ATMOSPHERIC DUST FROM THE PENNSYLVANIAN COPACABANA FORMATION (BOLIVIA): A HIGH-RESOLUTION RECORD OF CLIMATE AND VOLCANISM FROM NORTHWESTERN GONDWANA

Atmospheric dust from ice, oceanic, and continental (e.g. loess) records is a valuable climatic archive. Loess deposits of the late Paleozoic Ice Age (LPIA) are well known from tropical Pangaea, but less studied in extra-tropical regions. This study documents dust from Pennsylvanian carbonates of the Copacabana Formation, recovered in core (the “Manuripi X-1”) from the Madre de Dios basin (Bolivia), located within southern mid-latitudes of western Gondwana. The Copacabana Formation spans Pennsylvanian and Early Permian time, and thus formed coeval with ice centers and associated glacial deposits located at more southerly paleolatitudes in adjoining regions of Gondwana (e.g. the Parana and Tarija basins of Brazil). In Pennsylvanian time this unit formed isolated from fluvial-deltaic influx, and thus siliciclastic material here is interpreted to reflect atmospheric input. The 27.5 m study interval comprises open-marine fossiliferous wacke/packstone to restricted carbonate mudstone arranged in apparent upwardly shallowing cycles 1 – 3 m thick. Green volcanic ash horizons also occur, and new U-Pb zircon dating (Hamilton et al., this meeting) on one especially thick bed has yielded an age of 316 Ma. Cycles are bounded by surfaces recording either an abrupt basinward facies shift, or inferred subaerial exposure surfaces marked by, e.g. reddened microkarst, with a commonly silty matrix. Siliciclastic material extracted at 20 cm intervals (avoiding obvious ashes) varies from 0.56 – 43.89 wt % in carbonate facies and 15.13 – 63.94 wt % in microkarsted surfaces. The extraction process removes carbonate, organics, pyrite, and iron; any authigenic silica was removed manually. Grain size modes range from <1 to 97 µm, with coarser intervals generally correlating to peak dust content (wt%), and cycle bounding surfaces. Ash-rich samples exhibit elevated K/(Fe+Mg) and Ti/Al; relative to reddened silt-rich samples. These data, together with relative proportions of La-Th-Sc enable discrimination of ash-rich dust reflecting an arc source from continental (cratonal-sourced) dust inputs. These preliminary results suggest that cratonal dust influx generally peaked at cycle boundaries, coincident with lowstand (glacial) conditions during the LPIA.