Triple oxygen and sulfur isotope analyses of sulfate extracted from voluminous volcanic ashes in the Oligocene John Day Formation: insight into dry climate conditions and ozone contribution to supereruptions

Large volume pyroclastic silicic eruptions emit hundreds of megatons of SO₂ into the troposphere and stratosphere that is oxidized into sulfuric acid (H₂SO₄) by a variety of reactions with mass independent oxygen signatures (MIF), Δ¹⁷O>0. Sulfuric acid is then preserved as gypsum in parental volcanic deposits. Diagenic effects are mass dependent and can dilute, but otherwise do not affect MIF ratios. Pleistocene Yellowstone and Bishop tuffs and modern volcanic eruptions preserved under arid climate conditions in North American playa lakes, preserve small amounts of volcanic sulfate as gypsum. This gypsum’s Δ¹⁷O>0, in combination with isotopic variations of δ¹⁸O, δ³³S and δ³⁴S is distinct from sedimentary sulfate and reveals its original MIF sulfate isotopic signal and the effect of super eruptions on the atmosphere, and ozone consumption in particular. We use linear algebraic equations to resolve volcanic versus sedimentary (MIF=0) sources. We have found that many large volume ignimbrites have very high initial Δ¹⁷O in volcanic sulfate that can only be acquired from reaction with stratospheric ozone.

We here investigate nine thick (>2 m) ash beds ranging in age from ~33-23 Ma in the John Day Formation of central Oregon, including massive 28.6 Ma Picture Gorge tuff of newly identified Crooked River supercaldera. The 28.6 Ma Picture Gorge tuff (PGT) has the highest measured Δ¹⁷O of 3.5‰, and other tuffs (Tin Roof, Biotite, Deep Creek) have +1.3 to 3.4‰ Δ¹⁷O excesses. Sulfate from modern smaller tropospheric eruptions studied for comparison have a resolvable 0.4‰ range consistent with liquid-phase based H₂O₂ oxidation. The PGT is coeval with the ignimbrite flare-up in western N. America, the 28-29 Ma eruption of the 5000 km³ Fish Canyon tuff and the 28 Ma Never Summer Field eruption in Nebraska-Colorado that have the highest measured Δ¹⁷O of 6‰ (Bao et al. 2003). We speculate on the climatic/atmospheric effects of these multiple ~28 Ma supereruptions in the N America semiarid paleoclimate. The strong mass independent signal of the 28-29 Ma ignimbrite flare-up suggests these super eruptions must have reached well into the stratosphere to record interactions with ozone and peroxide or created dry fog tropospheric conditions.