Climate controls on spatial and temporal variations in the formation of pedogenic carbonate in the western Great Basin of North America

摘要

Pedogenic carbonate is common in arid and semiarid soils, and though its stable C and O isotope composition has been shown to reflect local environmental conditions, questions remain about the quantitative nature of this relationship and the implications for paleoclimate applications. To further address these questions, a climosequence of four Holocene soils in Fish Lake Valley, Nevada (western United States), was instrumented and examined over the course of more than a year. The annual precipitation along the transect ranges from similar to 80 mm yr(-1) to similar to 220 mm yr(-1). Results show that the C and O stable isotope compositions of soil CO2, H2O, and carbonate change systematically with elevation and climate. However, there was considerable temporal variability in the conditions that affected carbonate isotope values. At the lowest elevation, CO2 delta C-13 and H2O delta O-18 values were similar to that in equilibrium with carbonate nearly year-round. At the midelevation sites, spring through summer CO2 delta C-13 and H2O delta O-18 values appeared to most closely match the delta C-13 and delta O-18 values of pedogenic carbonate. At the highest elevation, the C and O isotopes of carbonate did not reflect the soil CO2 delta C-13 and H2O delta O-18 values measured during the period of study, but they did appear to reflect soil respiration rates during the late spring. The transect shows that arid soil carbonate delta C-13 values strongly reflect variations in soil respiration rates and the resulting concentrations of soil CO2. These results also indicate that depth profile sampling may be required to adequately constrain respiration rates, which affect the interpreted atmospheric CO2 concentrations.