Boulder, Colo., USA A Canadian team lead by Stephen Grasby reports the discovery of the highest latitude perennial spring known in the world. This high-volume spring demonstrates that deep groundwater circulation through the cryosphere occurs, and can form gullies in a region of extreme low temperatures and with morphology remarkably similar to those on Mars. The 2009 discovery raises many new questions because it remains uncertain how such a high-volume spring can originate in a polar desert environment.
Grasby and colleagues encountered the northernmost perennial spring in the world, which they have dubbed the Ice River Spring, on Ellesmere Island, Nunavut, Canadian High Arctic. The specific study area is north of Otto Fiord in a mountainous region underlain by carbonates of the Nansen Formation. The spring discharges at 300 m elevation from colluvium on a south-facing (21 incline) mountain slope. The unnamed mountain rises 800 m above sea level. Detailed recordings show that this spring flows year-round, even during 24 hours of darkness in the winter months, when air temperatures are as low as minus 50 degrees Celsius.
Detailed geochemistry shows that the waters originate from the surface and circulate down as deep as 3 km before returning through thick permafrost as a spring. This points to a much more active hydrogeological system in polar regions than previously thought possible, which is perhaps driven by glacial meltwater.
Another intriguing feature of the Ice River site is the remarkable similarity to mid-latitude gullies observed on Mars. The discovery of these features on Mars has led to suggestions that recent groundwater discharge has occurred from confined aquifers.
Deep groundwater circulation through the High Arctic cryosphere forms Mars-like gullies
Stephen E. Grasby et al., Geological Survey of Canada, Natural Resources Canada, 3303 33rd Street NW, Calgary, Alberta T2L 2A7, Canada, and Dept. of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada. Published online 9 June 2014; http://dx.doi.org/10.1130/G35599.1.
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Anatomy of an active submarine volcano
A.F. Arnulf et al., Scripps Institution of Oceanography, Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics, University of California-San Diego, La Jolla, California 92093, USA. Published online 9 June 2014; http://dx.doi.org/10.1130/G35629.1.
Geophysicists are always looking for new ways to study the internal workings of volcanoes. On land, obtaining decent coverage with active source seismology is challenging because of the inhospitable terrain, but with modern marine seismology acquisition and processing methods, Earth processes can be visualized with unprecedented fidelity, resolving features on the order of tens of meters. In their study published online for Geology on 9 June 2014, A.F. Arnulf and colleagues applied innovative methods to image the thickest magma reservoir observed, to date, beneath any ocean spreading center. Their results for Axial Volcano, a submarine volcano offshore of the Pacific Northwest, reveal a complex melt body beneath the summit caldera that is approx. 14 km long, 3 km wide, and up to 1 km thick. This 18- to 30-cubic-kilometer magma reservoir is comparable in size and volume with a famous California landmark: the Yosemite Valley. Previous studies imaged upper crustal melt lenses that are ~50-100 m thick and 1-2 km wide, which lie above a partially molten lower crust where magma cools and crystallizes to form the gabbroic lower crust. The larger magma chamber, greater size, and activity of Axial Volcano arise from the intersection of the Juan de Fuca Ridge with the Cobb hotspot chain.
Great tsunamigenic earthquakes during the past 1000 yr on the Alaska megathrust
Ian Shennan et al., Sea Level Research Unit, Dept. of Geography, Durham University, Durham DH1 3LE, UK. Published online 9 June 2014; http://dx.doi.org/10.1130/G35797.1.
This paper by Ian Shennan and colleagues uses evidence from sediments preserved in coastal marshes from the Kodiak archipelago, the fossils contained within the sediments, and radiocarbon dating to demonstrate that rupture patterns along the Alaska megathrust in the last few centuries differ to those observed during the 20th century. They have a much shorter recurrence interval than those used in current seismic hazard assessment maps. Shennan and colleagues combine new observations with previous geological, historical, and archaeological investigations. They suggest that in addition to multi-segment ruptures (Prince William Sound and Kodiak segments rupturing together) in 1964 and AD 1020 to 1150 (95% age estimate), a single segment rupture (Kodiak segment alone) occurred in 1788, with earthquake-induced land surface subsidence across much of Kodiak Island and a tsunami that is recorded in historical documents and in sediment sequences, and another, similar rupture of the same Kodiak segment AD 1440 to 1620. These indicate shorter intervals between ruptures of the Kodiak segment than previously thought, and are more frequent than for the Prince William Sound segment.
Giant middle Eocene bryozoan reef mounds in the Great Australian Bight
Alexander G.W.D Sharples et al., School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK. Published online 9 June 2014; http://dx.doi.org/10.1130/G35704.1.
A series of mid-Eocene reef mound complexes have been discovered in the Great Australian Bight, some 100 miles south of Australia. The greater than 500-km-long reef mound complexes are composed of bryozoan build ups and herald the birth of the largest cool-water carbonate province in the southern hemisphere. The reef mounds formed due to a complete shut-down of clastic sediment input from Australia and rising sea level caused by accelerated separation of Australia and Antarctica. The reef mounds were discovered some 500 m beneath the seafloor and mapped utilizing oil-industry and academic two-dimensional seismic profiles. Calibration was given by cuttings samples from an exploration borehole, Potoroo-1, which directly penetrates a mounded complex. Further insights regarding the growth and demise of the reef mound complexes and their paleo-biological and -oceanographic significance will require targeted seismic profiling and, ultimately, coring to retrieve continuous recovery.
Evolution of lumpy glacial landscapes
Robert S. Anderson, Dept. of Geological Sciences, and Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, Colorado 80303, USA. Published online 9 June 2014; http://dx.doi.org/10.1130/G35537.1.
At small scales, glaciated valleys are lumpy. They sport rocky knobs with smooth abraded up-valley surfaces and sharp, quarried down-valley edges. Using numerical models, I address the evolution of glacial landforms in order to explore the dependence of bed topography on both glacier and rock properties. Sliding of a glacier against its rocky bed, a prerequisite for both erosion processes of abrasion and quarrying, is governed by water pressures at the glacier bed. On daily timescales, variations in water pressure associated with snow and ice melt cycles result in expansion and collapse of water-filled cavities at the bed that in turn repetitively stress corners in the bed. Numerical models of glacial bed evolution at longer timescales incorporate both abrasion and quarrying of fracture-bound blocks. Using a rule in which the probability of block quarrying during a stress event depends inversely upon both block size and the depth of the niche in which it sits, up-glacier migrating rocky bumps inevitably emerge, reflecting efficient quarrying of blocks from down-valley facing steps in the bed. The relative importance of abrasion and quarrying is controlled by fracture spacing, and major steps in the valley floor are attributable to transitions in fracture spacing.
Dramatic effects of stress on metamorphic reactions
John Wheeler, Dept. of Earth, Ocean and Ecological Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 3GP, UK. Published online 9 June 2014; http://dx.doi.org/10.1130/G35718.1.
A new analysis of how mineral growth is controlled in the Earth shows that stresses may have an effect far larger than hitherto expected. This means that the ways we interpret the minerals observed in once deeply buried rocks require reappraisal. Stress in the Earth is a key aspect of its behavior and this theory paves the way for how ancient stress levels might be deduced from rocks in new ways. In the Earth, stresses result from, for example, movement of tectonic plates. As stress is applied slowly over time, rocks deform and change shape forming aligned mineral textures which are very common. Despite this it has previously been assumed the effects of stress on new mineral growth are small. New calculations show the effects of stress are much bigger and this means that current interpretations of mineral growth require modification; the theory presents a new quantitative way to think about mineral growth during deformation in rocks. It may also be of interest in metallurgy. Metals, like rocks, are actually interlocked crystals of different chemistries and are often processed by deformation which occurs in parallel with chemical change.
Rapid magma evolution constrained by zircon petrochronology and 40Ar/39Ar sanidine ages for the Huckleberry Ridge Tuff, Yellowstone, USA
Tiffany A. Rivera et al., Quaternary Dating Laboratory, Roskilde University, Universitetsvej 1, 4000 Roskilde, Denmark, and Dept. of Geosciences, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA. Published online 9 June 2014; http://dx.doi.org/10.1130/G35808.1.
The Huckleberry Ridge Tuff is the product of the largest eruption at Yellowstone. The mineral zircon, present within this volcanic deposit, has been analyzed for its chemical composition and crystallization temperature in order to deduce the path of magmatic evolution prior to eruption. Additionally, uranium-lead dating on the same zircon crystals allows for a time-stamp of the evolutionary process. Within the Huckleberry Ridge Tuff, zircon analyses demonstrate that some of the crystal cargo was derived from previously erupted volcanic deposits, and the remaining crystals formed over a period of about 10,000 years. To complement the zircon findings, 40Ar/39Ar age data on sanidine crystals from the same rock show similar evidence for recycling of previously formed crystals, and provide an eruption age of 2.079 million years. The findings of this study show that, despite the large volume of magma (~2500 cubic kilometers), differentiation and cooling is geologically rapid, with most crystals forming several millennia prior to eruption.
740 Ma vase-shaped microfossils from Yukon, Canada: Implications for Neoproterozoic chronology and biostratigraphy
Justin V. Strauss et al., Dept. of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. Published online 9 June 2014; http://dx.doi.org/10.1130/G35736.1.
Biostratigraphy, or the use of fossils to help constrain time in ancient sedimentary rocks, underpins the more recent geological time scale, but its application to pre-Ediacaran (> ~541 million years old) strata has remained limited because older fossil taxa commonly have poorly understood preservational biases and/or inadequate geological or geochronological context. Here, we report the discovery of abundant and well-preserved vase-shaped microfossils from a Neoproterozoic carbonate deposit in Yukon, Canada, that highlight the potential for biostratigraphic correlation of Proterozoic sedimentary successions. The fossiliferous horizon, dated here with Re-Os geochronology at 739.9 plus or minus 6.1 million years, shares multiple species-level taxa with a well-characterized assemblage from the Chuar Group, Grand Canyon, Arizona, dated with U-Pb on zircon from an interbedded tuff at 742 plus or minus 6 million years. The overlapping age and species assemblages from these two deposits suggests biostratigraphic utility, at least within Neoproterozoic basins of ancient North America, or Laurentia, and perhaps globally. The new Re-Os age also confirms the timing of a large negative carbonate carbon isotopic anomaly, which predates the onset of the Sturtian "Snowball Earth" glaciation by >15 million years. Together, these data provide global calibration of Neoproterozoic sedimentary, paleontological, and geochemical records.
Southward shift of the Intertropical Convergence Zone due to Northern Hemisphere cooling at the Oligocene-Miocene boundary
Kiseong Hyeong et al. (Boo-Keun Khim [corresponding]), Korea Institute of Ocean Science and Technology, 878 Haean-ro, Ansan 426-744, Republic of Korea (Khim: Dept. of Oceanography, Pusan National University, Busan 609-735, Republic of Korea). Published online 9 June 2014; http://dx.doi.org/10.1130/G35664.1.
The Mi-1 glaciation (approx. 23 million years ago), which marks the OligoceneMiocene boundary, was an aberrant cooling event that led to a build-up of Antarctic ice sheet reaching the near-modern volume or larger from its ephemeral or partial existence. In contrast, Northern Hemisphere (NH) glaciation has not been considered as a consequence of the event due to lack of definitive evidence. Here, we investigated the inter-hemispheric temperature contrast during Mi-1, by tracing the movement of the tropical maximum rainfall belt (TMRB) at a site (1031'N) in the East Pacific (IODP Site U1333), to understand NH cooling and possibility of NH glaciation. Our dust data indicate the southward displacement of the TMRB over Site U1333 during Mi-1 (~4N at the Oligocene-Miocene boundary). The TMRB shifts toward the warmer hemisphere. Thus our results suggest that the cooling during Mi-1 was more significant in the NH than the Southern Hemisphere which underwent the sudden expansion of its continental ice sheets. The published data suggest two possible mechanisms for NH cooling during the brief time interval: the extensive growth of NH ice sheets and/or changes in the production of North Atlantic-origin deep water.
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