Reconstructing regional BuRnt Areas from microcharcoal preserved In marine SEdiments – BRAISE

The BRAISE project focuses on marine sediments within 500 kms from the coastline on a North-South transect from the eastern Atlantic Ocean (northern Europe to southern Africa including the Mediterranean eco-region). This transect had been specifically selected because it provides good sampling of modern climate and vegetation space (from tundra to desert). It also provides a gradient in the level of fire, as some regions today are characterized by high levels of fire and others by lower levels. Thus, it should provide a reasonable basis for reconstructing broad-scale relationships between CHAR in marine realm and fire regimes on the continent. BRAISE method is based on the collection of charcoal data and metadata on marine surface samples and the creation of the BRAISE dataset; the compilation of continent-ocean Geographic Information System (GIS) and fire regime data from satellite observations; exploratory analyses of quantitative relationships between charcoal accumulation and fire regime metrics; and the production of two new charcoal records covering the Holocene on which the calibration can be applied in the future.

The abundance and shape of microcharcoal found in surface marine sediment samples collected off the Iberian Peninsula (Genet et al. 2021) and Africa (Haliuc et al. 2023) were examined and compared with satellite data on fires (number, size, intensity) as well as the types of vegetation burned and recent climatic conditions. Numerical modelling using the Hysplit wind transport model (wet and dry deposition) showed that the microcharcoal present in marine sediments reflects fire activity in the nearest river basin (Haliuc et al. 2023). High-intensity fires were identified by a high presence of small elongated microcharcoal in marine sediments off areas with mixed vegetation on the Iberian Peninsula (Genet et al. 2021) and mixed graminoid ecosystems in Africa (Haliuc et al. 2023).

Using this calibration on a sedimentary archive collected in deep waters off the Iberian Peninsula revealed a change in the fire regime beginning 8,000 years ago. This was marked by the emergence of rare yet intense fires, which were linked to the gradual degradation of the Mediterranean forest and the expansion of shrubland areas. Applying this calibration to a marine core collected off the coast of Namibia revealed significant and intense fires recurring over the last 190,000 years, which spread across southern Africa's open, grassy savannas (Daniau et al., 2023). Finally, applying this calibration to a marine sedimentary archive from the Gulf of Lions covering the last 8,200 years made it possible to reconstruct the evolution of palaeo-fire regimes in south-eastern France. This highlighted the constant influence of wind on fires in this region (Genet et al., 2025).

The results on the calibration of the entire western Mediterranean basin have just been presented (Genet et al. submitted, Quaternary Science Reviews). This work has proven that average elongation is a reliable factor for identifying the type of vegetation that burned. Furthermore, a higher concentration of microcharcoal for the same type of burned vegetation indicates an increase in the frequency and intensity of fires.

The abundance and morphology of microcharcoal can be used to trace changes in the intensity of fires in subtropical areas. These results offer new insights into interpreting paleofire records: variations in microcharcoal concentration signal changes in fire regimes for the same vegetation type, while variations accompanied by changes in vegetation indicate changes in the types of vegetation burned (rather than changes in fire regimes). Synthesising the results of this work raises new challenges:

Can we develop quantitative relationships with climatic variables to reconstruct annual or seasonal precipitation in subtropical regions?

Can we develop quantitative relationships with fire regime variables to reconstruct past fire intensity and burned areas, with a view to improving model-data comparisons of fires?

Daniau A.-L., Loutre M.-F., Swingedouw D., Laepple T., Bassinot F., Malaizé B., Kageyama M., Charlier K., Carfantan H. (2023) Precession and obliquity forcing of the South African monsoon revealed by sub-tropical fires. Quaternary Science Reviews, 310, 108128, ISSN 0277-3791, doi.org/10.1016/j.quascirev.2023.108128

Haliuc A., Daniau A.-L., Mouillot F., Chen W., Leys B., David V., Hanquiez V., Dennielou B., Schefuß E., Bayon G., Crosta X. (2023) Microscopic charcoals in ocean sediments off Africa track past fire intensity from the continent. Communications Earth & Environment, 4, 133, 1-11 - www.nature.com/articles/s43247-023-00800-x

Genet M., Daniau A.-L., Mouillot F., Hanquiez V., Schmidt S., David V., Georget M., Abrantes F., Anschutz P., Bassinot F., Bonnin J., Dennielou B., Eynaud F., Hodell D. A., Mulder T., Naughton F., Rossignol L., Tzedakis P., Sanchez-Goni M.F. (2021) Modern relationships between microscopic charcoal in marine sediments and fire regimes on adjacent landmasses to refine the interpretation of marine paleofire records: An Iberian case study. Quaternary Science Reviews, 270, 107148, doi.org/10.1016/j.quascirev.2021.107148

Large uncertainties remain in understanding the evolution of fire activity under projected warming scenarios because fire is a complex process to integrate into global modelling. Empirical models used for projections lack potential changes in the interaction between climate, vegetation and fire. Process-based models of the coupled vegetation-fire system provide new tools to address this issue. Evaluating those models against benchmark datasets from charcoal sediment records, outside of the modern climate conditions range, is necessary. Long marine charcoal records capture regional-scale biomass burning over a large range of natural climate variability, i.e. multiple warm and cold climate states. The development of comprehensive data-model comparisons is limited by the lack of common units between data and model output. BRAISE intends to develop a calibration which, applied on paleofire records, should provide new datasets of regional burnt areas for key periods in the past.