al 2013) following the recommendations of Petchey et al (2009), though a gradient from north to south is likely (Buntgen et al 2018) and a mixed Northern/Southern Hemisphere calibration may be more appropriate at certain time periods (cf., Marsh et al 2018). ∆R values calculated using SHCal13 (Hogg et al 2013) are also given in Table 1 for comparison.…”