LUMINESCENCE SIGNALS FROM SOIL AND SAPROLITE IN DEEPLY WEATHERED PROFILES- INTRIGUING NEW RESULTS FROM THE PIEDMONT OF NORTH CAROLINA (Invited Presentation)

Optical and infrared stimulated luminescence (OSL and IRSL) dating provides an age estimate for the last time sediment was exposed to light or heat. Generally, luminescence sensitivity (brightness and quality of signal) increases as a grain is exposed to repeated cycles of alternating sunlight exposure and radiation during burial. OSL/IRSL dating is typically employed for calculating dates and rates of deposition of sediment. Soils are generally avoided due to mixing of sediment with various sunlight exposure histories and non-stationary radio-isotope concentrations over time. OSL/IRSL signals have rarely been measured, however, in saprolite (in situ weathered bedrock) especially igneous or metamorphic terranes, given that sand-sized minerals in such bedrock have never been exposed to sunlight or sensitized. Here we seek to test the assumption that such grains would essentially exhibit zero signal by comparing luminescence between saprolite and overlying well-developed soil.

The Piedmont region in the southeastern US is generally characterized by well-developed, clay-rich soils (~2 m thick) grading downward into 10+ m of saprolite formed in structurally complex bedrock. Luminescence samples were extracted from vertical weathering profiles of soil and saprolite for both metagranitic and metavolcanic bedrock in summit profiles (cores) and steep north and south-facing hillslopes (soil pits) at the Redlair Observatory near Gastonia, NC. As expected, preliminary small-aliquot (multi-grain) results show that sand-sized quartz (150-250µm) from the upper 1m soil have higher sensitivity to luminescence compared to the underlying saprolite. We also find, however, that differentially-weathered saprolite (~6-16 m depth) has closely-related - non-zero - luminescence signals. These results possibly suggest that saprolite exhibits a baseline luminescence that should be measured and accounted for when calculating rates of soil mixing in the overlying regolith. Analysis of luminescence characteristics of minerals promises to be useful for identifying mobile regolith from in-situ weathered bedrock (saprolite) and can play a critical role in grain sourcing and mobility within the critical zone.