Validation and application of pyrogenic and vegetation biomarkers as proxies

Field: Earth and Planetary Sciences (Physical Sciences)

Fires and climate are active components of the Earth System, which strongly interact with each other. Projected climate change, induced by human activities, will significantly affect both fire regimes and hydroclimate, one of the key drivers of fires in the climate system. Human-climate synergies in the Holocene are exceptionally important for the understanding of future fire and hydroclimate regimes under a global climate warming scenario. To study past fire-climate interactions, proxies of climate, vegetation and fire incidence are needed. In this sense, the first main objective of this dissertation is to validate a new set of environmental proxies in Spain based on pyrogenic and higher plant biomarkers. The aim is to compare their spatial variability (characterized through the use of molecular organic geochemical techniques) with the corresponding environmental variability on fire occurrence and hydroclimatic parameters. This has been undertaken using an exhaustive data set of recent surface lake sediments and soils from natural and rural areas. The new molecular proxies on fire evaluated are based on the quantification of levoglucosan and polycyclic aromatic hydrocarbons (PAHs). These proxies have also been compared, whenever possible, with data from the established fire proxy charcoal, with the goal to increase the reliability of fire reconstructions using a multiproxy approach. Our results show that the recent regional biomass fire variability in Spain is qualitatively well explained by the distribution of pyrogenic PAHs, macrocharcoal and levoglucosan in lake sediments. In contrast, for the soils, the distribution of fire proxies can only be partially explained by the occurrence of fire across Spain. The spatial distributions of the molecular and charcoal fire proxies in the lake samples display some differences, which are interpreted as a reflection of the distinct nature of the proxies considered. Regarding the appraisal of hydroclimatic proxies, the study has been focused on the measurement of compound-specific deuterium/hydrogen ratios (δD) of terrestrial higher plant lipids (i.e. n-alkanes). We find that the relation between terrestrial higher plants n-alkane δD and rainwater δD values for our Spanish recent soil set is in very good agreement with the global regression dataset. Moreover, the small rain water δD range of our data, allows us to confirm a significant vegetation and aridity influence in the n-alkane δD signal of our soil data set. In this sense, when the variability of both factors is constrained, the results show that n-alkane δD signal can be used as quantitative paleo hydrological proxy. Finally, the second main objective of this thesis is to assess the application of some of the aforementioned proxies in a late Holocene record (for the last 1200 years) from Lake Montcortes (Southern Central Pyrenees), to further appraise the complementarity of a multiproxy fire-hydroclimate reconstruction approach. We show that n-alkane ACL (the average chain length) and specially Paq (a ratio that reflects the relative contribution of terrestrial and aquatic plants) is a successful tool for reconstructing qualitatively heavy rainfall events (HR) in Montcortes, although other factors such as vegetation changes are needed to take into account. Furthermore, the variability of the pyrogenic biomarker levoglucosan, as well as other related monosaccharide-anhydrides (MAs) successfully recorded the biomass fire incidence in Montcortes in comparison with a microcharcoal record. However, it seems that there is a positive correlation between deposition of soil organic matter by HR and the MAs content, which could affect both reconstructions. In conclusion, pyrogenic PAHs and levoglucosan have the potential to be used as regional biomass fire proxies in Spain and similar environments through the world, and the joint study of n-alkane proxies could be used to infer the role of hydroclimate as a fire driver.