Sr-isotopes as a tracer of Ca sources and mobility in profiles hosting regolith carbonates from southern Australia

Abstract

Regolith carbonates are common in soil profiles in the arid to semiarid areas of southern and central Australia. Extrinsic sources are now inferred to be the dominant source of Ca even as far inland as Alice Springs in central Australia. This study of Ca sources in a near coastal 2 m deep profile at Clarendon, South Australia, and two inland 4- and 3 m-deep profiles near Olary, South Australia uses Sr isotopes as a proxy for Ca. Our results show that up to 90% of Sr (and by association Ca) in both labile fractions and in soil carbonate is sourced from atmospheric input. The silicate weathering input is minor (<10%) and independent of the influence of different bedrock types. The Sr isotope composition throughout the profiles is relatively homogeneous with most values between 0.714 and 0.717, increasing only when in the saprolith, indicating that a process of continual reworking and mixing of the Sr (and Ca) goes beyond the near surface and continues throughout the profile.

KEY WORDS:

• regolith carbonates
• Sr isotopes
• ⁸⁷Sr/⁸⁶Sr
• Ca source
• calcrete

Acknowledgements

Support for this research was through a joint CRC LEME and University of Adelaide PhD scholarship awarded to R. Dart. Thanks are given to Exco Resources for access to the White Dam profiles. Assistance in sample preparation and TIMS operation was provided by David Bruce. We thank Ken McQueen and David Gray for their constructive comments and careful reviewing of this paper.