Variability in Bioavailable ⁸⁷Sr/⁸⁶Sr in the North American Midcontinent

Abstract

Strontium (Sr) isotope tracers are useful for understanding provenance and mobility in biological materials across multiple disciplines. However, the impact of these techniques is highly dependent on the construction of appropriate comparative baselines (i.e., an isoscape). We present the results of a systematic survey of ⁸⁷Sr/⁸⁶Sr values from grasses in the North American Midcontinent with a particular emphasis on sedimentary systems. Although ⁸⁷Sr/⁸⁶Sr values are highly variable across the region, the Sr isoscape shows multi-scalar patterns that are dependent on local-to-regional trends in surficial geology. High values are found in bedrock-dominated areas such as the Black Hills (SD) and Ozark Uplift (MO), or formerly glaciated areas where surface deposits are dominated by ice-transported Precambrian clasts. The lowest values are found in river valleys that incorporate eroded Neogene sediments into terrace formation. Intermediate values are found in upland loess and alluvial deposits which blanket much of the study area. We demonstrate trends in large-scale variability of the Midcontinent’s ⁸⁷Sr/⁸⁶Sr isoscape and suggest that future refinement focus on sub-regional trends in Sr isotope variability.