Geochemistry (δ13C, δ15N, 13C NMR) and residence times (14C and OSL) of soil organic matter from red-brown earths of South Australia: Implications for soil genesis

Soil forming processes important to the development of Red-Brown Earths (duplex soils) in southeastern Australia have been investigated by a combination of techniques, including isotopic (δ13C, δ15N, 14C), spectroscopic (13C NMR, MIR), optically stimulated luminescence dating (OSL) and phytolith analyses. A distinct increase in clay content, corresponding changes in the abundance of major elements, as well as changes in organic chemistry (13C NMR), stable isotope trends (δ13C, δ15N), and phytolith abundance, are apparent in the transition from the very sandy A horizon to the clayey B horizon in three soil profiles from the Coonawarra–Padthaway region of South Australia. These structural and chemical changes between the A and the B horizons are associated with an abrupt increase in both 14C (bulk soil organic matter) and OSL burial ages of individual quartz grains. While previous interpretations have promoted the formation of duplex red-brown earths as due to clay illuviation, we propose a two-stage soil formation, which may be related to paleoclimatic changes during and after the Last Glacial Maximum. Our data suggest that a major part of the A horizon was aeolian derived and was deposited over the last 10,000 years, whereas much of the B horizon, although originally aeolian, has been extensively modified over much longer periods of time (tens of thousands of years). These results indicate the influence of different substrates (sandy versus clayey), process and time for formation as well as paleoclimatic history on the physical properties of the soil and the chemical characteristics of the organic matter within the soil profile.