CARBONATE STABLE ISOTOPES | Non-Lacustrine Terrestrial Studies

The carbon- and oxygen-isotopic composition of surficial meteoric carbonate (soils, groundwater) is widely used in paleoenvironmental reconstruction in the Quaternary. Temperature, the oxygen-isotopic composition of rainwater, and evaporation are the main determinants of the oxygen-isotopic composition of surficial carbonates. At Devils Hole, Nevada, where a ∼500 kyr record of spring calcite is preserved, changes in temperature in the recharge area drive cyclic changes in oxygen isotopic values on glacial–interglacial timescales. Soil respiration rates and the fraction C3/C4 biomass determine the carbon-isotopic composition of soil carbonates. With other types of surficial carbonate such as spring tufas and some cave speleothems, rapid off-gassing of CO2 and water–rock interaction in closed-system conditions reduce their utility in environmental reconstruction. Over the Neogene, C4 biomass has expanded dramatically, as reflected in the large increases in carbon-isotopic composition of soil carbonates. At the same time, the oxygen-isotopic composition of soil carbonates has increased, likely due to global aridification. During the Pleistocene, changes in C4 biomass have been conservative on most continents except Africa, where large increases in the carbon-isotopic composition of soil carbonates are observed, denoting expansion of savanna ecosystems.