Strontium and carbon isotope tracers and the origins of soil carbonate in South Australia and Victoria

Calcium carbonate occurs in soils in association with a variety of rock types in coastal and inland South Australia and Victoria. The ⁸⁷Sr ⁸⁶Sr ratios of the coastal soil carbonates fall between 0.7094 and 0.7098, showing that the ocean is the principal source of calcium to nearly all soils near the coast, and that contributions from bedrock are generally small. An exception is the soil formed on the young volcano at Mt. Gambier in which the average ratio for carbonate is about 0.7070 compared to 0.7041 for the volcanic ash. The basaltic ash of the volcano is readily weathered and rich in calcium: roughly 40% of strontium and 10 to 20% of the calcium in the soil carbonate derives from the ash, and the balance from marine sources. The ⁸⁷Sr ⁸⁶Sr ratios from soil carbonate gradually increase inland, reaching values of 0.7152. These higher ratios reflect, at least in part, the increasing ⁸⁷Sr ⁸⁶Sr ratio of inland dust. In contrast, the ocean does not contribute significantly to the carbon pool of the soils we studied. The δ¹³C (PDB) values of soil carbonates on Kangaroo Island and the higher rainfall areas of eastern Victoria fall between -11.0 and -8.7‰, indicating that the carbonate formed in equilibrium with the C₃-dominated vegetation typical of the region. In the warmer inland areas to the north such as the Eyre Peninsula, the δ¹³C value of soil carbonate averages -5.3‰, reflecting the higher proportion of C₄ plants in the local biomass. Mixing with marine sources of carbonate cannot account for the observed carbon isotopic patterns. The flux of carbon into soils from the ocean therefore must be small compared to that derived from plant respiration and decay.