Calcium in regolith carbonates of central and southern Australia:Its source and implications for the global carbon cycle

The widespread regolith carbonates of the Australian continent are a potential sink for CO2. We have used Sr isotopes to investigate the source of the Ca in regolith carbonates that cover approximately 1.6 × 106 km2 of inland Australia. 87Sr/86Sr ratios for nearly all the carbonates were in the range 0.7094 to 0.7211. The results show that only about 10% of the Ca in regolith carbonates is derived from weathered bedrock, with the remaining component being derived from an external marine source. The exceptions are the most northerly samples with a range of 0.7270 to 0.7374, in which bedrock content may be as high as 30%. The likely Ca source areas are sedimentary carbonates located on the continental shelf and calcareous aeolianites that surround much of the southern and western Australian coastline. Winds passing over these carbonates continually rework and transport the material over the Australian continent where it has settled and formed the regolith carbonates we see today. The impact of this process is that, despite the immense area covered, Australian regolith carbonates do not capture any additional CO2; instead the carbonate is simply being remobilised from one pool (marine) to another (terrestrial).