T02_P21

¹⁴C ramp-pyrolysis and FTIR analysis of Trondheim CaCO₃ precipitates of atmospheric CO₂ samples: further investigations and insights

Santos G¹,  Leong C¹, Seiler M², Grootes P², Svarva H², Nadeau M²

¹University of California, Irvine, Irvine,, United States, ²The National Laboratory for Age Determination, NTNU University Museum, Trondheim, Norway

An archive of atmospheric CO₂ samples collected by the Trondheim Radiocarbon Laboratory (NTNU) since early 1960s, and precipitated as CaCO₃, has been evaluated by radiocarbon (¹⁴C) analysis for its reliability (Seiler et al. 2022). Results indicated the presence of a contaminant that was not removed by different chemical cleansing procedures. Here, we present a follow up investigation using a subset of the samples shown in Seiler et al. (2022). The selected CaCO₃ samples and several reference materials of carbonate and mixed matrix origins were subjected to ramp-pyrolysis at < 285ºC. Upon 24 hours, CaCO₃ samples as well as reference materials were transferred still hot to the vacuum line, flame torch sealed, combusted, and then brought through graphitization using standard protocols. Radiocarbon measurements of CaCO₃ samples performed at Keck Carbon Cycle Accelerator Mass Spectrometer match with those recently attained by NTNU. To determine the performance of heat treatment, Fourier-transform infrared (FTIR) spectroscopy were performed on ramp-pyrolysis treated and untreated CaCO₃ samples. While FTIR confirmed that water and chains of carbohydrates have been removed upon heating, their absence did not improve the ¹⁴C results of CaCO₃ powders. Maximum ¹⁴C depletion was still 11‰. This may indicate that the elusive contaminant is of carbonate origin, possibly due to reactivation of carbonation by moisture and changes in temperature during decades of poor storage conditions. These findings will be discussed.

Seiler et al. 2022. Radiocarbon. Accepted.