An alkaline spring system within the Del Puerto Ophiolite (California,USA): A Mars analog site

Mars appears to have experienced little compositional differentiation of primitive lithosphere, and thus much of the surface of Mars is covered by mafic lavas. On Earth, mafic and ultramafic rocks present in ophiolites, oceanic crust and upper mantle that have been obducted onto land, are therefore good analogs for Mars. The characteristic mineralogy, aqueous geochemistry, and microbial communities of cold-water alkaline springs associated with these mafic and ultramafic rocks represent a particularly compelling analog for potential life-bearing systems. Serpentinization, the reaction of water with mafic minerals such as olivine and pyroxene, yields fluids with unusual chemistry (Mg–OH and Ca–OH waters with pH values up to ～12), as well as heat and hydrogen gas that can sustain subsurface, chemosynthetic ecosystems. The recent observation of seeps from pole-facing crater and canyon walls in the higher Martian latitudes supports the hypothesis that even present conditions might allow for a rock-hosted chemosynthetic biosphere in near-surface regions of the Martian crust. The generation of methane within a zone of active serpentinization, through either abiogenic or biogenic processes, could account for the presence of methane detected in the Martian atmosphere. For all of these reasons, studies of terrestrial alkaline springs associated with mafic and ultramafic rocks are particularly timely. This study focuses on the alkaline Adobe Springs, emanating from mafic and ultramafic rocks of the California Coast Range, where a community of novel bacteria is associated with the precipitation of Mg–Ca carbonate cements. The carbonates may serve as a biosignature that could be used in the search for evidence of life on Mars.     

Search for H2 Emission from Disks around T Tauri and Herbig AE Stars

Preliminary results are presented of observations of the pure rotational lines of H2 toward T Tauri and Herbig Ae stars using the Short Wavelength Spectrometer (SWS) on the Infrared Space Observatory (ISO). The sources are selected to be isolated low- and intermediate-mass young stellar objects, for which the presence of a circumstellar disk has been established by millimeter interferometry. The lowest H2 S(0) and S(1) lines are detected in 3 out of 5 objects. The measured intensities indicate ∼ 0.01 M⊙ of warm (T ≈ 150 K) gas in the ISO beam. It is argued that for at least one case (HD 163296), the emission is probably dominated by the warm gas in the circumstellar disk rather than by shocked- or photon-heated gas in the surrounding envelope. Such observations can provide important constraints on the radial and vertical temperature profiles in circumstellar disks. This revised version was published online in August 2006 with corrections to the Cover Date.     

Modelling the dynamics and thermodynamics of volcanic degassing

 The rates of passive degassing from volcanoes are investigated by modelling the convective overturn of dense degassed and less dense gas-rich magmas in a vertical conduit linking a shallow degassing zone with a deep magma chamber. Laboratory experiments are used to constrain our theoretical model of the overturn rate and to elaborate on the model of this process presented by Kazahaya et al. (1994). We also introduce the effects of a CO₂–saturated deep chamber and adiabatic cooling of ascending magma. We find that overturn occurs by concentric flow of the magmas along the conduit, although the details of the flow depend on the magmas' viscosity ratio. Where convective overturn limits the supply of gas-rich magma, then the gas emission rate is proportional to the flow rate of the overturning magmas (proportional to the density difference driving convection, the conduit radius to the fourth power, and inversely proportional to the degassed magma viscosity) and the mass fraction of water that is degassed. Efficient degassing enhances the density difference but increases the magma viscosity, and this dampens convection. Two degassing volcanoes were modelled. At Stromboli, assuming a 2 km deep, 30% crystalline basaltic chamber, containing 0.5 wt.% dissolved water, the ∼700 kg s^–1 magmatic water flux can be modelled with a 4–10 m radius conduit, degassing 20–100% of the available water and all of the 1 to 4 vol.% CO₂ chamber gas. At Mount St. Helens in June 1980, assuming a 7 km deep, 39% crystalline dacitic chamber, containing 4.6 wt.% dissolved water, the ∼500 kg s^–1 magmatic water flux can be modelled with a 22–60 m radius conduit, degassing ∼2–90% of the available water and all of the 0.1 to 3 vol.% CO₂ chamber gas. The range of these results is consistent with previous models and observations. Convection driven by degassing provides a plausible mechanism for transferring volatiles from deep magma chambers to the atmosphere, and it can explain the gas fluxes measured at many persistently active volcanoes. Received: 26 September 1997 / Accepted: 11 July 1998     

The environments and clustering properties of 2dF Galaxy Redshift Survey selected starburst galaxies

We investigate the environments and clustering properties of starburst galaxies selected from the 2dF Galaxy Redshift Survey (2dFGRS) in order to determine which, if any, environmental factors play a role in triggering a starburst. We quantify the local environments, clustering properties and luminosity functions of our starburst galaxies and compare to random control samples. The starburst galaxies are also classified morphologically in terms of their broad Hubble type and evidence of tidal merger/interaction signatures. We find the starburst galaxies to be much less clustered on large (5–15 Mpc) scales compared to the overall 2dFGRS galaxy population. In terms of their environments, we find just over half of the starburst galaxies to reside in low to intermediate luminosity groups, and a further ∼30 per cent residing in the outskirts and infall regions of rich clusters. Their luminosity functions also differ significantly from that of the overall 2dFGRS galaxy population, with the sense of the difference being critically dependent on the way their star formation rates are measured. In terms of pin-pointing what might trigger the starburst, it would appear that factors relating to their local environment are most germane. Specifically, we find clear evidence that the presence of a near neighbour of comparable luminosity/mass within 20 kpc is likely to be important in triggering a starburst. We also find that a significant fraction (20–30 per cent) of the galaxies in our starburst samples have morphologies indicative of either an ongoing or a recent tidal interaction and/or merger. These findings notwithstanding, there remain a significant portion of starburst galaxies where such local environmental influences are not in any obvious way playing a triggering role, leading us to conclude that starbursts can also be internally driven.     

Neoproterozoic (Cryogenian) stromatolites from the Wolfe Basin,east Kimberley,Western Australia: Correlation with the Centralian Superbasin

Linella avis, an early to middle Neoproterozoic (Tonian to Cryogenian) stromatolite, occurs in the Eliot Range Dolomite, part of the Ruby Plains Group in the Wolfe Basin, east Kimberley. Previously, this dolomite was assigned to the Mesoproterozoic Bungle Bungle Dolomite in the Osmond Basin, which contains a different suite of stromatolites. Linella avis, which also occurs in the Neoproterozoic Bitter Springs Formation of the Amadeus Basin, central Australia, appears to be restricted to rocks aged around 850 to 800 Ma. The presence of L. avis indicates that the Ruby Plains Group is a probable correlative of the Heavitree Quartzite and Bitter Springs Formation, and is probably much younger than the Bungle Bungle Dolomite. If the correlation suggested here is correct, the Wolfe Basin, together with the Amadeus and Ngalia Basins, formed part of the Centralian Superbasin.