Stable carbon isotope analysis of mudstone deposits from the late Permian and early Triassic sequence at Lootsberg Pass, South Africa.

J.F. Thackeray, Transvaal Museum,

June 24, 2002

Stable carbon isotope ratios (delta ¹³C) , based on relative abundances of carbon isotopes ¹²C and ¹³C, have been used in palaeontological and geological contexts as indicators of palaeo-environmental change. Results obtained from tooth samples of Diictodon, a mammal-like reptile (therapsid) which is relatively common in Late Permian deposits in the South African Karoo basin, indicate an apparent gradual change in stable carbon isotope ratios considered to reflect global changes in climate towards the end of the Permian, about 245 million years ago (Thackeray et al, 1990). More recently, carbonates and therapsid tooth samples from late Permian and early Triassic sequences have been reported by MacLeod et al (2000). This study serves to present results obtained from Karoo mudstones from sediments associated with the very end of the Permian and early Triassic, supplementing data obtained by MacLeod et al (2000), Ward et al (2000) and Smith and Ward (2001), and confirming that a sudden change in delta ¹³C occurred at the very end of the Permian.

MATERIALS AND METHODS

Mudstones from the Lootsberg Pass were collected by B. Rubidge and F. Thackeray in 1994. Powdered samples were digested in 95% phosphoric acid in a vacuum, at 25.0 ^oC. The CO₂ produced from this reaction was then analysed for ¹²C and ¹³C ratios, using a SIRA mass spectrometer with triple collectors and a dual inlet system. Results are calibrated relative to the Vienna Peedee B (V-PDB) standard, and are expressed in units of delta ¹³C, in parts per thousand (per mil).

RESULTS AND DISCUSSION

Results are presented in Table 1.

Stable carbon isotope ratios from samples LBW, LBX and LBY at the top of the Balfour Formation show a relatively rapid decline in stable carbon isotope ratios, from 0.49 per mil (sample LBY), to -4.18 per mil (sample LBX) to an even more negative value of -11.3 per mil (sample LBW). All three of these samples are from the end of the Permian, and together show a decline in delta ¹³C of about 10 per mil within a depth of less than 20 metres. By contrast, samples LBS1, LBS2 and LBS3, from the base of the Triassic in the Katberg Formation, range from -6.85 for sample LBS2 to -9.56 for sample LBS3, with an intermediate value of -8.49 for sample LBW 1. These values show little variation within a depth of 100 metres of deposit that includes alternating mudstones and sandstones. Samples LBS5, LBS6, and LBS7 are a later set of values clustering between -3.23 and -5.54.

Of particular interest is the apparent rapid decline in delta¹³C values at the very end of the Permian, in the uppermost component of the Balfour Formation represented by deposits less than 20 metres in thickness. This is in marked contrast to the apparent gradual change in delta ¹³C values for a sequence of Diictodon tooth samples, spanning several million years represented by the accumulation of more than 1000 metres of deposit (Thackeray et al, 1990). The causes for these changes are not known with certainty, but are likely to relate to global events at the end of the Permian, contributing to mass extinctions. The point made here is that changes towards the end of the Permian, spanning several million years, may have been gradual in nature, whereas the very end of the Permian was more rapid.

ACKNOWLEDGMENTS

This work has been supported by the National Research Foundation. I would like to thank Professor Bruce Rubidge (Bernard Price Institute (Palaeontology, University of the Witwatersrand) for his assistance in collecting samples from the Lootsberg Pass in 1994, and Siep Talma (Quaternary Dating Research Unit, CSIR, Pretoria ) for assistance with stable carbon isotope ratio analyses.

REFERENCES

Macleod, K.G., Smith, R.M.H., Koch, P.L., and Ward, P.D. 2000. Timing of mammal-like reptile extinctions across the Permain-Triassic boundary in South Africa. Geology 28, 227-230.

Smith, R.M.H. and Ward, P.D. 2001. Pattern of vertebrate extinctions across an event bed at the Permian/Triassic boundary in the main Karoo basin of South Africa. Geological Society of America 29 (12), 1147-1150.

Thackeray, J.F., van der Merwe, N.J., Lee-Thorp, J.A., Sillen, A., Lanham, J.L., Smith, R., Keyser, A., and Monteiro, P.M.S. Changes in carbon isotope ratios in the late Permian recorded in therapsid tooth apatite. 1990. Nature 347, 751-753.

Ward, P.D., Montgomery, D.R., and Smith, R. 2000. Altered river morphology in South Africa related to the Permian-Triassic extinction. Science 289, 1740-1743.

Table 1. Stable carbon isotope ratios (delta ¹³C) for mudstone samples from the Balfour (B) and Katberg (K) Formations, Lootsberg Pass. Results are presented relative to the Vienna Peedee belemnite (V-PDB) standard. Samples collected by B. Rubidge and J.F. Thackeray, 1994. K: Katberg Formation. B: Balfour Formation. Datum represents the Permo-Triassic boundary. Depths relative to datum are approximate.