|13 April 2011
GSA Release No. 11-27
Director of Education, Communication, & Outreach
May Geology cover: Infaunal molt pattern generated by the trilobite Paciphacops argentinus Tomas, 1906, late Silurian, from the Los Espejos Formation, Argentine Precordillera, San Juan Province, Argentina. See "Infaunal molting in Trilobita and escalatory responses against predation," by Rustán et al.
MAY 2011 GEOLOGY HIGHLIGHTS
Boulder, CO, USA – Highlights of articles in the May issue of GEOLOGY are provided below. Topics include examination of freshwater fish fossils to understand the Neogene water cycle in Central Africa; a study proposing that Nuna was Earth's first supercontinent; ancient cave systems near the summits of the Allgau Mountains (Austria) that preserved the oldest radiometrically dated dripstones known in the European Alps; and a report from the GRACE satellite mission over River Nile, Niger, and Congo basins.
GEOLOGY is now regularly posting pre-issue publication content — finalized papers that are ready to go to press and not under embargo. GSA invites you to sign up for e-alerts and/or RSS feeds to have access to new journal content the minute it is posted online. Go to http://www.gsapubs.org/cgi/alerts and enter your e-mail address to manage your subscriptions for pre-issue postings, full tables of contents alerts, and more.
Keywords: Southern Westerly Winds, El Niño-Southern Oscillation, Antarctica, D/H ratios, Azolla, Arctic Ocean, end-Triassic mass extinction event, Central Atlantic Magmatic Province, Chadian fossil assemblages, Sahelanthropus tchadensis, Australopithecus bahrelghazali, Neogene, Central Africa, Aconcagua fold-and-thrust belt, sandbox modeling, supercontinent Nuna, speleothems, Allgau Mountains, Songliao Basin, China, Westerly Winds, South Island, New Zealand, Tibet, Tien Shan, subduction, cosmogenic nuclides, Amazon Basin, Cascadia subduction zone, Juan de Fuca plate, United Kingdom Atlantic margin, petroleum, GRACE satellite mission, belemnite extinction, cephalopods, Mashonaland igneous province, astronomical cycles, trilobites, Mount Etna, Plum Island Estuary, coastal wetland loss
Representatives of the media may obtain complementary copies of GEOLOGY articles by contacting Christa Stratton at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. GEOLOGY abstracts are available at http://geology.gsapubs.org/.
Non-media requests for articles may be directed to GSA Sales and Service, .
Zonally symmetric changes in the strength and position of the Southern Westerlies drove atmospheric CO2 variations over the past 14 k.y.
M.-S. Fletcher, and P.I. Moreno, Institute of Ecology and Biodiversity, University of Chile, Santiago, Las Palmeras 3425, Nunoa, Chile; doi: 10.1130/G31807.1.
M.-S. Fletcher and P.I. Moreno of the University of Chile at Santiago identify hemisphere-wide zonal symmetry in changes in the strength and position of the Southern Westerly Winds (SWW) between 14 and seven thousand years ago that correlate closely with changes in atmospheric carbon dioxide concentrations recorded in Antarctic ice cores. These results support the notion that a SWW-Southern Ocean coupled system influences global atmospheric carbon dioxide concentrations. A breakdown in zonal symmetry after seven thousand years is attributed to the onset of increased El Nino-Southern Oscillation variability.
A record of Antarctic surface temperature between 25 and 50 m.y. ago
Luigi Dallai and Ray Burgess, CNR (Consiglio Nazionale delle Ricerche), Istituto di Geoscienze e Georisorse, Via Moruzzi 1, 56124 Pisa, Italy; doi: 10.1130/G31569.1.
Luigi Dallai and Ray Burgess of the Consiglio Nazionale delle Ricerche, Italy, report a semi-quantitative record of environmental change within continental Antarctica at the Eocene-Oligocene transition (~30 million years ago), when Earth experienced the major climatic change from a greenhouse world to an ice-house one. Dallai and Burgess exploit the anomalous and low deuterium/hydrogen (D/H) ratios of hydrous (oxygen- and hydrogen-bearing) silicate minerals that interacted with polar waters to detect hydrothermal episodes produced by the emplacement of small plutons along the Antarctic coastline. These plutons provided the necessary heat for glacial waters (characterized by low D/H ratios) to interact with oxygen- and hydrogen-bearing minerals. Once the timing of pluton emplacement is defined by geochronological methods, the age of hydrothermal alteration and glacial water availability are indirectly constrained.
Orbitally forced Azolla blooms and Middle Eocene Arctic hydrology: Clues from palynology
Judith Barke et al., Biomarine Sciences, Institute of Environmental Biology, Dept. of Biology, Faculty of Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands; doi: 10.1130/G31640.1.
The high abundance of freshwater fern Azolla remains recovered from approximately 50-million-year-old central Arctic Ocean sediments have been related to ocean surface water freshening. The distribution of Azolla concentrations in the sediments throughout time are strongly cyclic and have been suggested to be astronomically driven. Judith Barke of Utrech University and colleagues evaluate the potentially underlying forcing mechanisms for these freshwater cycles by applying an integrated palynological and cyclostratigraphical approach. Their results show that Azolla concentrations fluctuate with a 1.2 m cyclicity, accompanied by in-phase changes in the terrestrial elements sensitive to hydrology (vegetation), and marine phytoplankton changes sensitive to salinity. This strong correlation suggests periods of enhanced rainfall and runoff to the Arctic Ocean during Azolla blooms. Through spectral analysis, Barke et al. link these 1.2 m cycles to the astronomical-type cycles of obliquity (Earth's axial tilt; approximately a 40-thousand-year cycle). Increased obliquity leads to higher summer insolation at high latitudes, which, in turn, enhances total annual precipitation in the Arctic region. Barke et al. conclude that sufficient freshening of the Arctic Ocean surface waters during obliquity maxima may have allowed colonization by Azolla of the ocean surface.
Multiple phases of carbon cycle disturbance from large igneous province formation at the Triassic-Jurassic transition
M. Ruhl and W.M. Kurschner, Palaeoecology, Laboratory of Palaeobotany and Palynology, Institute of Environmental Biology, Faculty of Science, Utrecht University, Budapestlaan 4, NL-3584 CD Utrecht, Netherlands; doi: 10.1130/G31680.1.
The end-Triassic mass extinction event (~200 million years ago) is one of the Big Five mass extinction events in earth history. It is marked by global extinction of up to 50% of all species living in the oceans and by large changes in ecosystems on the continents. The cause for the extinction event is related to strongly intensified volcanic activity and deposition of the largest basalt deposit on Earth, the Central Atlantic Magmatic Province. Recent studies show that the end-Triassic mass extinction event directly coincided with the onset of this volcanic period that lasted for ~600,000 years. The release of massive amounts of carbon dioxide from volcanism triggered a strong change in the global carbon cycle and likely had a strong impact on global climate. M. Ruhl and W.M. Kurschner of Utrecht University show, however, that a strong increase in atmospheric carbon dioxide of up to 1000 parts per million (today's values are ~350 parts per million) and a strong change in the global carbon cycle already occurred ~100,000 years before the onset of intense volcanism. This suggests that large amounts of carbon dioxide were already released by the thermal heating of organic-rich sediments by intrusion of magma bodies from the mantle on their way to Earth's surface.
Freshwater fish delta-18O indicates a Messinian change of the precipitation regime in Central Africa
Olga Otero et al., iPHEP (Institut de Paleoprimatologie, Paleontologie humaine: Evolution et Paleoenvironnements), UMR CRNS 6046, Universite Poitiers, SFA, 40 Avenue du Recteur Pineau, 86 022 Poitiers cedex, France; doi: 10.1130/G31212.1.
The Neogene has long been recognized as the epoch during which the modern world emerged, notably with global changes that led to the modern glacial regime and the origin and rise of the human lineage. However, Neogene environmental change remains unknown in numerous continental areas including in the hominid-bearing Central African region. There, the Neogene fossil record consists of four Chadian fossil assemblages dated ca. 7.04 million years ago, 5.26 million years ago, 3.96 million years ago, and 3.58 million years ago. The oldest and the youngest yielded, respectively, our oldest ancestor, Sahelanthropus tchadensis, and the westernmost australopithecine, Australopithecus bahrelghazali. For the first time, the environmental change recorded in the Chadian localities through time is evidenced. Olga Otero of iPHEP, France, and colleagues use stable isotope ratios of oxygen in fossils of fish with a known ecological range. This allows the comparison of the delta-18O derived from freshwater at the four epochs, ranging from the late Miocene to the lower Pliocene. The results reflect an unambiguous change in the water cycle in Central Africa during the late Neogene. It is interpreted as a constant drying trend along the four successive wet episodes registered in Chad, with a main shift during the Messinian in relation to an east-African massive uplift.
Erosional control of the kinematics of the Aconcagua fold-and-thrust belt from numerical simulations and physical experiments
Leonardo Cruz et al., Dept. of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA; doi: 10.1130/G31675.1.
Leonardo Cruz of Stanford University and colleagues investigate the impact of erosion on the geometry of the Aconcagua mountain belt in central Argentina using analytical, sandbox, and numerical models. This investigation uses a realistic erosion rule that combines well-known and established mountain-belt mechanics with an erosion rule that considers the effect of river erosion. Cruz and colleagues show that theoretical predictions of the mountain-belt geometry, as well as the geometry predicted by both sandbox and numerical experiments, are internally consistent and correctly predict the interpreted and measured field geometries. The results indicate that, to explain the observed and interpreted geometries in the Aconcagua mountain belt, the system requires relatively high erosion during the early stages of deformation and relatively low erosion during the latter stages. This is consistent with the progressive exposure of different rock types during the different stages of deformation.
Assembly and breakup of the core of Paleoproterozoic-Mesoproterozoic supercontinent Nuna
David A.D. Evans and Ross N. Mitchell, Dept. of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, Connecticut 06520-8109, USA; doi: 10.1130/G31654.1.
David A.D. Evans and Ross N. Mitchell of Yale University propose a reconstruction of the ancient supercontinent Nuna (sometimes called "Columbia") that existed between about 1.8 and 1.5 billion years ago. The reconstruction is guided by both geological and paleomagnetic data, and includes direct juxtapositions of Laurentia (North America plus Greenland), Baltica (northern Europe), and Siberia. More speculative connections with fragments of Australia, North China, West Africa, and the Amazon region are also proposed. Nuna might have been Earth's first supercontinent, assembling via numerous collisions of subcontinental-sized crustal fragments. Nuna's breakup and the tectonic transition to its successor landmass, Rodinia, appear to be less mobile than the subsequent, profound tectonic transition between Rodinia and Gondwanaland/Pangea.
Speleothems and mountain uplift
Michael C. Meyer et al., Institut fur Geologie und Palaontologie, Universitat Innsbruck, Innrain 52, 6020 Innsbruck, Austria; doi: 10.1130/G31881.1.
In one of his songs, Bob Dylan asks "How many years can a mountain exist before it is washed to the sea?" and thus poses an intriguing geological question for which an accurate answer is not easily provided. Mountain ranges are in a constant interplay between climatically controlled weathering processes and the tectonic forces that cause folding and thrusting, and thus thickening, of Earth's crust. While erosion eventually erases any geological obstacles, tectonic forces are responsible for piling and lifting up rocks, and thus for forming spectacular mountain landscapes such as the European Alps. In reality, climate, weathering, and mountain uplift interact in a complex manner and quantifying rates for erosion and uplift, especially for the past couple of millions of years, remains a challenging task. Michael C. Meyer of Universitat Inssbruck, Austria, and colleagues report on ancient cave systems discovered near the summits of the Allgau Mountains (Austria) that preserved the oldest radiometrically dated dripstones currently known from the European Alps. "These cave deposits formed about 2 million years ago and their geochemical signature and biological inclusions are vastly different from other cave calcites in the Alps," says Meyer. By carefully analyzing these dripstones and using an isotopic modeling approach, Meyer and colleagues were able to back-calculate both the depth of the cave and the altitude of the corresponding summit area at the time of calcite formation. They thus derived erosion and uplift rates for the northern rim of the Alps and - -most critically -- for a geological time period that is characterized by recurring ice ages and, hence, by intensive glacial erosion. "Our results suggest that 2 million years ago the cave was situated approximately 1500 meters below its present altitude and the mountains were probably up to 500 meters lower compared to today," says Meyer. These altitudinal changes were significant and much of this uplift can probably be attributed to the gradual unloading of the Alps due to glacial erosion.
High-precision 40Ar/39Ar age of the gas emplacement into the Songliao Basin
Hua-Ning Qiu et al., Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences Wuhan), Ministry of Education, Wuhan 430074, China; doi: 10.1130/G31885.1.
It is difficult to obtain an exact isotopic age of hydrocarbon emplacement because minerals suitable for dating with isotopic methods are lacking. The igneous quartz from the Cretaceous volcanic rocks that host the gas reservoir in the Songliao Basin (northeastern China), contains abundant potassium-rich secondary fluid inclusions with high methane pressures trapped during gas emplacement, providing an excellent closed system, well suited for 40Ar/39Ar dating. Three igneous quartz samples were measured by stepwise crushing to release the inclusions. All three samples yielded well-defined isochrons with ages in close agreement, precisely constraining the gas emplacement at 42.4 plus or minus 0.5 million years ago below the Daqing oil field in the Songliao Basin, extending possible gas reservoirs from the upper Cretaceous to the middle Eocene.
Abrupt millennial-scale changes in intensity of Southern Hemisphere westerly winds during marine isotope stages 2-4
Thomas E. Whittaker et al., Dept. of Chemistry, University of Waikato, Private Bag 3105, Hamilton, New Zealand; doi: .10.1130/G31827.1.
The strength of mid-southern latitude westerly atmospheric circulation plays an important role in global climate change. Because of a lack of long, continuous, high-resolution paleoclimate archives from mid-southern latitudes, it remains unclear what factors control changes in its intensity and how past changes affected climates of landmasses in their path. A stalagmite from South Island, New Zealand, reveals at decadal to centennial scale, how westerly intensity varied between 73 and 11 thousand years ago. The growth rate and geochemistry of the stalagmite are sensitive to changes in annual rainfall, a factor controlled by westerly intensity. These proxies display long-term trends supporting existing evidence that weaker westerlies predominated over South Island during the last glacial period and that climate was relatively dry. The short-term trends reveal for the first time that abrupt millennial-scale wetter periods frequently interrupted the drier climate, such that the stalagmite record resembles those from Greenland ice cores. Authors Thomas E. Whittaker of the University of Waikato and colleagues conclude that the near-synchronous timing of the nine most prominent wetter periods with massive iceberg discharge events in the North Atlantic suggests that increased westerly intensity is closely linked to North Atlantic cooling, and that climate change during the last glacial period probably was globally synchronous.
Did the growth of Tibetan topography control the locus and evolution of Tien Shan mountain building?
Alan R.A. Aitken, School of Geosciences, Monash University, Wellington Road, Clayton, Victoria 3800, Australia; doi: 10.1130/G31712.1.
Alan R.A. Aitken of Monash University presents a new hypothesis for the building of the Tien Shan in central Asia. This mountain range is unusual because it formed in Asia in response to the distant collision of India with Eurasia. A popular model involving north-directed indentation of the strong Tarim Basin region into the weak Tien Shan region fails to fully explain the location and timing of Tien Shan mountain building. It is well known that the growth of Tibet's northern margin to 5 km in elevation has caused cantilever-like bending of Earth's crust, recorded in the sedimentary rocks of the Tarim Basin. This bending generates stretching forces that inhibit mountain building within 500 to 600 kilometers of Tibet, and compressive forces that act to enhance mountain building along the Tibet parallel axis of the Tien Shan, located 600 to 700 kilometers from Tibet. Furthermore, the timing of major mountain-building events on the Tibetan Margin is echoed in the Tien Shan, and bending provides a causal basis for this synchronization. Bending of Earth's crust in response to mountain building is common, and this phenomenon may be of general importance in controlling where and when mountains are built within continents.
Numerical analysis of subduction initiation risk along the Atlantic American passive margins
Ksenia Nikolaeva et al., Dept. of Geosciences, Swiss Federal Institute of Technology (ETH-Zurich), CH-8092 Zurich, Switzerland; doi: 10.1130/G31972.1.
Cooling of the oceanic plate with time leads to an increase in its density. At one stage, the density of the plate exceeds the density of the material underneath it (the asthenosphere) and, thus, the plate can sink spontaneously under its own weight (subduction starts). However, despite the fact that some seafloors are about 170 million years old, an undeniable Cenozoic example of such sinking of the oceanic plate starting at the continental margin is unknown. As suggested by recent numerical experiments, spontaneous subduction initiation may have a hidden initial phase not expressed in diagnostic features such as trenches and magmatic arcs. Ksenia Nikolaeva of the Swiss Federal Institute of Technology and colleagues analyze the probability of subduction initiation along the Atlantic American passive margins based on their topography and internal structures. According to their experimental results, proper subduction will likely start during the next 10 to 20 million years along the southern part of the Brazilian margin, while other Atlantic margins of North and South America are stable under the present geodynamic conditions.
Recycling of Amazon floodplain sediment quantified by cosmogenic 26Al and 10Be
H. Wittmann et al., GFZ German Research Center for Geosciences, 14473 Potsdam, Germany; doi: 10.1130/G31829.1.
Rivers that drain floodplain areas incorporate both fresh sediment from the eroding mountain headwaters and old, weathered sediment from the floodplain, which is being recycled when the river changes course. The proportions of both reservoirs being mixed by the river can now be determined by using isotopes that are produced by cosmic rays in mineral grains. These are the cosmogenic, in situ-produced aluminum-26 and beryllium-10. The ratio of the nuclides is sensitive to sediment burial, as these nuclides decay at different rates. H. Wittmann of the German Center for Geosciences and colleagues found that sediment leaving the Amazon Basin is a mixture of never-buried sediment from the Andes and sediment that has spent up to several million years in storage in the floodplain -- the latter contributing surprisingly high amounts, up to 40%, of the total sediment flux discharged to the ocean by the Amazon River.
High pore pressures and porosity at 35 km depth in the Cascadia subduction zone
Simon M. Peacock et al., Dept. of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada; doi: 10.1130/G31649.
A major plate boundary, the Cascadia subduction zone, extends along the west coast of North America, from northern California to southern British Columbia. Along this plate boundary, the oceanic Juan de Fuca plate descends eastward beneath the North American continental plate at a rate of a few centimeters per year. Along the shallow part of the plate boundary, stresses accumulate over a period of centuries until these stresses are released violently in a large (magnitude 9) earthquake. At greater depths, around 35 kilometers, plate boundary stresses are released more gradually in a phenomenon called "episodic tremor and slip." Simon M. Peacock of the University of British Columbia and colleagues use seismic waves to illuminate the region of episodic tremor and slip. Combined with laboratory experiments, their results show that these crystalline rocks contain far more water than generally expected at these depths. The seismic data are best explained by a 5-km-thick layer with a porosity of 2.7 to 4.0 volume percent filled with water. These results provide insight into how plate boundaries work at depths far greater than can be reached by modern drilling techniques.
Re-Os geochronology and fingerprinting of United Kingdom Atlantic margin oil: Temporal implications for regional petroleum systems
Alexander J. Finlay et al., Dept. of Earth Sciences, Durham University, Science Laboratories, Durham DH1 3LE, UK; doi: 10.1130/G31781.1.
Alexander J. Finlay of Durham University and colleagues use the rhenium-osmium radioisotope system to date oil in the United Kingdom Atlantic margin. They show that rhenium-osmium geochronology records the timing of oil generation, and that oil was generated at about 68 million years ago. They also suggest that osmium isotopes can be used to identify the source of the oil.
Integration of GRACE (Gravity Recovery and Climate Experiment) data with traditional data sets for a better understanding of the time-dependent water partitioning in African watersheds
Mohamed Ahmed et al., Dept. of Geosciences, Western Michigan University, 1903 West Michigan Avenue, Kalamazoo, Michigan 49008, USA; 10.1130/G31812.1.
The Gravity Recovery and Climate Experiment (GRACE) satellite mission was launched to map temporal variations in Earth's global gravity field that are largely related to changes in the distribution of water and snow. Spatial analysis of monthly GRACE gravity field solutions acquired (August 2002 through July 2008) over the River Nile, Niger, and Congo basins, together with relevant data sets (e.g., topography, geology, remote sensing), showed (1) large persistent anomalies on standard deviation images over periods of 2 to 7 years; (2) that anomalous areas originate at mountainous source areas that receive high precipitation, extend down slope toward mountain foothills, and often continue along main channels, wetlands, or lakes that drain these areas; and (3) that seasonal mass variation lags behind seasonal precipitation from time-series analyses over anomalous areas. Results presented by Mohamed Ahmed of Western Michigan University and colleagues indicate that observed temporal mass variations are largely controlled by elements of the hydrologic cycle (e.g., runoff, infiltration, groundwater flow) and have not been obscured by noise as was previously thought, and that GRACE data could be used to investigate the temporal local responses of a much larger suite of hydrologic systems (watersheds, lakes, rivers, marshes, etc.) and domains (e.g., source areas, lowlands) within watersheds and subbasins worldwide.
Belemnite extinction and the origin of modern cephalopods 35 m.y. prior to the Cretaceous--Paleogene event
Yasuhiro Iba et al., Institut fur Geologie, Mineralogie und Geophysik, Ruhr-Universitat Bochum, 44801, Germany; doi: 10.1130/G31724.1.
Belemnites, a very successful group of Mesozoic cephalopods, flourished in Cretaceous oceans until the Cretaceous-Paleogene event, when they became globally extinct. Following this event, modern types of cephalopods (squid, cuttlefish, octopus) radiated in all oceans in the Cenozoic. In the North Pacific, however, a turnover from belemnites to modern types of cephalopods about 35 million years before the Cretaceous-Paleogene event documents a more complex evolutionary history of cephalopods than previously thought. Yasuhiro Iba of Ruhr-Universitat Bochum and colleagues show that the modern types of cephalopods originated and prospered throughout the Late Cretaceous in the North Pacific. The mid-Cretaceous cephalopod turnover was caused by cooling and the closure of the Bering Strait, which led to a subsequent faunal isolation of this area. In the Late Cretaceous, the former niches of the fast-swimming belemnites were taken over by modern cephalopods, which evolved endemically. The Cretaceous-Paleogene event allowed the modern cephalopods to spread globally and to take over the niches previously held by belemnites.
Paleomagnetic and geochronological evidence for large-scale post-1.88 Ga displacement between the Zimbabwe and Kaapvaal cratons along the Limpopo belt
Richard E. Hanson et al., Dept. of Geology, Texas Christian University, Fort Worth, Texas 76129, USA; doi: 10.1130/G31698.1.
Richard E. Hanson of Texas Christian University and colleagues present new isotopic dates for two intrusions within the Mashonaland igneous province, which was emplaced over a large part of an ancient crustal block present in what is now Zimbabwe, in southern Africa. These new dates, and other recently published data, indicate that the Mashonaland province was emplaced 1.88 billion years ago, at the same time as a major igneous province in a separate crustal block farther south, in what is now South Africa. At first glance, these data suggest that the two igneous suites are part of a single large igneous province that covered both crustal blocks. However, primary magnetization directions preserved in the igneous rocks indicate that the two crustal blocks were far apart at the time the igneous rocks were emplaced. This observation can best be explained by invoking >2000 kilometers of lateral displacement between the two crustal blocks at some time after 1.88 billion years ago. The displacement appears to have occurred along an ancient, deeply eroded mountain belt (the Limpopo belt), which separates the two crustal blocks. Hanson et al. provide the first evidence that significant plate motion occurred along the Limpopo belt in this time frame.
Shallow-water records of astronomical forcing and the eccentricity paradox
David B. Kemp, Dept. of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK; doi: 10.1130/G31878.1.
Periodic changes in Earth's orbit around the sun are often believed to be responsible for influencing climate and driving changes in sea level. Geologists can sometimes recognize these astronomically driven cycles of sea-level change in the geological record as cyclic changes in the type of sedimentary rock deposited. A paradox regarding this mechanism is that the type of astronomical cycles commonly found in the geological record often does not match very well with those that are known to influence climate. In particular, 100,000-year cycles in the eccentricity of Earth's orbit have only a weak influence on climate, but are sometimes strongly present in ancient rock successions. If astronomical cycles in sea level result in the periodic exposure and erosion of deposited sediment, however, then the preserved cyclicity will be distorted. These distortions can be shown to be capable of resulting in an increase in the strength of eccentricity cycles. In this study by David B. Kemp of the University of Cambridge, this effect is illustrated using a simple numerical model of sedimentation. It can be easily demonstrated, using models such as these, that the presence of gaps in the geological record has the potential to significantly influence the way climate signals are recorded.
Infaunal molting in Trilobita and escalatory responses against predation
Juan J. Rustán et al., CICTERRA-CIPAL (Centro de Investigaciones en Ciencias de la Tierra, Centro de Investigaciones Paleobiologicas, CONICET, Universidad Nacional de Cordoba), Avenida Velez Sarsfield 299, C.P. 5000 Cordoba, Argentina; doi: 10.1130/G31879.1.
Some intervals of geologic time are characterized by an intensification of adaptive trends among marine prey groups, driven by the macro-evolutionary impact of increased predation pressure. For these intervals, interpreted as representing paleoecological revolutions, the escalation hypothesis strongly predicts, among other kind of defenses, the evolution of active escape behavioral strategies in prey. Unraveling these evolutionary trends is a major challenge because evidence of behavior is extremely rare in the fossil record. Although trilobites are the main vagrant Palaeozoic invertebrates, and hence an ideal case to test these defensive skills, behavioral escalatory trends have yet to be recognized in this group. Juan J. Rustan of the Universidad Nacional de Cordoba, Argentina, and colleagues report compelling evidence of infaunal molting in a Late Silurian-Early Devonian trilobite lineage, which accounts for a hiding, anti-predatory adaptation. In addition, strengthening of the carapace and spinosity indicate an evolutionary trend toward morphological defensive strategies. These active and passive escalatory traits provide unequivocal insight into the ecological role of trilobites as a major prey group in the context of the global diversification of predators during the Middle Palaeozoic marine revolution bioevent 420 to 400 million years ago.
Experimental evidence for the alteration of the Fe3+/sigmaFe of silicate melt caused by the degassing of chlorine-bearing aqueous volatiles
Aaron S. Bell and Adam Simon, Dept. of Geoscience, and High Pressure Science and Engineering Center, University of Nevada, 4505 Maryland Parkway, Las Vegas, Nevada 89154-4010, USA; doi: 10.1130/G31828.1.
The temporal evolution of the redox conditions undergone by degassing magmas has petrologic implications for magmatic phase equilibria. Aaron S. Bell of the University of Nevada at Las Vegas and colleague Adam Simon present new experimental data that demonstrate that a chlorine-bearing magmatic volatile phase may affect the concentration and activity of ferrous iron in silicate liquid via the preferential mass transfer of Fe2+ into the volatile phase.
Body wave attenuation heralds incoming eruptions at Mount Etna
Pasquale De Gori et al., Istituto Nazionale di Geofisica e Vulcanologia, CNT (Centro Nazionale Terremoti), Via di Vigna Murata 605, 00143 Rome, Italy; doi: 10.1130/G31993.1.
In Earth's brittle crust, rock fracturing, fluid migration, and pore pressure buildup occur during the preparatory phases of an earthquake and a magmatic intrusion. These phenomena produce strong transient variations of the elastic parameters within wide rock volumes that can be studied by seismic wave propagation. The continuous volcanic and seismic activity at Mount Etna (Italy) make this volcano an important laboratory for seismological and geophysical studies. Using attenuation seismic tomography repeated in time, Pasquale De Gori of Italy’s Centro Nazionale Terremoti and colleagues have detected variations in the elastic properties of the volcanic structure, using data of different volcanic cycles during the July 2001 and October 2002 flank eruptions and during the inter-eruptive period. They observed a clear variation of attenuation of seismic body waves through time. In particular, De Gori and colleagues note that attenuation is very high when the intruding dike, which will feed the incipient eruption, is upraised in the shallow part of the volcano. Furthermore, from the analysis of temporal series of attenuation data, they observed a drastic increase in attenuation a few days before the 2001 eruption, while this occurred a few hours before the eruption occurred in 2003, delineating different magma upwelling and different conditions within the volcanic conduit for the two eruptions.
Rapid wetland expansion during European settlement and its implication for marsh survival under modern sediment delivery rates
Matthew L. Kirwan et al., Patuxent Wildlife Research Center, U.S. Geological Survey, Charlottesville, Virginia 22904, USA; doi: 10.1130/G31789.1.
Although most coastal wetlands are thought to have formed thousands of years ago during a time of slow sea-level rise, Matthew L. Kirwan of the U.S. Geological Survey’s Patuxent Wildlife Research Center and colleagues document a recent expansion of coastal wetlands that is unrelated to sea-level rise. Kirwan and colleagues propose that logging of forests in the 1700s and 1800s eroded soil, which was eventually deposited in estuaries and created marshland. Radiocarbon dating and numerical modeling from the Plum Island Estuary in Massachusetts indicates that about a third of the marshland there formed after European settlement. Kirwan and colleagues propose that because these marshes formed under artificially high sediment delivery rates, the loss of coastal wetlands today could be due to a reduction in sediment loads in rivers, rather than sea-level rise. If true, loss of coastal wetlands today could simply represent a return to more natural conditions.