QUARTZ LUMINESCENCE FROM SOIL AND SAPROLITE: A TEST CASE FROM THE WESTERN PIEDMONT OF NORTH CAROLINA

Red clay-dominated ultisol soils in the southeastern US Piedmont often exhibit indistinguishable characteristics when formed in fine-grained allochthonous surficial deposits and autochthonous weathered saprolite. Distinguishing sediment sources in these well-developed soils can be challenging without geochemical or geophysical analyses, and more refined granular details are needed to determine transport and mixing within mature soil profiles. Our goal is to determine if luminescence sensitivity may be used to distinguish transported (i.e. sediment) vs non-transported (i.e. in-situ weathered bedrock) material in soil profiles. We turn to linear-modulated optically stimulated luminescence (LM-OSL) in quartz grains from crystalline-bedrock-derived saprolite and immediately overlying soil horizons that contain no remnant bedrock structure.

Our study investigates soil and saprolite OSL samples from both shallow and deeply weathered soil profiles from 1-meter soil pits and 10+ m deep regolith cores in the Redlair Observatory near Charlotte, NC. We measure single-grain and small-aliquot LM-OSL to characterize luminescence signal components. Deconvoluting the luminescence signal is important for identifying the presence and intensity of the fast component and may indicate sedimentary maturity. The fast component takes time to evolve at the Earth’s surface, while non-fast components are not easily bleached by sunlight during sedimentary transport and are not preferred for luminescence age determination. Saprolite will not have undergone sedimentary transport or pedoturbation given the presence of in-situ bedrock foliation and thus we expect it to be dominated by non-fast signal components.

Our results show that LM-OSL measurements of saprolite have no fast component. Overlying soil horizons have variable luminescence intensity, with generally more signal from the fast component compared to saprolite. Upper soil horizons that have limited sensitivity and lack fast-dominated signals are either derived from in-situ materials, are less mixed, and/or recently eroded from saprolite. Similarly-classified soil horizons with more luminescence-sensitive quartz grains may suggest increased weathering, mixing, and/or greater input from distal sources.