GSA DATA REPOSITORY

Transcription

1 GSA DATA REPOSITORY Chase et al. Supplementary Methods Radiocarbon ages The hyraceum samples with lab code UBA were pretreated with 2% HCl for 1 hr at room temperature to remove potential carbonate contamination and dried at 60 C. They were then weighed into quartz tubes with an excess of CuO, sealed under vacuum and combusted to CO 2. The CO 2 was converted to graphite on an iron catalyst using the zinc reduction method (Slota et al., 1987). The 14 C/ 12 C ratio and 13 C/ 12 C were measured by accelerator mass spectrometry (AMS) at the 14 CHRONO Centre, Queen s University Belfast. The radiocarbon age and one standard deviation were calculated using the Libby half-life of 5568 years following the conventions of Stuiver and Polach (Stuiver and Polach, 1977). The ages were corrected for isotope fractionation using the AMS measured δ 13 C (not given) which accounts for both natural and machine fractionation and calibrated with the SHCal04 (up to 10, C yr BP (McCormac et al., 2004)) and IntCal09 (beyond 10, C yr BP (Reimer et al., 2009)) data and CALIB 6.0 software (Stuiver and Reimer, 1993). An offset was included for the southern hemisphere (McCormac et al., 2004) for ages calibrated with IntCal09 (Table DR1, Figure DR1). Stable isotopes Isotope ratios of bulk midden samples were measured at the Department of Archaeology, University of Cape Town on a Thermo-Finnigan Delta-Plus XP isotope ratio mass spectrometer. Samples were combusted at 1020ºC and the carbon and nitrogen converted into CO 2 and N 2 in a Thermo Flash EA 1112 elemental analyser. Gases were introduced into the mass spectrometer in a stream of helium, via a Conflo III gas control unit. The standards used were chocolate and DL Valine, and each has been calibrated against IAEA (International Atomic Energy Agency) standards. The standard deviation of repeated determinations of homogeneous material was less than 0.2 for both carbon and nitrogen. Carbon isotope results are expressed relative to Vienna PDB, with a correction for long-term variations in δ 13 CO 2 based on deviations from an estimated late Holocene average value of -6.5 (Smith et al., 1999) Nitrogen isotope results are expressed relative to atmospheric nitrogen. Stable carbon isotopes as indicators of climate Variations in the δ 13 C values of plants, and thus hyrax middens, are generally considered to be determined by: 1) the plant s photosynthetic pathway (C 4, C 3 or CAM) (Smith, 1972), 2) the canopy effect, which causes a depletion in 13 C abundance (Drucker et al., 2008), 3) variations in atmospheric CO 2 (Hedges et al., 2004; Polley et al., 1993; Richards and Hedges, 2003) and/or 1

2 δ 13 CO 2 (Arens et al., 2000; Hedges et al., 2006), and 4) changes in water-use efficiency determined by water availability and temperature (Ehleringer and Cooper, 1988; Pate, 2001). As the region of the Cederberg Mountains where the De Rif midden was found is dominated by Mountain Fynbos vegetation (a resilient C 3 ecosystem that consists primarily of shrubs and very few trees and thrives virtually unchanged under an exceptional range of climatic conditions (Cowling, 1983; Meadows and Sugden, 1991)) the first and second of these factors do not require detailed consideration. It is worth noting that some CAM plants are found in Mountain Fynbos, and that the ingestion of these plants would result in enrichment in midden δ 13 C. Considering however that the δ 13 C values from the midden vary between and -26.9, it is unlikely that CAM plants became an important part of the hyraxes diet during the period considered here. Additionally, as CAM plants represent the more xerophilic elements of the vegetation, enrichment in midden δ 13 C whether as a result in a change in diet or plant water-use efficiency would have the same significance in terms of palaeoenvironmental interpretation. Of potentially greater importance is atmospheric CO 2, which has changed considerably over the last 20 kyr (Indermühle et al., 1999; Monnin et al., 2001), and could have affected the midden δ 13 C record through influences on carbon isotope discrimination (Farquhar and Richards, 1984; Farquhar and Sharkey, 1982). The experimental work of Polley et al. (1993) identifies a strong negative relationship between changes in plant δ 13 C and CO 2 at CO 2 concentrations between 160 to 350 μmol mol -1. Predictions of δ 13 C variability over the course of midden deposition based on these data would, however, be in excess of 10 ; far more than the 2.3 recorded. This discrepancy supports the possibility that plants are able to adapt their stomatal conductance to changing atmospheric CO 2 concentrations, and thus maintain consistent carbon isotope discrimination (Ehleringer and Cerling, 1995). Combined with the poor correspondence between atmospheric CO 2 trends since the Last Glacial Maximum and the De Rif record (Figure DR2), this suggests that other factors are the primary determinants of the midden δ 13 C values. Indeed, some studies have now shown that atmospheric CO 2 variations do not have a significant effect on plant δ 13 C, and rather that it is atmospheric δ 13 CO 2 and environmental factors that primarily control δ 13 C fluctuations (Arens et al., 2000; Hedges et al., 2006). To control for the effect of changes in δ 13 CO 2, we have detrended our data based on deviations of δ 13 CO 2 values from the Taylor Dome ice core from an estimated late Holocene average δ 13 CO 2 value of ~-6.5 (Smith et al., 1999). The impact of this correction is not significant, and the same trends are apparent in both the raw and calibrated records (Figure DR3). More importantly, it exhibits the same strong similarity with the δ 15 N record, indicating that climatic parameters, and not changes in CO 2, are responsible for the variability evident in the record. This is further reinforced by the strong similarities between δ 13 C, δ 15 N and SE Atlantic SSTs (Figure 2 of the manuscript). 2

3 Table DR1. Radiocarbon ages and calibration information for the De Rif hyrax midden. Sample 14 C age yr B.P. 1 error Calibration data 95.4 % (2σ) cal age ranges Relative area under distribution UBA SHCal04 cal BP cal BP UBA SHCal04 cal BP cal BP UBA SHCal04 cal BP cal BP UBA SHCal04 cal BP cal BP cal BP UBA * 43* IntCal09 cal BP cal BP UBA * 44* IntCal09 cal BP UBA * 46* IntCal09 cal BP UBA * 51* IntCal09 cal BP UBA * 49* IntCal09 cal BP UBA * 85* IntCal09 cal BP *Adjusted for recommended SH offset of 56 +/- 24 (McCormac et al., 2004) 3

4 Figure DR1. δ 15 N and δ 13 C data and age-depth curves derived from radiocarbon ages for the De Rif hyrax midden. 4

5 Figure DR2. δ 13 C data from the De Rif hyrax midden compared with trends in atmospheric CO 2 as recorded in Antarctic ice cores from Taylor Dome (Indermühle et al., 2000) and Dome C (Monnin et al., 2001). Figure DR3. δ 13 C data from the De Rif hyrax midden, both in its raw form and once variations in δ 13 CO 2 (Smith et al., 1999) have been corrected for. 5

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