Variations of stable isotopes with depth in regolith calcite cements in the Broken Hill region, Australia: Palaeoclimate evolution signal?

Twenty two samples of calcretes from seven depth-profiles in the Menindee catchment, Broken Hill region, Australia were analysed for their inorganic and organic carbon contents and inorganic carbon and oxygen isotopes. The organic carbon content is very low (from 0.06 to 0.31 wt.%) while inorganic carbon (carbonate) is up to 3.9 wt.%. Both δ ¹³C and δ ¹⁸O become more positive closer to the surface. Carbon isotopes vary from - 8.5‰ to -5.5‰ PDB. Oxygen isotopes vary from - 6‰ to - 1.8‰ V-PDB. Depth-related δ ¹³C and δ ¹⁸O variations correlate over at least 15 km and show no significant variation along the flow path. δ ¹³C values increase by 3‰ and δ ¹⁸O values increase by 4‰ with decreasing depth in a 1.40 m thick soil profile. The variation is interpreted to indicate an increasingly elevated air temperature, greater water stress and subsequently an aridification of the area through time. The Broken Hill calcrete data confirm that climatic evolution can be deduced from isotopic series and be applied successfully to the Broken Hill region.