Luminescence chronology and thermometry studies of plant opal phytoliths

In this work we have been investigating the luminescence properties of plant opal phytoliths to assess their suitability for determination of age and/or thermometric information from soil and sediment sequences. Opal phytoliths, or bio-opal, form when monosilicic acid from soil-waters is taken up by plants and chemically altered to silica, producing intra- or extra- cellular structures that give grasses and stems their strength. Opal phytoliths are usually considered to be non-crystalline and referred to as silica mineraloid structures, with ~4-9% water, <5% other elements, and specific gravity ranging from ~1.5-2.3. They are known to be resistant to degradation and hence preserved in soil or sediment even after decomposition of organic matter. Our earlier work examined a <2.37 g/cm³ density fraction in parallel with quartz grains from samples collected from fluvial terraces and soil pits on Konza Prairie Biological Station native tall grass prairie a few km from Kansas State University. We observed generally similar luminescence characteristics from the phytolith fractions to quartz, with bright blue optically stimulated luminescence (OSL) signals and good single-aliquot regenerative-dose characteristics. In two hours the OSL signal is ~90% bleached by white light, whereas red fluorescence lab lighting has a negligible effect over the same exposure time. Thermoluminescence (TL) data suggested the presence of feldspatic-like minerals or perhaps thermal degradation of the phytoliths during TL measurement; the phytolith fractions were also stimulated by low-temperature infrared stimulated luminescence (IRSL₅₀) perhaps also indicating presence of contaminant minerals. Initial SEM analyses identify what appear to be weathered silica grains, but also highly weathered, pitted concretions with silicate-like structures according to element mapping but actual mineral identification is presently unclear.

Most recently we have begun analyzing samples collected from a suite of stratified paleosols from the mid-continent stream type-site of Claussen, Mill Creek, Wabaunsee County, Kansas. This site has documented phytolith examples and a radiocarbon framework. We are continuing luminescence characterization studies, incorporating screening of prepared fractions with SEM and IRSL₅₀ evaluation, and pulsed time domain analysis measurements are being explored.

We think luminescence from opal phytoliths shows great promise as an alternative target to quartz or feldspar, but moreover as a sensitive recorder of climatic change or fire exposure on plant communities. This presentation will review our earlier work on phytoliths and discuss most recent findings from the Claussen site.