Glaciation and extinction at the Devonian-Carboniferous boundary; high palaeolatitude record of the Bolivian Altiplano

The Late Devonian and Mississippian record the transition from Greenhouse into a cooler global climate punctuated by glaciations. Evidence for glaciation in the latest Devonian is primarily derived from diamictites deposits in South America. The event was broadly coincident with eustatic changes and mass extinction associated with the Hangenberg Crisis. To understand the record of glaciation in more detail a stratigraphic, palynostratigraphic and chemostratigraphic field-study has been undertaken near Lake Titicaca, Bolivia. Three primary sections were measured; Chaguaya, Villa Molino and CR5. These record a 1.1 km thick Devonian-Carboniferous (D/C) boundary interval. Glacigenic strata are represented by the Cumaná Formation, dated to within the lepidophyta-nitidus (LN) miospore zone. Additional sections were measured along a strike-parallel topographic ridgeline, which revealed that the Cumaná Formation consists of a single, preserved advance of glacigenic strata into a marine shelfal environment. It is associated with incised palaeo-relief, which is >7 km in length and 110 m deep. The D/C boundary in the study area is defined on the loss of Retispora lepidophyta and a collapse in marine phytoplankton diversity during initial post-glacial transgression. Hence, palynological extinctions represent a proxy for the D/C boundary in South America that is tied to wider palaeoclimatic and glacioeustatic changes. A 2 ‰ positive carbon isotope excursion is associated with palynological extinctions at the D/C boundary. Palynofacies were unable to quantify the marine vs. terrestrial ratio of preserved organic matter as amorphous organic matter was preferentially lost during standard palynological processing techniques. However the excursion is likely related to a reduction in the supply of terrestrial and marine organic matter at a time of environmental stress. These combined results provide a high-palaeolatitude glacial perspective on debates regarding mass-extinction, eustatic and environmental change around the D/C boundary. The glacigenic Cumaná Formation is interpreted to represent the glacioeustatic control sea-level falls observed globally at the D/C boundary