DIFFERENTIAL LUMINESCENCE PROPERTIES AND SENSITIVITY IN SAPROLITE AND SOIL AT REDLAIR OBSERVATORY NEAR CHARLOTTE, NC

Optically stimulated luminescence dating utilizes a trapped charge phenomenon in quartz grains to determine the amount of time since removal from sunlight. A key factor for luminescence sensitization is the number of electron-trapping crystallographic defects and chemical impurities within a grain. Original defects and impurities form during crystallization and are commonly saturated by light-insensitive charge. Through repeated exposure to sunlight and low-level ionizing radiation, luminescence-producing vacancies may become available. This follows that through weathering, transport, and deposition, luminescence sensitivity in quartz sand grains can increase over time.

Soil, saprolite, and weathered bedrock are ubiquitous in the Piedmont of the southeastern U.S. Ultisols formed in fine-grained surficial deposits and in weathered saprolite can have similar appearance and characteristics. Distinguishing allochthonous and autochthonous sediment in these often red, clay-rich, well-developed soils may be difficult without additional geochemical analyses even though the distinction plays a key role in understanding Quaternary landscape evolution.

Our study investigates deeply weathered soil profiles in the Redlair Observatory near Charlotte, NC. We measure luminescence sensitivity in quartz sand from saprolite derived from crystalline bedrock and from the overlying soil horizons. Our goal is to determine if luminescence sensitivity may be used to distinguish transported vs non-transported material in soil profiles. We find that saprolite, which has not undergone sedimentary transport, contains a small but measurable amount of light-sensitive trapped charged. This can be attributed to the defects and impurities created during crystallization. The overlying soil has luminescence sensitivity that varies from comparable to three times greater than the underlying saprolite. This variability suggests that processes which bring grains up to the surface then back down the profile may be an aid to further sensitization of soil grains derived from saprolite. In other words, quartz grains liberated from saprolite that become part of the active soil evolve toward luminescence maturity. Additionally, pre-sensitized sediment grains of external origin may also be present in these soil profiles.