Historical experience tells us that volcanic eruptions can directly cause short-term (few years) global climate perturbations through the release of gases to the atmosphere. What is less clear is the capacity for very large eruptions to produce catastrophic and/or long-lasting climate change. Such eruptions lie outside recorded human experience but their products are evident in the geologic record. Among this class of events are the gigantic piles of frozen lava flows that make up continental flood basalt regions. Individual flood lava eruptions may be hundreds of times bigger than the largest historically-erupted lavas, with, potentially, proportionately severe effects on climate. The youngest and best-studied continental flood basalt province is the Columbia River Basalts (CRB), which formed around 16 million years ago and now cover about 200,000 square kilometers of the Pacific Northwest region of the United States. Previous studies of gas release associated with the eruption of these lavas have focused on late flows that post-date the main outpouring of basalt. This project employs a new approach that targets glassy volcanic ash found in the areas around the volcanic vents, rather than the equivalent crystallized, 'stony' lavas themselves. The glasses, and the crystals that they hold, preserve a higher fidelity record of eruption outgassing than do the lavas, allowing estimates of atmospheric pollution caused by the most intense phase of CRB activity. These estimates will constitute 'ground truth' that can help constrain, for instance, models for regional and global climate disturbance at the time of eruption.
Before now, studies of degassing from CRB have focused on the porphyritic Wanapum lavas (6% of the whole CRB), which post-date the peak activity of flood volcanism represented by the Grande Ronde lavas (70% of the whole CRB). The principle barrier to study of the Grande Ronde has been a perception that they are aphyric, i.e. they do not contain melt inclusion-bearing 'phenocrysts' (crystals that grew in the magma prior to eruption). Melt inclusions in crystals preserve the original gas content of the magma (chiefly H2O, CO2, S species) that can then be compared with fully degassed groundmass basalt to gain an estimate of gas loss. However, study of naturally water-quenched basaltic glasses from the vent areas of Grande Ronde lava flows shows that they do indeed contain melt inclusion-bearing phenocrysts, small, but nonetheless amenable to analysis by microbeam methods. Additionally, some of the glassy ash fragments have been quenched prior to degassing and can also be analyzed to gain an estimate of gas loss. The data will be used to estimate fluxes of S, H2O and CO2 to the atmosphere from CRB activity. This in turn will provide constraints on the potential of the CRB for cooling (due to S pollution of the atmosphere) vs. warming (due to CO2 release to the atmosphere) the Miocene climate, both of which hypotheses have been advanced by previous workers, but in both cases supported by little data. A further area of study is afforded by the fact that the glasses preserve a broader range of overall chemical compositions than do the equivalent lavas, potentially providing new insights into the ultimate origins of the flood basalt magmas.
