RECONCILING INDICATORS OF COLD AND WARMTH IN LATE PALEOZOIC TROPICAL PANGAEA: HIGH-MAGNITUDE GLACIAL-INTERGLACIAL CLIMATE CHANGE?

Recent research documents the possibility of remarkably cool temperatures, even glaciation, in tropical Pangaea. The most direct evidence comes from the Unaweep-Cutler system of the Paradox basin (Ancestral Rocky Mountains), which records a paired late Paleozoic glacial valley and proglacial system that persisted to relatively low elevations. Unaweep Canyon exhibits the distinctive geomorphology of a glacially carved canyon, and harbors late Paleozoic aged diamictite containing quartz grains with distinctive crushing features and microstriae. The Cutler depositional system that onlaps the canyon has long been interpreted as an alluvial fan, but bears dropstones, turbidites and clastic varves more characteristic of a proglacial lacustrine system that ultimately reached to sea level within 50 km of the ice terminus. Furthermore, the greater ARM region of western tropical Pangaea houses the biggest dust deposits on Earth, preserved as both true eolian silt, or loess, and as eolian silt ultimately deposited in many other environments. The most efficient mechanism for production of this material is glacial grinding. Albeit a seemingly unlikely mechanism for the vast tropical silt of Pangaea, we hypothesize that widespread glaciation in the Pangaean tropics provides the most parsimonious explanation for this remarkably non-uniformitarian distribution of loess. Key attributes of the silt, e.g. its volume, textural and mineralogical immaturity, minimal chemical alteration, and first-cycle, local- to regional provenance, are all most compatible with a glacial or periglacial derivation. But this evidence has been controversial because it seems at odds with indicators long thought indicative of tropical warmth, such as photozoan carbonates, commonly intercalated with loess. We suggest that a possible solution to this conundrum may be high-magnitude climate change that enabled large shifts in temperature between glacial and interglacial states, but that evidence for such shifts becomes lost when we time-average geologic indicators of deep-time climate systems. Earth's deep-time record, viewed at high resolution, might reveal tales of abrupt and high-magnitude climate change that likely dwarf those of Earth's near-time record.