Abstract

Heuweltjies are large (20 m diameter) raised mounds, spread throughout much of the South-West Cape region of South Africa. Many heuweltjies have a thick basal calcrete layer. Our analyses of this calcrete layer indicate at the landscape level it contains several tonnes per hectare of Ca, Mg, Na and K. These amounts are surprisingly large considering that the underlying Table Mountain Group sedimentary rocks contain low concentrations of these elements. The low present-day input rates of these elements from rainfall suggest that the calcrete must have formed under a different climate and depositional regime. We propose that some of these elements are derived from marine aerosol deposits. This is supported by the mean 87Sr/86Sr ratios obtained from the calcrete layer of 0.71303, which is much lower than that of the immediate underlying rocks (mean 0.74936). The δ13C values for calcretes (−4.5 to −8‰ PDB ) suggest that Cape heuweltjies are sinks of atmospheric CO2 as the calcrete 13C is different from local limestones and dolomites, which have δ13C values close to zero.

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