BOULDER, Colo.- The Geological Society of America's April issue of GEOLOGY contains a number of newsworthy items. Topics include new evidence supporting the cool early Earth hypothesis; the discovery of an underground magma source for a Pacific Ocean volcano near Washington state; research revealing that the Beijing area was covered by a sea within the last 80 ky -- about 30 times younger than was thought; the definition of the location of the heretofore “elusive” Nubia-Somalia plate boundary; and the discovery of a natural 'plastic wrap' that beautifully fossilized certain Silurian ocean-going animals.
Highlights from GEOLOGY and a summary of the science article for the April GSA TODAY are provided below. The GSAT topic is a timely one: The volcanic risks of the Yucca Mountain site. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in stories published. Contact Ann Cairns at acairns@geosociety.org for copies of articles and for additional information or other assistance.
Carboniferous-Triassic subduction and accretion in the western Kunlun, China: Implications for the collisional and accretionary tectonics of the northern Tibetan Plateau.
Wen Jiao Xiao et al. Laboratory of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China. Pages 295-298.
The western Kunlun mountain range in Southwest China provides new and important information on the tectonic evolution of the northwestern margin of the Tibetan plateau. From ca. 350-200 Ma (in Carboniferous to Triassic time) a huge (250 km by 500 km) subduction-accretion complex, the Mazar accretionary prism, formed during closure of the Paleotethyan Ocean. The accretionary prism is similar to those that are forming underwater off the eastern side of Japan today. Such an accretionary prism forms when pieces of oceanic crust and overlying sediments are scraped off a subducting oceanic slab and are dumped into a trench in the throat of a subduction zone. The Mazar accretionary prism demonstrates that a large part of the western Kunlun developed by hitherto unrecognized subduction-accretion processes.
Jiggling the tropical thermostat in the Cretaceous hothouse.
Richard D. Norris et al. Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543-1541, USA. Pages 299-302.
Computer simulations of modern climate have been used to suggest that Earth's surface temperature is set by a thermostat produced by feedback systems between incoming radiation from the sun, cloud formation, and radiated heat from Earth's surface. In theory, average tropical surface temperatures should not climb much above 30 °C (86 °F). Paleoclimate studies have previously shown that some subtropical locations on Earth regularly experienced sea surface temperatures of ~30 °C or even a few degrees higher during warm periods in Earth history. These results raise the possibility that tropical temperatures (which should be warmer than the subtropics) exceeded the thermostat value predicted by climate models. The authors present the first solid evidence that Earth's thermostat was, as of 90 Ma, set ~5 °C warmer than it is today. Their data come from analysis of exceptionally well preserved planktic foraminifera (a kind of marine microfossil) from a deep-sea drill site in the tropical Atlantic. The authors’ analysis of the light stable isotope geochemistry of the foraminifer shells suggest that tropical sea surface temperatures were ~35 °C (95 °F) during the Cenomanian-Turonian “hothouse.” The authors conclude that Earth's thermostat may not be so reliable after all.
Climatic conditions during marine oxygen isotope stage 6 in the eastern Mediterranean region from the isotopic composition of speleothems of Soreq Cave, Israel.
Avner Ayalon et al. Geological Survey of Israel, 30 Malchei Israel Street, Jerusalem 95501, Israel. Pages 303-306.
At several times during marine oxygen isotope stage 6, the Eastern Mediterranean region was influenced by two extreme climatic systems: the large ice sheet over northern Europe and the wet tropics associated with African monsoons. During this interval, two major climatic events occurred in the region ca. 176 ka and ca. 151 ka. The isotopic composition of Soreq Cave speleothems seems to record these events as very low delta18O-delta13C values, which are typical of interglacial intervals, but here they were recorded during a glacial interval. Such low peaks indicate that in this part of the Eastern Mediterranean region, i.e., Israel, the rainfall amount increased dramatically. Moreover, the isotopic record of the speleothems also shows that during the entire stage 6, although the climate was as cold as during large parts of the last glacial, the conditions were never as dry.
Cathodoluminescence record of K-metasomatism in ash-flow tuffs: Grain-scale mechanisms and large-scale geochemical implications.
James R. Rougvie and Sorena S. Sorensen, Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560-0119, USA. Pages 307-310.
Volcanic rocks such as ash-flow tuffs are susceptible to alteration in low-temperature near-surface environments. Potassium metasomatism is a process that drastically alters rock compositions. It occurs when saline fluids add certain elements (such as potassium) and remove others from rocks. It is widespread in the western United States and may be a primary pathway of alteration of the upper crust of the earth in arid lands. Cathodoluminescence petrography of K-metasomatized ash-flow tuffs quickly yields information about the mineralogical changes and reaction mechanisms that take place when saline waters flow through volcanic rocks, and links these properties to geochemical changes in the rocks. Tertiary volcanic rocks from Creede, Colorado, Socorro, New Mexico, and the Harcuvar Mountains, Arizona, were altered in different geologic settings, yet have very similar cathodoluminescence textures and chemistry.
Analogue modeling of arc and backarc deformation in the New Hebrides arc and North Fiji Basin.
W.P. Schellart et al. Epsilon Laboratory, Australian Crustal Research Centre, School of Geosciences, P.O. Box 28E, Monash University, Melbourne, Victoria 3800, Australia. Pages 311-314.
Backarc basins are continental or oceanic basins up to several kilometers deep that have horizontal dimensions of hundreds to several thousands of kilometers. They occur on the concave side of arc-shaped topographic features. Backarc basins form in overall convergent tectonic settings in the overriding plate during rollback of the subducting plate. In most backarc basins extension is perpendicular to the arc. Thus individual spreading ridges or rift segments run approximately parallel to the arc. In the North Fiji Basin, however, several ancient and active spreading ridges strike 70°- 90° to the New Hebrides arc. These high-angle spreading ridges relocated southward during the asymmetric opening of the North Fiji Basin. In order to better understand these remarkable features, the authors have simulated the structural development of the North Fiji Basin and the New Hebrides arc with small-scale laboratory models (analogue models), the results of which have inspired them to come to several tentative conclusions. The authors interpret the orientation of the high-angle spreading ridges to be related to the asymmetric opening of the backarc basin around a hinge, where they form close to the hinge. Relocation of these spreading ridges is most likely related to subduction of a buoyant topographic feature (the West Torres Plateau) along the New Hebrides Trench. This resulted in localized collision between the New Hebrides arc and the plateau, retarded rollback of the subducting slab along the northwest corner of the trench, and reduced extension and shearing in the northwest corner of the North Fiji Basin. Backarc extension continued in the rest of the North Fiji Basin due to continued rollback of the southern part of the subducting slab. Here, active extension was separated from the slightly- or nonextending northwest corner by a zone striking at high angle to the New Hebrides arc, i.e., the Hazel Holme extensional zone. Moreover, impingement of another topographic feature (the d'Entrecasteaux Ridge) into the overriding plate led to local deformation and fragmentation of the arc.
Migration of methane gas through the hydrate stability zone in a low-flux hydrate province.
Andrew R. Gorman et al. Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA. Pages 327-330.
New high-resolution seismic imaging of the Blake Ridge (off the southeast coast of the United States) shows evidence for the migration of methane gas through the uppermost ~500 m of seafloor sediments. This is remarkable because methane in this environment (called the hydrate stability zone) should either be dissolved in pore fluids or tied up in an ice-like structure called hydrate. The new seismic images suggest that methane gas travels upward along faults or vertical chimney pathways for several hundred meters before being frozen into hydrate. The significance of this observation is that in places where appropriate migration pathways exist, it is likely that a lot of gas would find its way from the region below the hydrate stability zone into the ocean. The cycling of methane between seafloor sediments, the ocean, and the atmosphere (where methane is a strong greenhouse gas) is an important part of understanding long-term climate variations, the potential of seafloor methane as a natural resource, and the risk of hydrates to conventional oil and gas production in the world's oceans.
Increase in evenness and sampled alpha diversity through the Phanerozoic: Comparison of early Paleozoic and Cenozoic marine fossil assemblages.
Matthew G. Powell, Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, USA, and Michal Kowalewski, Department of Geological Sciences, Virginia Tech, Blacksburg, Virginia 24061, USA. Pages 331-334.
How the diversity of life has changed through time remains one of the most fundamental scientific questions about the history of life on our planet. Therefore, achieving a diversity curve that accurately estimates changes in species diversity through time is of central importance to paleontology. Previously compiled diversity curves have contributed a wealth of information, such as the timing and magnitude of mass extinctions, the dynamics of postextinction recovery, and how organisms have responded to changes in the physical environment, to name but a few. However, complicating the issue is the fact that all estimates of diversity are compiled from fossil samples, which may be subject to various biases. One of the key and previously unexplored biases is the dependency of a diversity estimate on the underlying abundance distribution of species (the way in which the number of individuals are distributed among the sampled species). This study uncovers a systematic difference in the abundance distribution of species between early Paleozoic (545-420 Ma) and Cenozoic (65-0 Ma) fossil samples. Early Paleozoic samples are dominated by few genera (used as a proxy for species here), whereas Cenozoic samples are more evenly distributed. This pattern may reflect fundamental changes through time in the way ecosystems are organized, but it may also reflect various biases that mar the fossil record. Whatever its origin, this systematic change in the diversity structure of the fossil samples may lead to different estimates of sampled diversity even if the actual diversity does not change. Corrections for differences in diversity structure may ultimately give rise to a different interpretation of the marine Phanerozoic history of diversity.
New evidence of magmatic-fluid-related phyllic alteration: Implications for the genesis of porphyry Cu deposits.
Anthony C. Harris and Suzanne D. Golding, Department of Earth Sciences, University of Queensland, Queensland 4072, Australia. Pages 335-338.
Porphyry ore deposits form within and adjacent to porphyritic intrusions that are apophyses to larger stocks that have been water saturated within the upper several kilometers of the crust. The intrusions form huge hydrothermal systems of exsolved magmatic fluids with a carapace of convectively circulating meteoric water. They are important not only because they are the world’s largest source of Cu but also because they provide evidence for how magmas and their hydrothermal systems evolve at shallow depths. Since the 1970s debate has raged over the role meteoric water plays in the development of alteration assemblages. Typically, potassic alteration is related to high-temperature saline magmatic fluid, while the phyllic alteration that develops on the periphery is believed to be the result of mixing between this magmatic fluid and lower temperature meteoric waters. Here the authors report new stable isotope studies for the E26N porphyry Cu-Au deposit (Australia) that confirm high-temperature saline magmatic fluids are responsible for both potassic and mineralized phyllic alteration. These results are consistent with other recent studies (e.g., El Salvador, Chile, Far Southeast, Philippines, and Panguna and Porgera, Papua New Guinea) and suggest that, rather than these results being unusual, a major revision of porphyry Cu genetic models is required.
Location of the Nubia-Somalia boundary along the Southwest Indian Ridge.
James Lemaux et al. Department of Earth Science, Rice University, Houston, Texas 77005, USA. Pages 339-342.
When J. Tuzo Wilson published the first paper on plate tectonics in 1965, he argued convincingly that the boundaries between Earth's tectonic plates could not end abruptly. In conflict with his argument, however, Wilson’s world map of plate boundaries showed a single exception to this rule: the East African rift. The boundary between the Nubian (West African) and the Somalian (East African) plates ended abruptly at the southern end of the rift valley. For more than 35 years, the location of this boundary beyond the southeastern coast of Africa has remained a mystery. The authors’ analysis of geophysical data from the seafloor of the Indian Ocean finds the answer. The study shows that the enigmatic East African rift becomes a strike-slip boundary south of the African coast and eventually intersects a mid-ocean plate boundary known as the Southwest Indian Ridge, which marks the northern boundary of the Antarctic plate. The authors locate the boundary between the two African plates and the Antarctic plate within a 100-kilometer region of the ridge known as the Andrew Bain Fracture Zone. They also show why the exact location of the plate boundary remained a mystery for so long. The rate of movement between the African plates is very slow--only about two millimeters per year. Seafloor spreading typically ranges from 10-160 millimeters per year. The movement is slower between the African plates because both are moving away from the Antarctic plate at almost the same speed.
Clingfilm preservation of spiraliform graptolites: Evidence of organically sealed Silurian seafloors.
Helen Jones et al. Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK. Pages 343-346.
Small, delicate animals often fossilize beautifully. But why? Thimble-sized, spring-shaped, 400 Ma fossils of extinct ocean-going animals called spiraliform graptolites provide a clue. Preserved in marine mudrocks, these three-dimensional animals are typically flattened onto a single plane. It is as if they had been wrapped in clingfilm (plastic wrap) prior to burial, preventing mud and silt from entering the open spiral of the fossil. The authors consider that this ancient clingfilm was a combination of microbial films and gelatinous marine snow, and think that this now-vanished organic matter carpeted large areas of seafloors then. The nearest modern comparison can be found at the unpleasantly stagnant bottom of the Black Sea.
Phenocrysts versus xenocrysts in the youngest Toba Tuff: Implications for the petrogenesis of 2800 km3 of magma.
James E. Gardner et al. Geophysical Institute and Department of Geology and Geophysics, University of Alaska, Fairbanks, Alaska 99775-7320, USA. Pages 347-350.
Large explosive eruptions of thousands of cubic kilometers of magma represent the most horrific volcanic hazards to man. Understanding how such large bodies of magma accumulate beneath volcanoes could provide valuable insight into such eruptions. The authors have investigated the petrogenesis of the 2800 km3 of erupted magma at the Toba Tuff ~74 ka in Sumatra, Indonesia, using a combination of experimental petrology and 40Ar/39Ar dating of crystals from the tuff. The authors found that some crystals present in the magma do not crystallize experimentally from the magma at known temperatures and pressures. Those same crystals are also not in isotopic equilibrium with others that do grow experimentally. It thus appears that a large population of crystals in the tuff were foreign to the magma. Common grouping of the foreign crystals into clots implies that they came from the same source, most likely the walls of the Toba magma body. Isotopic ages and diffusive modeling suggest that the source of the foreign crystals was at least 1.5 Ma, and that some were entrained as recently as 10 years before the eruption. A new estimate of the storage conditions of the magma based on a revised crystal assemblage suggests that the magma resided at pressures of 100 -150 MPa. Previously it was thought that the Toba magma had accumulated at pressures as high as 400 MPa, but that estimate was derived from the composition of the foreign crystals.
A cool early Earth.
John W. Valley et al. Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA. Pages 351-354.
The time of earliest Earth was a period of mystery. There are no rocks known to have formed during the first 500 m.y. after the formation of the solar system and Earth. Only a few tiny isolated crystals of the mineral zircon have been dated to come from this period. It has commonly been assumed that the reason for this absence is that the surface of Earth was molten and subject to intense meteorite bombardment. However, the chemistry of these rare zircons tells another story. The chemical and isotopic composition of these ancient zircons suggests that liquid water was stable on the surface of Earth and that continents were beginning to form. Possibly there were oceans as early at 4,400 Ma. These results are consistent with the hypothesis of a cool early Earth, where the meteorite flux was less intense than previously thought. It is possible that life first evolved at this time, 900 m.y. earlier than the oldest known fossils.
Shallow-crustal magma chamber beneath the axial high of the Coaxial segment of Juan de Fuca Ridge at the source site of the 1993 eruption.
William Menke et al. Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA. Pages 359-362.
In 1993 a volcanic eruption occurred beneath the floor of the Pacific Ocean off the coast of Washington State. Molten rock first traveled almost 25 miles horizontally through Earth's crust, along the crest of the Juan de Fuca ridge, a prominent submarine mountain range. It then poured out onto the seafloor, heating the seawater much above its usual icy temperatures and causing a brief flowering of deep-sea life. The source of the lava has been a subject of considerable debate among volcanologists and geophysicists. Some scientists thought that it must have come from a volcano. However, other scientists argued that Axial volcano--the nearest to the eruption--was too far away and that, in addition, the chemistry of the two erupted lavas didn't really match. Results from a recent experiment resolve this issue, and show that a second, heretofore undetected volcanic system is responsible for the 1993 eruption. This underground magma reservoir was detected using a technology that is a scaled-up version of medical ultrasound imaging. The reservoir contains enough magma for several more eruptions. These results help us to better understand how undersea ridges like the Juan de Fuca form.
Towering sponges in an Early Cambrian Lagerstätte: Disparity between nonbilaterian and bilaterian epifaunal tierers at the Neoproterozoic-Cambrian transition.
Xunlai Yuan et al. Nanjing Institute of Geology and Palaeontology, Academia Sinica, Nanjing 210008, China. Pages 363-366.
This paper describes articulated sponge fossils from an exceptionally preserved biota in the Lower Cambrian--the Hetang biota from southern Anhui Province of South China. The Hetang biota is ca. 535-520 Ma--older than the celebrated Chengjiang and Burgess Shale biotas. Unlike the Chengjiang and the Burgess Shale, however, the Hetang biota is dominated by extraordinarily preserved sponge body fossils, towering sponges that dwarf many bilaterally symmetrical animals (bilaterians) living on the ocean floor in the Early Cambrian. The authors propose that nonbilaterians such as sponges and cnidarians were important players in benthic epifaunal communities at the Precambrian-Cambrian transition; it was not until Ordovician time, at least 50 m.y. later, that bilaterians advanced into the high tiers in benthic communities. This ecological shift parallels the reptile-mammal shift across the Mesozoic-Cenozoic transition and, according to the authors, was probably caused by a radiation of bilaterian animals in the Ordovician.
Fate of reflux brines in carbonate platforms.
Gareth D. Jones et al. School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK. Pages 371-374.
Groundwater circulation in carbonate rocks is important in understanding topics as diverse as contaminant transport, management of groundwater resources, ore genesis, hydrocarbon migration, and carbonate reservoir diagenesis. Reflux is a style of groundwater circulation set up by increasing the density of seawater, through restriction and evaporation, on carbonate platforms like the Great Bahama Bank. The authors’ numerical simulations demonstrate that brines of reflux origin continue to drive groundwater circulation when environmental conditions on the platform-top change and waters are no longer restricted. They term this previously unrecognized style of reflux "latent reflux" (latent in the sense of hidden or concealed). The discovery of latent reflux has implications for understanding the formation of replacement dolomite, which forms some of the worlds most prolific, but poorly understood hydrocarbon reservoirs. In addition, the authors’ simulation results provide a fresh perspective and a new model for the long-standing debate regarding the origin of brines in sedimentary basins.
Oxygen and neodymium isotope evidence for recycling of juvenile crust in northeast China.
Chun-Sheng Wei et al. Department of Earth and Space Sciences, Laboratory for Chemical Geodynamics, University of Science and Technology of China, Hefei 230026, China. Pages 375-378.
It has been long recognized from radiogenic neodymium and strontium isotopes that depleted mantle sources consist of recycled oceanic materials, but difficulty was encountered in identifying this signature by means of oxygen isotopes because of significant postemplacement hydrothermal alteration. Zircon is expected to preserve this signature because its oxygen is resistant to high-temperature hydrothermal alteration. This effect is illustrated by a combined Sm-Nd and O isotope study of whole-rock and mineral samples from a Mesozoic A-type granite in northeastern China. This A-type granite shows positive initial Nd isotope ratios with young Nd model ages of Neoproterozoic for whole-rock, and lower oxygen isotope ratios of zircon than the normal mantle. These results point to a magma derivation from partial melting of a newly mantle derived material that underwent surface-fluid hydrothermal alteration before magma generation. The combined O and Nd isotopic study argues for the recycling of former ocean crust, hydrated with seawater into the continent. This provides the first geochemical evidence for the recycling of juvenile crust by subduction and thus has important implications for understanding the formation and evolution of granitoids and young continental crust. Furthermore, oxygen isotope analyses of zircon and the common rock-forming minerals reveal that the granite underwent pre- and postmagmatic water-rock interactions in the history of its formation and evolution. The results demonstrate the potential use of O versus Nd isotopes in tracking the magma source and its evolution in granite petrology. This study also demonstrates how important it is to know zircon oxygen isotope compositions for igneous rocks that underwent a secondary alteration.
Beijing inundated by the sea within the past 80 k.y.: Nannofossil evidence.
Wuchang Wei, Scripps Institution of Oceanography, University of California, San Diego, California 92093, USA. Pages 379-381
Examination of published data reveals that a marine bed in Beijing can be dated as 80 Ka or younger on the basis of abundant nannofossils. This age is ~30 times younger than that published previously on the basis of magnetostratigraphic and biostratigraphic interpretations. The abundant nannofossils and foraminifers suggest that the sea inundated Beijing within the last 80 k.y. The very recent nature of this marine transgression has profound societal and geological implications and thus calls for new studies and thorough evaluation of all relevant data.
GSA TODAY
Episodic Volcanism and Hot Mantle: Implications for Volcanic Hazard Studies at the Proposed Nuclear Waste Repository at Yucca Mountain, Nevada
Eugene I. Smith et al. University of Nevada, Las Vegas, gsmith@ccmail.nevada.edu.
Since 1987, when Congress amended the Nuclear Waste Policy Act, the U.S. Department of Energy (DOE) has been studying only one site for storage of high-level radioactive waste. This site is at Yucca Mountain, about 160 km northwest of Las Vegas, Nevada. If built, this site will house 70,000 metric tons of spent nuclear fuel from U.S. commercial nuclear power plants and high-level radioactive waste from the DOE’s nuclear weapons. This material will be buried 300 m below the surface at Yucca Mountain. The waste needs to be contained for at least 10,000 years because of the extreme hazard to public health and the environment associated with these radioactive materials. 2002 is a critical time for scientific studies at Yucca Mountain. This year, the DOE recommended the site for licensing as a nuclear waste repository. President Bush approved this request. The next steps involve the possible congressional override of Nevada’s veto, a license application to the Nuclear Regulatory Commission, and, if a license is granted, construction (which could begin in 2006) and waste acceptance (in 2010). No matter what events occur, the site should be deemed safe in terms of possible release of contaminants in groundwater and possible future volcanic eruptions at the site.
This paper addresses the volcanic risks of the Yucca Mountain site. The region is still volcanically active; basaltic volcanism has occurred in the last 10 million years in the area and eight alkali basalt volcanoes ranging in age from approximately 1 million to 80,000 years have erupted within 50 km of the proposed repository. The way the probability of another eruption is calculated depends on whether this volcanism is episodic or not and the period of cyclicity of volcanism. These authors propose that the volcanoes near Yucca Mountain are part of a larger zone of basaltic volcanism that stretches from Death Valley, California, to the Lunar Crater field in central Nevada. Within this larger zone, volcanism is episodic and sustained by hot mantle, with three peaks of volcanism occurring: one between 9.5 and 6.5 million years ago (Ma), the second between 4.5 and 3.5 Ma, and the last between 1.5 and 0.5 Ma. At the present time, volcanism in the region is relatively quiet with only three eruptions occurring in the last 80,000 years. The authors make petrologic arguments that suggest that recurrence rates of volcanism used by the DOE may be underestimated.
Moreover, if models of hot mantle are correct, then volcanism is not dead and another eruption peak is possible. The authors suggest that future calculations of volcanic risk take into account higher recurrence rates and patterns of volcanism directly determined by examining the geological record. Despite decades of work on volcanism and other areas, there are still many unanswered questions related to the suitability of Yucca Mountain to store nuclear waste. Sound science should take precedence over politics and program requirements when making the decision whether to place a repository at Yucca Mountain.
To view the abstracts for these articles, go to: http://www.gsajournals.org.
To obtain a complimentary copy of any GEOLOGY article, contact Ann Cairns at acairns@geosociety.org.
Geological Society of America
Release No. 02-20
Contact: Ann Cairns
303-357-1056
acairns@geosociety.org