Boulder, Colo., USA - New Geology articles cover using the architecture of ancient lava-fed deltas to estimate paleo-water levels and past ice thicknesses; bubbles and bubble haloes in lava; iron-silicate microgranules; the importance of durable, biomineralized hard parts; the link between wastewater disposal and earthquakes; shells, ocean pH, and atmospheric CO2; a SWEET hypothesis for mound-building on Mars; marine oxygenation may have preceded oxygenation on land; analysis of fossil plant tissues from Pakistan; and imaging the Transition fault.
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'A'a lava-fed deltas: A new reference tool in paleoenvironmental studies
John L. Smellie et al., Dept. of Geology, University of Leicester, Leicester LE1 7RH, UK. Posted online 18 March 2013 as part of the April 2013 issue; http://dx.doi.org/10.1130/G33631.1.
Knowing ice thickness is essential for making informed estimates of past polar ice sheet volumes and assessing potential changes in eustatic sea levels due to global climate change. An extraordinarily useful tool for determining paleo-water levels and past ice thicknesses is the architecture of ancient lava-fed deltas. The deltas are distinctive volcanic constructs that form when lavas flow into water, including meltwater lakes created by eruptions in ice sheets. Until now, only one type of lava delta has been recognized, fed by smooth-surfaced slow-moving lava (called pahoehoe). John J. Smellie and colleagues report on lava deltas fed by rough-surfaced faster-moving lava (called 'a'a) and present the first dynamic model for their formation. 'A'a deltas are very different in appearance from pahoehoe-fed deltas, and failure to recognize them can have profound consequences for paleoenvironmental reconstructions of past glacial environments. 'A'a deltas are therefore important tools to use in all paleoenvironmental investigations involving volcanic rocks.
Convection in a volcanic conduit recorded by bubbles
Rebecca J. Carey et al., ARC Centre of Excellence in Ore Deposits, and School of Earth Science, University of Tasmania, Sandy Bay, 7005 Tasmania, Australia. Posted online 18 March 2013 as part of the April 2013 issue; http://dx.doi.org/10.1130/G33685.1.
Microtextures of juvenile pyroclasts from Kilauea's early 2008 explosive activity record the velocity and depth of convection within the basaltic magma-filled conduit. Rebecca J. Carey and colleagues use X-ray microtomography to document the spatial distribution of bubbles. They find small bubbles (radii from 5 to 70 micrometers) in a halo surrounding larger millimeter-sized bubbles. This suggests that dissolved water was enriched around the larger bubbles -- the opposite of what is expected if bubbles grow as water diffuses into the bubble. Such volatile enrichment implies that the volatiles within the large bubbles were redissolving into the melt as they descended into the conduit by the downward motion of convecting magma within the lava lake.
Iron silicate microgranules as precursor sediments to 2.5-billion year-old banded iron formations
Birger Rasmussen et al., School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. Posted online 18 March 2013 as part of the April 2013 issue; http://dx.doi.org/10.1130/G33828.1.
Banded iron formations (BIFs) are chemical sedimentary rocks comprising alternating layers of iron-rich and silica-rich minerals that have been used to infer the composition of the early Precambrian ocean and ancient microbial processes. However, the identity of the original sediments and their formation is a contentious issue due to postdepositional overprinting and the absence of modern analogues. Petrographic examination of the approx. 2.5-billion-year-old Dales Gorge Member of the Brockman Iron Formation (Hamersley Group), Western Australia, reveals the presence of abundant silt-sized microgranules composed of stilpnomelane. The microgranules are most common in the least-altered BIF, where they define sedimentary laminations, implying a depositional origin. Study authors Birger Rasmussen and colleagues suggest that the precursor mineral was an iron-rich silicate that formed either in the water column or on the seafloor and that the microgranular texture may have developed due to clumping of amorphous mud, forming silt-sized floccules. They propose that for most of the early Precambrian, the persistence of ferruginous oceans with elevated silica concentrations favored the widespread growth of iron silicate minerals, which in environments starved of continental sediments formed extensive deposits of the precursor sediment to iron formation.
Origin and impact of the oldest metazoan bioclastic sediments
L.V. Warren et al., Instituto de Geociências, Universidade de São Paulo (USP), Rua do Lago, 562, São Paulo 05508-080, Brazil. Posted online 18 March 2013 as part of the April 2013 issue; http://dx.doi.org/10.1130/G33931.1.
The emergence of soft-bodied metazoans and the radiation of the earliest skeletal organisms substantially changed the ecological dynamics of Ediacaran environments, leading to the genesis of biogenic hard-part deposits for the first time in Earth's history. The impact of bioclasts origin on sedimentary processes is analyzed herein, focusing on the sedimentology and taphonomy of shell concentrations dominated by the Ediacaran index-fossil Cloudina from the Itapucumí Group, Paraguay. At that time, Cloudina was the critical source of durable biomineralized hard parts in an environment nearly free of other bioclasts. Despite their Precambrian age, the simple fabric and geometry of these accumulations are typical of Cambrian-style shell-beds. Furthermore, these deposits indicate that the establishment of the Phanerozoic style of marine substrates and preservation was determined more by the acquisition of hard parts rather than environmental changes.
Potentially induced earthquakes in Oklahoma, USA: Links between wastewater injection and the 2011 Mw 5.7 earthquake sequence
Katie M. Keranen et al., ConocoPhillips School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd Street, Norman, Oklahoma 73069, USA. Posted online ahead of print of 26 March 2013; http://dx.doi.org/10.1130/G34045.1.
In this study, Katie M. Keranen and colleagues demonstrate a relationship between the 2011 magnitude 5.7 Oklahoma earthquake sequence with wastewater disposal in nearby wells. Keranen and her co-authors use the locations of nearly 800 aftershocks to delineate the faults that ruptured, showing progressive rupture on three separate fault segments. The first rupture in this sequence was within ~650 feet of active wastewater disposal wells, in the same sedimentary rocks into which disposal occurs. Earthquake triggering by fluid injection occurs if pore pressure at the fault increases beyond a critical threshold, allowing the fault to slip. In this case, earthquakes began 17 years after injection commenced, and the study highlights that fluid pressure can rise slowly in confined reservoirs, causing delayed seismicity. The authors recommend modifying the criteria for induced earthquakes based on these results. The Oklahoma sequence was also important in that the volume and rate of fluid injection were relatively small, but the authors show that rupture of even a small fault was capable of triggering a cascading sequence of earthquakes leading up to the magnitude 5.7. Because of this possibility of sequential triggering, the risk of humans inducing large earthquakes from injection activities may be higher than previously considered.
Cenozoic boron isotope variations in benthic foraminifers
Markus Raitzsch (corresponding) and Bärbel Hönisch, Dept. of Earth and Environmental Sciences and Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, New York 10964, USA. Posted online ahead of print of 26 March 2013; http://dx.doi.org/10.1130/G34031.1.
Because carbon dioxide (CO2) in the atmosphere tends to equilibrate with the oceans, an increase in atmospheric CO2 leads to increased uptake of CO2 by the oceans, resulting in lowered pH. Hence, past changes in CO2 levels on geological time scales can be inferred if past ocean pH is known. One approach to estimate pH is the use of boron isotopes archived in fossil carbonate shells of surface-dwelling foraminifera. The ratio between the two boron isotopes incorporated into the carbonate lattice is governed by pH at the time when the shell was built. However, the boron isotopic composition of the carbonate is also dependent on that of seawater. Since the latter was not constant over time, this unknown was constrained in this study by using boron isotope ratios of foraminifera from the deep sea, where changes in pH are smaller compared to the sea surface. The variations in boron isotopes found in these deep-sea foraminifera are consistent between different oceans and water depths, suggesting that the obtained record reflects changes in secular variations in the seawater boron isotopic composition. This record will help to more accurately constrain past seawater pH and hence atmospheric CO2 concentrations.
Growth and form of the mound in Gale Crater, Mars: Slope wind enhanced erosion and transport
Edwin S. Kite et al., Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA. Posted online ahead of print of 26 March 2013; http://dx.doi.org/10.1130/G33909.1.
Ancient sediments provide archives of climate and habitability on Mars. Gale Crater, the landing site for the Mars Science Laboratory (MSL), hosts a 5-km-high sedimentary mound (Mount Sharp/Aeolis Mons). Hypotheses for mound formation include evaporitic, lacustrine, fluviodeltaic, and aeolian processes, but the origin and original extent of Gale's mound is still unknown. Here Edwin S. Kite and colleagues show new measurements of sedimentary strata within the mound that indicate ~3-degree outward dips oriented radially away from the mound center, inconsistent with the first three hypotheses. Moreover, they find that the Gale mound's current form is close to its maximal extent. Thus, Kite and colleagues propose that the mound's structure, stratigraphy, and current shape can be explained by growth in place near the center of the crater mediated by wind-topography feedbacks. Their model shows how sediment can initially accrete near the crater center far from crater-wall katabatic winds, until the increasing relief of the resulting mound generates mound-flank slope winds strong enough to erode the mound. The slope wind enhanced erosion and transport (SWEET) hypothesis indicates mound formation dominantly by aeolian deposition with limited organic carbon preservation potential, and a relatively limited role for lacustrine and fluvial activity. Morphodynamic feedbacks between wind and topography are widely applicable to a range of sedimentary and ice mounds across the Martian surface, and possibly other planets.
Anoxia in the terrestrial environment during the late Mesoproterozoic
Vivien M. Cumming et al., Dept. of Earth Sciences, Durham University, South Road, Durham DH1 3LE, UK. Posted online ahead of print of 26 March 2013; http://dx.doi.org/10.1130/G34299.1.
A significant body of evidence suggests that the marine environment remained largely anoxic throughout most of the Precambrian. In contrast, little attention has been paid to the oxygenation history of terrestrial aquatic environments -- despite the significance of such settings for early eukaryote evolution. Vivien M. Cumming and colleagues address this lack by providing a geochemical and isotopic assessment of sediments from the late Mesoproterozoic Nonesuch Formation of central North America. They utilize rhenium-osmium (Re-Os) geochronology to yield a depositional age of 1078 million years (plus or minus 24 million years), while Os isotope compositions support existing evidence for a lacustrine setting. Fe-S-C systematics suggest that the Nonesuch Formation was deposited from an anoxic Fe-rich (ferruginous) water column. Thus, similar to the marine realm, anoxia persisted in terrestrial aquatic environments in the Middle to Late Proterozoic, but sulfidic water column conditions were not ubiquitous. The data suggest that oxygenation of the terrestrial realm was not pervasive and may not have preceded oxygenation of the marine environment, signifying a major requirement for further investigation of links between the oxygenation state of terrestrial aquatic environments and eukaryote evolution.
Evidence for atmospheric carbon injection during the end-Permian extinction
Elke Schneebeli-Hermann et al., Palaeoecology, Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, Netherlands. Posted online ahead of print of 26 March 2013; http://dx.doi.org/10.1130/G34047.1.
One of the big five mass extinction in Earth's history occurred at the end of the Permian, about 252 million years ago. The enormous loss of biodiversity was accompanied by a profound perturbation of the carbon cycle. Because land plants incorporate ambient atmospheric CO2 into their tissues during photosynthesis, fossil cuticle and wood fragments can be used to investigate ancient changes in atmospheric carbon isotope composition. Fossil plant tissue samples from a Permian-Triassic boundary section in Pakistan have been investigated. The data show that the carbon isotope composition of atmospheric CO2 dropped by ~5.5 parts per million close to the Permian-Triassic boundary. This profound change in atmospheric carbon isotope composition is synchronous with the end-Permian mass extinction. It indicates the input of a large amount of isotopically light CO2 into the atmosphere, directly or indirectly associated with the Siberian Trap volcanism.
Seismic images of the Transition fault and the unstable Yakutat-Pacific-North American triple junction
S.P.S. Gulick et al., Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712, USA. Posted online ahead of print of 26 March 2013; http://dx.doi.org/10.1130/G33900.1.
In southern Alaska, the oceanic Pacific plate and a thick oceanic plateau, the Yakutat Terrane, both underthrust (subduct beneath) the North America plate along the Aleutian Trench and the leading fault of the Pamplona Zone. These Pacific and Yakutat plates are also sliding past each other at minimal rates along the Transition fault located at the base of the continental shelf. As the faults of the Pamplona Zone stepped eastward during the last few million years, the intersection of these three plates (Pacific, Yakutat, and North America) became unstable. Four recent images into the subsurface reveal that the Transition fault changes from a single fault east of the Pamplona Zone to three strands that step increasingly seaward as they approach the intersections with the Aleutian Trench. These southern two faults actually break the Pacific crust, and some vertical motion via thrusting has occurred on the southern one within the last million years and continuing today. S.P.S. Gulick and colleagues propose that this deformation of the Pacific plate is an attempt to re-attain stability, which can only be reached by creating a tectonic boundary that lines up with the leading fault of the Pamplona Zone, which is more northeasterly that the current trend of the Aleutian Trench. For a junction of three plates that includes faults of these types (two plates underthrusting a third) to migrate together through time and thus be "stable," the two faults where the underthrusting occur (i.e., along the Aleutian Trench) must be in a line on Earth's surface. Thus, Gulick and colleagues observe plate reorganization in progress in southeast Alaska and note that this reorganization will result a portion of the Pacific crust underthrusting itself in a rarely observed initiation of subduction event. The net effect of this new underthrusting is that a portion of the Pacific crust will be attached to the North American plate, which may be a mechanism over geologic time that oceanic material can be permanently added to our continental margins.
Journal
Geology