PROVENANCE, TEXTURE, AND RELATIVE AGE CONTROLS ON THE CHEMICAL INDEX OF ALTERATION (CIA) IN PLEISTOCENE TILLS, MINNESOTA USA

The chemical index of alteration (CIA) quantifies the degree of chemical weathering on sediment source materials, and has been utilized to infer climatic influences on a range of modern and ancient sediments including pre-Pleistocene tillites. Yet relatively few investigations explore CIA signatures in Pleistocene tills, where wide ranging lithologies may influence the value of the index. Here we report CIA values for Pleistocene tills from a series of samples from Minnesota, USA, representing a diverse range of textures and provenance. We calculated CIA from samples digested with buffered acetic acid, and compared these with values calculated from a method using a geochemical correction (excess CaO relative to Na₂O) to account for carbonate content. Both the <2mm and <63 μm grain size fractions were considered. The CIA values from samples processed with acid digestion were not comparable with CIA values calculated from untreated samples using a geochemical correction. As noted in previous studies, the finer grained size fraction yields a higher CIA value, with the offset relative to the coarser fraction is ranging from 2 to 7 index points in our samples. Lacustrine samples were shifted towards lower CIA values relative to the fine fraction of tills of the same provenance. Late Pleistocene tills sourced from sedimentary basins had higher CIA values relative to tills of similar age sourced from the crystalline rocks of the Canadian Shield. Our highest CIA value came from an old (mid-Pleistocene?) till sourced from a sedimentary basin, but its CIA was statistically indistinguishable from a sample derived from the last-Glacial maximum Des Moines lobe till also sourced largely from sedimentary materials. Comparing CIA data with trace element and radiogenic isotope (Sr and Nd) compositions from the same samples further underscores the strong provenance control on the geochemistry of these tills. Our results suggest that the carbonate content in tills must be accurately quantified if CIA calculations are to be of value, and that processes impacting provenance and texture must be carefully accounted for in order to make a meaningful interpretation of glacial sediment CIA. Properly contextualized, the CIA of glacial till represents a powerful tool for unlocking the dynamics of ancient ice sheets and climates.