Résumé
Projected warming scenarios suggest fire risk increases in certain regions, in particular in the Mediterranean region. However, potential changes in the interactions between climate, vegetation and fire are neglected by models based on modern-day statistical relationships. In addition, process-based models must be tested not only with modern observations but also with observations from climate conditions very different from today similar to the range of climate variability projected for the next centuries (Hantson et al. 2016). Marine sediments are a source of past fire history data that can provide such information. Long-term temporal marine charcoal records capture regional-scale biomass burning over a large range of natural climate variability, i.e. multiple warm and cold climate states. For example, Daniau et al (2007) showed that values of biomass burning of the southwestern Iberian Peninsula were larger during the Holocene and the Eemian, compared to the last glacial period. However, "biomass burning" records describe only the response in the relative level of fire and are not yet linked to changes in fire regime metrics. This study aims at developing the calibration of charcoal preserved in marine sediments. Microcharcoal concentration and morphology were quantified from surface sediment samples of modern ages from the East Atlantic Ocean off the Iberian Peninsula. Microcharcoal concentration and elongation ratio were then compared with different parameters linked to the microcharcoal production source area (burnt area, fire size, frequency, and intensity, net primary productivity, burnt vegetation, watershed size), and to the transport/deposition of microcharcoal particles (bathymetry, sedimentary discharge). Our results show a great heterogeneity in the spatial distribution of microcharcoal concentration. However, a general mean increase in microcharcoal concentration is observed from the north to the south of the Iberian margin. This pattern reflects well the spatial pattern of fire regime, climate, productivity and burnt vegetation type on land. Our results show also the influence of the bathymetry on the distribution of concentration in ocean. Those results help interpreting the Holocene changes observed in the biomass burning record from the southwestern Iberian Peninsula. A similar approach is being applied to surface sediment samples from the Mediterranean Sea off the Gulf of Lion to understand the late Holocene biomass burning in the southeast of France.