Quenched glasses, asteroid impacts, and ancient life on Mars

Quenched glasses formed by asteroid impacts can encapsulate and preserve biological material for millions of years on Earth, and can also serve as a substrate for microbial life. These impact glasses are thus an important target to search for signs of ancient life on Mars, but until now they have not been definitively detected on the martian surface.

In this study, Kevin Cannon and John Mustard used orbital remotely sensed data to investigate spectral signatures of geologic units on Mars that were formed during impacts (impactites).

Using spectral mixture modeling, they found that glass is in fact present in these units, mixed with other minerals like olivine and pyroxene. This modeling approach allows for the identification glass signatures that are not otherwise obvious when glass is present in a mixture.

The glass-rich impactites Cannon and Mustard have identified have been preserved on billion-year timescales, old enough to date back to more clement surface conditions on Mars. Their preservation is likely due to the current cold and dry surface environment; therefore, fossilization in glass, as proposed previously, seems to be a promising target to search for possible ancient martian biological activity.

FEATURED ARTICLE
Preserved glass-rich impactites on Mars
Kevin M. Cannon and John F. Mustard, Brown University, Providence, Rhode Island, USA. Published online ahead of print on 5 June 2015; http://dx.doi.org/10.1130/G36953.1.

Other GEOLOGY articles posted online on 5 June 2015 are listed below.

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Hydrological changes facilitated early rice farming in the lower Yangtze River Valley in China: A molecular isotope analysis
Robert Patalano et al., Laboratory for Terrestrial Environments, Bryant University, Smithfield, Rhode Island, USA. Published online ahead of print on 5 June 2015; http://dx.doi.org/10.1130/G36783.1.

Rice domestication and early farming are key events in agriculture revolution that sustained the development of civilization in Asia. Rice is the world's largest food crop with China currently being the world's largest producer and consumer of the domesticated rice. The Yangtze River valley is recognized as the cradle of rice domestication and a key region to explore the interaction between environmental, particularly climatic, changes and rice domestication. This study represents the first high resolution dual molecular isotopes (carbon and hydrogen) record that characterized hydrological changes at the well-known Neolithic Tianluoshan site in eastern China where early process of rice domestication has been previously documented. Robert Patalano and colleagues identified two major dry and high evaporation events during critical stages of early rice farming periods, around 7.0 ka and 6.4 ka respectively, which were previously unknown to this region. Contrary to conventional ideas, their new isotope data suggest that these short-duration dry climate events, while combined with marine transgression/regression, have facilitated the expansion of rice farming during subsequent humid climate regimes. Regression under these dry events were critical to open up new habitats where pioneer grass plants, including rice, developed; drought induced fire further facilitated the land use while adding nutrients to the soil to sustain extensive rice farming.

Is the present the key to the past? A global characterization of modern sedimentary basins
Björn Nyberg and John A. Howell, University of Aberdeen, Old Aberdeen, UK; Research CIPR (Centre for Integrated Petroleum Research), Bergen, Norway; and University of Bergen, Bergen, Norway. Published online ahead of print on 5 June 2015; http://dx.doi.org/10.1130/G36669.1.

The old adage “the present is the key to past” is one of the fundamental cornerstones of geology. The idea that we should use modern systems to help us understand the past and we should not invoke any processes to explain the ancient that we can't see happening today is the underlying principal of uniformitarianism. The advent of large volumes of publically available remote sensing data through services like Google Earth has revolutionized our study of the modern and how it relates to the ancient. In this study, Björn Nyberg and John Howell have mapped the parts of the present-day land surface that lie within sedimentary basins. These basins are the areas where the crust is subsiding and where sediment that is deposited by rivers, lakes, and sand dunes may be preserved and ultimately become part of the rock record. Their study shows that only 16% of the subaerial land surface lies in areas where the crust is subsiding. Furthermore, these areas are biased toward arid climates in large cratonic settings. Where a quarter of the present land surface lies within deserts, more than half of the basins are arid. These results illustrate an incomplete and potentially biased distribution of modern analogues.

Spherulites as in-situ recorders of thermal history in lava flows
Kenneth S. Befus et al., The University of Texas at Austin, Austin, Texas, USA; published online ahead of print on 5 June 2015; http://dx.doi.org/10.1130/G36639.1.

Kenneth S. Befus and colleagues use compositional gradients surrounding spherulites, spherical growths of alkali feldspar and quartz crystals, to assess the cooling rate of an obsidian lava flow from Yellowstone National Park (USA). The presence and form of compositional gradients surrounding spherulites are largely controlled by the cooling rate of the host lava and the nature of spherulite nucleation and growth. Befus and colleagues use the cooling rate to infer the minimum longevity and flow speed of Pitchstone Plateau lava flow, a prehistoric lava flow at Yellowstone that is an order of magnitude larger than any scientifically monitored obsidian lava flow.

Boron during meteoric diagenesis and its potential implications for Marinoan snowball Earth {delta}11B-pH excursions
Joseph A. Stewart et al., National Oceanography Centre, University of Southampton, Southampton, UK. Published online ahead of print on 5 June 2015; http://dx.doi.org/10.1130/G36652.1.

Measurements of boron isotopes in ancient marine carbonates are a promising tool for reconstructing past changes in ocean pH and identifying times of ocean acidification in the geological record. Yet, the interpretation of boron isotope data as records of seawater pH is predicated on the carbonate boron chemistry remaining unaltered since formation of these rocks from their host seawater. Here we show carbonate rocks that have interacted with fresh waters become highly altered in both their boron content and isotopic composition. These secondary changes in boron chemistry could be easily misinterpreted as intervals of major past ocean acidification. We recommend the use of additional trace-element measurements of ancient carbonates as screening tools to discount late alteration of these rocks. Chemical screening in conjunction with physical observations will allow a more confident assessment of the pH of ancient oceans.

Imaging high-pressure rock exhumation in eastern Taiwan
D. Brown et al., Barcelona, Spain. Published online ahead of print on 5 June 2015; http://dx.doi.org/10.1130/G36810.1.

Brown and colleagues present geophysical and petrophysical data that images, for the first time, an actively exhuming high-pressure rock terrane. They go on to use earthquake data to investigate how this terrane is deforming internally. High-pressure rocks are rare, but wide-spread rocks that provide important information about the physical and chemical processes that take place deep in subduction zones (places where one tectonic plate is sliding beneath another). Detailed studies of a large number of these subduction-related high-pressure rock terranes, in combination with numerical computer modeling, indicate that various mechanisms may explain how these terranes are exhumed to the Earth's surface after being carried down a subduction zone. Despite noteworthy advances having been made, there is still debate, and a significant number of unknowns, about the mechanisms of exhumation. To help narrow the gap on these unknowns, a number of important objectives for future research have recently been proposed, among which is the imaging of actively exhuming high-pressure terranes, something that Brown and others have accomplished in this paper.

Continental rifting and sediment infill in the northwestern Gulf of Mexico
Harm J.A. Van Avendonk et al., University of Texas Institute for Geophysics, Austin, Texas, USA; Published online ahead of print on 5 June 2015; http://dx.doi.org/10.1130/G36798.1.

ABSTRACT: The opening of the Gulf of Mexico was an important Mesozoic tectonic event that provides new insight in the role of magmatism and lithospheric stresses in the initiation of continental rifting. A new seismic velocity profile based on seismic refraction data in the northwestern Gulf of Mexico offshore Texas, where the basin started opening in the Early Jurassic, shows a rifted margin with strong lateral heterogeneity beneath the shelf and slope. The structure of the thinned crust is consistent with large-scale extensional faulting and moderate amounts of synrift magmatism before continental breakup. These new seismic constraints do not indicate the presence of a volcanic margin along the Texas coast, as has sometimes been proposed based on magnetic data. The Laurentian continental lithosphere of central Texas may have been too thick at the onset of rifting (>100 km) to let magmatic diking control the extension. In contrast, the continental lithosphere of the northeastern Gulf of Mexico may have been thinner, such that magma-assisted rifting formed a volcanic margin there later in the Jurassic.

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