The time over which a soil has developed since the parent materials were deposited and subaerially exposed, referred to here as soil development time, is of considerable interest to pedologists, geomorphologists, geologists, archeologist, and paleoclimatologists. Soil development time has been estimated both indirectly, based on the degree of soil development, and directly, by radiocarbon dating of soil organic matter, optically stimulated luminescence (OSL) dating of parent material deposition, and through exposure dating using cosmogenic nuclides. Here we propose a new model for estimating the soil development time by subtracting the low molecular weight (LMW) or pyrolysis-volatile 14C age of the uppermost A horizon of a soil from the OSL age of the C horizon of the parent material [soil time=OSL age(parent C horizon)-14C age(LMW fraction of uppermost A horizon)]. The LMW organic compound fraction represents the most mobile carbon compounds in the soil organic carbon pool, and is least resistant to physical, chemical, and biodegradation in the soil environment, potentially yielding the youngest 14C age. OSL dating of the C horizon provides an estimate of the time since deposition of unconsolidated parent material. Thus, the difference in the two ages can be used to estimate the time that has elapsed for pedogenic alteration from deposition of the parent material to the most recent additions of soil organic carbon. We applied this new approach in four scenarios: (1) in a modern soil that developed downward in loess; (2) in a buried soil developed in aggrading loess (a cumulative soil which grows upward); (3) in a buried soil formed in dune sand; and (4) in a counterexample showing that simple subtraction is not always appropriate because of changes in soil's parent materials from loess to sand. These case studies reveal that this approach constrains soil development time more reasonably than can be commonly done with either OSL or radiocarbon dating alone. The difference in 14C ages between the uppermost and lowermost sola of a soil profile almost certainly underestimates the time of soil development, and the difference in OSL ages taken from sediment units overlying and underlying a buried soil most likely overestimates it. A combination of OSL and radiocarbon dating better constrains soil development time, broadening the applications of these two dating methods beyond their use for cross-checking the accuracy of the other dating method.
- Low molecular weight (LMW) organic compound
- Optically stimulated luminescence dating
- Radiocarbon dating
- Soil development time
ASJC Scopus subject areas
- Soil Science