QUARTZ LUMINESCENCE: DISTINGUISHING SEDIMENT AND SAPROLITE IN REGOLITH

In the southeastern US Piedmont, regolith can share similar traits, regardless of whether it originates from fine-grained allochthonous sedimentary deposits (e.g., alluvium) or autochthonous weathered bedrock (i.e., saprolite). Sediment and saprolite must be distinguished for critical-zone research like the adsorption of rare earth elements in regolith and geomorphic change through time. Here we demonstrate how optically stimulated luminescence (OSL) methods can differentiate quartz sand sourced from transported sediment versus in-situ weathered bedrock material.

Our study examines samples from hillslope and floodplain sediment, and saprolite obtained from 1-meter-deep soil pits and 10-meter Geoprobe cores at the Redlair Observatory near Charlotte, NC. We utilize single-grain and small-aliquot linear-modulated OSL (LM-OSL) techniques to analyze luminescence signal components derived from quartz sand grains from these samples. Disentangling the signal helps identify the presence and intensity of the fast component, which constitutes the most light-sensitive signal and might indicate sedimentary maturity. The fast component takes time to develop at the Earth’s surface, while non-fast components (i.e., medium and slow decay signals) are less susceptible to sunlight bleaching during sedimentary transport.

Our findings reveal that LM-OSL measurements of saprolite expectedly lack a fast component, given that it has not undergone sedimentary transport or pedoturbation. Where original bedrock texture is missing, LM-OSL can be key to identifying otherwise unclear geology. In contrast, overlying soil horizons exhibit varying luminescence intensity, typically displaying more signal from the fast component compared to saprolite. Soil horizons with limited sensitivity and an absence of fast-dominated signals are likely derived from in-situ materials, experiencing less mixing, or are recently exhumed from saprolite. Similar soil horizons that show heightened luminescence-sensitive quartz grains consistently are found in deposits already identified as sediment using stratigraphic and sedimentologic evidence, and more closely match those collected in floodplain sediments. Results indicate that OSL signals from quartz provide a new fingerprint of mineral source and history.