Boulder, Colo. - Topics in the November issue of GEOLOGY include: a challenge to the use of banded iron formations as markers for the rise of oxygen in the oceans; new model of changes in seawater composition over time; factors in the rise of atmospheric methane; unmixing of magma into immiscible liquids; and evolution of organic molecules on early Earth. The GSA TODAY science article addresses global events accompanying the transition from primitive Earth to modern, oxygenated Earth.
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Novel observations on biomineralization processes in foraminifera and implications for Mg/Ca ratio in the shells
Samuel Bentov and Jonathan Erez, The Hebrew University of Jerusalem, Institute of Earth Sciences, Jerusalem 91904, Israel. Pages 841-844.
Atmospheric CO2 increase causes global changes that are of major environmental concern. Modeling efforts to predict the effects of these changes are based on paleoceanographic reconstructions. Foraminifera are unicellular microorganisms that build calcareous shells and accumulate in huge quantities on the ocean floor. Information hidden in the chemistry of their shells provides the basis for paleoceanographic reconstructions. However, the accuracy of these studies depends on understanding the processes by which these organisms precipitate their shells. Bentov and Erez describe unique and novel microscopic observations on the biomineralization process in live foraminifera. They discovered that foraminifera precipitate two types of calcareous shell with different Mg content. This finding has important implications for reconstructions of past oceanic temperatures based on the Mg content of foraminiferal shells.
Rapid microplate rotations and backarc rifting at the transition between collision and subduction
Laura Wallace, Institute of Geological and Nuclear Sciences,Lower Hutt, North Island 6009, New Zealand; et al. Pages 857-860.
One type of deformation that occurs at tectonic plate boundaries is subduction, which is where two tectonic plates collide and one sinks beneath the other. Interestingly, even though the whole plate boundary should be under compression, areas of crustal extension, or backarc rifting, can be found; why this occurs is unknown. Wallace et al. have used Global Positioning System data to track tectonic plate movements in Papua New Guinea, New Zealand, Tonga, Vanuatu, and the Marianas. These data have enabled them to develop a new model that explains how the initiation and maintenance of rifting at convergent plate boundaries may occur. Their model proposes that the Earth's crust is pulled apart as a result of rapid rotation of the overriding tectonic plate. In this model, these rotations are caused by the collision of unusually thick, buoyant areas of crust on the subducting plate. These areas of crust could, for example, be oceanic plateaus, seamount chains, or continental fragments. This work provides new insights into the physical processes producing backarc rifting and rapid tectonic rotations at subduction zones.
Deposition of banded iron formations by anoxygenic phototrophic Fe(II)-oxidizing bacteria
Andreas Kappler, Caltech, GPS, Pasadena, CA 91125, USA; et al. Pages 865-868.
Dating the rise of oxygen is a great challenge for Earth scientists and is the subject of intense debate. Classically, banded iron formations (BIFs) are used by geologists to date the appearance of oxygen in the oceans based on the assumption that the iron in BIFs was oxidized by oxygen produced by cyanobacteria. Kappler et al. suggest that a completely different mechanism that does not require oxygen was more likely to have been responsible for their formation in a stratified ancient ocean. This challenges researchers to find other markers for the rise of oxygen.
Methane-driven late Pleistocene ?13C minima and overflow reversals in the SW Greenland Sea
Christian Millo, University of Kiel, Geosciences, Kiel, D-24118, Germany; et al. Pages 873-876.
A sediment core transect across the southwestern Greenland Sea reveals coeval events of extremely negative planktic and benthic ?13C excursions between 40 and 87 ka. The most pronounced event, Event 1, began at peak Dansgaard-Oeschger stadial 22 (85 ka) with a duration of 18 k.y. During this episode, incursions of Atlantic Intermediate Water caused a bottom water warming of up to 8 °C. Amplitude, timing, and geographic pattern of the ?13C events suggest that this bottom water warming triggered clathrate instability along the East Greenland slope and a methane-induced depletion of ?13CDIC (DIC--dissolved inorganic carbon). Since ?13C Event 1 matches a major peak in atmospheric CH4 concentration (Blunier and Brook, 2001), this clathrate destabilization may have contributed to the rise in atmospheric CH4 and thus to climate warming over marine isotope stage 5.1.
Model of seawater composition for the Phanerozoic
Robert Demicco, State University of New York, Department of Geological Sciences and Environmental Studies, Binghamton, NY 13902-6000, USA; et al. Pages 877-880.
During the past decade it has become apparent that the major element chemistry of the oceans varied throughout the history of the Earth. Demicco et al. set out the details of a computer model that is an attempt to sort out the causes of these variations. Their idea is that there are three "faucets" that pour into the oceans, each with its own chemistry, and it is the mixing of the waters from these three faucets that makes up the complex cocktail called seawater.
Immiscible iron- and silica-rich melts in basalt petrogenesis documented in the Skaergaard intrusion
Jakob Jakobsen, University of Aarhus, Department of Earth Sciences, Aarhus C, - 8000, Denmark; et al. Pages 885-888.
Like oil and water, glowing hot magmas of Earth and earthlike planets may exsolve into immiscible liquids. Tiny pockets of liquid captured in crystals growing from hot magma below a 55-million-year-old volcanic system in Greenland document for the first time two immiscible liquids at depth. This process is important in the formation of basalt lavas, the most widespread rock type of Earth's crust. One of the exsolved liquids explains the formation of granite in the ocean floor. The other liquid is extremely rich in iron and sinks to the base of the crust.
First paleoseismological constraints on the strongest earthquake in France (Provence) in the twentieth century
Dominique Chardon, Institut de Recherche Pour Le Developpement, Nouvelle – Caledonie 98848, France; et al. Pages 901-904.
In Western Europe, active faults are elusive because of their very small displacement rates and low seismic activity. New field topographic and geological investigations led to the identification of the fault that produced the strongest earthquake recorded in France since the late 19th century. This earthquake led to considerable damage in the Aix-en-Provence region, Provence (the magnitude 6, 1909 Lambesc earthquake). Besides potentially identifying the trace of the 1909 earthquake, observations made in a trench opened across this fault indicate that repeated seismic events of magnitude larger than 6 have ruptured the land surface over the last 300,000 years. These results provide an initial firm basis for assessing seismic hazard around Aix-en-Provence, with potential return period for earthquakes of magnitude higher than 6 of 700 to 5,000 years.
Organic molecules formed in a primordial womb
Lynda Williams, Arizona State University, Department of Geological Sciences, Tempe, AZ 85287, USA; et al. Pages 913-916.
Expandable clay minerals can act like a primordial womb to protect and promote synthesis of organic molecules. On the early Earth, volcanic gases were emitted onto the seafloor from vents where carbon dioxide and hydrogen interact with metallic minerals to form methanol, a one carbon organic molecule. The temperature of this environment (>300°C [475°F]) would have been too high for most organic molecules to survive. However, clay minerals are commonly found in these volcanic vents, and expandable clay minerals (smectite), with silicate sheets stacked like a deck of cards, will adsorb and protect organics between the sheets. The clay mineral surfaces often promote organic reactions. Therefore, Williams et al. tested the potential for clays to not only protect methanol in hot water, but to synthesize new compounds needed as building blocks for more complex biomolecules. Volcanic vent simulations showed that certain expandable clays react quickly at 300°C to a non-expandable form, and the mineralogical changes coincide with production of a large number of complex organic molecules, primarily ring-structures (e.g., hexamethyl-benzene), some with up to 20 carbon atoms. The importance of this work is that it shows a potential mineralogical control on organic molecule evolution that could have been an important step in the origin of life.
A process-based model linking pocket gopher (Thomomys bottae) activity to sediment transport and soil thickness
Kyungsoo Yoo, University of California, Ecosystem Sciences Division, Berkeley, CA 94720-3110, USA; et al. Pages 917-920.
For the first time, researchers have modeled just how gophers and their habitat choices reshape landscapes. Researchers quantified how much energy a typical pocket gopher (Thomomys bottae) spends moving dirt in a year (9 Kilo Joules, or enough to keep a 100-watt bulb lit for 90 seconds). The gophers do not randomly spend the energy; they prefer to dig in thicker soils with abundant food and shelter. The scientists have proposed that such gopher burrowing may lead to landscapes that are shaped differently from ones formed by purely physical erosive processes. Since it is not just gophers that move soil, it is likely that earthworms, ants, and even trees affect soil thickness and hillslopes, and Yoo's model may have wider applications for describing organisms' roles in shaping the earth surface.
GSA TODAY Science Article
Emergence of the aerobic biosphere during the Archean-Proterozoic transition: Challenges of future research
Victor A. Melezhik, Geological Survey of Norway, Trondheim, Norway, et al. Pages 4–11.
Emergence of modern Earth: The earth that is familiar to us today emerged over the past 4.5 billion years and was marked by critical milestones such as the appearance of animals in the fossil record nearly 540 million years ago. One of the most important milestones in deep time was the development of a fully oxygenated atmosphere and ocean between 2.5 and 2.0 billion years ago, which changed forever how organisms operated on Earth. In a new paper appearing in GSA Today, Victor Melezhik and colleagues tackle this enigmatic but critical time period. They use the well-preserved rock record from the Fennoscandian Shield to demonstrate some of the remarkable global events that accompanied the transition into an oxygenated Earth: the oldest global glaciation, changes in the chemistry of seawater and atmosphere, and the first known example of significant petroleum generation. This suite of global scale biogeochemical changes marks a fundamental but poorly understood transition from a primitive Earth to one that operated similar to modern Earth based on oxygen.
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