Glacial buzz-saws, gold in fool's gold, fingerprints in sea water, and fluvial iron

(Press-News.org) Contact information: Kea Giles
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Geological Society of America
Glacial buzz-saws, gold in fool's gold, fingerprints in sea water, and fluvial iron New Geology articles posted online ahead of print 16 October 2013 Boulder, Colo., USA – New article postings for Geology cover glacial erosion and glacial slip; the work of marine organisms in changing the face of Earth; collisional shortening in the Central Alps; changes in sediment transport in Taiwan after typhoon Morakot in 2009; a new type of iron formation, dubbed "fluvial iron formation"; kimberlites in South Africa; using fossil marine plankton records in 70-million-year-old sediments as indicators of sea ice formation and retreat; and Greenland Ice Sheet behavior.

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Selective glacial erosion on the Norwegian passive margin
Adrian M. Hall et al., School of Geography and GeoSciences, University of St Andrews, Irvine Building, North Street, St Andrews KY16 9AL, Fife, Scotland, UK; amh22@st-andrews.ac.uk. Co-authors: Karin Ebert, Johan Kleman, Atle Nesje, and Dag Ottesen. Posted online ahead of print on 16 Oct. 2013; http://dx.doi.org/10.1130/G34806.1.

Glaciers cut down and cut deep, carving deep valleys in mountains. This efficient erosion -- the glacial buzz-saw -- operates best at the snowline where glaciers are thickest and fastest moving. But can glaciers also cut horizontally to create low angle surfaces or plateaus? That's what has been claimed recently to have happened during the Ice Age on the west coast of Norway. We provide evidence that the plateaus have been cut into by and so are older than cirques and valley glaciers. We also find no relationship to cirque distribution or to Pleistocene snowlines. The gentle, high elevation surfaces of this and other glaciated passive margins are largely inherited from Neogene non-glacial, fluvial environments. Yet Pleistocene glacial erosion has done far In Norway than to cut its magnificent fjords -- many hundreds of meters of soft rocks must have been removed from the coastal and inshore zone to account for the huge sediment volumes offshore.

Does gold in orogenic deposits come from pyrite in deeply buried carbon-rich sediments?: Insight from volatiles in fluid inclusions
Damien Gaboury, Laboratoire de Métallogénie Expérimentale et Quantitative (LAMEQ), Université du Québec à Chicoutimi (UQAC), 555 Boulevard de l'Université, Chicoutimi, Québec G7H 2B1, Canada; dgaboury@uqac.ca. Posted online ahead of print 16 Oct. 2013; http://dx.doi.org/10.1130/G34788.1

Orogenic gold deposits form an important class of hydrothermal deposits distributed in metamorphic volcano-sedimentary belts worldwide. It is accepted that gold-bearing fluids are generated by metamorphic dehydration reactions at depth (about 5 to 12 km) following mountain building during tectonic collisional events. However, the source of gold remains speculative and because of that, key criteria for selecting favorable areas for exploration is lacking. Recently, it was proposed that gold in primary nodular pyrite hosted in organic matter-rich shale was the source. Results presented here by Damien Gaboury provide an independent validation of this model. It was postulated that if gold-bearing fluids are derived from organic-rich material, fluids involved in the formation of gold deposits should contain some hydrocarbon species. Fluid inclusions are microscopic bubbles of trapped fluids in minerals. Fluid inclusions from selected deposits around the world were analyzed for volatile composition by solid-probe mass spectrometry following a unique technique developed by the author. It is demonstrated that ethane (C2H6) is sourced from thermally degraded organic matter, hence providing a reliable tracer. The C2H6 content is recorded in fluids from Meso-Archean to Cretaceous gold deposits, providing support for a general model where fluids and gold were sourced from deeply buried, carbon-rich, and pyrite-gold-bearing sedimentary rocks.

Tectonic forcing of Early to Middle Jurassic seawater Sr/Ca
Clemens V. Ullmann et al., University of Copenhagen, Department of Geosciences and Natural Resource Management, and Nordic Center for Earth Evolution (NordCEE), Øster Voldgade 10, 1350 Copenhagen-K, Denmark; c.v.ullmann@gmx.net. Co-authors: Stephen P. Hesselbo, and Christoph Korte. Posted online ahead of print 16 Oct. 2013; http://dx.doi.org/10.1130/G34817.1.

Earth's surface changes slowly, because of plate tectonic processes, long-term changes in climate, and even due to the activities of living organisms. On timescales of millions of years, these acting forces leave specific fingerprints in the chemical composition of seawater. Marine organisms record information about seawater composition in their shells. Fossil shell remains of such organisms can be used to estimate the variations of seawater composition through time and to evaluate the acting forces, leading to the observed variability. Around the Triassic-Jurassic transition (~201 million years ago), fundamental changes in the plate tectonic setting commenced and one of the most severe mass extinction events known from the geologic record occurred. The evolving seawater composition during ~37 million years after this important transition was tracked here by measuring the concentration and isotopic composition of strontium in the shells of oyster-like bivalves and belemnites -- extinct, marine predators. The observed patterns are attributed to an overall decreasing importance of strontium from weathering continental rocks with respect to strontium derived from Earth's mantle at the Mid Ocean Ridges. A major impact on seawater composition, related to the changing and recovering ecosystems, and the spreading of calcite producing nannoplankton, however, is not indicated.

Three-dimensional insight into Central-Alpine collision: Lower-plate or upper-plate indentation?
Claudio L. Rosenberg, UPMC University of Paris 6, ISTEP, F-75005, Paris, France; claudio.rosenberg@upmc.fr. Co-author: Eduard Kissling. Posted online ahead of print 16 Oct. 2013; http://dx.doi.org/10.1130/G34584.1.

Accommodation of collisional shortening in the Central Alps varies dramatically along strike. In the western Central Alps, 90% of shortening is accommodated in the thickened lower plate. In the eastern Central Alps, 90% of shortening is accommodated in the upper plate. In the central part of the Central Alps shortening is almost equally partitioned between the two plates. Whereas the upper plate indents into the thickened accreted lower plate in the Simplon section, it is the lower plate that indents an intensely deforming upper plate in the Engadine section. In the west, the Ivrea mantle body increases the strength of the Adriatic upper plate and Barrovian metamorphism weakens the lower plate. Therefore, along-strike transfer of shortening from one plate to the other appears to be a manifestation of along-strike changes of rheology deep in the crust.

Altered regional sediment transport regime after a large typhoon, southern Taiwan
Michelle Y.-F. Huang, Research Center for Environmental Changes, Academia Sinica, Taipei 115, Taiwan; michelleyfhuang@gmail.com; ae2612@hotmail.com. Co-author: David R. Montgomery, Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA. Posted online ahead of print 16 Oct. 2013; http://dx.doi.org/10.1130/G34826.1.

Analyses of river-suspended sediment response to record-breaking regional rainfall in southern Taiwan during typhoon Morakot, 7-9 August 2009, reveal systematic changes in the regional sediment transport regime as characterized by rating curve parameters. These changes result in much greater sediment concentration, and thus sediment transport, in subsequent low-flow events after the typhoon, an effect that amplifies and extends the influence of such extreme events through increased low-flow sediment transport. Findings by Michelle Huang and David Montgomery show that the rating curve exponent is not constant contrasts with the conventional assumption that large events influence sediment yields through increased intercept values, thereby supporting the interpretation that basin sediment delivery influences both rating parameters, and increases post-event low-flow sediment transport. Surveys of landslide density and riverbed grain sizes before and after typhoon Morakot support the interpretation that the observed changes reflect an altered sediment transport regime and a shift from channel migration and bank erosion to reworking of landslide debris and enhanced bed mobility as the dominant processes supplying fluvial sediment.

Late Cretaceous winter sea ice in Antarctica?
Vanessa C. Bowman et al., School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK; v.c.bowman@leeds.ac.uk or vcbowman@gmail.com. Co-authors: Jane E. Francis, and James B. Riding. Posted online ahead of print 16 Oct. 2013; http://dx.doi.org/10.1130/G34891.1.

Scientists have found evidence of sea ice at a time in the geological past when forests covered Antarctica and the climate was thought to be very warm. Geologists from the University of Leeds, UK, and the British Geological Survey, supported by the British Antarctic Survey, undertook fieldwork and laboratory analysis over a period of four years. They interpreted the record of fossil marine plankton in Cretaceous sediments (70 million years ago) as indicators of sea ice formation and retreat. This implies that ice sheets may have existed on Antarctica under a high CO2 climate. These results help us reconstruct the history of the Antarctic ice sheet in order to understand how the ice sheet is responding to climate warming today and how it may behave in the future.

A re-evaluation of the Pleistocene behavior of the Scoresby Sund sector of the Greenland Ice Sheet
Jan Sverre Laberg et al., Department of Geology, University of Tromsø, N-9037 Tromsø, Norway; jan.laberg@uit.no. Co-authors: Matthias Forwick, Katrine Husum, and Tove Nielsen. Posted online ahead of print 16 Oct. 2013; http://dx.doi.org/10.1130/G34784.1.

The Greenland Ice Sheet is the largest ice sheet outside of Antarctica. The amount of water stored equals seven meters of global sea-level rise so that the future behavior of the ice sheet is of global concern. However, limited data on the past dynamics of the Ice Sheet exceeding the ice-core records have led to partly contradictory reconstructions. Whereas the Scoresby Sund sector of the Greenland Ice Sheet has been suggested to be stable and not much larger than at present during the peak Pleistocene glaciations, the southeastern sector of the Ice Sheet has been inferred to be much more dynamic. Jan Sverre Laberg, Matthias Forwick, Katrine Husum, and Tove Nielsen present seismic data showing that glaciogenic debris-flow deposits dominate the earlier than ca. 2.58 Ma succession of the Scoresby Sund Trough Mouth Fan on the East Greenland continental margin, suggesting much more frequent expansions of the Greenland Ice Sheet to the shelf break than found previously. This rapid response of the glacier to climate forcing indicates how dynamic is the glacier ice front and what one might expect of the glacier as the influences of climate warming become more pronounced.

Riverine mixing and fluvial iron formation: A new type of Precambrian biochemical sediment
Peir K. Pufahl et al., Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada; peir.pufahl@acadiau.ca. Co-authors: Franco Pirajno, and Eric E. Hiatt. Posted online ahead of print 16 Oct. 2013; http://dx.doi.org/10.1130/G34812.1.

The deposition of Precambrian iron formation is perhaps one of the least understood phenomena in the Earth sciences. This iron-rich sedimentary rock precipitated in some way from seawater and therefore holds important clues about the composition of the early oceans and atmosphere as well as the evolution of life. For example, the appearance of large, economically important iron formations reflects photosynthetic oxygenation of the oceans and atmosphere approximately 2.5 billion-years-ago. In this paper, Peir Pufahl, with Franco Pirajno and Eric Hiatt, introduce a new type of iron formation, fluvial iron formation, which formed by mixing river discharge and seawater in coastal environments. Their results are significant because it shifts the locus of known iron formation precipitation processes into estuarine settings, providing a new window for understanding ocean-atmosphere development on the early Earth.

Kimberlite (U-Th)/He dating links surface erosion with lithospheric heating, thinning, and metasomatism in the southern African Plateau
Jessica R. Stanley et al., Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA; jessica.stanley@colorado.edu. Co-authors: Rebecca M. Flowers, and David R. Bell. Posted online ahead of print 16 Oct. 2013; http://dx.doi.org/10.1130/G34797.1.

Documenting the response at Earth's surface to processes at depth is a central challenge in continental tectonics. Kimberlites are ultramafic magmas derived from a depth greater than 150 km, famous for bringing diamonds to the surface, as well as other information about the state of the lithosphere in the form of mantle xenoliths. Though less well studied, kimberlites also contain xenoliths from sedimentary units present at the time of eruption, which may have since eroded away. In this study, Jessica Stanley, Rebecca Flowers, and David Bell combine constraints from sedimentary xenoliths with cooling histories derived from apatite (U-Th)/He dating of kimberlites to constrain 1-2 km of Mesozoic erosion across the interior of South Africa. This erosion pulse is contemporaneous with heating, thinning, and metasomatism of the lithosphere below documented in the mantle xenoliths from these same pipes. Thus, the data presented here appear to record the surface response to active processes in the mantle.

Glacier slip and seismicity induced by surface melt
Peter L. Moore et al., Department of Geological and Atmospheric Science, Iowa State University, Ames, Iowa 50011, USA; pmoore@iastate.edu. Co-authors: J. Paul Winberry, Neal R. Iverson, Knut A. Christianson, Sridhar Anandakrishnan, Miriam Jackson, Mark E. Mathison, and Denis Cohen. Posted online ahead of print 16 Oct. 2013; http://dx.doi.org/10.1130/G34760.1.

Peter Moore and colleagues installed instruments for measuring seismic activity, sliding, and stress at the bottom of Engabreen, a glacier in northern Norway where a hydro-power facility allows human access to the glacier bed. The instruments measured pronounced glacier response to meltwater input during the onset of summer melt in May 2010 and 2011. This paper documents three separate episodes in which surface melt or precipitation introduced a pulse of water to the glacier bed and our instruments measured the glacier's response. The added water briefly enhanced glacier sliding as it pressurized cavities between the ice and underlying bedrock, locally lifting the ice. Enhanced sliding ceased, however, when the increased cavity size and connectivity allowed the water to be evacuated from the cavities more efficiently -- a few hours in each case. During each of these events, broadband seismometers in the tunnels a few meters below the glacier bed detected tilt, interpreted as a slight deformation of the rock as water pressurized basal cavities. Though measurements also suggest that enhanced sliding at the bed exploited a frictional interface, no seismic activity directly associated with this slip could be resolved.

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