Phytoliths (plant-derived mineral bodies) as geobiological and climatic indicators in arid environments

Abstract

Phytoliths are plant-derived mineral bodies . They have been used in a variety of archaeological, environmental, and climate studies to infer paleoclimate from the vegetation types represented by diagnostic phytolith morphologies . Phytoliths can be recovered from modern plants, soils, sediments, lacustrine deposits, eolian deposits, archaeological sites, plant fossils, and potentially allochthonous sediment deposits in caves . In order to interpret such data, documentation of modern plant-derived minerals is needed, especially in arid regimes . Four goals were attempted in this study, 1) provide a morphological description of the different types of biomineralization products from the modern plant species of creosote and horsetail, two Southwestern plants with significantly different environmental requirements; 2) describe the elemental chemistry and mineralogy of the phytoliths from these plants; 3) describe morphological changes to biominerals during experimental mechanical weathering and abrasion processes, and; 4) compare all of these results to identified fragments of potential phytoliths and preserved plant-derived minerals from Fort Stanton Cave sediments and Miocene-aged plant fossils from the Rainbow Loop Flora of the Barstow Formation . This study examines biominerals from two modern key indicator species in the arid Southwest U . S . A . : Larrea tridentata (DC . ) Coville (creosote), which is characteristic of North American hot arid regimes, and Equisetum hyemale L . (horsetail), a known silica accumulating plant from wet environments and an indicator of riparian areas within desert environments . Creosote and horsetail produce biominerals that are different from one another, and possibly unique to the species or genus level of identification . We conducted a series of observations, analyses, and experiments including: 1) analysis of plant tissues with scanning electron microscopy (SEM) with energy dispersive x-ray (EDX) and x-ray diffraction (XRD); 2) investigation of preservation potential by documenting morphological changes to phytoliths after simulated mechanical weathering for different lengths of time; and 3) modern and weathered biominerals from creosote and horsetail were compared to sediments and fossils of different ages from two study sites, Fort Stanton Cave, Lincoln County, New Mexico and the Barstow Formation, San Bernardino County, California . Both study sites revealed preserved phytoliths indicating potential long-term preservation and the potential for application of these structures as vegetation paleoclimate indicators . All phytolith types from modern plant material were still distinguishable after simulated weathering treatments .