THE CONTRIBUTION OF VOLCANIC AEROSOLS TO EARTH SYSTEM BEHAVIOR IN THE LATE PALEOZOIC

The late Paleozoic ice age (LPIA) is a well-known icehouse interval, driven in large part through relatively low values of atmospheric pCO₂. However, the relationship between icehouse conditions, recorded in the frequency of occurrence of glacial deposits, and pCO₂ bears some inconsistencies, suggesting the possible influence of another factor and associated feedbacks in initiating and modulating climate during this time. Examination of ~200 My of Earth history, from the greenhouse of the Early-Middle Devonian through the greenhouse of the Triassic indicates a sustained increase of explosive (felsic-intermediate) volcanism in tropical and extratropical latitudes during the LPIA. The foci of volcanism during this interval were largely intraplate regions of the southern mid latitudes (Australia) and the orogenic collapse accompanying the equatorial Variscan system (western Europe). The geologic evidence of this activity is recorded in the occurrences of ignimbrites and calderas as well as general pyroclastic fallout, including fallout detected in dusts of the remote equatorial Panthalassic Ocean. This volcanic activity peaked at the same time as peak icehouse conditions. Comparison with the Quaternary record of explosive volcanism suggests that this peak corresponded to a frequency of eruptions injecting substantial sulfur to the stratosphere, almost an-order-of-magnitude greater than the present day. Modeling of the radiative effects implied by the pCO₂ and explosive volcanism records suggests that increasing sulfate aerosol loading at low, but gradually increasing CO₂ levels best explains the LPIA, with sulfate producing an especially strong effect at peak icehouse (~298-295 Ma). In addition to the direct (negative) radiative forcing accompanying sulfate aerosols, sustained volcanic injection would have also promoted atmospheric acidity, with accompanying effects on aerosol chemistry. Most notably, acidity increases solubility and thus bioavailability of both Fe and P, key nutrients for primary production. We are exploring the possibility that increased reactivity of dust-borne iron relates to this volcanic effect, with further implications for both the occurrence of organic-rich facies and the onset and widespread proliferation of redbeds in Permian time.