image: This image shows a swim traceway from Capitol Reef National Park. view more
Credit: Tracy J. Thomson and Mary L. Droser, <i>Geology</i>, 5 Feb. 2015.
Boulder, Colo., USA - Vertebrate tracks provide valuable information about animal behavior and environments. Swim tracks are a unique type of vertebrate track because they are produced underwater by buoyant trackmakers, and specific factors are required for their production and subsequent preservation. Early Triassic deposits contain the highest number of fossil swim track occurrences worldwide compared to other epochs, and this number becomes even greater when epoch duration and rock outcrop area are taken into account.
This spike in swim track occurrences suggests that during the Early Triassic, factors promoting swim track production and preservation were more common than at any other time. Coincidentally, the Early Triassic period follows the largest mass extinction event in Earth's history, and the fossil record indicates that a prolonged period of delayed recovery persisted throughout this time period.
During this recovery interval, sediment mixing by animals living within the substrate was minimal, especially in particularly stressful environments such as marine deltas. The general lack of sediment mixing during the Early Triassic was the most important contributing factor to the widespread production of firm-ground substrates ideal for recording and preserving subaqueous trace fossils like swim tracks.
FEATURED ARTICLE
Swimming reptiles make their mark in the Early Triassic: Delayed ecologic recovery increased the preservation potential of vertebrate swim tracks
Tracy J. Thomson and Mary L. Droser, University of California, Riverside, California, USA. Published online ahead of print on 5 Feb. 2015; http://dx.doi.org/10.1130/G36332.1.
Other recently posted GEOLOGY articles (see below) cover such topics as
1. The discovery of a new marine CO2 archive; and
2. Bubbles and eruptions;
3. Extraterrestrial material in the Kuruman Iron Formation.
GEOLOGY articles published online 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 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.
Coralline algal Mg-O bond strength as a marine pCO2 proxy
Maren Pauly et al., University of Waterloo, Waterloo, Ontario, Canada. Published online ahead of print on 5 Feb. 2015; http://dx.doi.org/10.1130/G36386.1. This article is OPEN ACCESS.
Anthropogenic CO2 releases are expected to affect marine chemistry and the associated biosphere via ocean acidification (OA). An understanding of historic rates of acidification is needed not only to help better project future rates of OA but also to determine how marine biota will respond in the real world. We have discovered a new marine CO2 archive, a proxy for determining historic marine CO2 concentrations. This proxy utilizes the skeleton bonding strength of a marine calcifying type of plant and will enable shallow-water OA reconstructions (e.g., around coral reefs) and also the reconstruction of historic atmospheric CO2 concentrations. These are vital for understanding how atmospheric CO2 concentrations have responded to natural and anthropogenic CO2 forcing.
The role of bubbles in generating fine ash during hydromagmatic eruptions
E.J. Liu et al., University of Bristol, Bristol, UK. Published online ahead of print on 5 Feb. 2015; http://dx.doi.org/10.1130/G36336.1.
Recent volcanic eruptions in Iceland raised awareness of the hazards posed by fine-grained volcanic ash, and highlighted gaps in our knowledge of mechanisms responsible for fine fragmentation. We address this gap by quantifying the shapes of volcanic ash produced by the 2011 subglacial eruption of Grímsvötn, Iceland, where the early phase of activity involved eruption through glacial water. We find that ash-particle morphology is controlled primarily by the original size distribution of bubbles in the melt, and, more fundamentally, that the bubble size distribution exerts a strong control on ash size. This result is unexpected because it contrasts with findings from studies of "dry" (magmatic) basaltic eruptions (e.g., K?lauea, Hawaii), where individual volcanic fragments are much larger than the constituent bubble population. We hypothesize that the mechanism by which bubbles control the shapes and sizes of ash particles is through stresses created by rapid quenching of basaltic melt by glacial water. Our results demonstrate that accurate predictions of fragmentation and ash generation during hydromagmatic eruptions need to incorporate the abundance and size distribution of the pre-fragmental bubble population. This is important for modelling ash dispersion, which is sensitive to the size and shape of the particles.
Magnetotelluric images of magma distribution beneath Volcán Uturuncu, Bolivia: Implications for magma dynamics
Matthew J. Comeau et al., University of Alberta, Edmonton, Alberta, Canada. Published online ahead of print on 5 Feb. 2015; http://dx.doi.org/10.1130/G36258.1.
Volcán Uturuncu is located in the Bolivian Andes and is unusual because the surface has been moving upward for the past 20 years at a rate of 1 to 2 cm per year. It is located in a region where a number of very large volcanic eruptions have taken place in the geologically recent past, and other geophysical studies show that a large magma body is present in the crust. In this paper, Matthew J. Comeau of the University of Alberta and colleagues employ a geophysical technique called magnetotellurics to look at the properties of the crustal rocks in this area. This method uses low-frequency radio waves and is very sensitive to the presence of fluids, such as magma and water. The results show that a layer of partially molten rock is located in this region at a depth of 15 km below sea level. Beneath Volcán Uturuncu, several low-resistivity fingers extend toward the surface and are likely to be regions of molten rock moving toward the surface. This motion of magma can explain the observed uplift of the surface and may represent the formation of a pluton.
First detection of extraterrestrial material in ca. 2.49 Ga impact spherule layer in Kuruman Iron Formation, South Africa
Bruce M. Simonson et al., Oberlin College, Oberlin, Ohio, USA. Published online ahead of print on 5 Feb. 2015; http://dx.doi.org/10.1130/G36225.1.
Bruce M. Simonson and colleagues have found extraterrestrial material in a thin layer of the Kuruman Iron Formation of South Africa. Their geochemical analyses prove that a thin layer in the Kuruman Iron Formation, which is rich in sand-size, formerly molten spherules, contains extraterrestrial material. This means the layer was created by the impact of an asteroid striking Earth some 2.49 billion years ago. Since the layer is thicker than the one that coincided with a mass extinction 66 million years ago, the object responsible for the much older layer must have been more than 10 km across. The Kuruman layer closely matches a spherule layer of similar age in an iron formation in Western Australia compositionally, suggesting the Kuruman and Australian (Dales Gorge) spherule layers were produced by the same impact. Because spherules ejected by such a large impact quickly settle back to Earth, these layers can be used to recognize strata formed on different continents within hours of one another, even if they are billions of years old. Finally, the absence of any dramatic changes in the strata deposited before and after the Kuruman and Dales Gorge spherule layers suggests large impacts are not sufficient to change Earth's surface environments unless conditions are already near a tipping point.
Why cold slabs stagnate in the transition zone
Scott D. King et al., Virginia Tech, Blacksburg, Virginia, USA, and Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany. Published online ahead of print on 5 Feb. 2015; http://dx.doi.org/10.1130/G36320.1.
One of the outstanding questions of plate tectonics is what happens at the boundary where two plates collide. Seismologists can image Earth's interior at these boundaries to see the fate of the downgoing plate using the energy associated with earthquakes much like a medical imaging scan. They find that instead of sinking straight down, the path of the downgoing plate, or slab, is bent and contorted. One theory for the contorted shape is that material in the cold slab remains in a low-pressure material phase long after the rest of Earth's interior has transformed to a higher-pressure phase. We use numerical modeling to address whether the delay in transformation of material phase within cold slabs explains the slab shape. Our theory predicts that the coldest slabs should be the most deformed. This is somewhat counter-intuitive because the coldest slabs should be the most dense slabs and therefore the most likely to sink without deforming. We test our theory by comparing observations of subducted slab shape and parameters that should control the temperature of slabs and find the observations are with our theory that cold slabs inhibit the change in material phase.
Selenium isotopes support free O2 in the latest Archean
Eva E. Stüeken et al., University of Washington, Seattle, Washington, USA. Published online ahead of print on 5 Feb. 2015; http://dx.doi.org/10.1130/G36218.1.
Selenium, an essential micronutrient for life, has a wide range of isotopes with ratios that are reset according to the amount of oxygen available. Thus, it can potentially monitor when oxygen first became abundant in Earth's environment. We investigated selenium abundances and isotopes in a drill-core from northwestern Australia that sampled shales spanning the 2.5-billion-year-old Archean-Proterozoic boundary before the atmosphere became permanently oxygen-rich. Spikes in both abundance and isotopes indicate that at least some oxygen was temporarily present in the upper ocean and probably on land as well, as a so-called "whiff of oxygen." This indicates that oxygenic photosynthesis, the source of almost all atmospheric oxygen, had evolved well before the "Great Oxidation Event" around 2.35 billion years ago.
Ancient depletion and mantle heterogeneity: Revisiting the Permian and Jurassic paradox of Alpine peridotites
Anders McCarthy and Othmar Müntener, University of Lausanne, Lausanne, Switzerland. Published online ahead of print on 5 Feb. 2015; http://dx.doi.org/10.1130/G36340.1.
Abyssal peridotites and mid-ocean ridge basalts (MORB) have been found, in certain cases, to be in isotopic equilibrium, thus confirming the genetic link between mantle depletion and formation of MORB. However, within Alpine-Apennine peridotites, which represent a preserved section of the magma-poor (ultra)slow-spreading Jurassic Ligurian Tethys, this genetic link has been recently called into question. McCarthy and Müntener provide new geochemical evidence indicating that isotopic decoupling between depleted peridotites found in Alpine-Apennine settings and MORB can be ascribed to ancient partial melting events. In this specific case, these partial melting events are related to fundamentally different crust-forming processes in Permian times and therefore unrelated to MORB genesis. They propose that mantle isotopic heterogeneity found within certain modern (ultra)slow-spreading systems and ocean-continent transition zones is related to the exhumation of variably depleted rafts of subcontinental mantle during continental break-up and rifting.
Seafloor silicification and hardground development during deposition of 2.5 Ga banded iron formations
Birger Rasmussen et al., Curtin University, Bentley, Western Australia, Australia. Published online ahead of print on 5 Feb. 2015; http://dx.doi.org/10.1130/G36363.1.
ABSTRACT: Banded iron formations (BIFs) are important archives of the ancient oceans, atmosphere, and biosphere, but fundamental questions remain about their origin. It is widely assumed that BIFs were derived from layers of ferric oxyhydroxides and silica that precipitated directly from a water column that was enriched in dissolved iron and silica. The reported lack of current-generated structures and clastic particles beyond mud grade, and the perceived basin-scale extent of laminae, is regarded as evidence for uninterrupted pelagic settling with no sedimentary reworking. New sedimentological and petrographic results show that laminated cherts in the 2.5 Ga Dales Gorge Member of the Brockman Iron Formation, Western Australia, preserve textures indicative of in situ brecciation immediately below the seafloor and the deposition of intraformational sandstones composed of chert clasts in a chert matrix. Chert intraclasts have two sedimentary components: silt-sized microgranules and submicron-sized particles, indicating that the original sediment comprised iron rich silicate muds that were cemented on or just below the seafloor by pore-filling silica. Silicified muds were episodically eroded by density currents, and the resulting detritus was transported as sand-sized clasts and locally deposited in a matrix of microgranules and mud. Our results support the hypothesis that high concentrations of silica in early Precambrian seawater favored episodic silica cementation of sediments on the seafloor. We suggest that competition between sediment accumulation and seafloor silica cementation, with subsequent differential compaction, explains primary layering in BIFs between beds of relatively thickly laminated chert and beds of thinly laminated, iron-rich minerals. The thickest laminated chert beds are interpreted to represent intervals when seafloor silicification outpaced deposition of hydrothermal muds, forming the equivalent of Phanerozoic hardgrounds at sequence boundaries.
A genetic linkage between subduction- and collision-related porphyry Cu deposits in continental collision zones
Zengqian Hou et al., Chinese Academy of Geological Sciences, Beijing 100037, P.R. China and Centre for Exploration Targeting and Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (CCFS), University of Western Australia, Perth, Australia. Published online ahead of print on 5 Feb. 2015; http://dx.doi.org/10.1130/G36362.1. This article is OPEN ACCESS.
Most porphyry Cu deposits (PCDs) form in worldwide magmatic arcs resulted from subduction of oceanic lithosphere, recently, such PCDs have been also found in continent collisional zones. However, genesis of collision-related PCDs remains controversial; it is unclear whether a genetic linkage exists between collision- and subduction-related PCDs. Here we studied Jurassic subduction-related Cu-Au and Miocene collision-related Cu-Mo porphyry deposits in south Tibet. The Jurassic PCDs only developed in the western segment of the Jurassic arc, which have the depleted mantle-like isotopic compositions. By contrast, no Jurassic PCDs have been found in its eastern segment with more radiogenic isotopic compositions. These results imply that the involvement of crustal components during underplating of the Jurassic magmas induced copper accumulation as sulfides at the base of the eastern Jurassic arc. The Miocene PCDs are spatially confined within the Jurassic arc, only giant PCDs cluster in its eastern segment, showing complementary metal Cu endowment between subduction- and collision-related magmatic suites. Miocene mineralized porphyries have young Hf model ages and Sr-Nd-Hf isotopic compositions overlapping with the Jurassic rocks in the eastern segment, whereas contemporaneous barren porphyries outside the Jurassic arc have abundant zircon inheritance and crust-like Sr-Nd-Hf isotopic compositions. These data suggest that remelting of the low-crustal sulfide-bearing Cu-rich Jurassic cumulates, triggered by Cenozoic crustal thickening and subsequent slab or breaking-off, led to the formation of giant Miocene PCDs. The spatial overlap and complementary metal endowment between subduction- and collision-related magmas may be used to evaluate the mineral potential for such deposits in other orogenic belts.
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Journal
Geology