Boulder, Colo., USA - The origin and early evolution of animals have been a fascinating topic since Charles Darwin. Definite early animal fossils largely appear from early Cambrian, so the fossil records are often interpreted as documenting a "Cambrian explosion" of animals. The phosphatized embryo-like fossils displaying cellular and subcellular structures from Ediacaran Doushantuo Formation, South China, provide an unparalleled snapshot of life before the Cambrian.
Because the fossils are from the interval in which animal clades were diversifying, according to molecular estimates, they were thought to have great potential to reveal the evolution of animals 600 million years ago. However, the affinities of these fossils are contentious.
In their article for Geology, Zongjun Yin and colleagues report new Doushantuo embryo-like fossils. They used high-resolution synchrotron radiation X-ray microtomography to reconstruct three-dimensional structures of the fossils, and the results demonstrate that these fossils preserve unique features directly comparable to living animal embryos that utilize a special kind of cell division pattern known as discoidal cleavage. Given that discoidal cleavage only occurs in animal embryos, the biological affinities of these fossils are probably animals.
This result substantiates the conclusion derived from molecular estimates that animal lineages had evolved by the mid-Ediacaran after the termination of the Marinoan Glaciation, if not earlier.
Meroblastic cleavage identifies some Ediacaran Doushantuo (China) embryo-like fossils as metazoans
Zongjun Yin et al., State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China. This paper is online at http://geology.
GEOLOGY articles are online http://geology.
Other recently posted GEOLOGY articles are highlighted below:
Evidence for a reducing Archean ambient mantle and its effects on the carbon cycle
Sonja Aulbach, Institut für Geowissenschaften, Goethe-Universität, 60323 Frankfurt am Main, Germany; and Vincenzo Stagno, Earth Science Dept., Sapienza University of Rome, 00185 Rome, Italy. This article is OPEN ACCESS online at http://geology.
It has been widely debated when the oxidation state of Earth's mantle, which affects the speciation of volatiles (e.g. carbon) not only in the planetary interior but also during volcanism and degassing to the atmosphere, reached its present value. So far, the most accepted hypothesis has been that Earth's interior has been at its present oxidation state for at least 3.8 billion years. This implies that the mantle played no role in the marked transition from a reducing to an oxidizing atmosphere composition -- the Great Oxidation Event, ca. 2.4 billion years ago. In this study, Sonja Aulbach and Vincenzo Stagno use carefully screened Archean eclogites (aged greater than 2.5 billion years), which formed by recrystallization of mid-ocean-ridge basalts at high pressure and temperature, as samples of the convecting mantle in order to unravel its redox history. They show that the geochemical composition of these eclogites record significantly more reducing conditions than their modern equivalent. These results suggest that magmatic gases crossed the threshold that allowed a build-up in atmospheric O2 levels ca. 3000 Ma. In addition, the generation of carbonated silicate melts by redox melting beneath mid-ocean-ridges would provide a possible mechanism of primordial CO2 transfer from the uppermost mantle to the seafloor.
Osmium isotope evidence for two pulses of increased continental weathering linked to Early Jurassic volcanism and climate change
L.M.E. Percival et al., Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK. This article is OPEN ACCESS online at http://geology.
Episodes of abrupt climate change punctuate the geological record, and can provide insights into how such events are caused, and what their wider effect on environmental and biological systems might be. This study compares geological proxy data for volcanism, the global carbon cycle, and continental weathering during an interval of the Early Jurassic (183 million years ago). The purpose of this comparison is to understand how these phenomena interacted during two episodes of geologically rapid climate change, which have been previously linked to widespread extinctions of marine fauna and global warming associated with carbon cycle disruption. Crucially, this is the first study to compare data for the carbon cycle, volcanism and weathering from a single rock record; removing the problem of correlating between different records that has plagued previous studies. The new data show a repeatable sequence of events, whereby volcanism increased atmospheric CO2, causing global warming, which in turn was alleviated by elevated weathering rates of continental rocks (which absorbs atmospheric CO2 and buries it in ocean sediments). The rapidity and severity of the two events may have been linked to episodic volcanic activity associated with a previously identified volcanic 'province' that extended across present-day South Africa and Antarctica.
A matter of minutes: Breccia dike paleomagnetism provides evidence for rapid crater modification
Luke M. Fairchild et al., Dept. of Earth and Planetary Science, University of California, Berkeley, California 94709, USA.This article is online at http://geology.
When meteoroids collide with planetary surfaces they form geologic features known as impact craters. While the smallest impact craters are simple bowl shaped depressions, impact craters that are more than a couple miles across on Earth are more complex and the initial bowl shape changes right after the impact through rock in the center rising up to form a central peak. Computer simulations of impacts suggest that this modification occurs within several minutes, but there are few observations that can confirm these rapid timescales. This study focuses on an impact crater whose central peak is exposed as the Slate Island archipelago in Lake Superior. The research team used the magnetic signatures of lithic breccia dikes -- fragmented rock injected into the subsurface as the crater was being excavated during the impact event -- to constrain the formation of the central peak to a maximum timeframe of six minutes. Breccia dikes were injected at high temperature, resulting in their full remagnetization. As these injections cooled, their magnetizations would have recorded local rotations. The alignment of breccia dike magnetization directions across the Slate Islands impact structure indicates that no rotations were occurring by the time breccias had cooled sufficiently to record their magnetizations. In the thinnest sampled breccia dike, this cooling occurred approximately six minutes after its injection. This short time span represents the maximum duration of crater modification in the Slate Islands -- a rare case in which a geological process can be resolved on the time scale of minutes.
Synchronous diversification of Laurentian and Baltic rhynchonelliform brachiopods: Implications for regional versus global triggers of the Great Ordovician Biodiversification Event
Sarah Trubovitz, Dept. of Geological Sciences, Ohio University, 316 Clippinger Laboratories, Athens, Ohio 45701, USA; and Alycia L. Stigall, Ohio Center for Ecology and Evolutionary Studies, Ohio University, Athens, Ohio 45701, USA. This article is online at http://geology.
A new study has documented, for the first time, the timing of rapid increase in the number of species of shelly animals during the Great Ordovician Biodiversification Event in North America. The Great Ordovician Biodiversification Event, which occurred between 470 and 460 million years ago) was an interval of rapid diversity increase of Earth and is considered to be the greatest increase in species and genus level diversity during the history of animal life. Until now, it was unknown whether diversification occurred at the same or different times among continents. This new study demonstrates that diversification occurred at the same time in North America and Baltica (modern Scandinavia, Estonia, and western Russia). These continents were located in different regions (tropical vs. temperate) during the Ordovician. Therefore, the synchronous timing in diversification was facilitated by a trigger with global influence, such as global cooling and increased plankton supply. Understanding diversification controls during the Great Ordovician Biodiversification Event can help scientists better understand how diversity is produced more broadly.
Fault zone characteristics and basin complexity in the southern Salton Trough, California
Patricia Persaud et al., Seismological Laboratory, California Institute of Technology, Pasadena, California 91125, USA. This article is online at http://geology.
As part of the 2011 Salton Seismic Imaging Project, a collaborative project between U.S. and Mexican institutions, we installed seismometers at 4125 locations in southeastern California, and Mexico. Our overall goal was to measure the speed and amplitude of seismic waves that traveled from a set of 126 shallow underground explosions to the seismometers, and use this information to image the style of rifting and the distribution of new crust formed by rifting in this area. In this study, we use the travel times of the first seismic waves recorded from the explosions to constrain seismic wave speeds in the Imperial Valley south of the Salton Sea. This allowed us to obtain a detailed 3-D seismic velocity model down to 10 km depth, including the region of the Brawley Geothermal area. Our improved seismic velocity model and identification of important unmapped faults or buried interfaces will help refine the seismic hazard for parts of Imperial County, California, where the population is projected to almost double by 2060. Our model reveals previously unrecognized NE-SW oriented faults that are interacting with the dominant NW-oriented faults to control deformation along the active Pacific-North America plate boundary.
Rifting under steam -- How rift magmatism triggers methane venting from sedimentary basins
C. Berndt et al., GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1-3, 24148 Kiel, Germany. This article is online at http://geology.
We have discovered an active hydrothermal vent within a young rift basin in the Gulf of California that resembles black smokers at mid-ocean ridges but is far larger (almost a km long and 80 m high). This discovery in itself is striking as it is far bigger than any active hydrothermal mound reported for a rift basin before. The striking similarity of the seismic image of this vent and eye structures observed in seismic data that were collected off Norway suggests that the thousands of eye structures off Norway may in fact all be extinct hydrothermal vents that have formed during the break-up of the Atlantic Ocean. In this case they would have released large amounts of carbon high up into the water column and may have contributed to rapid global warming back in the Early Eocene.
Silicate weathering and North Atlantic silica burial during the Paleocene-Eocene Thermal Maximum
Donald E. Penman, Geology & Geophysics, Yale University, New Haven, Connecticut 06520, USA. This paper is online at http://geology.
For several decades it has been thought that Earth's climate is stabilized by the chemical weathering (by reaction with atmospheric carbon dioxide) of silicate rocks that make up the continents. This theory posits that when the climate warms, the rate of silicate weathering increases, which draws down CO2 and cools the climate - creating a negative feedback. A previously unexplored side-effect of this mechanism has been the impact that this dissolution of silicates has on the marine geochemical cycling of dissolved Si, an important nutrient for marine plankton. A new paper in Geology examines that impact in response to a well-documented ancient episode of global warming - the Paleocene-Eocene Thermal Maximum, or PETM, 56 Million years ago - and argues that the marine Si-cycle must have been significantly perturbed during the event. The paper then details new records of silica accumulation (in the form of chert) in PETM-aged marine sediments recovered from the North Atlantic, which appear to represent the burial of excess silica derived from elevated weathering - thus balancing the ocean's sources and sinks of silica. This paper confirms the operation of a silicate-weathering feedback on climate, and also points to a potential new archive - the silica content of marine sediments - for records of global weathering processes over geologic time.
Pleistocene onset of rapid, punctuated exhumation in the eastern Central Range of the Taiwan orogenic belt
Wei-Hao Hsu et al., Dept. of Geosciences, National Taiwan University, Taipei 106, Taiwan, Republic of China. This article is online at http://geology.
The Taiwan orogenic belt is often treated as a steady, southward-propagating orogenic system with an essentially constant erosion rate of 4-6 mm/yr over the past 5 m.y. We present four new age-elevation transects from the Central Range based on 19 new and 86 previously published fission track and (U-Th)/He dates of completely reset detrital zircon and apatite grains. The age-elevation curves and thermal models imply slow cooling prior to ca. 2-1.5 Ma (at exhumation rates of ~0.1 mm/yr), an increase in exhumation rates from ca. 2-1.5 Ma to ca. 0.5 Ma (2-4 mm/yr), and possibly a further acceleration in exhumation from ca. 0.5 Ma to present (4-8 mm/yr). Three transects from three different latitudes in the eastern Central Range yield similar results, each showing punctuated exhumation with progressively faster rates.
Three-dimensional textural and quantitative analyses of orogenic gold at the nanoscale
M. Sayab et al., Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland. This article is online at http://geology.
Orogenic gold deposits are among the major source of world gold production and are typically formed in multiply deformed and variably metamorphosed rocks of all ages. In this regard, much of our knowledge about the textural characteristics of orogenic gold comes directly from two dimensional (2-D) trace element mapping and image analysis, leaving a considerable gap in our understanding of ore- or gold-forming processes in three dimensions (3-D). High-resolution X-ray computed micro- and nanotomography are emerging, versatile, non-destructive technologies for visualizing ore textures at the micro- and nanoscales in 3-D and provide a holistic approach to study ore-forming processes. In this study, we first analyzed micro-scale textures of drill cores from the world-class Suurikuusikko orogenic gold deposit of northern Finland using lab-based X-ray computed microtomography. For 3-D nanotomography, individual sulfide crystals were separated and scanned using the X-ray nanoprobe beamline at the European Synchrotron Radiation Facility, France. This ultra-high resolution technique illustrated the 3-D distribution of micron to nano-scale gold inclusions inside sulfide crystals. The workflow, from micro- to nanotomography, outlined in this study offers an indispensable new technique in quantifying and characterizing 3-D textural settings of ores, which are otherwise impossible with conventional 2-D imaging devices.
Shock-metamorphosed rutile grains containing the high-pressure polymorph TiO2-II in four Neoarchean spherule layers
Frank C. Smith et al., Department of Geological Sciences, University of Delaware, 255 Academy Street, Newark, Delaware 19716, USA. This article is online at http://geology.
About 17 distinctive sedimentary layers deposited between about 3.47 and 2.49 billion years ago occur in Western Australia and South Africa. These layers contain varying amounts of millimeter-sized, formerly molten spherules. Several lines of evidence support the interpretation that these "spherule layers" were formed as a result of large impact events. A long-standing conundrum is the fact that only one shock-metamorphosed grain (quartz) had previously been identified in these layers. During large impact events, high-pressure shock waves cause distinctive "shock-metamorphic" effects in rocks and mineral grains. These effects provide definitive physical evidence for an impact event. We searched for shock-metamorphosed grains in four spherule layers deposited between about 2.63 and 2.54 billion years ago. In the four layers, we discovered 34 grains containing rutile (titanium dioxide) and TiO2-II, a high-pressure form (polymorph) of titanium dioxide. We interpret them as shock-metamorphosed rutile grains that provide unambiguous physical evidence to support an impact origin for these layers. To our knowledge, this is the first report of a shock-induced, high-pressure polymorph formed by an impact event on the Earth prior to 2.5 billion years ago. Our results provide a new tool in the search for shock-metamorphosed grains in very ancient rocks.
Biogenic overgrowth on detrital pyrite in ca. 3.2 Ga Archean paleosols
Sami Nabhan et al., Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstrasse 74-100, 12249 Berlin, Germany. This article is online at http://geology.
The Archean Eon is a time of dramatic environmental changes such as cratonic development, atmospheric variation and the rise of an active biosphere. Paleosols are excellent recorders of environmental conditions at the interface between rocks and the atmosphere, hydrosphere and biosphere. The Barberton Greenstone Belt in northeast southern Africa is one of two localities with Paleoarchean marginal marine and terrestrial strata with potential to preserve paleosols. Its youngest part, the ca. 3.2 Ga old Moodies Group contains in its lower part a series of paleosols that have as their most prominent feature nodules of carbonatic and sulfatic origin. The paleosols contain also heavy mineral laminations, mainly composed of pyrite. The shape and distribution of the nodules already allows us to interpret the paleosols as Aridisols that formed in a semiarid climate. The joint occurrence of sulfate nodules and authigenic sulfide, their nearly contemporaneous formation, the macro- and microscopic relations of both and the sulfur isotopic composition of the pyrite shows the involvement of biogenic processes in the formation of the paleosols. The S-isotope values are the lowest and oldest reported for an Archean terrestrial setting to date, documenting that life on land dates back to at least 3.22 Ga.
Paleomagnetic constraints on the Mesozoic drift of the Lhasa terrane (Tibet) from Gondwana to Eurasia
Zhenyu Li et al., Key Laboratory of Continental Collision and Plateau Uplift (LCPU), Institute of Tibetan Plateau Research, Chinese Academy of Sciences (ITPCAS), Beijing 100101, China. This article is online at http://geology.
The Mesozoic plate tectonic history of Gondwana-derived crustal blocks of the Tibetan Plateau is hotly debated, but so far, paleomagnetic constraints quantifying their paleolatitude drift history remain sparse. Here, we compile existing data published mainly in Chinese literature and provide a new, high-quality, well-dated paleomagnetic pole from the ca. 180 Ma Sangri Group volcanic rocks of the Lhasa terrane that yields a paleolatitude of 3.7 degrees S +/- 3.4 degrees. This new pole confirms a trend in the data that suggests that Lhasa drifted away from Gondwana in Late Triassic time, instead of Permian time as widely perceived. A total northward drift of ~4500 km between ca. 220 and ca. 130 Ma yields an average south-north plate motion rate of 5 cm/yr. Our results are consistent with either an Indian or an Australian provenance of Lhasa.
Groundwater controls on episodic soil erosion and dust emissions in a desert ecosystem
J.M. Kaste et al., Geology Department, College of William & Mary, Williamsburg, Virginia 23187, USA. This article is online at http://geology.
Groundwater depletion from human overuse and climate change is a global problem, but the ecological and geomorphic effects of water table fluctuations are poorly understood. We show that soil erosion in Owens Valley, California is most significant in places when the water table drops below the rooting depth of native vegetation (>2 m) but remains shallow enough (<6 m) so that groundwater capillary action can maintain loose erodible sediment at the surface. Our work applies novel methods for calculating erosion rates over multiple timescales and indicates that erosion rates in Owens Valley were highest during 1987-1991 when water extractions were the most significant, vegetation cover dropped at the steepest rate, soils were driest, and very high windspeeds were recorded.
Relationship link between landward vergence in accretionary prisms and tsunami generation
Nadaya Cubas et al., ISTeP (Institut des Sciences de la Terre de Paris), Université Pierre et Marie Curie, 4 place Jussieu, 75005 Paris, France. This article is online at http://geology.
The very large slip up to the subduction front of the 2011 Tohoku-Oki (Japan) earthquake has challenged our classic view of the subduction interface undergoing only aseismic slip at shallow depth. Furthermore, the enhancement of tsunamis during frontal rupturing has increased concern about tsunami risks. Recent geophysical surveys from the Sumatra subduction zone have shown frontal landward-vergent thrust faults in the accretionary prism in the area of supposed shallow ruptures and enhanced tsunamis. Using a mechanical approach, this study shows that sudden and successive shear stress drops along the plate interface are required to form landward-vergent thrust faults. These decreases are most likely caused by dynamic weakening mechanisms related to the propagation of earthquakes to the seafloor. Therefore, landward vergence in accretionary prisms is indicative of past seafloor frontal ruptures and consequent tsunamis. The presence of landward-verging structures in the Cascadia and Sumatra subduction zones might indicate future frontal rupture of the shallowest portion of the plate interface, increasing the tsunami risk.