Opportunistic bivalves, high-flying diatoms, mirror-like faults, and petit-spot volcanism

(Press-News.org) Boulder, Colo., USA – New Geology studies posted online ahead of print on 6 Sept. 2013 cover faulting and strain; mineralogy; tsunamigenic earthquakes; the formation of banded iron formations by microbial processes; stalagmites in Vanuatu; garnets; the world's largest saltpan complex and one of the world's largest inland deltas; estuaries beneath ice sheets; volcanism; erosion; mirror-like faults; the Baltic Sea dead zone; and the first real-time record of a turbidity current associated with the 2011 Mw 9.0 Tohoku-Oki tsunami.

Highlights are provided below. Geology articles published ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary Geology articles by contacting Kea Giles 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. Contact Kea Giles for additional information or assistance.

Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.



Incipient faulting near Lake Pillsbury, California, and the role of accessory faults in plate boundary evolution
Amanda M. Thomas et al., Berkeley Seismological Laboratory, Dept. of Earth and Planetary Science, University of California, Berkeley, 307 McCone Hall, Berkeley, California 94720, USA. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34588.1.

Amanda M. Thomas and colleagues compare the spatiotemporal progression, geometry, and earthquake source characteristics of a zone of anomalous swarm seismicity between the Maacama and Bartlett Springs faults (California, USA) within the northern San Andreas fault system to both laboratory studies of fracture initiation and structural field observations of fault formation. The similarities between laboratory and field studies of incipient faulting and the earthquake swarms suggest that the seismic lineament on which the swarms occur is a fault in an early stage of development. Kinematic descriptions of faulting and models of fault system development suggest that the ability of existing faults to accommodate deformation across plate boundaries is governed by the length scales over which they accommodate stress. Many of the characteristics of juvenile fault zones, such as segmentation, geometric complexity, and the depth extent of faulting, act to reduce this length scale; this requires the reactivation of existing faults or production of accessory faults to accommodate deformation across the plate boundary.



Water in nominally anhydrous minerals: Implications for partial melting and strain localization in the lower crust
S.J. Seaman et al., Dept. of Geosciences, University of Massachusetts, 611 North Pleasant Street, Amherst, Massachusetts 01003, USA. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34435.1.

Nominally anhydrous minerals (NAMs) are those in which molecular water (H2O) or hydroxide (OH−) is not an essential part of the mineral structure. Even nominally anhydrous minerals such as quartz and feldspar, however, host trace amounts of molecular water or hydroxide, either occupying sites in the crystal structure or in fluid inclusions. In this contribution, by S.J. Seaman and colleagues, the effect of water stored in the abundant quartz and feldspar crystals of the lower crust is explored. Water that is liberated from the structure of these minerals during crustal deformation may collect on grain boundaries and allow the neighboring minerals to melt at a lower temperature than they would have in the absence of melting. Samples taken across a strain gradient in the 2.6-billion-year-old Stevenson granite from northern Saskatchewan show that with increasing deformation, water concentration in feldspar and quartz crystals decreases, and water concentration on grain boundaries increases. Thin, very fine-grained, multi-mineral films on crystals in highly strained samples is suggested to be the remnants of melt films that were generated as a result of lowering of melting temperature along grain boundaries and resulting coating of mineral grains by partial melt. Melt-rich grain boundaries may have facilitated further lower crustal deformation.



Localized tsunamigenic earthquakes inferred from preferential distribution of coastal boulders on the Ryukyu Islands, Japan
Kazuhisa Goto et al., International Research Institute of Disaster Science, Tohoku University, Aoba 6-6-40-102, Aramaki, Aoba-ku, Sendai 980-8579, Japan. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34823.1.

The occurrence of large earthquakes and tsunamis along the Ryukyu Trench is a subject of continuing interest, the key to which is the long-term geological record. Kazuhisi Goto and colleagues describe the clast size and spatial distributions of ~2,900 boulders on the reefs of the Ryukyu Islands, Japan, as markers of paleo-tsunamis and causative tsunamigenic earthquakes. Boulders of tsunami origin were observed only at a specific island group at the southern end, suggesting the local occurrence of tsunamigenic earthquakes there. In contrast, in the central to northern Ryukyu Islands, no evidence exists of tsunamis larger than those at the southern end of the Ryukyu Islands during the past 2,000-3,000 years. These islands have numerous boulders deposited by storm waves during the past 2,300 years or prior. Their spatial distribution has not been disturbed by large tsunamis. Results suggest that large tsunamis did not strike this area during that period, nevertheless that these regions are seismically active. Results show that coastal boulder deposits present great potential not only to ascertain the histories and effects of paleo-tsunamis but also to constrain the fault models of the causative earthquakes.



Iron formation carbonates: Paleoceanographic proxy or recorder of microbial diagenesis?
Clark M. Johnson et al., Dept. of Geoscience, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA; and National Aeronautics and Space Administration Astrobiology Institute, USA. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34698.1.

Banded iron formations, chemical marine precipitates that largely formed 2.5 billion years ago and supply the majority of the world's iron, have been used as proxies for ancient surface conditions on Earth. This proposal has been tested using Rb-Sr isotopes, which show that these rocks did not form in equilibrium with ancient oceans, isolated from ambient marine conditions. This in turn supports previous suggestions than the largest banded iron formations formed through record microbial processes after deposition, indicating that the majority of the iron used by humans in their daily lives has a "microbial fingerprint."



Multidecadal rainfall variability in South Pacific Convergence Zone as revealed by stalagmite geochemistry
J.W. Partin et al., Institute for Geophysics, Jackson School of Geosciences, University of Texas, Austin, Texas 78712, USA. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34718.1.

A new reconstruction of climate in the South Pacific during the past 446 years shows rainfall varied much more dramatically before the start of the 20th century than after. The finding is based on an analysis of a cave formation called a stalagmite from the island nation of Vanuatu. Vanuatu lies within the largest rain band in the southern hemisphere, the South Pacific Convergence Zone. The climate reconstruction shows a roughly 50-year cycle of rainfall in Vanuatu, toggling between wet and dry periods. The 50-year cycle of rainfall in Vanuatu does not appear to be linked to any external forces, such as changes in solar intensity. No correlation was found with the sun's regular 11-year cycle of intensity or the Little Ice Age, a multi-decade change in climate possibly caused by solar dimming. Instead, the researchers propose that the 50-year cycle, or Pacific Decadal Variability (PDV), arises from natural fluctuations in Earth's climate. The PDV causes the South Pacific Convergence Zone to shift northeast and southwest over time. At times, the zone is over Vanuatu (corresponding to wet times) and at others, it is farther to the northeast (corresponding to dry times).



Multistage garnet in high-pressure metasediments: Alpine overgrowths on Variscan detrital grains
Paola Manzotti and Michel Ballèvre, Géosciences Rennes, UMR-CNRS 6118, Université de Rennes 1, 35042 Rennes Cedex, France. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34741.1.

Garnet, a common mineral that is also sometimes used in jewelry, can record details of the history of the earth crust. Specifically, it grows when pressure and temperature increases, and some grains can even record several episodes of growth (these are called multistage garnets). One limitation of the use of multistage garnets is explored here. The rock studied has been sampled in a mountain belt (in the Italian Alps) resulting from the collision between two continents after subduction of the intervening ocean. In this rock, the second episode of garnet growth took place around detrital grains in a former sandy layer, when this layer was buried at large depths during the subduction/collision process.



Paleo-megalake and paleo-megafan in southern Africa
Joel E. Podgorski et al., Institute of Geophysics, ETH, Sonneggstrasse 5, 8092 Zurich, Switzerland. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34735.1.

The near juxtaposition of the Makgadikgadi Basin (Botswana), the world's largest saltpan complex, with the Okavango Delta, one of the planet's largest inland deltas, has intrigued explorers and scientists since the middle of the 19th century. The sometimes-hostile Makgadikgadi Basin, with its seasonal changes from vast barren wasteland to a complex of shallow lakes, and the remarkable Okavango Delta, with its rich biological diversity, both lie within the Kalahari Basin. For most of the year, the Makgadikgadi Basin is desiccated, with sand and large salt pans dominating the landscape. In contrast, the Okavango Delta comprises a large network of river channels, wetlands, and islands. It was clear from early observations that the Makgadikgadi Basin once contained a huge lake, paleo-Lake Makgadikgadi. Several authors have since speculated that this lake also covered wide regions to the north and west of the Makgadikgadi Basin. The interpretation presented here by Joel Podgorski and colleagues using unusually high-quality airborne geophysical data indicates that paleo-Lake Makgadikgadi extended northwestward at least into the region presently occupied by the Okavango Delta. The total area of paleo-Lake Makgadikgadi exceeded 90,000 square kilometers, larger than Earth's most extensive freshwater body today, Lake Superior. Airborne data also provide evidence for a paleo-megafan underlying paleo-Lake Makgadikgadi sediments.



Estuaries beneath ice sheets
Huw J. Horgan et al., Antarctic Research Centre, Victoria University of Wellington, Wellington, New Zealand. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34654.1.

Interactions between subglacial water and the ocean make the existence of estuaries beneath ice sheets likely. In some cases, subglacial water will flow only in one direction, in a manner akin to a river entering the ocean via a waterfall. In other cases, water and sediment will be exchanged both upstream and downstream of the ice sheet-ice shelf transition zone, and it is in this situation that the biological abundance and unique sedimentary deposition observed in subaerial environments is likely to be replicated, and additional phenomena peculiar to the cryosphere are likely to occur. Huw J. Horgan and colleagues present newly acquired geophysical data from the transition zone between the West Antarctic Ice Sheet and the Ross Ice Shelf. Using radio echo sounding, active source seismology, and kinematic GPS data, they image a likely estuary located beneath approx. 800 m of ice and many hundreds of kilometers from the open ocean.



Primary productivity controls on opportunistic bivalves during Early Jurassic oceanic deoxygenation
Bryony A. Caswell, Dept. of Environment, Earth and Ecosystems, Centre for Earth, Planetary, Space and Astronomical Research, The Open University, Milton Keynes MK7 6AA, UK; and Angela L. Coe Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34819.1; OPEN ACCESS online.

Results from 36,000 fossil shells in the Jurassic rocks near Whitby, England, show how marine bivalves were impacted by a period of rapid global warming and reduced oxygen levels 183 million years ago. Scientists from the Open University and Liverpool University have shown that whilst many seafloor animals disappeared or even became extinct during this event, one species, the bivalve Pseudomytiloides dubius was able to survive. This bivalve endured because it was small, reached sexual maturity quickly, and reproduced in large numbers. These adaptations are similar to the bivalve Mulinia lateralis that inhabits low oxygen areas of the seafloor today. The study shows that changes in bivalve body-size are linked with seawater molybdenum concentrations. This is likely to be because the molybdenum enhanced nitrogen-fixation by blue-green algae, stimulating growth of the algal community and thus the food available to Pseudomytiloides dubius. Over the last 50 years marine pollution and climate change have caused increases in the number of zones of low oxygen and this now affects 7% of ocean area. This research provides a case study for the long-term impacts of low oxygen and shows that it will probably cause changes in marine food webs, reduced body-size and decreased biodiversity.



Carbon dioxide emission to Earth's surface by deep-sea volcanism
Satoshi Okumura, Division of Earth and Planetary Materials Science, Dept. of Earth Science, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan; and Naoto Hirano, Center for Northeast Asian Studies, Tohoku University, Sendai 980-8576, Japan. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34620.1; OPEN ACCESS online.

Large amounts of carbon dioxide are transferred from Earth's interior to the surface by volcanism. On a geological time scale, the rate of carbon dioxide emission has controlled the evolution of Earth's atmosphere and climate as well as the dynamic processes that take place in the mantle and core. The total rate of natural carbon dioxide emission from Earth has been estimated on the basis of carbon dioxide flux from arc, mid-ocean ridge, and hotspot volcanism. However, previous estimates have overlooked the carbon dioxide emitted from a recently discovered type of volcanism -- petit-spot volcanism -- that occurs on the deep sea floor. Here, Satoshi Okumura and Naoto Hirano measure the carbon dioxide and water contents of glassy basalts produced by petit-spot volcanism. Based on these values and magma flux of petit-spot volcanism, Okumura and Hirano show that the rate of carbon dioxide emission from petit-spot volcanoes is a few percent of the carbon dioxide emissions from arc and mid-ocean ridge volcanism and up to 14% from hotspot volcanism. Thus, the contribution to the carbon cycle on Earth of the large amounts of carbon dioxide that have been emitted from the deep sea floor by petit-spot volcanism has not previously been recognized.



Shaping post-orogenic landscapes by climate and chemical weathering
Oliver A. Chadwick et al., Dept. of Geography, University of California–Santa Barbara, Santa Barbara, California 93106, USA. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34721.1.

The spacing of hills and valleys is a characteristic feature of Earth's landscapes. Geomorphologists explain differences in their spacings as a change in the balance between processes that smooth hills and processes that cut valleys. However, geologists' powers of explanation are limited because they do not know how these processes are modified by differences in rock type or vegetation or rainfall. In this study, Oliver Chadwick and colleagues isolate rainfall as a contributor to differences in hill-valley spacing by studying a climate gradient on granite rock in Kruger National Park, South Africa. The only major factor that varies among the sites they studied was rainfall. Chadwick and colleagues found that increasing rainfall favored the broadening and lengthening of hills such that wetter sites had fewer streams per unit area than drier ones. When rain falls on drier, less vegetated hills more of it runs off along the surface creating streams that erode into the sides of hills, whereas in wetter sites rainfall is intercepted by vegetation and more of it sinks into the soil and slowly oozes through the soil to stream channels. In the latter instance, stream cutting tends to be relegated to main channel ways rather than the sides of hills.



Mirror-like faults and power dissipation during earthquakes
Michele Fondriest et al., Dipartimento di Geoscienze, Università degli Studi di Padova, 35131 Padua, Italy Istituto Nazionale di Geofisica e Vulcanologia (INGV), 00143 Rome, Italy. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34641.1.

Shiny, "mirror-like" faults that reflect sunlight are widely observed geological features, especially in carbonate rocks, but their origin has never been established. In this paper, Michele Fondriest and colleagues demonstrate that these faults (1) form during earthquakes and (2) can be used as markers of seismic hazard in areas of active faulting. This conclusion is supported by (1) the analysis of natural faults with mirror-like surfaces in the Italian Southern Alps; (2) experiments performed with the most powerful earthquake simulator in use today (SHIVA: http://www.roma1.ingv.it/laboratori/laboratorio-hp-ht/usems-project/the-shiva-apparatus), which successfully reproduced mirror-like faults by imposing deformation conditions typical of earthquakes; and (3) the similarity between natural and experimental mirror-like faults. The conclusions presented here show that mirror-like faults are markers of seismic faulting, and this provides a definitive answer to a long-standing problem. Fondriest and colleagues note that "we are hopeful that it will lead to significant breakthroughs in our understanding of the structure and dynamics of seismic sources in the shallow crust. Moreover because active faults can also creep aseismically, the presence of mirror-like faults could be used to assess seismic hazard in areas with incomplete earthquake catalogues."



The influence of reefs on the rise of Mesozoic marine crustaceans
Adiël A. Klompmaker et al., Dept. of Geology, Kent State University, 221 McGilvrey Hall, Kent, Ohio 44242, USA; and Florida Museum of Natural History, University of Florida, 1659 Museum Road, P.O. Box 117800, Gainesville, Florida 32611, USA. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34768.1.

Modern reef ecosystems are under intense stress due to anthropogenic and natural causes, including ocean acidification, coral bleaching, storms, and diseases. A key question is what will happen to the associated fauna as reefs continue to decline in many parts of the world. This study by Adiel Klompmaker and colleagues investigates the causes of biodiversity of decapod crustaceans (crabs, lobsters, shrimp) during the Mesozoic Era (252-66 million years ago), a period with huge fluctuations in reef abundance. During this period, a revolution took place within the Decapoda: the biodiversity of shrimp and lobsters increased substantially, while true crabs, hermit crabs, and squat lobsters appeared for the first time in the fossil record and also diversified dramatically. The diversity of decapods is influenced by reef abundance: during the Late Jurassic, reef abundance and decapod diversity were both high. The subsequent collapse of reefs resulted in the extinction of many species of decapods, especially true crabs. This suggests a high extinction vulnerability of present-day, reef-associated decapods as modern reefs continue to deteriorate. Today, a substantial part of the biodiversity of decapods (perhaps thousands of species) is associated with coral reefs, and many species are obligatorily associates. Thus, this study highlights the importance of reef conservation.



Rapid high-amplitude variability in Baltic Sea hypoxia during the Holocene
Tom Jilbert and Caroline P. Slomp, Dept. of Earth Sciences (Geochemistry), Faculty of Geosciences, Utrecht University, P.O. Box 80.021, 3508 TA Utrecht, Netherlands. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34804.1.

The Baltic Sea today contains one of the ocean's largest "dead zones," with an area the size of West Virginia experiencing hypoxia (dissolved oxygen concentrations less than 2 ml/l). The extreme conditions in the Baltic today are thought to be caused by inputs of nutrients from wastewater and agricultural runoff during the 20th century. Using chemical data from sediment cores, a new study by Jilbert and Slomp shows that hypoxic conditions in the Baltic Sea have waxed and waned repeatedly throughout the last 8000 years. The authors suggest that climate variability may be responsible for these changes, but that "feedbacks" in the phosphorus cycle also play a role. However, the study also shows that the shift from pristine oxic conditions to widespread hypoxia in the 20th century occurred extremely rapidly, confirming the influence of human activities on the delicate ecosystem of the Baltic Sea.



High-flying diatoms: Widespread dispersal of microorganisms in an explosive volcanic eruption
Alexa R. Van Eaton et al., School of Geography, Environment and Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34829.1.

The products of explosive volcanism are not traditionally thought to contain primary fossils. However, researchers at Victoria University of Wellington in New Zealand have found that some microorganisms hitch a ride in airborne volcanic plumes, leaving behind a unique record of the volcanic source in far-traveled ash deposits. Alexa Van Eaton and colleagues provide evidence that the world's youngest super-eruption (Oruanui, New Zealand) transported significant amounts of microscopic, glass-shelled algae (diatoms) into the atmosphere from a lake overlying the volcanic vent area. By matching the diatom species present in ash deposits to those in larger clasts of erupted lake sediment, the study demonstrates that ~0.6 cubic kilometers of diatom remains were released into the atmosphere at the time of eruption (a volume comparable to the amount of magma discharged during the Mount St. Helens eruption in 1980). The study indicates that microbe dispersal may be an overlooked feature of many ancient and modern explosive eruptions involving water or sediment interaction. Furthermore, discovery of cell contents preserved in diatoms transported ~850 km downwind suggests that live microorganisms may survive the eruption process and spread through windborne plumes of volcanic ash. These findings provide a novel tool to characterize volcanic deposits in terms of their microfossil content, shedding light on source areas, environmental settings, extent of magma-water interaction, and potentially clarifying the origins of enigmatic volcanic ash layers.



Abundant bacterial magnetite occurrence in oxic red clay
Toshitsugu Yamazaki, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan; and Takaya Shimono, Atmosphere and Ocean Research Institute, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34782.1.

Magnetotactic bacteria produce chains of intracellular magnetite crystals and respond to an ambient magnetic field. Toshitsugu Yamazaki and Takaya Shimono demonstrate that magnetites produced by magnetotactic bacteria dominate magnetic mineral assemblages of pelagic red clay in the South and North Pacific. Red clay, which occupies ~40% of the global ocean floor, is the end-member sediment with the slowest depositional rate and minimal supply of organic matter, and thus the sediment column contains abundant dissolved oxygen and no oxic-anoxic transition zone exists. These finding implies that magnetotactic bacteria inhabit red clay, which conflicts with widespread interpretations of the ecology of magnetotactic bacteria, namely that they require low levels of oxygen to grow and produce magnetite, and that they sense and utilize the geomagnetic field to help them find optimal positions in a strong vertical chemical gradient near the oxic-anoxic transition zone. Previously, it was considered that bacterial magnetites occur only in sediments or water columns with a strong vertical chemical gradient.



Tsunami-generated turbidity current of the 2011 Tohoku-Oki earthquake
Kazuno Arai et al., Dept. of Earth Sciences, Graduate School of Science, Chiba University, 1-33 Yayoicho, Inage-ku, Chiba 263-8522, Japan; [Hajime Naruse (corresponding), Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan]. Posted online 6 Sept. 2013; http://dx.doi.org/10.1130/G34777.1.

Kazuno Arai and colleagues show the first real-time record of a turbidity current (sediment-laden deep-sea flow) associated with the 2011 Mw 9.0 Tohoku-Oki tsunami offshore Japan. After the 2011 Tohoku-Oki event, an anomalous event on the seafloor consistent with a turbidity current was recorded by sensors deployed off Sendai. Freshly emplaced deposits were collected from a wide area of seafloor off the Tohoku coastal region. Arai and colleagues analyzed these measurements and sedimentary records to determine conditions of the modern tsunami-induced turbidity current. The authors note that they "anticipate our discovery to be a starting point for the identification of tsunami-induced turbidity current deposits to estimate paleo-earthquake and paleo-tsunami recurrence intervals from geologic records."



INFORMATION:

http://www.geosociety.org