Macroscopic charcoal analysis from lacustrine sediments as a methodology to reconstruct fire history: first results from Santa Cruz (50°-52°S), Argentina.

QUINTANA, F. A; BIANCHI, M.M.

Viena, Austria

Workshop; 2nd International ICDP-Workshop PASADO; 2010

This research is aimed to determine the relationship between the characteristics of modern fire events (magnitude, severity, location) and macroscopic charcoal particles deposition in lakes of Southern Santa Cruz. This calibration constitutes the first step for reconstructing the fire history of this region based on macro charcoal data obtained from cores within PASADO project. Fire episodes, climatic variability and human activity relationships at millennial to centennial scales have been studied in sedimentary records in Patagonia showing the importance of fire as an agent operating since the postglacial period. However, no records far beyond the Last Glacial Maximum have been recovered yet and most of the obtained records have been acquired from sites located in the Subantartic Forest and forest-steppe ecotone. Therefore, charcoal records of PASADO would give the unique opportunity to contrast trends of fire regimes in other ecosystems than the forest in the region. Due to the current low frequency of lightening in this area, the origin of past fires is still under discussion (climate variability vs. human activity). Past fire occurrences has been attributed to prehistoric inhabitants activities. However, other authors suggest that the Holocene climatic variability would have influenced the natural agents of fire production (lightening frequency). The charcoal analysis of PASADO records would give the opportunity to asses this controversial issue. The current methodology includes the size and quantity analysis of macro-particles of charcoal along with pollen content in modern and fossil lacustrine sediment samples. A method to examine macroscopic charcoal content (>0.125mm) instead of microscopic charcoal on pollen slides is proposed. Microscopic charcoal counting has been pointed out as non-adequate for calculating fire frequency because is dependent on the resolution of pollen analysis and the source area (local, regional) is ambiguous. As a first approach, the macroscopic charcoal content of surface sediments from fifteen lakes along an east-west gradient has been analyzed and a number of known fire events in this area have been mapped. This constitutes the first calibration step in order to correlate size and quantity of charcoal particles in lakes with the proximity and spatial characteristics of known fire events. Current fire records from 2000-2007 (Consejo Agrario Provincial, Santa Cruz) point out that most of fires occur in the steppe due to human causes. In the forest, records of Parque Nacionales indicate that a fire event per year occur also due to human causes. Preliminary results indicate that concentration of particles bigger than 63µ in the steppe (2,8-187,2part/cm3) is higher than in the forest-steppe ecotone (11,4-49part/cm3) and the forest (8,6-74,8part/cm3). Concentration of particles bigger than 125µ and 250µ display high values in the forest (1,4-32,8part/cm3; 1,4part/cm3) and forest-steppe ecotone (1,8-26 part/cm3; 0,6-1,4 part/cm3), and low values in the steppe (1,6-18,4 part/cm3y 0,2-0,4 part/cm3). This proposal is in the line of Proyecto Interdisciplinario Patagonia Austral (PIPA, PICT 2006-02338) which one of the major aim is to construct a modern biodiversity data base of organism and organic remains sensitive to or indicatives of environmental variability and its relationship with physic and chemic characteristics of lakes and climatic factors between 50º-52ºS, in Santa Cruz Province.