BIOSILICATE RECONSTRUCTION OF THE BRADY SOIL IN SOUTHWESTERN NEBRASKA: THE IMPORTANCE OF NON-SHORT CELL PHYTOLITHS FOR PALEOCLIMATE INTERPRETATIONS

The Old Wauneta Roadcut site near Wauneta, Nebraska exhibits a 1.2 meter-thick exposure of the Brady Soil, an accretionary paleosol that began to form ~14 ka within the uppermost part of the LGM Peoria Loess and was subsequently extinguished ~ 9 ka by the onset of Holocene-age Bignell Loess deposition. Analysis of short cell phytoliths reveals quantitative plant taxa shifts from Pooideae (C₃) dominant grasses, with relatively large numbers of arboreal dicot spheres and a few Cyperaceae (sedge) present in the Bølling-Allerød climatic period (~14.6 ka to 12.9 ka), to Chloridoideae (C₄) dominant grasses in the early Holocene. Stipa-type Pooideae, a cool-season grass preferring drier soil conditions, marks the onset of the Younger Dryas (~12.9 ka to 11.7 ka). Beyond the quantified changes in vegetation assemblages seen with short cell phytolith analysis, large-cell phytoliths such as long cells, bulliforms, and trichomes, along with other biosilicates, have been shown to be notable climatic indicators in this study. Collective decreases in bulliform, trichome and long cell counts within both C₃- and C₄-dominated temperature phases indicate periods of lower moisture availability and play a major role in aridity index calculations. Bulliforms are produced during high evapotranspiration episodes and, therefore, when compared to long cell counts, a water stress index can be computed and used to formulate potential evapotranspiration evaluations. Other biosilicates such as diatoms, algal statospores, and sponge spicules typically provide indications of standing water environments. In this study, however, these have been identified primarily within the uppermost solum during the onset of Bignell Loess deposition indicated by lower phytolith concentrations and soil morphology changes. The combination of biosilicate information and concentration values indicates loess influx to the upland from a nearby stream valley rather than standing water occurring at the sample site. These examples illustrate the need for all grass phytoliths and other biosilicates to be identified, which allow for a more accurate and detailed paleoclimatic reconstruction than would have been possible with a short cell phytolith analysis alone.