Assessing the dead carbon proportion of a modern speleothem from central Brazil

Geographic and temporal variations in atmospheric carbon isotopic ratios are recorded in environmental proxies. In temperate regions, this may assist in the construction of datasets for the calibration of radiocarbon ages. Over the tropics, the high growth rate of speleothems combined with high precision ²³⁰ Th dating provides potential records for atmospheric ¹⁴ C reconstruction. In this preliminary work, we investigate the isotopic composition (δ ¹⁸ O and δ ¹³ C) of a speleothem from central Brazil, precisely dated by the multi-collector inductively coupled plasma mass spectrometry technique (MC-ICP-MS). Radiocarbon Accelerator Mass Spectrometry (AMS) measurements were compared to the ¹⁴ C concentration in the atmosphere during the nuclear tests period based on the Bomb13SH1-2 curve. Our results show that the speleothem dead carbon proportion is less than 20% between 1932 and 1992 AD while δ ¹³ C values vary between −14 and −9‰. Given that the empirical radiocarbon calibration curves for the Southern Hemisphere were constructed based on limited records, we discuss the potential and limitations of Brazilian speleothems for the evaluation of atmospheric ¹⁴ C concentrations over the last millennia.