New Geology postings illustrate complex systems and innovative results

June 8th, 2012
This shows a limestone roche moutonnée in front of Tsanfleuron Glacier in the western Bernese Alps, Switzerland. Credit: Neal Iverson
Analysis of the water content of hydrous minerals in martian meteorites shows that Mars' interior is as wet or even wetter than Earth's mantle; detailed examination of well-preserved organic structures in Ediacaran specimens illustrates the "dawn of skeletogenesis"; a study of stromatolites in Nevada suggests that complex ecological phenomena such as reef-building began sooner than previously thought; and new findings regarding coral reef systems call for a "re-think" of prevailing models of reef growth dynamics.

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Heavy metal, sex, and granites: Crustal differentiation and bioavailability in the mid-Proterozoic

John Parnell et al., School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK. Posted online 8 June 2012; doi: 10.1130/G33116.1.

A fundamental advance in life on Earth was the evolution from simple single-celled organisms (prokaryotes) to more complex, multi-celled organisms (eukaryotes) about 1.5 billion years ago. This advance has been linked by biologists to the increased availability of trace metals, especially zinc, copper and molybdenum. The metals are built into proteins, which can take on more complex and diverse functions. A critical aspect of this more complex behavior was the development of sexual reproduction, which allowed mixing of genes to create variation, and the natural selection that underpins evolution. Geologists have suggested that the key metals became available due to changes in ocean chemistry. John Parnell and colleagues argue, alternatively, that the metals appeared in abundance due to a major expansion of Earth's crust, involving the widespread formation of granite. The granite brought huge volumes of new matter from deep in the Earth, including metals. The metals became concentrated as ore deposits at/near Earth's surface (many being mined today in Australia), where they were weathered and entered surface environments to become utilized by increasingly complex life. This formation of granite, which introduced metals and prompted the evolution of sex, was therefore a seminal event in the development of life on Earth.

Precipitation of barite by marine bacteria: A possible mechanism for marine barite formation

M.T. Gonzalez-Muñoz et al., Departamento de Microbiología, Universidad de Granada, Campus Fuentenueva, 18002 Granada, Spain. Posted online 8 June 2012; doi: 10.1130/G33006.1.

This manuscript describes a novel research study on microbial barite precipitation. Barite (BaSO4) is a mineral that has been used for decades for reconstructing ocean chemistry and export productivity in the ocean. Barite forms in the oceanic water column in relation to sinking organic matter and a possible direct biogenic formation of barite has prompted a search for the identity of Ba-secreting organisms. However, no living organisms that could account for the barite abundance in the ocean have been identified, and mechanisms for barite formation in seawater are not fully understood. Here it is shown for the first time that marine bacteria have the ability to precipitate barite through a metabolically mediated biomineralization process. M.T. Gonzalez-Muñoz of the Universidad de Granada and colleagues precipitated barite in laboratory experiments in the presence of several strains of marine bacteria and propose that bacterial mediation of barite precipitation can explain barite distribution in the water column and the occurrence of barite crystals in organic rich sinking aggregates where bacteria are concentrated. This finding has novel and significant implications for the use of barite and Ba proxies in paleoceanographic research and provides new insights on biomineralization processes in marine environments.

The dawn of animal skeletogenesis: Ultrastructural analysis of the Ediacaran metazoan Corumbella werneri

L.V. Warren et al., Instituto de Geociencias, Universidade de Sao Paulo, Rua do Lago, 562, Sao Paulo 05508-080, Brazil. Posted online 8 June 2012; doi: 10.1130/G33005.1.

For the first time in Precambrian paleontology is described the presence of well-preserved organic structures in a carapace of Corumbella werneri, one of the first animals capable of producing exoskeleton. The unprecedented quality of preservation in the Ediacaran specimens evaluated here by L.V. Warren and colleagues confers a unique status for the samples analyzed. Observation of features like plates, pores, and papillae offered a first opportunity to understand skeletogenesis, a key issue of the evolution of first animals. Also, this material provides an important contribution on paleoenvironmental and evolutionary implications for the end of Neoproterozoic Era.

A theory of glacial quarrying for landscape evolution models

Neal R. Iverson, Dept. of Geological and Atmospheric Sciences, Iowa State University, Ames, Iowa 50011, USA. Posted online 8 June 2012; doi: 10.1130/G33079.1.

Erosion by glaciers results in some of Earth's most spectacular landscapes and affects the tectonic uplift of mountain belts. Efforts to mathematically model this erosion use a relationship between glacier movement and rock erosion rate that is guessed, rather than derived from the principal process of glacial erosion, quarrying. During this process, rock fragments ranging in size from pebbles to house-sized boulders are dislodged from bedrock under glaciers due to their sliding motion, thereby lowering the bedrock surface. Neal R. Iverson presents a theory of quarrying that provides the required relationship between the rate of glacier movement and rock erosion rate. The form of this relationship is sensitive to both fractures in bedrock that predate glaciation and water pressure in cavities under glaciers where sliding ice separates from rock. This theory anchors, for the first time, numerical models of large-scale glacial erosion to the primary small-scale process that these models hope to simulate. The theory can help explain how movement of glaciers, water flow under them, and characteristics of bedrock fractures combined during past glaciations to create the striking landscapes of Alpine regions.

Bowers Ridge (Bering Sea): An Oligocene-Early Miocene island arc

Maren Wanke et al., Helmholtz Centre for Ocean Research Kiel (GEOMAR), Wischhofstrasse 1-3, 24148 Kiel, Germany. Posted online 8 June 2012; doi: 10.1130/G33058.1.

Bowers Ridge is an approx. 700-km-long arcuate ridge behind the central Aleutian Arc in the Bering Sea. The lack of age and geochemical data for the ridge has hampered the development of geodynamic models for the evolution of the North Pacific and the Aleutian-Bering Sea region. Maren Wanke and colleagues present the first geochemical and age data for the volcanic basement of Bowers Ridge and a seamount from the western end of the ridge sampled during R/V SONNE cruise SO201-1b. These data imply highly oblique subduction along the northern part of Bowers Ridge in its present-day configuration, consistent with an in-situ origin of Bowers Ridge as an Oligocene-Early Miocene island arc in the Bering Sea.

Eclogite breccias in a subducted ophiolite: A record of intermediate depth earthquakes?

S. Angiboust et al., ISTEP, UMR CNRS 7193, UPMC Sorbonne Universités, F-75005 Paris, France. Posted online 8 June 2012; doi: 10.1130/G32925.1.

Eclogite facies breccias (strongly fractured rocks), discovered in exhumed ophiolitic rocks from Western Alps (Italy), constitute a plausible fossilized relict of deep earthquakes occurring at 80-90 km depth in subduction zones. This finding by S. Angiboust and colleagues provides important constraints on deformation processes and fluid circulation patterns and enlightens present-day subduction zones geophysical observation.

Paleoecology and geochemistry of Early Triassic (Spathian) microbial mounds and implications for anoxia following the end-Permian mass extinction

Pedro J. Marenco et al., Dept. of Geology, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010, USA. Posted online 8 June 2012; doi: 10.1130/G32936.1.

The largest mass extinction in the history of life occurred ~252 million years ago. It took ~5 million years during the Early Triassic for life to recover following this mass extinction. During this interval, Earth experienced a resurgence of rock structures called stromatolites that were likely formed by microbial communities. Stromatolites were common before the evolution of animals but became less common once animals evolved. It has been suggested that low oxygen levels (anoxia) in seawater suppressed animal life and fostered the resurgence of stromatolites. Pedro J. Marenco and colleagues report their study of the most well-known of the Early Triassic stromatolites -- patch reef-forming build-ups from Nevada. They measured elements whose abundance in marine rocks is directly proportional to the degree of anoxia in seawater and found that the abundances of these compounds were too low to support the hypothesis that seawater was anoxic when these patch reefs were formed. Furthermore, they discovered that some of the patch reefs were built by both microbes and sponges and provided habitat for other animals which would have required oxygen. The results presented by Marenco and colleagues falsify the dominant hypothesis for the formation of these stromatolites and suggest that complex ecological phenomena such as reef-building began sooner than previously thought.

Evidence of very rapid reef accretion and reef growth under high turbidity and terrigenous sedimentation

C.T. Perry et al., Geography, College of Life and Environmental Sciences, University of Exeter, Exeter EX4 4RJ, UK. Posted online 8 June 2012; doi: 10.1130/G33261.1.

Globally, coral reef ecosystems are reported to be in major decline and are threatened by a wide range of environmental stresses, all of which may modify marine environmental parameters to levels that exceed coral tolerance levels. One such major environmental threat results from high rates of terrestrially derived sediment and nutrient inputs, and these are often linked to changed catchment land-use practices. Under deteriorating water quality regimes, reduced coral cover and species diversity are commonly inferred, and impaired reef development and lower reef accretion rates are an assumed consequence. C.T. Perry and colleagues present evidence that clearly challenges the overly simplistic assumption that high rates of terrestrial sediment input necessarily inhibit reef development and suppress reef accretion rates. In contrast, they show that these environments can in fact be sites of very high-rate reef accretion and that these rates can be sustained over centennial timescales. Data presented by Perry and colleagues also suggests that measured rates of accretion can match or exceed those reported in more "optimal" marine settings. These findings thus necessitate a re-think of prevailing models of reef growth dynamics both generally, and specifically within "at risk" inner-shelf and nearshore environments.

Hydrologic forcing of ice stream flow promotes rapid transport of sediment in basal ice

M. Bougamont and P. Christoffersen, Scott Polar Research Institute, University of Cambridge, Cambridge CB2 1ER, UK. Posted online 8 June 2012; doi: 10.1130/G33036.1.

The geologic record from high-latitude continental margins show that soft-bedded ice streams are capable of eroding, transporting and depositing large volumes of sediment. Interpretation of these records relies on correct understanding of how sediments were transferred. Based on recent observations of a 15 m-thick debris-bearing basal ice layer (BIL) in Kamb Ice Stream, Antarctica, Bougamont and Christoffersen propose that sediment entrainment by freeze-on, followed by englacial transport and eventually melt-out represent efficient mechanisms whereby ice streams erode their bed and re-distribute sediments. Using a numerical model of ice flow and basal mechanics, they set up experiments in which ice stream flow was characterized by oscillations between fast and stagnant modes. They show that there is a strong coupling between the amplitude of the flow oscillations and the amount of sediment eroded and transferred out of the modeled system due exclusively to the formation and advection of a BIL. Moreover, increased incorporation of water from a basal water system amplifies the oscillations and thus the growth of the BIL. The patterns of ice stream flow, and associated modeled sediment fluxes and transport, are consistent with modern Antarctic ice streams and with the seemingly erratic behavior of paleo-ice streams during the last deglaciation.

The influence of a mantle plume head on the dynamics of a retreating subduction zone

Peter G. Betts et al., School of Geosciences, Monash University, Clayton, VIC 3800, Australia. Posted online 8 June 2012; doi: 10.1130/G32909.1.

Earth's subduction zones are where two geological plates of the outer Earth converge and the dense ocean crust sinks into the mantle. Subduction zones form an important component of mantle convection. Mantle plumes are hot buoyant material that rises from deep in the interior of Earth and interact with Earth's crust. When plumes interact with ocean crust, they can form large areas of buoyant ocean floor topography. Subduction zones can migrate backwards and interact with mantle plumes causing the subduction zone to change behavior. Peter G. Betts and colleagues modeled this geological situation and discovered that subduction zone-plume interactions can cause massive geological damage at the edges of plates. The buoyant plume head hinders subduction and causes the subduction zone to migrate forward causing intense deformation in the adjacent geological plate. The subducting oceanic plate is also damaged and large tears can form allowing the plume to migrate across plate boundaries. The Yellowstone hotspot may be an ancient example of this process.

Evidence for end-Permian ocean acidification from calcium isotopes in biogenic apatite

Jessica L. Hinojosa et al., Dept. of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA. Posted online 8 June 2012; doi: 10.1130/G33048.1.

The end-Permian mass extinction, ca. 252 million years ago, was the most severe crisis in the history of animal life. More than 90% of the species in the oceans were lost at this time. Animals with thick calcium carbonate shells were preferentially victimized. Several scenarios involving various changes to ocean and atmosphere chemistry have been proposed to account for these observations. Jessica L. Hinojosa and colleagues measured the calcium isotope composition of microfossils to distinguish among these several possibilities. They observed a decrease in the ratio of calcium-44 to calcium-40 in the microfossils, which is best interpreted to reflect a change in the calcium isotope composition of seawater during and after the mass extinction event. These findings are most easily explained an interval of ocean acidification coincident with the mass extinction event, suggesting this ancient catastrophe bears some commonalities with anticipated changes to the global oceans over the next century.

Hydrous melting of the martian mantle produced both depleted and enriched shergottites

Francis M. McCubbin et al., Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico 87131, USA. Posted online 8 June 2012; doi: 10.1130/G33242.1.

There have been numerous studies that have confirmed the past presence of liquid water on the surface of Mars; however, until now, the origin of this water was highly uncertain. Francis M. McCubbin and colleagues analyzed the water contents of hydrous minerals in martian meteorites that formed from cooling magma that originated deep within the martian interior. These data were used to estimate the water content of the mantle source region from which the magmas were derived, and McCubbin and colleagues report that the interior of Mars is as wet or even wetter than the Earth's mantle. Furthermore, the ancient mantle reservoir from which the water was derived indicates that Mars stored its mantle water very early, most likely at the time of planet formation. This early storage of water in the planet is not likely unique to Mars and could explain early water storage in other planetary bodies like the Moon and early Earth. These results bode well for the formation of habitable environments early in Mars' geologic history and should reinvigorate efforts regarding the search for life on Mars.

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