22 research outputs found
Precessional pacing of early Proterozoic redox cycles
Regularly alternating reduction-oxidation (redox) patterns attributed to variations in the Earth's orbit and axis (Milankovitch cycles) are widely recorded in marine sediment successions of the Phanerozoic and attest to a dynamic history of biospheric oxygen in response to astronomically forced climate change. To date, however, such astronomical redox control remains elusive for much older, Precambrian intervals of the geological record that were characterized by a globally anoxic and iron-rich ocean, i.e., prior to Earth's atmospheric oxygenation (ca. 2.4–2.2 billion years ago). Here we report a detailed cyclostratigraphic and geochemical investigation of marine-sedimentary redox cycles identified in the ca. 2.46 billion-year-old Joffre Member of the Brockman Iron Formation, NW Australia, suggesting the imprint of Earth's climatic precession cycle. Around the base and top of regularly intercalated mudrock layers, we identify sharp enrichments in redox sensitive elements (Fe, S, Ca, P) that appear to represent chemical reaction fronts formed during nonsteady state diagenesis. Using a reactive transport model, we find that the formation of characteristic double S peaks required periods of increased organic matter deposition, coupled to strongly declining Fe2+ concentrations in the overlying water column. This combination, in turn, implies a periodic deepening of the iron chemocline due to enhanced oxygenic photosynthesis in marine surface waters, and is interpreted as the result of precession-induced changes in monsoonal intensity that drove variations in runoff and associated nutrient delivery. Our study results point to a dynamic redox evolution of Precambrian oceanic margin environments in response to Milankovitch forcing, and offer a temporal framework to investigate linkages between biological activity and the early build-up of oxygen in Earth's ocean-atmosphere system
Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere
We interpret the three-dimensional seismic wave-speed structure of the Australian upper mantle by comparing its
azimuthal anisotropy to estimates of past and present lithospheric deformation. We infer the fossil strain field from
the orientation of gravity anomalies relative to topography, bypassing the need to extrapolate crustal measures, and
derive the current direction of mantle deformation from present-day plate motion. Our observations provide the depth
resolution necessary to distinguish fossil from contemporaneous deformation. The distribution of azimuthal seismic
anisotropy is determined from multi-mode surface-wave propagation. Mechanical anisotropy, or the directional
variation of isostatic compensation, is a proxy for the fossil strain field and is derived from a spectral coherence
analysis of digital gravity and topography data in two wavelength bands. The joint interpretation of seismic and
tectonic data resolves a rheological transition in the Australian upper mantle. At depths shallower than V150^200
km strong seismic anisotropy forms complex patterns. In this regime the seismic fast axes are at large angles to the
directions of principal shortening, defining a mechanically coupled crust^mantle lid deformed by orogenic processes
dominated by transpression. Here, seismic anisotropy may be considered ‘frozen’, which suggests that past
deformation has left a coherent imprint on much of the lithospheric depth profile. The azimuthal seismic anisotropy
below V200 km is weaker and preferentially aligned with the direction of the rapid motion of the Indo-Australian
plate. The alignment of the fast axes with the direction of present-day absolute plate motion is indicative of
deformation by simple shear of a dry olivine mantle. Motion expressed in the hot-spot reference frame matches the
seismic observations better than the no-net-rotation reference frame. Thus, seismic anisotropy supports the notion
that the hot-spot reference frame is the most physically reasonable. Independently from plate motion models, seismic
anisotropy can be used to derive a best-fitting direction of overall mantle shear
The deep structure of the Australian continent inferred from surface wave tomography
We present a new model of 3-D variations of shear wave speed in the Australian upper mantle, obtained from the
dispersion of fundamental and higher-mode surface waves. We used nearly 1600 Rayleigh wave data from the portable
arrays of the SKIPPY project and from permanent stations (from AGSO, IRIS and GEOSCOPE). AGSO data have not been used
before and provide better data coverage of the Archean cratons in western Australia. Compared to previous studies we
improved the vertical parameterization, the weighting scheme that accounts for variations in data quality and reduced the
influence of epicenter mislocation on velocity structure. The dense sampling by seismic waves provides for unprecedented
resolution of continental structure, but the wave speed beneath westernmost Australia is not well constrained. Global
compilations of geological and seismological data (using regionalizations based on tectonic behavior or crustal age) suggest
a correlation between crustal age and the thickness and composition of the continental lithosphere. However, the age and the
tectonic history of crustal elements vary on wavelengths much smaller than have been resolved with global seismological
studies. Using our regional upper mantle model we investigate how the seismic signature of tectonic units changes with
increasing depth. At large wavelengths, and to a depth of about 200 km, the inferred velocity anomalies corroborate the
global pattern and display a progression of wave speed with crustal age: slow wave propagation prevails beneath the
Paleozoic fold belts in eastern Australia and wave speeds increase westward across the Proterozoic and reach a maximum in
the Archean cratons. The high wave speeds associated with Precambrian shields extend beyond the Tasman Line, which
marks the eastern limit of Proterozoic outcrop. This suggests that parts of the Paleozoic fold belts are underlain by
Proterozoic lithosphere. We also infer that the North Australia craton extends off-shore into Papua New Guinea and beneath
the Indian Ocean. For depths in excess of 200 km a regionalization with smaller units reveals that some tectonic subregions
of Proterozoic age are marked by pronounced velocity highs to depths exceeding 300 km, but others do not and,
surprisingly, the Archean units do not seem to be marked by such a thick high wave speed structure either. The Precambrian
cratons that lack a thick high wave speed ‘‘keel’’ are located near passive margins, suggesting that convective processes
associated with continental break-up may have destroyed a once present tectosphere. Our study suggests that deep
lithospheric structure varies as much within domains of similar crustal age as between units of different ages, which hampers
attempts to find a unifying relationship between seismic signature and lithospheric age
Spatio-spectral localization of isostatic coherence anisotropy in Australia and its relation to seismic anisotropy : Implications for lithopsheric deformation
We investigate the two-dimensional (2-D) nature of the coherence between Bouguer
gravity anomalies and topography on the Australian continent. The coherence function or isostatic response is commonly assumed to be isotropic. However, the fossilized strain field recorded by gravity anomalies and their relation to topography is manifest in a degree of isostatic compensation or coherence which does depend on direction. We have developed a method that enables a robust and unbiased estimation of spatially, directionally, and wavelength-dependent coherence functions between two 2-D fields in a computationally efficient way. Our new multispectrogram method uses orthonormalized Hermite functions as data tapers, which are optimal for spectral localization of nonstationary, spatially dependent processes, and do not require solving an eigenvalue problem. We discuss the
properties and advantages of this method with respect to other techniques. We identify
regions on the continent marked by preferential directions of isostatic compensation in two wavelength regimes. With few exceptions, the short-wavelength coherence anisotropy is nearly perpendicular to the major trends of the suture zones between stable continental domains, supporting the geological observation that such zones are mechanically weak.
Mechanical anisotropy reflects lithospheric strain accumulation, and its presence must be
related to the deformational processes affecting the lithosphere integrated over time. Threedimensional models of seismic anisotropy obtained from surface wave inversions provide an independent estimate of the lithospheric fossil strain field, and simple models have been proposed to relate seismic anisotropy to continental deformation. We compare our measurements of mechanical anisotropy with our own model of the azimuthally anisotropic seismic wave speed structure of the Australian lithosphere. The correlation of isostatic
anisotropy with directions of fast wave propagation gleaned from the azimuthal anisotropy of surface waves decays with depth. This may support claims that above circa 200 km, internally coherent deformation of the entire lithosphere is responsible for the anisotropy
present in surface wave speeds or split shear waves
Climate control on banded iron formations linked to orbital eccentricity
Astronomical forcing associated with Earth’s orbital and inclination parameters (Milankovitch forcing) exerts a major control on climate as recorded in the sedimentary rock record, but its influence in deep time is largely unknown. Banded iron formations, iron-rich marine sediments older than 1.8 billion years, offer unique insight into the early Earth’s environment. Their origin and distinctive layering have been explained by various mechanisms, including hydrothermal plume activity, the redox evolution of the oceans, microbial and diagenetic processes, sea-level fluctuations, and seasonal or tidal forcing. However, their potential link to past climate oscillations remains unexplored. Here we use cyclostratigraphic analysis combined with high-precision uranium–lead dating to investigate the potential influence of Milankovitch forcing on their deposition. Field exposures of the 2.48-billion-year-old Kuruman Banded Iron Formation reveal a well-defined hierarchical cycle pattern in the weathering profile that is laterally continuous over at least 250 km. The isotopic ages constrain the sedimentation rate at 10 m Myr−1 and link the observed cycles to known eccentricity oscillations with periods of 405 thousand and about 1.4 to 1.6 million years. We conclude that long-period, Milankovitch-forced climate cycles exerted a primary control on large-scale compositional variations in banded iron formations
Climate control on banded iron formations linked to orbital eccentricity
Astronomical forcing associated with Earth’s orbital and inclination parameters (Milankovitch forcing) exerts a major control on climate as recorded in the sedimentary rock record, but its influence in deep time is largely unknown. Banded iron formations, iron-rich marine sediments older than 1.8 billion years, offer unique insight into the early Earth’s environment. Their origin and distinctive layering have been explained by various mechanisms, including hydrothermal plume activity, the redox evolution of the oceans, microbial and diagenetic processes, sea-level fluctuations, and seasonal or tidal forcing. However, their potential link to past climate oscillations remains unexplored. Here we use cyclostratigraphic analysis combined with high-precision uranium–lead dating to investigate the potential influence of Milankovitch forcing on their deposition. Field exposures of the 2.48-billion-year-old Kuruman Banded Iron Formation reveal a well-defined hierarchical cycle pattern in the weathering profile that is laterally continuous over at least 250 km. The isotopic ages constrain the sedimentation rate at 10 m Myr−1 and link the observed cycles to known eccentricity oscillations with periods of 405 thousand and about 1.4 to 1.6 million years. We conclude that long-period, Milankovitch-forced climate cycles exerted a primary control on large-scale compositional variations in banded iron formations
Climate control on banded iron formations linked to orbital eccentricity
Astronomical forcing associated with Earth’s orbital and inclination parameters (Milankovitch forcing) exerts a major control on climate as recorded in the sedimentary rock record, but its influence in deep time is largely unknown. Banded iron formations, iron-rich marine sediments older than 1.8 billion years, offer unique insight into the early Earth’s environment. Their origin and distinctive layering have been explained by various mechanisms, including hydrothermal plume activity, the redox evolution of the oceans, microbial and diagenetic processes, sea-level fluctuations, and seasonal or tidal forcing. However, their potential link to past climate oscillations remains unexplored. Here we use cyclostratigraphic analysis combined with high-precision uranium–lead dating to investigate the potential influence of Milankovitch forcing on their deposition. Field exposures of the 2.48-billion-year-old Kuruman Banded Iron Formation reveal a well-defined hierarchical cycle pattern in the weathering profile that is laterally continuous over at least 250 km. The isotopic ages constrain the sedimentation rate at 10 m Myr−1 and link the observed cycles to known eccentricity oscillations with periods of 405 thousand and about 1.4 to 1.6 million years. We conclude that long-period, Milankovitch-forced climate cycles exerted a primary control on large-scale compositional variations in banded iron formations
Intravascular ultrasound-guided versus coronary angiography-guided percutaneous coronary intervention in patients with acute myocardial infarction: A systematic review and meta-analysis
Background: Intravascular ultrasound (IVUS) can overcome the intrinsic limitations of coronary angiography for lesion assessment and stenting. IVUS improves outcomes of patients presenting with stable or complex coronary artery disease, but dedicated data on the impact of IVUS-guided percutaneous coronary intervention (PCI) in patients with acute myocardial infarction (AMI) remains scarce. Methods: We systematically searched Embase, MEDLINE, Web of Science Core Collection, Cochrane Central Register of Controlled Trials and Google Scholar for studies that compared clinical outcomes for IVUS- versus angio-guided PCI in patients with AMI. The primary endpoint was all-cause mortality and the secondary endpoint major adverse cardiovascular events (MACE). Mantel-Haenszel random-effects model was used to calculate pooled risk ratios (RR) with 95% confidence intervals (CI). Results: Nine studies (8 observational, 1 RCT) with a total of 838.902 patients (796.953 angio-guided PCI, 41.949 IVUS-guided PCI) were included. In patients with AMI, IVUS-guided PCI was associated with a significantly lower risk of all-cause mortality (pooled RR: 0.70; 95% CI, 0.59–0.82; p < 0.01), MACE (pooled RR: 0.86; 95% CI, 0.74–0.99; p = 0.04) and target vessel revascularization (TVR) (pooled RR: 0.83; 95% CI, 0.73–0.95; p < 0.01). In the subset of patients presenting with ST-segment elevation, IVUS-guided PCI remained associated with a reduced risk for both all-cause mortality (pooled RR: 0.79; 95% CI, 0.66–0.95, p = 0.01) and MACE (pooled RR: 0.86; 95% CI, 0.74–0.99, p = 0.04). Conclusions: This is the first systematic review and meta-analysis comparing IVUS- versus angio-guided PCI in patients with AMI, showing a beneficial effect of IVUS-guided PCI on all-cause mortality, MACE and TVR. Results of ongoing dedicated prospective studies are needed to confirm these findings
A Present-Day \u201cGlass Bead Game\u201d: A Framework for the Education of Prospective Informatics Teachers Inspired by a Reflection on the Nature of the Discipline
Informatics has an intrinsically multifaceted nature.
The usual approaches to plan classwork in the high school are organized around concepts, principles, tools.
However, each concept, principle, or tool can be seen under diverse perspectives, in particular those held by mathematicians, scientists and engineers, with quite different implications from a conceptual, methodological and epistemological point of view.
Accordingly, the learning issues and the pedagogical means to address them are also different.
Hence, it may be worth considering a complementary approach in order to shift the focus from the notions to learn to the (cognitive, methodological, creative) processes required from the students, the latter being tightly connected to the aforementioned general perspectives.
The framework outlined in this paper is the basis of a core module as part of the renewed program offered by the University of Udine for the education of prospective teachers of informatics.
This module addresses in an integrated way the nature of our discipline and the teaching of programming as a key activity to appreciate the distinctive features of each perspective. It aims at deepening the teachers' awareness about the variety of coexisting views and their pedagogical implications
Hypothermia for Cardioprotection in Patients with St-Elevation Myocardial Infarction: Do Not Give It the Cold Shoulder Yet!
The timely revascularization of an occluded coronary artery is the cornerstone of treatment in patients with ST-elevation myocardial infarction (STEMI). As essential as this treatment is, it can also cause additional damage to cardiomyocytes that were still viable before reperfusion, increasing infarct size. This has been termed "myocardial reperfusion injury". To date, there is still no effective treatment for myocardial reperfusion injury in patients with STEMI. While numerous attempts have been made to overcome this hurdle with various experimental therapies, the common denominator of these therapies is that, although they often work in the preclinical setting, they fail to demonstrate the same results in human trials. Hypothermia is an example of such a therapy. Although promising results were derived from experimental studies, multiple randomized controlled trials failed to do the same. This review includes a discussion of hypothermia as a potential treatment for myocardial reperfusion injury, including lessons learned from previous (negative) trials, advanced techniques and materials in current hypothermic treatment, and the possible future of hypothermia for cardioprotection in patients with STEMI