|21 October 2011
GSA Release No. 11-72
Director - GSA Communications & Marketing
November 2011 Geology Highlights: New Research Posted 5 October 2011
Boulder, Colo., USA – Another packed issue of GEOLOGY, The Geological Society of America’s premier journal and the top-most cited geoscience journal in the world, is online in pre-issue publication. Topics include the San Andreas Fault and SAFOD; the 1944 Tonankai earthquake, Japan; Bonin Island beach sands; the fluid properties of ice; fracturing of the Panamanian Isthmus; diatoms from Ocean Drilling Program Site U1304; China’s Loess Plateau; deglaciation and climate change in Turkey; an ancient Death Valley landslide; and cephalopod hatchling habits.
Keywords: Nankai Trough, Tonankai earthquake, large igneous province, Olympic Dam, Gawler Range Volcanics, Mesoproterozoic, Panamanian Isthmus, thermochronology, Boston Blue Clay, Ocean Drilling Program Site U1304, Northern Atlantic, Red Sea Rift, Alpine fault, New Zealand, San Andreas, glaciers, Chinese Loess Plateau, Tibet, Qaidam basin, ocean carbonate chemistry, Atlantic, inoceramids, OAE 2, Vallecillo, Mexico, San Andreas fault, SAFOD, Younger Dryas, Turkey, end-Permian mass extinction, Guadalupian extinction, iron cycling, cephalopods, Phanerozoic, South America monsoon, Brazil, North AMOC, Bond events, Red Sea, Levantine corridor, Aqaba, Jordan, paleoflood, OSL dating, storm surge, Netherlands, Tecopa paleolake, Death Valley
Highlights are provided below. Representatives of the media may obtain complementary copies of GEOLOGY articles by contacting Christa Stratton at the address above. Abstracts for the complete issue of GEOLOGY are available at http://geology.gsapubs.org/.
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Progressive illitization in fault gouge caused by seismic slip propagation along a megasplay fault in the Nankai Trough
Asuka Yamaguchi et al., Dept. of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan; doi: 10.1130/G32038.1.
The question of whether coseismic ruptures along megasplay faults in accretionary prisms (i.e., large landward-dipping thrust faults branching from the plate boundary) reach the seafloor is critical for assessing the risk of tsunami disaster. Asuka Yamaguchi of the University of Tokyo and colleagues present geochemical and mineralogical data of drilled core samples obtained from the shallow portion of the megasplay fault that coincides with the rupture area of the A.D. 1944 Tonankai earthquake in the Nankai Trough. Chemical and mineralogical features of the localized slip zone may reflect a transformation in clay mineralogy caused by frictional heating, and suggest that the seismic slip can propagate to very shallow levels along megasplay fault systems.
Osmium behavior in a subduction system elucidated from chromian spinel in Bonin Island beach sands
Katsuhiko Suzuki et al., Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine Earth Science and Technology (JAMSTEC), 2-15 Natsushima, Yokosuka 237-0061, Japan; doi: 10.1130/G32044.1.
Osmium (Os) isotopes are a potential tracer of a recycled crust in mantle or volcanic rocks because of the significant contrast between Os isotope ratios of crust and mantle. However, possible contamination of crust with the high Os isotope ratio to magma during magma ascent may overprint this original signal of the Os isotopic ratios. Katsuhiko Suzuki of JAMSTEC and colleagues explored the use of Os isotopes in chromian spinel (Cr-spinel) as a discriminator of primitive magma Os isotopic compositions in the Izu-Bonin arc, using beach sands as composite samples of the boninite and tholeiite magmas. Cr-spinel is an early-stage crystal that preserves its isotopic composition even during later crustal contamination. Suzuki et al. found very low Os isotope ratios in Cr-spinels from boninites, suggesting that they represent primitive magmas with no Os recycled from the subducting slab during the boninite generation in the infant arc stage. Conversely, the high Os isotopic ratios in Cr-spinels from tholeiites likely reflect the contribution from a slab-derived component, because more oxidative conditions in the subarc mantle probably allowed Os to mobilize from the subducting slab during the transitional arc stage. Although contamination of crustal components may overprint the original signature of magma during its ascent, Cr-spinel allows us to compensate such possibilities.
The fluorine link between a supergiant ore deposit and a silicic large igneous province
Jocelyn McPhie et al., ARC Centre of Excellence in Ore Deposits and School of Earth Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia; doi: 10.1130/G32205.1.
Olympic Dam is a supergiant ore deposit that is mined for copper, uranium, gold, and silver. These metals occur in small grains of sulfide and oxide minerals within granite and volcanic rocks that have been intensely broken up. The metal-rich minerals were precipitated from hot, metal-rich water that permeated through the broken-up rocks. Jocelyn McPhie of the University of Tasmania and colleagues have found that originally, the granite and volcanic rocks contained a lot of fluorine, and fluorine-rich minerals are also abundant in the ore deposit. They propose that the hot, metal-rich water was able to concentrate and transport metals because it inherited high fluorine content from the rocks through which it circulated. Further, McPhie and colleagues infer that hydrofluoric acid, the most corrosive acid known, was partly responsible for breaking up the granite and volcanic rocks by dissolving them along fractures and faults. It is no accident that the world's largest hydrothermal ore deposit occurs in granite and volcanic rocks that are fluorine-rich.
Fracturing of the Panamanian Isthmus during initial collision with South America
David W. Farris et al., Florida State University, Dept. of Earth, Ocean, and Atmospheric Sciences, Tallahassee, Florida 32306, USA; doi: 10.1130/G32237.1.
David W. Farris of Florida State University and colleagues report evidence that tectonic collision between South America and Panama began approximately 23-25 million years ago. This is much earlier than previously thought, and is significant because it ultimately led to development of the Panamanian Isthmus, which in turn had wide-ranging oceanic, climatic, biologic, and tectonic implications. The interpretation of earlier collision is based on distinct changes in the chemistry of Panama volcanic arc rocks and synchronous exhumation of both Panama and the adjacent northern Andes in Colombia. Exhumation on both the Panama and South American sides of the collision is what would be expected once the collision began, and this is what is observed at 23-25 million years ago. Farris and colleagues also present a new tectonic model in which Panama fractured and rotated in several pieces to produce the modern curved shape of the Panamanian Isthmus that we observe today.
Insights into pore-scale controls on mudstone permeability through resedimentation experiments
Julia Schneider et al., Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78713, USA; doi: 10.1130/G32475.1.
Fine-grained marine sediments have limited ability to flow water. This ability is strongly dependent on the fraction of coarser-grained sediments that are present. These properties are difficult to interpret from sediments taken in natural settings because of natural variability and sample disturbance. Instead, Julia Schneider of the Jackson School of Geosciences and colleagues follow a systematic approach, adding coarse material to fine-grained sediment in different proportions to test the effect of composition on flow. They prepared the samples using a new technique termed resedimentation that avoids sample variation and disturbance. Schneider and colleagues show that by replacing 50 percent of the fine-grained sediment with coarse material, the ability of fluid to flow through the sample (permeability) is increased by an order of magnitude. Coarser samples have larger pores and fewer aligned particles than finer-grained samples providing easily accessible pathways for fluids. This behavior is described by a dual-porosity model where one rock fraction is dominated by silt, large pore throats are present, and the majority of flow occurs, and another fraction is dominated by fine-grained particles, where limited flow occurs. These results are important because fine-grained sediments act as seals for both carbon dioxide and hydrocarbons in the subsurface, behave as gas reservoirs, and can potentially cause harm in offshore drilling.
A highly productive Subarctic Atlantic during the Last Interglacial and the role of diatoms
O.E. Romero et al., Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada, 18002 Granada, Spain; doi: 10.1130/G32454.1.
Most paleoclimatic and paleoceanographic studies have focused on coarse time scales that have a more academic and less immediate appeal. A new development is the increasing recognition of the occurrence of laminated sediments composed of diatom mats in open-ocean, deep-sea environments. Because of the laminated nature of these deposits, origins have been ascribed previously to implausible occurrences of reduced oxygen conditions. Using an expanded section of the Last Interglacial (LIG), gained at Integrated Ocean Drilling Program Site U1304 in the Subarctic Atlantic, O.E. Romero of the University of Granada and colleagues demonstrate that the early interglacial stage 5e was marked by oceanographic conditions conducive for high diatom production and accumulation. Though the Subarctic Front provided the physical conditions for high diatom production and deposition, these processes alone are insufficient to explain the high rates of siliceous productivity and the formation of diatomaceous sediments. Instead, the additional presence of an increased nutrient pool provided by Subantarctic Mode Waters played the decisive role in initiating and sustaining diatom production. These observations by Romero and colleagues provide new insight into the workings of the marine biological silica and carbon pumps during early LIG and have important implications for understanding variability in diatom productivity during other glacial/interglacial transitions.
Initial burst of oceanic crust accretion in the Red Sea due to edge-driven mantle convection
Marco Ligi et al., Istituto di Scienze Marine, Consiglio Nazionale delle Ricerche, Via Gobetti 101, 40129 Bologna, Italy; doi: 10.1130/G32243.1.
Throughout Earth's history, the continents periodically assemble in a single supercontinent, then fragment and disperse again in cycles each lasting about 500 million years. Rupturing of a continent and the birth of a new ocean are taking place now in the Red Sea, where Arabia is separating from Africa. Geophysical surveys and bottom rock samples were obtained in a recent expedition in the central Red Sea, where continental breakup and accretion of new oceanic crust are just starting. Data collected by Marco Ligi of Italy’s Consiglio Nazionale delle Ricerche suggest that as soon as the lid of continental crust breaks, an initial strong burst of oceanic crust generation takes place, with intense basaltic volcanism and fast opening velocity. This initial active pulse fades then into steady, more passive, slower accretion. The initial pulse may be triggered by secondary convective movements in Earth's mantle below the rift, caused by a strong horizontal thermal gradient between the cold walls of the continental plates and the hot, axially upwelling subrift mantle. This mechanism of initial continental breakup may have acted also during the earliest stages of the opening of the Atlantic Ocean more than 100 million years ago.
Inverted metamorphic sequences in Alpine fault mylonites produced by oblique shear within a plate boundary fault zone, New Zealand
Alan F. Cooper and Richard J. Norris, Geology Dept., University of Otago, Dunedin 9016, New Zealand; doi: 10.1130/G32273.1.
The east-dipping Australia-Pacific plate boundary in southern New Zealand, the Alpine fault, is bounded on its eastern side by a sequence of mylonites whose metamorphic grade is lower than the hanging-wall sequence that overlies them. Rather than being caused by an inverted thermal gradient, this inverted metamorphic sequence is attributed to lateral and vertical shear within the mylonite zone translating low-grade rock from its source area, 110 km to the south west. As documented by Alan F. Cooper and Richard J. Norris of the University of Otago, New Zealand, this transpressional shear occurred subsequent to the post-late Miocene inception of oblique convergence on the plate boundary.
Stress modulation on the San Andreas fault by interseismic fault system interactions
John P. Loveless and Brendan J. Meade, Dept. of Geosciences, Smith College, Northampton, Massachusetts 01063, USA; doi: 10.1130/G32215.1.
Earthquake hazard is directly related to the level of stress on major faults. In the branching southern California fault system, the rate at which stress increases on one fault is affected by activity throughout the entire fault system. By calculating the interseismic (between earthquakes) stressing rate on all faults in southern California, John P. Loveless and Brendan J. Meade of Smith College find that neighboring faults amplify the stress accumulation rate on the Mojave and San Bernardino sections of the San Andreas fault. Since the 1857 Mw = 7.9 Fort Tejon earthquake, the last large earthquake on the San Andreas fault, stress accumulated interseismically on these sections due to interactions with nearby faults is about three times larger than the coseismic stress changes induced by the Landers and Hector Mine earthquakes. The amplification of stress accumulation rates on the San Andreas fault serves as a type example of the way in which fault system interactions may affect physics-based seismic hazard estimates.
How nonlinear is the creep deformation of polar ice? A new field assessment
K.M. Cuffey, Dept. of Geography, University of California, Berkeley, California 94720-4740, USA; and J.L. Kavanaugh, Dept. of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada; doi: 10.1130/G32259.1.
Glacier ice flows across Earth's surface much in the same way that pancake batter spreads when poured onto a skillet, or syrup spreads when poured on a pancake. The fluid properties of ice are very different from those of syrup or pancake batter, however. The purpose of this study by K.M. Cuffey of the University of California at Berkeley and J.L. Kavanaugh of the University of Alberta, Canada, is to determine the fluid properties of ice. To accomplish this task, they analyzed measurements of flow and thickness of a large Antarctic glacier together. Information derived from this study will help geoscientists to understand how glaciers in polar regions, including the vast continental ice sheets, are responding to climate change.
Qaidam Basin and northern Tibetan Plateau as dust sources for the Chinese Loess Plateau and paleoclimatic implications
Alex Pullen et al., Dept. of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA; doi: 10.1130/G32296.1.
The Chinese Loess Plateau (central Asia) is one of Earth's most complete terrestrial records of wind-derived sediment for the past 2.5 million years. Alex Pullen of the University of Rochester and colleagues analyze detrital zircon crystals from glacial-period loess strata of the Loess Plateau to better understand dust storm sources and general wind patterns over central Asia during Quaternary glacial periods. Using the data collected during this project and observations of modem-interglacial dust storm tracks, Pullen et al. note a substantial southward shift in the source areas and tracks of dust-generating storms between modem-interglacial and glacial periods. The ages of zircon crystals included in the loess suggest the Qaidam Basin and northern Tibetan Plateau were significant source areas for the loess. This is at odds with modem-interglacial observations that suggest source areas to the north. Pullen and colleagues interpret this shift to represent an equatorward shift in the mean annual position of the subtropical jet stream during glacial periods. This work suggests that the southerly jet position inhibited the northward movement of precipitation associated with the East Asian monsoon into central East Asia during glacial periods.
Modern and late Pleistocene B/Ca ratios of the benthic foraminifer Planulina wuellerstorfi determined with laser ablation ICP-MS
M. Raitzsch et al., MARUM, Center for Marine Environmental Sciences, University of Bremen, Leobener Strasse, 28359 Bremen, Germany; doi: 10.1130/G32009.1.
The ocean, the largest carbon reservoir besides the solid earth, plays a major role in governing the carbon dioxide concentration in the atmosphere. The carbon dioxide concentration of the deep sea is part of the oceanic carbonate chemistry regulating the carbonate saturation state, a term for the degree of calcium carbonate preservation and dissolution. Benthic foraminifers, microorganisms dwelling on the sea floor, may record this saturation state of deep waters in the composition of their calcite shells, specifically, the ratio of the elements boron to calcium (B/Ca). Hence, B/Ca of fossil shells of foraminifers can be used to reconstruct past variations of the deep oceanic carbonate chemistry, which is essential for understanding past atmospheric carbon dioxide changes. M. Raitzsch of the University of Bremen, Germany, and colleagues measure samples from sediments accumulated in the past 135 thousand years in the equatorial Atlantic. It was already known that this region was alternately influenced by different deep water masses with different carbonate chemistries, which is clearly reflected in their new B/Ca data. However, the exact mechanism of boron incorporation into biogenic calcite is still largely unknown. Their new data demonstrates that boron is heterogeneously distributed within single shells, suggesting that the boron uptake is biologically influenced by the foraminifer metabolism.
Fluctuations of the oxygen minimum zone at the end of Oceanic Anoxic Event 2 reflected by benthic and planktic fossils
Christina Ifrim et al., Institute of Geosciences, Ruprecht Karls University, 69120 Heidelberg, Germany; doi: 10.1130/G32161.1.
Oceanic Anoxic Events (OAEs), oceanic crises resulting from ocean acidification and a drastic reduction of oxygen dissolved in the water, repeatedly occurred in Earth history. They appear to be related to extreme Greenhouse climate. The study of OAEs’ effects on marine organisms is thus of increased interest due to comparison with the recent climatic changes in climate and the increase of carbon dioxide in the atmosphere. The fossil record shows that shelled organisms suffered from these rapid chemical oceanic perturbations. One specific OAE occurred in the early Late Cretaceous about 94 million years ago, and caused massive faunal extinction on a global scale. One group of Cretaceous bivalves, the inoceramids, benefitted. They conquered hostile bottom environments at the seafloor. However, quantitative analysis of data from a section in northeastern Mexico by Christina Ifrim of Ruprecht Karls University, Germany, and colleagues suggests that inoceramid larvae remained vulnerable to the lack of oxygen. The team suggests that the abundance of these bivalves is a paleoproxy for oxygen levels in the seawater. Their data indicate that a stepwise increase of oxygen levels toward the end of the OAE led to a recovery of the marine ecosystem about 93 million years ago, including a fast evolution of the inoceramidae into numerous new species. This data thus provides insight into the paleobiology of these exceptional bivalves.
High pore pressure, or its absence, in the San Andreas Fault
Chi-yuen Wang, Dept. of Earth and Planetary Science, University of California, Berkeley, California 94720, USA; doi: 10.1130/G32294.1.
The weakness of the San Andreas Fault Zone, as revealed by a number of indirect observations, has major implications on the mechanics of earthquakes and the forces that move the lithospheric plates. Yet the mechanism for this weakness is not well understood. Several hypotheses have been proposed, each implying different faulting mechanisms, making it important to determine which is correct. Drilling projects into active faults have been conducted around the world to obtain direct information on the constitution and pore pressure in the fault. The SAFOD (San Andreas Fault Observatory at Depth) drilling project near Parkfield, California, is the best documented and reported. Observations made during drilling led some scientists to conclude that there was no evidence of high pore pressure in the fault cores. If the conclusion holds, it would have important implications on our understanding of earthquake mechanisms. Chi-yuen Wang of the University of California at Berkeley shows, however, that these observations may not be sufficient to reject the hypothesis of high pore pressure in the San Andreas fault; definite knowledge of pore pressure in the fault zone may require long-term monitoring at the SAFOD site. The result may also be useful for interpreting results from other active fault-drilling projects around the world.
Remarkably extensive glaciation and fast deglaciation and climate change in Turkey near the Pleistocene-Holocene boundary
Marek Zreda et al., Dept. of Hydrology and Water Resources, University of Arizona, Tucson, Arizona 85721, USA; doi: 10.1130/G32097.1.
Looking at how climate changed in the geological past can provide a useful perspective for studying modern climate change and for predicting climate changes in the next century. Marek Zreda of the University of Arizona and colleagues use mountain moraines to reconstruct the former glaciers and to determine climatic changes in Turkey at the beginning of the Holocene, the current interglacial epoch. They found that the glaciers were unusually large for that time, with snow lines lower than present by more than 1400 meters, implying a temperature 9 degrees Celsius lower than modern long-term average temperature. The main glacier melting phase lasted 500 years during which the ice margin retreated at the average rate of 1700 meters per century, which is higher than modern glacier retreat rates computed over comparable time. This corresponds to the temperature increase at the rate of 1.4 degrees Celsius per century, which exceeds the global warming trend of the past century, 0.6 degrees Celsius, showing that natural causes can lead to fast and large climate changes, and that the magnitude and the rate of climate change observed in the past century are not unprecedented.
Seismically induced slump on an extremely gentle slope (<1°) of the Pleistocene Tecopa paleolake (California)
Francisco J. García-Tortosa et al., Departamento Geología, Facultad Ciencias, Universidad de Jaén, Campus Las Lagunillas, 23071 Jaén, Spain; doi: 10.1130/G32218.1.
The primary effect of earthquakes is ground shaking; however, much of the local destruction is actually caused by secondary effects from landslides, liquefaction, or tsunamis. Francisco García-Tortosa of Universidad de Jaen, Spain, and colleagues describe a large, ancient landslide (slump) which occurred in a nearly flat lake plain in the Death Valley region (California). Usually landslides occur on steep slopes, but specific types of subsurface sediments will allow sudden movements on terrain with less than 1 degree of slope. They show that this condition requires sediments rich in some clay (sepiolite) that will behave like a liquid when shaken. This allows the overlying land surface to move, buckling into ridges in the front of the slump and creating sudden trenches and hollow areas at the back of the slump. Great care should be taken in siting buildings in seismic-prone valleys that have these unstable subsurface characteristics.
Acidification, anoxia, and extinction: A multiple logistic regression analysis of extinction selectivity during the Middle and Late Permian
Matthew E. Clapham, Dept. of Earth and Planetary Sciences, University of California, 1156 High Street, Santa Cruz, California 95064, USA; and Jonathan L. Payne, Dept. of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305, USA; doi: 10.1130/G32230.1.
Ancient environmental crises and extinction events can provide valuable clues into stresses facing modern marine organisms. The end-Permian mass extinction (250 million years ago), the largest extinction event in the history of animals, has been attributed to some combination of reduced oxygen levels in the ocean, ocean acidification, and rapid climate warming -- all stresses affecting marine animals today. The precise kill mechanism of the Permian extinction has remained more elusive, however. Matthew Clapham of the University of California and Jonathan L. Payne of Stanford University quantified extinction selectivity, the direct fingerprint of the kill mechanism, using a large database of marine fossil occurrences to show that animals with low metabolic rates and shells made from calcium carbonate were significantly more likely to become extinct at the end of the Permian. This selectivity is most consistent with ocean acidification as the primary kill mechanism, although reduced oxygen levels could have acted in combination to exacerbate the stresses. Although Late Permian ocean chemistry and the composition of Earth's biosphere were different from today, the extreme severity of the extinction (nearly 80 percent of shelly marine animals went extinct) highlights the potential dangers of ocean acidification.
Extracting storm surge data from coastal dunes for improved assessment of flood risk
Alastair C. Cunningham et al., Delft University of Technology, Faculty of Applied Sciences, Mekelweg 15, NL-2629 JB Delft, Netherlands; doi: 10.1130/G32244.1.
Alistair Cunningham of Delft University of Technology and colleagues have found geological evidence of a major storm surge occurring in historical times. The evidence was found in coastal dunes of the Netherlands, which trap sediment from high-magnitude events. They dated the event to the late 18th century by using tiny light signals emitted from the sediments in the laboratory. This research has two principle benefits for the understanding of storm-surge risk. First, it allows storm-surge data to be obtained from events that occurred before instrumental records began. It is vital to obtain such data, because instrumental records are seldom long enough to contain the high-magnitude (low-frequency) events. Second, storm surges of the past may have occurred under subtly different climatic conditions. Studying past storm surges allows for a better appreciation of how storm surges respond to climate parameters, useful information given the changes in climate patterns and sea level that are likely in the near future.
Freshwater on the route of hominids out of Africa revealed by U-Th in Red Sea corals
Boaz Lazar, Institute of Earth Sciences, Hebrew University of Jerusalem, Jerusalem 91904, Israel; and Mordechai Stein, Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel; doi: 10.1130/G32257.1.
The uplifted coral reefs on the shores of the northern Red Sea south of Aqaba, Jordan, contain several terraces that were recrystallized from the original aragonite mineralogy into calcite. The extensive recrystallization indicates that the reefs were uplifted through well-developed phreatic freshwater coastal aquifer. Applying an open-system uranium-thorium (U-Th) dating methodology to the calcitic corals, Boaz Lazar of Hebrew University and Mordechai Stein of the Geological Survey of Israel date the freshwater recrystallization to the last interglacial at about 140 thousand years B.P. It is generally accepted that at this period, anatomically modern hominids migrated out of Africa through the Levantine corridor. The presence of freshwater in the hyperarid Arabian deserts during the last interglacial may solve a fundamental issue in human culture evolution concerning the conditions during anatomically modern hominids' migration out of Africa.
Abrupt variations in South American monsoon rainfall during the Holocene based on a speleothem record from central-eastern Brazil
Nicolás M. Stríkis et al., Institute of Global Environmental Change, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China; doi: 10.1130/G32098.1.
Well-dated high-resolution oxygen isotope records of speleothems in central-eastern Brazil spanning from 1.3 to 10.2 thousand years ago reveal that the occurrence of abrupt variations in monsoon precipitation is not random. They show a striking match with Bond events and a significant pacing at ~800 years, a dominant periodicity present in sea surface temperature records from both the North Atlantic and equatorial Pacific Oceans that is possibly related to periods of low solar activity. The precipitation variations over central-eastern Brazil are broadly antiphased with the Asian and Indian Monsoons during Bond events and show marked differences in duration and structure between the early and late Holocene. The results presented by Nicolas Strikis of Xi’an Jiaotong University and colleagues suggest that these abrupt multicentennial precipitation events are primarily linked to changes in the North Atlantic meridional overturning circulation (AMOC). Anomalous cross-equatorial flow induced by negative AMOC phases may have modulated not only the monsoon in South America but also affected El Nino-like conditions in the tropical Pacific during the Holocene.
Sympatric speciation drove the macroevolution of fossil cephalopods
Ryoji Wani, Graduate School of Environment and Information Sciences, Yokohama National University, 79-7, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan; doi: 10.1130/G32317.1.
Sympatric speciation, new species derived from a biological barrier to gene exchange without any spatial segregation, is highly controversial. Allopatric speciation is thought to be the prevalent mode of speciation in modern animals. Cephalopods, limited in this study by Ryoji Wani of Yokohama National University, Japan, to ammonoids and nautiloids, underwent repeated episodes of diversification and extinction, related to Earth's environmental change. The relationship between egg sizes and modes of hatchling habit in modern cephalopods (planktotrophic or non-planktotrophic) was applied by Wani to fossils, and their hatchling habits were reconstructed. The temporal changes of reconstructed hatchling habits demonstrated planktotrophic habits in ammonoids and a planktotrophy to non-planktotrophy trend in nautiloids. The correlation of ammonoid richness with sea-level changes and the lack of correlation in nautiloids with non-planktotrophic hatchlings, contradict the general theory that animals with planktotrophic larvae have lower speciation rates and extinction probability. This contradiction suggests that allopatric speciation, which would be generally enhanced with non-planktotrophic larvae, is not the primary mode of speciation in fossil cephalopods but sympatric speciation was a plausible dominant process of their macroevolution. This contradiction relative to prevalent theory that allopatric speciation is the prevalent mode of speciation in modern animals suggests that speciation is a more complicated process than previously thought.
Successive detachment faults and mantle unroofing at magma-poor rifted margins
Tim J. Reston and Ken G. McDermott, School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham B152TT, UK; doi: 10.1130/G32428.1.
When continents are pulled apart to form ocean basins, the crust is thinned, stretched, and eventually broken. In some locations, this process occurs with little or no magmatic activity. At these magma-poor rifted margins, broad expanses of mantle rocks are brought to the seafloor. As these rocks are found on both sides of the ocean, the process appears to be broadly symmetrical. Furthermore, seismic data reveal dipping structures in the mantle that dip landwards. Tim Reston and Ken McDermott of the University of Birmingham, UK, propose that these observations are caused by multiple generations of normal faults that develop repeatedly in the center of the rift. These faults are responsible for the thinning of the crust, for the eventual breaking and separation of the crust to expose the underlying mantle, and the subsequent symmetrical unroofing of more mantle. Each new fault cuts through the preceding structure, leaving the deep dipping zone of the earlier faults as landward-dipping structures. Repetition of the process produces a symmetric zone of unroofed mantle dominated by the landward-dipping root zones of these abandoned faults.
Trace element cycling through iron oxide minerals during redox-driven dynamic recrystallization
Andrew J. Frierdich et al., Dept. of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130, USA; doi: 10.1130/G32330.1.
A.J. Frierdich of Washington University and colleagues have identified a previously unknown trace element cycle caused by Fe(II)-catalyzed recrystallization of Fe(III) oxide minerals. The recrystallization process repartitions trace elements among the mineral bulk, mineral surface, and aqueous solution, and unexpectedly represents an iron redox process that alters the speciation of redox-inactive trace elements. Frierdich and colleagues explain previous observations in biogeochemical systems, such as enhanced iron oxide retention of metals when Fe(II) is present. They also suggest that important proxies for ocean composition on early Earth may be invalid, identify new processes controlling micronutrient availability in soil, sedimentary, and aquatic ecosystems, and point toward a mechanism for trace element mobilization during diagenesis and enrichment in geologic fluids.