4. THEORETICAL BACKGROUND
4.3 Charcoal analysis
40 derived from the Constrained Incremental Sum of Squares (CONISS) function in Psimpoll (Bennett, 2005).
4.2.2.6 Palaeo-environmental reconstruction and interpretation
Much evidence used for palaeo-environmental reconstruction is stratigraphic in nature, and therefore clues regarding environmental conditions at specific time periods are investigated.
Changes in the stratigraphic sequence over an extended period of time are used to infer changes which occurred in the environment (Anderson et al., 2007). There are two general interpretative approaches that may be taken: an individualistic approach and an assemblage approach. The use of the assemblage approach has more strength in fossil pollen analysis as assemblages of fossils are used for comparison, whereas the individualistic approach is better suited to plant macrofossils that can be identified to species level (Birks and Birks, 2005).
When interpreting pollen diagrams, it is important to consider the contemporary vegetation composition, as this is key to explaining vegetation patterns and changes, and may be achieved by collecting modern pollen samples of known vegetation communities (Williams et al., 1998). Fossil pollen assemblages can then be compared with known ecological tolerances, to reconstruct past environments (Birks and Birks, 2005).
A factor complicating the interpretation of pollen assemblages is the impact of humans in the recent past. Although anthropogenic impacts may be observed in pollen assemblages and provide insight regarding historical human activity, some vegetation assemblages reflect human interference rather than climatic change. It is essential that an understanding of such influences is gained before interpretation is attempted (Williams et al., 1998).
41 they determine the degree of charcoal preservation for environmental interpretations (Scott and Damblon, 2010). The use of charcoal in describing and understanding past environments was limited until the 1970s, however developments in the methods have resulted in an increase in the use of this proxy (Carcaillet, 2007).
Charcoal is formed as a result of the combustion of vegetation and has a high carbon content of 60-90% (Scott, 2010). In Quaternary studies, there has been some debate as to the transportation mechanisms of micro- and macro-charcoal fragments: microscopic charcoal fragments can be waterborne or wind-blown, and macroscopic charcoal fragments can be transported via water or erosional processes (Scott et al., 2000). Scott (2002) states that the most suitable sites for charcoal analysis are those where transportation of charcoal fragments are not through fluvial processes. Transportation and reworking of allochthonous charcoal fragments in the sediment samples may prove to be problematic in analyses, as such particles may not necessarily be of the same age as the atmospheric charcoal component in the sample.
The assumption can therefore be made that atmospheric micro-fragments of charcoal from burning of surrounding vegetation can provide accurate results of the charcoal ratios of a site (Scott, 2002). The charcoal fragments possess a number of characteristics: they are able to preserve the anatomy of the plant (allowing for taxanomic identification); are fairly inert; and can be well-preserved in the fossil record (Scott, 2010).
Natural fires are largely controlled by climatic conditions, and although it results in vegetation disturbance, can also increase productivity, vegetation diversity and nutrient cycling (Daniau et al., 2010). Vegetation can therefore track climate change directly (lightning ignition, fuel moisture and prevalence of fire weather) and indirectly (changes in vegetation and productivity) (Daniau et al., 2010). In summer rainfall regions, biomass becomes dry and more ignitable during dry winter periods (Scott, 2002). Although interpretation should be done with caution, charcoal records may therefore provide us with an indication of when drier conditions were prevalent, and fire frequency more regular (Scott, 2002). However, lightning is characteristic of subtropical conditions, where it can be associated with rainy conditions. It is therefore important to use a multi-proxy analysis when attempting to reconstruct palaeo-environmental conditions, as the use of pollen data in conjunction with charcoal records will assist in establishing a relationship between
42 vegetation, natural burning, climate change and anthropogenic impacts (Ritchie, 1995; Scott et al., 2000; Whitlock and Larsen, 2001; Scott, 2002)
Some difficulties are experienced in the analytical methods and the interpretation of charcoal data, as outlined by Ritchie (1995). Issues regarding poor chronology in many sediment types hinder the investigation of the history of local fires, once again emphasising the importance of appropriate sites and sediment type. Charcoal taphonomy raises questions regarding the identification of airborne and water-borne sources, particularly in lacustrine deposits (Ritchie, 1995). A further limitation of reconstructing an accurate fire-history is the use of sediment samples at a lower resolution. As the occurrence of natural fires does not occur frequently, the fire history obtained from the sediment samples will only be as accurate as the sediment samples are able to provide (Clark, 1982). To obtain a more detailed fire history, the sediment core would have to be analysed at a very high resolution.
Scott (2002) provides a summary of fire fluctuations in South Africa during the Holocene, although it is clear that correlations in fire history between distant sites are not necessarily expected. Differences in charcoal patterns at various sites indicate that fire events are merely an indication of local fires which occurred in close proximity to the site (Scott, 2002). An interesting relationship can be seen in records from Lake Eteza and Port Durnford, two coastal swamp sites in KwaZulu-Natal. The records contain zones where charcoal concentrations and Podocarpus pollen concentrations are simultaneously high (Scott, 2002).
Although the reasons for this are unclear and requires further analysis, a possible explanation could be the availability of large amounts of grass fuel (Scott, 2002). At Wonderkrater spring, an increase in fire frequency can be observed at a time which coincides with the arrival of Iron Age culture in the region, and the charcoal peaks can be attributed to local burning. When examined in conjunction with pollen data from the site, it can be inferred that the charcoal is as a result of burning where Zea cultivation took place, as Zea pollen (which must have been produced locally) was not (as Scott (2002) argues), a result of long-distance dispersal. However, at Lake Funduzi in Limpopo Province, charcoal concentrations during the period of Iron Age people are very low (Scott, 2002). This may be an indication that people did not initiate fires in the area. Pollen records show that there may have been impacts from human activities and changes in hydrology, however this does not coincide with charcoal peaks and may be an indication that human presence followed at a later stage. An
43 increase in charcoal concentrations with the arrival of the Iron Age people would be expected, although the charcoal records do not indicate similar situations (Scott, 2002). The importance of using palaeo-environmental information and archaeological evidence in conjunction with charcoal data is therefore essential. The use of charcoal analysis does have potential for estimating fire frequency, although there are constraints within the technique and potentially a need for more sophisticated charcoal analysis techniques (Scott, 2002).