Assessing the carbonisation temperatures recorded by ancient charcoals for δ13C-based palaeoclimate reconstruction

Ancient charcoal fragments, produced by the use of wood as fuel in archaeological contexts or during natural or anthropic forest fires, persist in soil and sediments over centuries to millennia. They thus offer a unique window to reconstruct past climate, especially palaeo-precipitation regimes thanks to their stable carbon isotope composition. However, the initial delta C-13 of wood is slightly modified as a function of the carbonisation temperature. Carbonisation-induced C-13 fractionation is classically investigated through a transfer function between experimental carbonisation temperatures and the carbon content. This approach assumes that the carbon content is conservative through time in ancient charcoals and neglects the potential impact of post-depositional oxidation occurring in soils and sediments. In the present study, we first show that post-depositional oxidation can lead to a large underestimation of past carbonisation temperatures, thereby minimising the estimation of carbonisation-induced C-13 fractionations and possibly biasing delta C-13-based climate reconstructions. Secondly, by combining carbon content, Fourier-transform infrared and Raman spectroscopy, we propose a new framework to assess the carbonisation temperatures registered in ancient charcoals. This new framework paves the way to reassessing delta C-13-based climate reconstruction.

Automated summary

Ancient charcoal fragments provide a unique opportunity to reconstruct past climate, especially precipitation regimes, through stable carbon isotope analysis. However, the carbonisation temperature can affect the initial carbon isotope composition, and post-depositional oxidation in soils and sediments can lead to an underestimation of past carbonisation temperatures.