[1] Sulfuric acid aerosols produced in the stratosphere following massive volcanic eruptions possess a massindependent sulfur isotopic signature, acquired when volcanic SO 2 experiences UV photooxidation. The volcanic data are consistent with laboratory SO 2 photooxidation experiments using UV light at 248 nm (maximum absorption of ozone), whereas sulfur isotopic anomalies previously observed in Archean samples are consistent with photodissociation at 190 -220 nm. A mechanism of SO 2 photooxidation, occurring in the early stage of a stratospheric volcanic plume, in the range of 220 -320 nm (weak band absorption of SO 2 ), is also proposed. Since mass-independent sulfur isotope anomalies in stratospheric volcanic sulfate appear to depend on the exposure of SO 2 to UV radiation, their measurements might therefore offer the possibility to determine the degree of UV penetration in the ozone-absorption window for the present and past atmospheres. They can also be used to determine the stratospheric or tropospheric nature of volcanic eruptions preserved in glaciological records, offering the possibility to reassess the climatic impact of past volcanic eruptions.

Mass‐independent (MI) sulfur isotopic anomalies recently observed in Precambrian rock samples have been attributed to photochemistry and used to establish oxygen levels in Earth's early atmosphere [Farquhar et al., 2000b, 2000a]. Here we report the first MI sulfur isotopic compositions in the present atmosphere from samples of Northern Hemispheric aerosol sulfate. The MI sulfur isotopic compositions do not correlate with MI oxygen signatures previously observed in the same samples [Lee and Thiemens, 2001; Lee, 2000; Bao et al., 2001]. Antarctic dry valley soil sulfate, which is atmospheric in origin and has a MI oxygen signature [Bao et al., 2000], is mass‐dependent (MD) with respect to sulfur. A different process than that proposed to explain the MI oxygen signature in sulfate [Savarino et al., 2000] is therefore required. Possible sources of the anomaly are discussed, as well as potential applications to global climate and implications for Archean geology and the Earth's early atmosphere.

This study describes the eruption characteristics of the Pleistocene Cienega Cinder Cone. This volcano is located in the southeastern part of the Cerros del Rio volcanic field (CdR) west of Santa Fe, NM. The CdR is the largest (>700 km2) of several middle Pliocene to Pleistocene basaltic volcanic fields of the axial Rio Grande Rift in northern New Mexico. Eruptive centers are typically central vent volcanoes, ranging from low-relief shields to steep-sided, breached cinder and spatter cone remnants. The Cienega Cinder Cone is actually a volcanic complex that consists predominantly of tephra fall deposits as well as several vents, multiple intrusions, and numerous lava flow sequences. A slightly eroded northern vent is 230m in diameter and consists of inward bedded crater facies and periclinally bedded wall facies. A smaller southern vent is 95m in diameter and composed of steep N-dipping pyroclastic layers that suggest the development of a late-stage shallow cryptodome. Vent facies include vesiculated fragments, oxidized cinders, and spatter agglutinate interbedded with lava flows. Proximal wall facies are moderately sorted with a high proportion of coarse scoria and bombs while the distal wall facies are very well sorted with a high proportion of fine lapilli. Fluvial sand and gravel deposits as well as aeolian sand deposits within some of the pyroclastic layers illustrate the development of stream channels and exposed surfaces in between eruptions. One major N-striking anastomosing dike (>10 m long by 4 m wide) as well as several minor N-striking dikes (< 2 m long by 1 m wide) intrude the southern complex. Macrostructures (slickenlines, chatter marks, and Reidel shears) consistently show wall rock deformation having a North-sense of shear, indicating S-moving magma towards the inflating southern vent. Sample from all volcanic facies (vent, lava flows, proximal wall, and distal wall) contain major olivine (1-3%), pyroxene (1-3%), and plagioclase (5%) phenocrysts in an aphanitic matrix. Scoria cinders contain 20-50% vesicles in a holohyaline matrix. Our observations show that the Cienega Cinder Cone is a monogenetic volcanic complex that developed by endogenic and exogenic dome growth with short eruptive events that likely were derived from a rapidly evolving reservoir-conduit system.