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GEOLOGY
Evidence for extensive, olivine-rich bedrock on Mars
Victoria E. Hamilton, University of Hawai'i, Hawai'i Institute of Geophysics and Planetology, Honolulu, HI 96822, USA; and Philip R. Christensen, Arizona State University, Department of Geological Sciences, Tempe, AZ 85287-1404, USA. Pages 433-436.
Adjacent to Syrtis Major, one of Mars's largest volcanoes, a large (~30,000 km2) ancient region was identified by the Mars Global Surveyor Thermal Emission Spectrometer as having an enrichment in the mineral olivine (the dominant mineral component of several Martian meteorites) relative to typical Martian basalts. Using new, high spatial resolution infrared data from the Mars Odyssey spacecraft's Thermal Emission Imaging System, geologists from the University of Hawaii and Arizona State University have discovered that the area of olivine enrichment is nearly four times larger than previously estimated. They find evidence that these are olivine-rich basalts that appear to be present in the form of in-place, layered rock units, and that they are being exposed by tectonic uplift and the erosion of younger rocks. These basalts may represent the early, primitive source of the Syrtis Major basalts (which contain less olivine), or a magmatic source distinct from that of other basaltic rocks in the region.
North Atlantic warming during global cooling at the end of the Cretaceous
Kenneth G. MacLeod, University of Missouri, Department of Geological Sciences, Columbia, MO 65211-1380, USA; et al. Pages 437-440.
This study demonstrates that regional expressions of climate can be markedly divergent on long time scales and that the geographic resolution of paleoclimatic data are now good enough to investigate these phenomena. Whereas regional climatic variability (e.g., patterns of drought and flooding, heat waves and cold snaps) are well-documented for historic climatic fluctuations, the possibility of regional differences other than latitudinal gradients have been largely ignored in studies of ancient greenhouse climates. Contrary to the expectation of global warming or global cooling implicit in most treatments of climate evolution over millions of years, this paper shows that the North Atlantic warmed by up to 6 °C during the interval from ~71 million years ago to 65 million years ago (the end of the Cretaceous greenhouse interval) when the South Atlantic cooled by about the same amount. Such regional differences are clearly important in decadal-scale climate fluctuations today and should be considered in studies of ancient greenhouse climates as well.
Interaction of pyroclastic density currents with human settlements: Evidence from ancient Pompeii
Lucia Gurioli, INGV Pisa, Pisa 56126, Italy; et al. Pages 441-444.
The 79 AD eruption of Vesuvius, Italy, caused widespread damage to Roman settlements, killing thousands in Pompeii where pyroclastic currents of hot ash, moving at hundreds of kilometres an hour, devastated the city. Integrated geologic and geophysical measurements permitted modelling of pyroclastic currents moving through the town. Buildings divided the pyroclastic current into streams following features such as walls and roads. Vortices formed down flow of obstacles or inside cavities, inducing localized but drastic temperature reductions due to ambient air entrainment. Although these perturbations affected only the lower part of the current and were localized, they could represent, in certain case, cooler zones within which chances of human survival are increased.
Cyclic climate fluctuations during the last interglacial in central Europe
Ulrich C. Müller, University of Tuebingen, Institute of Geosciences, Tuebingen 72076, Germany; et al. Pages 449-452.
This paper shows that natural cyclic changes in mean winter climates affected Central European environments during the last interglacial (127,000–110,000 years ago). By analogy with recent findings from the present Holocene interglacial, the trigger of the climate variability may have been changes in solar activity. The findings suggested natural cyclic changes to be a persistent feature of interglacial climates.
Linking desert evolution and coastal upwelling: Pliocene climate change in Namibia
Lydie M. Dupont, Universität Bremen, Geosciences, Bremen D-28334, Germany; et al. Pages 461-464.
Tracing vegetation change in Namibia during the initiation of the Northern Hemisphere glaciations showed strong linkages of the development of the Namib desert with changes in the Namibian (Benguela) Upwelling System and the latitudinal position of the South Atlantic high pressure cell. Dupont et al. show a high-resolution continuous record of Pliocene vegetation development in southern Africa. The study covers a period of almost 2 million years (from 3.5–1.7 million years ago) in which large changes in climate took place triggering rapid evolution.
U-Pb sensitive high-resolution ion microprobe ages from the Doushantuo Formation in south China: Constraints on late Neoproterozoic glaciations
Shihong Zhang, China University of Geosciences, School of Earth Sciences and Resources, Beijing 100083, China; Ganqing Jiang, University of Nevada–Las Vegas, Geosciences, Las Vegas, Nevada 89154-4010, USA; et al. Pages 473-476.
Severe Neoproterozoic glaciations were thought to have provided extreme environmental forces for the first appearances of animals in Earth history and were interpreted as the origin for the unusual Neoproterozoic negative carbon isotope excursions. The new U-Pb age data from the Neoproterozoic Doushantuo Formation in south China by Zhang et al. challenge these traditional interpretations. The age of ca. 621 Ma from the ash beds immediately above the Doushantuo cap carbonate directly overlying the Nantuo glaciogenic diamictite provides the upper limit for the younger of the two Neoproterozoic glaciations in south China, and, in combination with the existing global data set, implies that the traditionally interpreted Marinoan glaciation may not be globally synchronous. The age of ca. 555 Ma from the ash beds near the Doushantuo/Dengying boundary where a strongly negative carbon isotope excursion was documented challenges the interpretation that such an isotopic anomaly is related to global glaciation, as evidenced by (1) the proliferation of organisms within strata encompassing the negative ?13C excursion in south China and globally, (2) the lack of a ca. 555 Ma glacial record, and (3) the absence of stratigraphic evidence for sea-level change. The data call for alternative paleoceanographic models for the origin of Neoproterozoic ?13C excursions not clearly related to glaciation.
Constraints on landscape evolution from slope histograms
Matthew A. Wolinsky and Lincoln F. Pratson, Duke University, Earth and Ocean Sciences, Durham, NC 27708, USA; Pages 477-480.
The rugged topography of mountainous landscapes is sculpted by erosion over geologic time. This erosion is caused by surface processes acting on the landscape, such as rainfall runoff and landsliding, with different types of processes resulting in different landscape textures. Wolinsky and Pratson develop a method to identify which processes are acting in a particular landscape, using analysis of topographic slope statistics. This method can help geologists to deduce the imprint of past environments on a landscape.
Global dominance of coralline red-algal facies: A response to Miocene oceanographic events
Jochen Halfar, Universität Stuttgart, Institut für Geologie und Paläontologie, Stuttgart, BW 70174, Germany; and Maria Mutti, University of Potsdam, Geosciences, Potsdam, Brandenbourg 14415, Germany. Pages 481-484.
Faunal or floral community replacements are commonly observed throughout geologic history and can be attributed to a variety of climatic, plate tectonic, oceanographic, biogeographic, and evolutionary controls. Sea-level fluctuations and changes in temperature and nutrients are the most important controls over short- and medium-term time scales affecting shallow water carbonate environments. In their paper, Halfar and Mutti describe an example of a community replacement involving Miocene carbonate settings, which are characterized by a worldwide bloom of coralline red algae. Global distributions illustrate that from 16–11 Ma coralline red algal dominated settings reached peak abundances and commonly replaced coral-reef environments. Under similar temperatures, corals are well known to exist preferably in nutrient-poor settings, whereas coralline red algae can also be abundant under elevated nutrient levels. The dominance of red algae over coral reefs appears to have been triggered by globally strengthened temperature gradients associated with the establishment of the East Antarctic Ice Sheet leading to enhanced upward transport of nutrient-rich deep waters, introducing nutrients into the shallow oceans. Globally cooler temperatures following a climatic optimum in the early middle Miocene contributed to sustain the dominance of red algae and prevented the recovery of coral reefs, which are adapted to warm temperatures. The global shift in nearshore shallow-water carbonate producers to groups tolerant of higher nutrient levels provides further evidence for increased nutrient levels during that time interval and shows the sensitivity of shallow-water carbonates as indicators of past oceanographic conditions.
Interior channels in Martian valley networks: Discharge and runoff production
Rossman P. Irwin III, Smithsonian Institution, Center for Earth and Planetary Studies, National Air and Space Museum, Washington, DC 20013-7012, USA; et al. Pages 489-452.
Mars is now a cold, dry desert, but robotic satellites and rovers have returned new evidence of a warmer and wetter climate more than 3.5 billion years ago, when conditions may have been more favorable for life. Geologists at the National Air and Space Museum's Center for Earth and Planetary Studies, working with colleagues at the University of Virginia, have discovered 21 river channels in the dry Martian valleys, which provide new clues to this ancient climate. The researchers have determined that Martian rivers were about the same size as their counterparts on Earth, suggesting similar amounts of runoff from thunderstorms or rapid snowmelt. Until the discovery of channels, scientists could not determine the amount of water that had flowed through these valleys. Larger, periodic floods carve wider river channels, so by measuring the width of a channel, geologists can estimate the size of the flood that carved it. Larger watersheds gave rise to larger river channels, as they do on Earth. Even using a conservative method to estimate the amount of water discharged through the rivers, the Martian rivers matched their terrestrial counterparts in terms of the volume of water per second during these ancient episodes of flow. Previously, only eight river channels had been found in Martian valleys, two of these by Irwin and Howard in 2002.
Turnover of larger foraminifera during the Paleocene-Eocene thermal maximum and paleoclimatic control on the evolution of platform ecosystems
C. Scheibner, Universität Bremen, Institut für Geowissenschaften, Bremen 28334, Germany; et al. Pages 493-496.
A platform-to-basin transect in Egypt provides new data on timing and causal mechanisms of the extreme radiation of larger foraminifera at the Paleocene-Eocene transition. Because of the excellent outcrops it was possible to recalibrate the larger foraminifera biozonation with the planktic biozonations. This recalibration shows that the radiation of the larger foraminifera at the shallow-water platforms coincided with the Paleocene-Eocene Thermal Maximum at which an extreme carbon isotopic excursion occured. Simultaneously it came to an extinction of smaller benthic foraminifera in the open ocean, to perturbations in the planktic ecosystem and to a rejuvenation of terrestrial vertebrates. However, early Paleogene evolution of larger foraminifera must primarily be seen in the context of long-term processes. These processes are (1) the reorganization of the larger foraminifera after the Cretaceous-Paleogene crisis, and (2) the following long-term trends toward oligotrophy and higher sea-surface-temperatures, the latter leading to the demise of corals in low latitudes.
Evidence for pervasive petroleum generation and migration in 3.2 and 2.63 Ga shales
Birger Rasmussen, University of Western Australia, School of Earth and Geographical Sciences, Perth, Western Australia 6009, Australia. Pages 497-500.
This paper reports the oldest evidence for in situ petroleum generation and expulsion (by more than one billion years) from ~3.2 and 2.63 billion-year old shales from the Pilbara Craton in Australia. The results are important because questions have been raised in recent years about the antiquity of life and the origin of ancient hydrocarbons. Indeed, it has even been suggested that carbonaceous residues (formerly hydrocarbons) were generated solely by abiogenic processes. In contrast, this paper presents compelling evidence for in situ generation of petroleum from ancient organic-rich shales, indicating that the thermal decomposition of organic matter, the main source of petroleum in Proterozoic and Phanerozoic deposits, operated in sedimentary basins on the early Earth. The results imply the existence of a vast marine photosynthetic, and probably chemosynthetic, biomass as early as 3.2 billion years ago.
Anomalous uplift and subsidence of the Ontong Java Plateau inferred from CO2 contents of submarine basaltic glasses
Julie Roberge and Paul J. Wallace, University of Oregon, Geological Sciences, Eugene, OR 97403-1272, USA; et al. Pages 501-504.
The submarine Ontong Java Plateau in the western Pacific is the world's largest volcanic oceanic plateau. Such plateaus are basically the submarine equivalents of enormous continental flood basalt provinces such as the Columbia River Basalt and Deccan Traps but are even larger than their continental counterparts. Most of the enormous volume of the Ontong Java Plateau appears to have formed very rapidly at about 122 million years ago from submarine eruptions of basaltic lava. It is believed that the plateau formed as the oceanic lithosphere passed over a high temperature mantle plume originating deep within the mantle. In this paper Roberge et al. present new data on the water and carbon dioxide contents of basaltic glasses from Ontong Java Plateau, and they use the data to estimate the water depths at which lavas on different parts of the plateau originally erupted. The value of these data is that they can estimate how much uplift was caused by passage of the lithosphere over the purported high temperature mantle plume, and how much subsidence of the lithosphere has occurred since the plateau moved away from the plume. Their results show that the Ontong Java Plateau is highly anomalous compared with other oceanic plateaus because of its relatively small amounts of initial uplift and postformation subsidence. Typically oceanic plateaus undergo so much uplift that they form large landmasses above sea level before later subsiding below sealevel again, but the Ontong Java Plateau never formed such a landmass. This finding presents a serious inconsistency with the mantle plume hypothesis, and Roberge et al. evaluate some alternative possibilities, such as the proposition that Ontong Java formed as a result of a giant meteorite impact. However, none of the possible alternatives to the mantle plume hypothesis appear to be totally viable either, and thus they conclude that the origin of the largest magmatic event on Earth in the last 200 million years remains an enigma.
Kinematics of the northern Walker Lane: An incipient transform fault along the Pacific–North American plate boundary
James E. Faulds, University of Nevada–Reno, Nevada Bureau of Mines and Geology, Reno, NV 89557-0088, USA; et al. Pages 505-508.
It has long been known that the San Andreas fault does not accommodate all of the motion between the Pacific and North American plates. Global Positioning System observations indicate that a system of right-lateral faults in the western Great Basin accommodates 15%–25% of the Pacific–North American plate motion. The northern end of this system in northwest Nevada and northeast California is geologically very young, having developed within the past several million years, and therefore offers insight into how strike-slip fault systems develop. Offset segments of a west-trending 30 million old paleovalley suggest ~20–30 km of cumulative right lateral displacement across this region. The pattern of discontinuous en echelon faults distributed over a relatively narrow zone indicates that a through-going fault exists at depth, possibly in the lower crust and/or upper mantle. Considering that the San Andreas fault has a history of stepping inland with time, the youthful strike-slip fault system in the western Great Basin may reflect the birth of a lithospheric-scale transform fault and presage a bold eastward jump in the plate boundary.
Holocene loess deposition in Iceland: Evidence for millennial-scale atmosphere-ocean coupling in the North Atlantic
Matthew G. Jackson, Woods Hole Oceanographic Institution, Geology and Geophysics, Woods Hole, MA 02543-1525, USA; et al. Pages 509-512.
In order to understand recent climate trends, it is important to place them in the context of long-term natural climate variability. Warmth and stability are usually associated with climate during the Holocene (approximately the past 10,000 years), but several regional cool periods have punctuated this epoch. Iceland is well suited to studying these cooling events because the island is exposed to the rapidly changing North Atlantic climate system that greatly influences the Northern Hemisphere. Wind can be a sensitive indicator of climate, and grain-size variation in a loess sequence from south Iceland provides us with a proxy for wind strength throughout the Holocene. Windy periods, inferred from coarser soil deposition in the Icelandic soil sequence, are associated with periods of cooling in the North Atlantic region that are indicated by glacial advance in Northern Europe, increased deposition of sea spray on the Greenland ice cap, and advance of sea ice and icebergs to southerly latitudes. This observation is important because it suggests that Iceland has not been climatically isolated during the Holocene. Previous work on Icelandic terrestrial climate proxies has shown little correlation with other climate proxies in the North Atlantic region, particularly those derived from the Greenland ice cores. Correlations between windiness in Iceland and other climate records from the North Atlantic suggest that regionally coherent climate trends in the North Atlantic have characterized the Holocene. The mechanism responsible for the climate coherence in the North Atlantic during the Holocene is still unknown, but the atmosphere-ocean link that Jackson et al. observe in the Holocene may provide an important clue.
Exsolved magnetite inclusions in silicates: Features determining their remanence behavior
Joshua M. Feinberg, University of California–Berkeley, Earth and Planetary Science, Berkeley, California 94720, USA; et al. Pages 513-416.
The paleomagnetic time scale is a tremendously useful scientific tool for studying the rate of plate tectonics, evolution, and basin development. Despite its utility, the existing paleomagnetic time scale covers only the last 7% of Earth's history. If it were possible to uncover the other 93% of Earth's magnetic history, geologists could address questions about the evolution of the liquid core and origin of the solid inner core as well as the style of plate tectonics during early Earth history. For this early paleomagnetic window to open, a rock type is needed with the capability of maintaining a recording of the direction and intensity of Earth's magnetic field over billions of years. The research reported in this data describes one such material: needle-shaped magnetite inclusions exsolved in silicate minerals. Using a technique called magnetic force microscopy to directly image the magnetizations of their samples Feinberg et al. discovered that the two factors that most affect the magnetic memory of this material are (1) the diameter of the inclusions, and (2) microstructures within the inclusions related to mineral unmixing. This improved understanding of the remanence behavior of silicate-hosted magnetite inclusions will allow researchers to begin using them in paleomagnetic studies and continue expanding the paleomagnetic timescale further into Earth's history.
Bubble growth and rise in soft sediments
Bernard P. Boudreau, Dalhousie University, Department of Oceanography, Halifax, NS B3H 4J1, Canada; et al. Pages 517-520.
While most people have frequently observed bubbles in fluids (like soda pop, beer, etc.), bubbles also form in solids. A natural example is the growth of methane bubbles in sediments with large amounts of decomposing organic matter. (Another appears to be bread!) How such bubbles form has been a bit of a mystery. Boudreau et al. have used an industrial quality CT scanner, normally employed to image the fine-structure of opaque solids, to view a bubble they injected into a soft sediment. The bubble is not even remotely round, as those seen in a liquid. The bubble looks like a corn flake and mathematically is well approximated as a highly flattened disk. You can also grow bubbles in gelatin, which is another soft solid, and these bubbles are also very flattened disks. Roberge et al. can explain this peculiar shape if they assume that soft sediments and gelatin react as elastic solids that must fracture to let a bubble grow. With this idea they constructed a mathematical model that predicts the shape and the thickness-to-radius ratio of the disk-like bubble, and they found that observation and prediction are very close indeed. Bubbles in sediments also appear to rise by fracture, that is to say extending a crack. With this in mind, it is much easier to understand how gas can be trapped as relatively large pockets in a sediment, which can catastrophically blow-out to produce pits on the ocean floor called pockmarks.
Three-dimensional geologic map of the Hayward fault, northern California: Correlation of rock units with variations in seismicity, creep rate, and fault dip
R.W. Graymer, U.S. Geological Survey, Menlo Park, CA 94025, USA. Pages 521-524.
In order to better understand mechanisms of active faults, Graymer et al. studied relationships between fault behavior and rock units along the Hayward Fault in the San Francisco Bay Area using a three-dimensional geologic map. They describe how they made that map, and how it provided a geologic map of each fault surface, showing rock units on either side of the fault truncated by the fault, and how the two fault-surface maps were superimposed to create a rock-rock juxtaposition map. The three maps were compared with various aspects of fault behavior, including the distribution of small earthquakes and the shape of the fault. Fault behavior appears correlated to the fault-surface maps, but not to the rock-rock juxtaposition map, suggesting that properties of individual wall-rock units play an important role in fault behavior. Although preliminary, these results suggest that any attempt to understand the distribution of earthquakes or other behavior along a fault should include consideration of the rock types that abut the fault surface, including the physical properties of the rock bodies that intersect the fault at depth.
Late Cenozoic uplift of southeastern Tibet
M.K. Clark, Caltech, Division of Geological and Planetary Sciences, Pasadena, CA 91125, USA; et al. Pages 525-528.
The age of the high topography of the Tibetan Plateau is important for understanding how the continental crust deforms in response to plate tectonic motions and how tectonic and climate systems interact. The age of plateau growth in southeastern Tibet is currently unknown, but the initiation of large river canyons that cut into the eastern plateau margin today can be used to determine plateau uplift. The southeastern plateau margin is mantled by an elevated, low-relief landscape that formed at a time of slow erosion at low elevation, prior to uplift of the eastern plateau. Deep river canyons that cut this older, paleolandscape reflect increased erosion rates related to plateau uplift. Low temperature thermal histories from rocks within the river canyons record a change from slow to rapid erosion, signaling the acceleration of river incision between 13–9 million years ago. A rapid increase in elevation in eastern Tibet during this time supports tectonic-climate models that correlate the lateral (eastern) expansion of high topography in Tibet with the intensification of the Indian and east Asian moonsoons.
To view the complete table of contents for the June issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-current-toc&issn=0091-7613.
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