1. Unravelling The Record:
The Need to Combine Carbon Isotopes with Biological Proxies when Reconstructing African Palaeovegatation during the Quaternary Period
Rachael Lem*1
Carbon Isotopes
as Geological Proxies
The grouping of species according to these pathways has particular relevance to species response to climatic variables such as temperature and precipitation.
Terrestrial Higher Plants C3 C4
Calvin-Benson Pathway
Hatch-Slack Pathway
δ13C values: -24‰ to -36‰
δ13C values: -9‰ to -17‰
Trees
Herbs
Sedges
Grasses
Tropical Grasses
Shrubs
Variation in δ13C values (Figure 2) can provide information on the relative proportion of C3 and C4 plants in an environment due to the concept that differences in photosynthetic pathways result in distinct isotopic signatures for each group.
Typical δ13C values for C3 plants:
-24‰ to -36‰
Av. Value: ‰
Typical δ13C values for C4 plants:
-9‰ to -17‰
Av. Value: -12‰
(Roberts et al., 2013)
Figure 2: δ13C values of individual grasses (Cerling et al., 1999).
The majority of terrestrial higher plants follow two photosynthetic pathways due to the way in which atmospheric CO2 is assimilated (Figure 1).
The Need for Biological Proxies
δ13C isotopic analysis fails to recognise the vegetation types within each photosynthetic pathway, allowing palaeovegetation to only be reconstructed in broad terms, i.e. grassland vs. mixed forest. Here we identify two instances where δ13C must be correlated with biological proxies.
Latitude
Altitude
Modern environment studies show a distinct altitudinal gradient in the distribution of C3 & C4 plants in Africa (Fig 3).
Studies during the Late Pleistocene and the LGM have shown that broad altitudinal zones have changed significantly in terms of the C3 & C4 components, resulting in a mix of both high and low altitude vegetation at different times (Rucina et al., 2009).
Likely to result in geographical crossovers between plants that are both C3 & C4 e.g. Amaranthaceae, Cyperaceae and Poaceae (Feakins, 2013) making the distinction between these boundaries difficult without correlation from species specific techniques such as pollen or phytolith analyses.
δ13C isotopic analysis is unable to account for families which utilise both photosynthetic pathways e.g. Cyperaceae and Poaceae, with each pathway being preferentially favoured at different times during the Late Quaternary (Wooller et al., 2001).
Stock et al. (2004) documented that the geographical distribution of C4 taxa in the Poaceae family did not dovetail the spread of species from other families throughout South Africa.
C4 grasses occurred more abundantly in austral summer rainfall areas however, Cyperaceae, following the same photosynthetic pathway, did not show the same climatic lineage, highlighting a species specific response to climatic change.
Figure 3: The changing distribution of C3 & C4 vegetation along an altitudinal gradient in Lesotho (After Roberts et al., 2013).
Pioneering Steps Forward
Figure 4: Combined pollen & isotope record (Feakins et al., 2013).
A pioneering study undertaken by Feakins et al. (2013) used a combination of pollen and δ13C wax to examine the evolution of C4 grasslands over the last 12 ka.
Opposing trends were found between the pollen and geochemical data; however, they were explained by a greater proportion of C3 grasses than had previously been assumed.
The combination of both proxies (Figure 4) was argued to provide insights that neither proxy could afford on an individual basis.
The study sets the precedence for further palaeoclimatic research and illustrates the need for the combination of biological and isotopic data in the reconstruction of African climate during the Quaternary Period (Scott et al., 2012).
The utilisation of multiple proxies in palaeoclimatic reconstructions, more specifically the combination of
biological and geochemical data, allows for a comprehensive view of past climatic changes to be determined.
Pollen Analysis
Phytolith Analysis
Through the identification of species-specific vegetation types within each pathway, pollen analysis provides direct evidence of palaeovegatation changes across the continent & has unveiled long-term patterns of vegetation change in conjunction with climatic cycles (Dupont et al., 2007; Dupont et al., 2013; Gilson et al., 2004).
Phytolith analysis enables the identification
of subfamilies at both higher and lower altitudes.
Adopting a Multi-Proxy Approach
Pollen assemblages are not complete indicators of past vegetation assemblages due to differential dispersal patterns between species. Pollen analysis does not allow for the differentiation of Poaceae beyond the family level (Cerling et al., 2011; Fægri et al., 1989).
Pooideae (C3 high elevation grasses)
Panicoideae (C4 tall grasses)
Chloridoideae (C4 short grasses)
The ratios of the specific phytoliths within an assemblage provides information that it is not possible to gain from either palynological or carbon isotopic data (Barboni & Bremond, 2009).
Data from palynological studies show that climate change can induce a species specific response in vegetation types that follow both C3 and C4 photosynthetic pathways.
It is therefore that we highlight the need utilise a multi-proxy approach, and to correlate carbon isotope data with biological proxies such as pollen and phytoliths, in order to accurately reconstruct and assess changes in palaeovegetation structure
during the African Quaternary.
1 Dept. of Geography
University of Liverpool UK
S. Hoare2 & F. Marret Dept. of Archaeology
Figure 1: Schematic highlighting two photosynthetic pathways of terrestrial higher plants.
Abstract
Carbon Isotopes have been successfully utilised to document past C3 and C4 vegetation distributions across Africa during the Quaternary Period. Variation in carbon isotopic (δ13C) values can provide information on the relative proportion of C3 and C4 plants in an environment due to the concept that differences in photosynthetic pathways result in distinct isotopic signatures for each group.
However, caution is needed when utilising it as a lone proxy as it fails to recognise the individual vegetation types within each pathway. Changes in temperature, precipitation and atmospheric carbon dioxide can induce a species specific response to climate change in species, such as grasses, that follow both the C3 and C4 pathways.
Here we convey the need to combine carbon isotopic data with biological proxies, namely palynological and phytolith records, to better understand species specific response to palaeoclimatic change.
Conclusions
Carbon isotopes have been widely used to reconstruct past variations in the distribution of C3 and C4 vegetation due to their distinct isotopic signature.
References
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