QA_Vol 24_No 1_July 2007 - Australasian Quaternary Association

Research ArticleCarbon isotope discrimination by C 3 pasturegrasses along a rainfall gradient in South Australia:Implications for palaeoecological studiesF. Donald Pate, Evelyn KrullF. Donald Pate (corresponding author) Email: donald.pate@flinders.edu.auDepartment of Archaeology, Flinders University, Adelaide, SA 5001, AustraliaEvelyn Krull, CSIRO Land and Water, Private Bag No. 2, Glen Osmond, SA 5064, AustraliaAbstractSamples of C 3 pasture grasses (Barley grass, Hordeumleporinum) were collected at five field sites in southeasternSouth Australia with varying mean annualrainfall in order to address variability in grass tissuestable carbon isotope composition in open, arid-landhabitats. Mean annual rainfall at the collection sitesranges from 1045 mm at Bridgewater in the southernAdelaide Hills to 238 mm at Morgan in the arid north.Stable carbon isotope values of C 3 grasses become lessnegative with decreasing mean annual rainfall. Meanδ 13 C values vary from -31.5‰ at Bridgewater to -27.7‰at Morgan, a 3.8‰ difference correlated with a changeof mean annual rainfall of 807 mm. The greatestmagnitude of grass tissue δ 13 C change (2.5‰) occursbetween the semi-arid (345 mm rainfall) and arid (238mm rainfall) zones. Although the least negative C 3 grassδ 13 C values occur in the arid zone, effects on the stablecarbon isotope composition of bone collagen in grazerswill be minimal because C 4 grasses dominate the aridlandscape. In a majority of the temperate and semi-aridhabitats occupied by grazers, mean δ 13 C values in C 3grasses differ by only 0.3 to 1.0‰. The quantification ofvariability in δ 13 C values within C 3 and C 4 plants associatedwith environmental variables such as soil moisturestress, aridity, and high light levels is essential toa range of ecological and palaeoecological studiesemploying stable isotope analyses of tissues fromplants, herbivores and human consumers.IntroductionStable isotopic analyses are employed regularly inecological studies to address environmental and dietaryvariability in a range of modern and past ecosystems.Isotopic analyses of soils and tissues from humans,animals, and plants have been employed to addressdietary composition, trophic level, photosyntheticpathway, habitat use, water stress, nutritional stress,humidity, and climate in marine and terrestrial ecosystems(Peterson and Fry, 1987; Rundel et al., 1989;MacFadden and Bryant, 1994; Pate, 1994, 1997;Bocherens et al., 1999; Witt, 2002).Past research addressing dietary variability in terrestrialmammals via stable carbon isotope analysis focusedon isotopic signatures in plant foods associated withvarious habitats and environmental conditions. Theseisotopic differences in plants are passed on to thetissues of herbivore consumers (Schwarcz and Schoeninger,1991; Pate, 1994). Due to slow biochemicalturnover rates, bone stable isotope values are relatedto long-term patterns of food consumption and habitatuse. In most vertebrates, stable carbon isotopes provideinformation about lifetime dietary averages (Libby etal., 1964; Stenhouse and Baxter, 1979).Initial studies focused on the bimodal distribution ofδ 13 C values in plants as determined by various photosyntheticmechanisms (Park and Epstein, 1961; Smithand Epstein, 1971; Downton, 1975; Raghavendra andDas, 1978; O’Leary, 1988). Stable carbon isotope analysisof animal bones permitted the identification ofbrowsing and grazing fauna in many regions of theworld in both modern and fossil systems (Ambrose &DeNiro 1986, van der Merwe 1986, 1989, Cormie andSchwarcz, 1994; Quade and Cerling, 1995; Grocke, 1997;Cerling et al., 1999; Pate and Schoeninger, in review).Because the carbon isotopic composition of soils andplants shows spatial variations within regional foodwebs, herbivore bone has the potential to provide anaverage isotopic signal for particular environments(Tieszen et al., 1979; Teeri et al., 1980; Cavagnaro, 1988;Korner et al., 1988; van der Merwe et al., 1988; Cormieand Schwarcz, 1994). Hattersley (1983) reports the geographicdistribution of C 3 and C 4 grasses in Australiain relation to climatic zones. The southern temperateregions of South Australia are dominated by C 3 grasses;C 4 grasses become dominant in the arid northern areas.Thus, there is a distinct geographic distribution of C 3and C4 grasses in Australia that may be employed toaddress variations in climate and associated herbivoredietary variability.29 | Quaternary AUSTRALASIA 24 (2)