Variability in Bioavailable &lt;sup&gt;87&lt;/sup&gt;Sr/&lt;sup&gt;86&lt;/sup&gt;Sr in the North American Midcontinent

Chris Widga,Andrew Boehm,J Douglas Walker

doi:10.5334/oq.32

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.

Highlights

Isotopic tracers are useful in provenance and mobility applications across many different disciplines (Bentley, 2006; Crowley et al, 2015)
Because the Rb to Sr ratio is so low and the time involved in studies so short, the 87Sr/86Sr ratio does not evolve by radioactive decay and can be treated as essentially a stable isotope (Bowen and West, 2008)
The lowest 87Sr/86Sr values are found in the western part of the study area, where valley alluvium and uplands are dominated by Neogene sources and loess-covered landscapes along the Missouri River and the eastern prairies (0.7079–0.7120)

Summary

Introduction

Isotopic tracers are useful in provenance and mobility applications across many different disciplines (Bentley, 2006; Crowley et al, 2015). The impact of these techniques is highly dependent on the quality of baseline comparative data (i.e., isoscapes), complicating its use in areas dominated by allochthonous surface sediments of alluvial, eolian, or glacial origin. There have been previous attempts to model regional 87Sr/86Sr composition in the region (Beard and Johnson, 2000; Bataille and Bowen, 2012), the resolution of surface geological mapping and the wide variability in surface geomorphology, even within small areas, have complicated efforts to resolve a Sr isoscape for surface materials. Direct surface sampling of plants, animals, and sediments (Price et al, 2007; Hedman et al, 2009; Widga et al, 2010; Slater et al, 2014) offers some insight into regional Sr distribution patterns, these efforts have been limited to small parts of the study area and are opportunistic surveys undertaken independent of local geomorphic information

Methods

Results

Discussion

Conclusion