The chemical index of alteration (CIA) as a proxy for climate change during glacial-interglacial transitions in Earth history

Abstract

The glacial-interglacial transitions of ice ages are associated with large, rapid changes in climate, which potentially can be reconstructed from stratigraphic profiles of the chemical index of alteration (CIA). Here, we present a case study based on high-resolution CIA profiles of Neoproterozoic glacial deposits from South China (paleolatitude ~30° N) as a record of the climate transition at the termination of the Sturtian Glaciation (~663 Ma). Drillcore ZK2115 (Gaodi Deposit) exhibits a progressive upsection increase in mean CIA over a ~50-m interval, from 58 (range 52–65; note: all ranges are given as 16th–84th percentiles) in the synglacial upper Tiesi'ao Formation to 67 (range 64–69) in the deglacial basal Datangpo Formation cap carbonates, and to 68 (range 66–70) in the postglacial Datangpo Formation black shales. A coeval section from the Lijiawan Deposit also exhibits an increasing CIA trend upsection within shallow-water Mn-carbonate facies. These CIA patterns show no relationship to lithology and are interpreted to reflect climatic warming and intensified chemical weathering during the Sturtian deglaciation. Similar large increases in postglacial CIA values are seen in Paleoproterozoic and Late Paleozoic successions, and at least moderate increases in CIA values are recorded during warm stages of the late Quaternary (e.g., the Bølling-Allerød and Holocene) relative to the cold stages (e.g., the Last Glacial Maximum and Younger Dryas). The rapidity of CIA changes in Quaternary systems (~1 to 10 kyr) suggests that weathering intensity may have changed at similarly rapid rates in more poorly dated deep-time glacial successions, with potential implications for Snowball versus Slushball models of Cryogenian glaciations. These examples show that CIA can be a robust proxy for climate changes (i.e., enhanced chemical weathering intensity) during glacial-interglacial transitions of both ancient and recent ice ages.