Rare sulfur and triple oxygen isotope geochemistry of volcanogenic sulfate aerosols

We present analyses of stable isotopic ratios O-17/O-16, O-18/O-16, S-34/S-32, and S-33/S-32, S-36/S-32 in sulfate leached from volcanic ash of a series of well known, large and small volcanic eruptions. We consider eruptions of Mt. St. Helens (Washington, 1980, similar to 1 km(3)), Mt. Spurr (Alaska, 1953, < 1 km(3)), Gjalp (Iceland, 1996, 1998, < 1 km(3)), Pinatubo (Phillipines, 1991, 10 km(3)), Bishop tuff (Long Valley, California, 0.76 Ma, 750 km(3)), Lower Bandelier tuff (Toledo Caldera, New Mexico, 1.61 Ma, 600 km(3)), and 3 3 Lava Creek and Huckleberry Ridge tuffs (Yellowstone, Wyoming, 0.64 Ma, 1000 km(3) and 2.04 Ma 2500 km, respectively). This list covers much of the diversity of sizes and the character of silicic volcanic eruptions. Particular emphasis is paid to the Lava Creek tuff for which we present wide geographic sample coverage. This global dataset spans a significant range in delta S-34, delta O-18, and Delta O-17 of sulfate (29(0)/(00), 30(0)/(00), and 3.3(0)/(00), respectively) with oxygen isotopes recording mass-independent (Delta O-17 > 0.2(0)/(00)) and sulfur isotopes exhibiting mass-dependent behavior. Products of large eruptions account for most of these isotopic ranges. Sulfate with Delta O-17 > 0.2(0)/(00) is present as 1-10 mu m gypsum crystals on distal ash particles and records the isotopic signature of stratospheric photochemical reactions. Sediments that embed ash layers do not contain sulfate or contain little sulfate with Delta O-17 near 0(0)/(00), suggesting that the observed sulfate in ash is of volcanic origin. Mass-dependent fractionation of sulfur isotopic ratios suggests that sulfate-forming reactions did not involve photolysis of SO2, like that inferred for pre-2.3 Ga sulfates from Archean sediments or Antarctic ice-core sulfate associated with few dated eruptions. Even though the sulfate sulfur isotopic compositions reflect mass-dependent processes, the products of caldera-forming eruptions display a large delta S-34 range and exhibit fractionation relationships that do not follow the expected equilibrium slopes of 0.515 and 1.90 for S-33/S-32 VS. S-34/S-32 and S-36/S-32 vs. S-34/S-32, respectively. The data presented here are consistent with modification of a chemical mass-dependent fractionation of sulfur isotopes in the volcanic plume by either a kinetic gas phase reaction of volcanic SO2 with OH and/or a Rayleigh processes involving a residual Rayleigh reactant-volcanic S02 gas, rather than a Rayleigh product. These results may also imply at least two removal pathways for S02 in volcanic plumes. Above-zero Delta O-17 values and their positive correlation with delta O-18 in sulfate can be explained by oxidation by high-delta O-18 and high-Delta O-17 compounds such as ozone and radicals such as OH that result from ozone break down. Large caldera-forming eruptions have the highest Delta O-17 values, and the largest range of delta O-18, which can be explained by stratospheric reaction with ozone-derived OH radicals. These results suggest that massive eruptions are capable ofcausing a temporary depletion of the ozone layer. Such depletion may be many times that of the measured 3-8% depletion following 1991 Pinatubo eruption, if the amount of sulfur dioxide released scales with the amount of ozone depletion. (C) 2007 Elsevier Ltd. All rights reserved.