Boulder, Colo., USA: Article topics and locations include a fragile record of fleeting water on Mars; boosting the capabilities of the good old petrographic microscope; fossils of chemotrophic microbes preserved in seep carbonates from Vestnesa Ridge; no plate tectonics necessary to explain Eoarchean rocks at Isua (Greenland); and implications for the evolution of two supercontinents. These Geology articles are online at https://geology.geoscienceworld.org/content/early/recent.
Volcanic origin of the mercury anomalies at the Cretaceous-Paleogene transition of Bidart, France
Eric Font; Jiubin Chen; Marcel Regelous; Anette Regelous; Thierry Adatte
Abstract: The timing and mechanisms of the climatic and environmental perturbations induced by the emplacement of the Deccan Traps large igneous province (India) and their contribution to the Cretaceous-Paleogene (K-Pg) mass extinction are still debated. In many marine sediment archives, mercury (Hg) enrichments straddling the K-Pg boundary have been interpreted as the signature of Deccan Traps volcanism, but Hg may also have been derived from the Chicxulub (Mexico) impact. We investigated the Hg isotope composition, as well as the behavior of iridium (Ir) and other trace elements, in K-Pg sediments from the Bidart section in southwest France. Above the K-Pg boundary, Ir content gradually decreases to background values in the Danian carbonates, which is interpreted to indicate the erosion and redistribution of Ir-rich fallouts. No significant enrichment in Ir and W, or Zn and Cu, is observed just below the K-Pg boundary, excluding the hypothesis of downward remobilization of Hg from the boundary clay layer. Positive Δ199Hg and slightly negative values in the upper Maastrichtian and lower part of the early Danian are consistent with the signature of sediments supplied by atmospheric Hg2+ deposition and volcanic emissions. Up section, large shifts to strongly negative mass-dependent fractionation values (δ202Hg) result from the remobilization of Hg formerly sourced by the impactor or by a mixture of different sources including biomass burning, volcanic eruption, and asteroid impact, requiring further investigation. Our results provide additional support for the interpretation that the largest eruptions of the Deccan Traps began just before, and encompassed, the K-Pg boundary and therefore may have contributed to the K-Pg mass extinction.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49458.1/608428/Volcanic-origin-of-the-mercury-anomalies-at-the
Formation of low-gradient bedrock chutes by dry rockfall on planetary surfaces
Zhongheng Sun; Thomas P. Ulizio; Jade N. Fischer; Janette N. Levin; Alexander R. Beer ...
Abstract: Channel-like forms are ubiquitous on steep hillslopes on Earth, Mars, and other planetary bodies. On Earth and Mars, these landforms are commonly attributed to water activity, especially for slopes below the angle of repose (~30°) where dry granular flows are considered ineffective. While the angle of repose characterizes loose sediment stability, it is unclear whether dry rockfall can traverse and erode channels in bedrock or cemented substrates. We used a large-scale experiment to show that bedrock chutes can form spontaneously at low gradients from dry rockfall. Our results, combined with observations of rocky outcrops and boulders on Mars, indicate that rockfall can be an important bedrock degradation process that can produce low-gradient channels in the absence of water.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49286.1/608429/Formation-of-low-gradient-bedrock-chutes-by-dry
Polychromatic polarization: Boosting the capabilities of the good old petrographic microscope
Bernardo Cesare; Nicola Campomenosi; Michael Shribak
Abstract: Polychromatic polarizing microscopy (PPM) is a new optical technique that allows for the inspection of materials with low birefringence, which produces retardance between 1 nm and 300 nm. In this region, where minerals display interference colors in the near-black to gray scale and where observations by conventional microscopy are limited or hampered, PPM produces a full spectrum color palette in which the hue depends on orientation of the slow axis. We applied PPM to ordinary 30 μm rock thin sections, with particular interest in the subtle birefringence of garnet due both to non-isotropic growth or to strain induced by external stresses or inclusions. The PPM produces striking, colorful images that highlight various types of microstructures that are virtually undetectable by conventional polarizing microscopy. PPM opens new avenues for microstructural analysis of geological materials. The direct detection and imaging of microstructures will provide a fast, non-destructive, and inexpensive alternative (or complement) to time-consuming and more costly scanning electron microscope–based analyses such as electron backscatter diffraction. This powerful imaging method provides a quick and better texturally constrained basis for locating targets for cutting-edge applications such as focused ion beam-transmission electron microscopy or atom probe tomography.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49303.1/608430/Polychromatic-polarization-Boosting-the
Focused fluid-flow structures potentially caused by solitary porosity waves
Viktoriya M. Yarushina; Lawrence Hongliang Wang; David Connolly; Gábor Kocsis; Ingrid Fæstø ...
Abstract: Gas chimneys, fluid-escape pipes, and diffused gas clouds are common geohazards above or below most petroleum reservoirs and in some CO2 storage sites. However, the processes driving the formation of such structures are poorly understood, as are the time scales associated with their growth or their role as long-term preferential fluid-migration pathways in sedimentary basins. We present results from a multidisciplinary study integrating advanced seismic processing techniques with high-resolution simulations of geological processes. Our analyses indicate that time-dependent rock (de)compaction yields ascending solitary porosity waves forming high-porosity and high-permeability vertical chimneys that will reach the surface. The size and location of chimneys depend on the reservoir topology and compaction length. Our simulation results suggest that chimneys in the studied area could have been formed and then lost their connection to the reservoir on a time scale of a few months.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49295.1/608431/Focused-fluid-flow-structures-potentially-caused
Putative fossils of chemotrophic microbes preserved in seep carbonates from Vestnesa Ridge, off northwest Svalbard, Norway
Tobias Himmler; Antoine Crémière; Daniel Birgel; Richard Wirth; Victoria J. Orphan ...
Abstract: The microbial key players at methane seeps are methanotrophic archaea and sulfate-reducing bacteria. They form spherical aggregates and jointly mediate the sulfate-dependent anaerobic oxidation of methane (SD–AOM: CH4 + SO42– → HCO3– + HS– + H2O), thereby inducing the precipitation of authigenic seep carbonates. While seep carbonates constitute valuable archives for molecular fossils of SD–AOM-mediating microbes, no microfossils have been identified as AOM aggregates to date. We report clustered spherical microstructures engulfed in 13C-depleted aragonite cement (δ13C values as low as –33‰) of Pleistocene seep carbonates. The clusters comprise Mg-calcite spheres between ~5 μm (single spheres) and ~30 μm (clusters) in diameter. Scanning and transmission electron microscopy revealed a porous nanocrystalline fabric in the core area of the spheres surrounded by one or two concentric layers of Mg-calcite crystals. In situ measured sphere δ13C values as low as –42‰ indicate that methane-derived carbon is the dominant carbon source. The size and concentric layering of the spheres resembles mineralized aggregates of natural anaerobic methanotrophic archaea (ANME) of the ANME-2 group surrounded by one or two layers of sulfate-reducing bacteria. Abundant carbonate-bound 13C-depleted lipid biomarkers of archaea and bacteria indicative of the ANME-2-Desulfosarcina/Desulfococcus consortium agree with SD–AOM-mediating microbes as critical agents of carbonate precipitation. Given the morphological resemblance, in concert with negative in situ δ13C values and abundant SD–AOM-diagnostic biomarkers, the clustered spheres likely represent fossils of SD–AOM-mediating microbes.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49620.1/608432/Putative-fossils-of-chemotrophic-microbes
A prolonged, two-step oxygenation of Earth’s early atmosphere: Support from confidence intervals
Malcolm S.W. Hodgskiss; Erik A. Sperling
Abstract: The Great Oxidation Event (GOE), among Earth’s most transformative events, marked the sustained presence of oxygen above 10–5 times the present atmospheric level. Estimates of the onset of the GOE span 2501–2225 Ma and are based primarily on the loss of mass-independent fractionation of sulfur isotopes (MIF-S) in pyrite. To better constrain the timing of the GOE, we apply probabilistic techniques to estimate the confidence intervals of four proxies: MIF-S, redox-sensitive detrital minerals, "red beds," and I/(Ca + Mg). These GOE proxies are drawn from a highly fragmentary geologic record, and consequently, estimates of the 95% confidence intervals span tens to hundreds of millions of years—orders of magnitude larger than suggested by radiometric constraints on individual successions. Confidence interval results suggest that red beds and nonzero I/(Ca + Mg) values may have appeared earlier than 2480 Ma and 2460 Ma, respectively, whereas redox-sensitive detrital minerals and MIF-S may have disappeared after 2210 Ma and 2190 Ma, respectively. These data suggest a delay of potentially >300 m.y. between initial and permanent oxygenation of the atmosphere and a delay of tens of millions of years between onset of the Lomagundi-Jatuli carbon isotope excursion and permanent oxygenation of the atmosphere.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49385.1/608433/A-prolonged-two-step-oxygenation-of-Earth-s-early
A fragile record of fleeting water on Mars
Ari H.D. Koeppel; Christopher S. Edwards; Andrew M. Annex; Kevin W. Lewis; Gabriel J. Carrillo
Abstract: The light-toned sedimentary layers that outcrop widely throughout Mars’ Southern Highlands have long been an enigma in uncovering the climatic history of Mars. Although these units seem to contain unique records of fluctuating surface conditions, the role of water in their formation is debated. A distinctive property of many such deposits is their elevated thermal inertia relative to that of surrounding materials. This temperature-controlling trait is often interpreted to indicate induration resulting from aqueous processes. However, prevalent erosional landforms suggest that the deposits host much weaker materials than neighboring units. We address this apparent contradiction by disentangling the relationships between thermal inertia, mineralogy, and erosion susceptibility and by quantifying the cohesion of layered deposits in the Arabia Terra region. We demonstrate that variations in dust cover associated with relative erodibility and eolian abrasion can be inconspicuous controls on apparent thermal inertia. We ultimately find that these deposits are not as cohesive as would be expected from a high water-to-rock ratio setting either during or after deposition. If water-rich surface conditions existed in the region after the Noachian, these deposits suggest they may have only been intermittent and fleeting.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49285.1/608434/A-fragile-record-of-fleeting-water-on-Mars
Miocene to modern hydrothermal circulation and high topography during synconvergent extension in the Cordillera Blanca, Peru
Tyler A. Grambling; Micah J. Jessup; Dennis L. Newell; Katharina Methner; Andreas Mulch ...
Abstract: The Cordillera Blanca detachment in the highest elevations of the Peruvian Andes has been accommodating synconvergent extension since the late Miocene. Stable isotope analysis of synkinematic mica from its exhumed footwall shear zone provides new constraints on deep meteoric-hydrothermal circulation during ductile deformation and regional paleoelevation. Muscovite and biotite that deformed and/or grew synkinematically in the shear zone have δ2H values of –131‰ to –58‰ and –149‰ to –98‰ (versus Vienna standard mean ocean water, VSMOW), respectively. The δ2H value difference between coexisting muscovite and biotite is consistent with equilibrium fractionation of the same fluid at the same temperature. Calculated δ2H values of water (–107‰ to –78‰) in equilibrium with these micas are indistinguishable from those of present-day, deeply circulated (9–11 km) hot spring waters emanating from the fault. Such low-δ2H fluids indicate circulation of meteoric water to the depths of the brittle-ductile transition that cannot be explained by other mechanisms. Average recharge paleoelevation for water entering the shear zone based on hydrogen isotopes was 3400 + 500/–700 m (1σ). This is near, but ~500 m below, the present-day mean elevation of the catchments feeding modern hot springs of 3965 ± 880 m, and ~700 m below the 4200 + 700/–900 m mean recharge elevation derived from δ2H values of modern surface and thermal water. The consistency between modern and ancient fault-assisted hydrothermal systems and elevation suggests that high topography, steep relief, and meteoric-hydrothermal circulation have persisted throughout the history of the Cordillera Blanca detachment system.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49263.1/608435/Miocene-to-modern-hydrothermal-circulation-and
No plate tectonics necessary to explain Eoarchean rocks at Isua (Greenland)
Hugh Rollinson
Abstract: Trace element and isotopic data for basalts from the Isua greenstone belt, West Greenland, indicate that they were derived from a range of mantle reservoirs that included depleted lower mantle, the mantle transition zone, and a primitive mantle reservoir probably located in the shallow upper mantle. Modeling of trace element compositions indicates that the Isua basalts were formed through the mixing between and refertilization of these diverse sources and their resultant melts and that this took place in the shallow upper mantle. It is proposed that the melting and mixing were driven by the heat transferred from hot deep mantle sources. This geochemical interpretation leads to a geodynamic model in which deep mantle domains rise to melt in the shallow mantle where there is mixing between a range of sources and melts. There is no evidence for material descending from the shallow to deeper mantle and no necessity for the involvement of crustal materials. These processes imply the activity of a mantle plume and/or heat pipe.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49278.1/608450/No-plate-tectonics-necessary-to-explain-Eoarchean
Lateral variation in slab window viscosity inferred from global navigation satellite system (GNSS)–observed uplift due to recent mass loss at Patagonia ice fields
Raymond M. Russo; Haipeng Luo; Kelin Wang; Boudewijn Ambrosius; Victor Mocanu ...
Abstract: The geographic coincidence of the Chile Ridge slab window and the Patagonia ice fields offers a unique opportunity for assessing the effects of slab window rheology on glacial isostatic adjustment (GIA). Mass loss of these ice fields since the Little Ice Age causes rapid but variable crustal uplift, 12–24 mm/yr around the North Patagonia ice field, increasing to a maximum of 41 mm/yr around the South Patagonia ice field, as determined from newly collected or processed geodetic data. We used these observational constraints in a three-dimensional Maxwell viscoelastic finite element model of GIA response above both the subducting slab and slab window in which the upper-mantle viscosity was parameterized to be uniform with depth. We found that the viscosity of the northern part of the slab window, ~2 × 1018 Pa·s, is lower than that of the southern part by approximately an order of magnitude. We propose that this along-strike viscosity contrast is due to late Cenozoic ridge subduction beneath the northern part of the slab window, which increases asthenospheric temperature and reduces viscosity.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49388.1/608221/Lateral-variation-in-slab-window-viscosity
South Tarim tied to north India on the periphery of Rodinia and Gondwana and implications for the evolution of two supercontinents
Peng Wang; Guochun Zhao; Peter A. Cawood; Yigui Han; Shan Yu ...
Abstract: Constraining the positions of, and interrelationships between, Earth’s major continental blocks has played a major role in validating the concept of the supercontinent cycle. Minor continental fragments can provide additional key constraints on modes of supercontinent assembly and dispersal. The Tarim craton has been placed both at the core of Rodinia or on its periphery, and differentiating between the two scenarios has widespread implications for the breakup of Rodinia and subsequent assembly of Gondwana. In the South Tarim terrane, detrital zircon grains from Neoproterozoic–Silurian strata display two dominant populations at 950–750 and 550–450 Ma. Similarly, two main peaks at 1000–800 and 600–490 Ma characterize Neoproterozoic–Ordovician strata in northern India. Moreover, the two dominant peaks of South Tarim and north India lag two global peaks at 1200–1000 and 650–500 Ma, which reflect Rodinia and Gondwana assembly, arguing against a position within the heart of the two supercontinents. Ages and Hf isotopes of Tarim’s detrital zircons argue for a position on the margin of both supercontinents adjacent to north India with periodic dispersal through opening and closing of small ocean basins (e.g., the Proto-Tethys). Alternating tectonic transitions between advancing and retreating subduction in North Tarim coincide with periodic drift of South Tarim from north India in Rodinia and Gondwana, emphasizing the importance of retreating subduction in supercontinent dispersal. Moreover, the Rodinia-related orogenic belts spatially overlap the Gondwana-related orogenic belts in the two blocks, indicating no significant relative rotation of India and Tarim during the evolution from Rodinia to Gondwana.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49238.1/608222/South-Tarim-tied-to-north-India-on-the-periphery
Spatio-temporal evolution of the Christiana-Santorini-Kolumbo volcanic field, Aegean Sea
J. Preine; J. Karstens; C. Hübscher; P. Nomikou; F. Schmid ...
Abstract: The Christiana-Santorini-Kolumbo volcanic field (CSKVF) in the Aegean Sea is one of the most active volcano-tectonic lineaments in Europe. Santorini has been an iconic site in volcanology and archaeology since the 19th century, and the onshore volcanic products of Santorini are one of the best-studied volcanic sequences worldwide. However, little is known about the chronology of volcanic activity of the adjacent submarine Kolumbo volcano, and even less is known about the Christiana volcanic island. In this study, we exploit a dense array of high-resolution marine seismic reflection profiles to link the marine stratigraphy to onshore volcanic sequences and present the first consistent chronological framework for the CSKVF, enabling a detailed reconstruction of the evolution of the volcanic rift system in time and space. We identify four main phases of volcanic activity, which initiated in the Pliocene with the formation of the Christiana volcano (phase 1). The formation of the current southwest-northeast–trending rift system (phase 2) was associated with the evolution of two distinct volcanic centers, the newly discovered Poseidon center and the early Kolumbo volcano. Phase 3 saw a period of widespread volcanic activity throughout the entire rift. The ongoing phase 4 is confined to the Santorini caldera and Kolumbo volcano. Our study highlights the fundamental tectonic control on magma emplacement and shows that the CSKVF evolved from a volcanic field with local centers that matured only recently to form the vast Santorini edifice.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49167.1/608223/Spatio-temporal-evolution-of-the-Christiana
Evidence for enhanced fluvial channel mobility and fine sediment export due to precipitation seasonality during the Paleocene-Eocene thermal maximum
Eric A. Barefoot; Jeffrey A. Nittrouer; Brady Z. Foreman; Elizabeth A. Hajek; Gerald R. Dickens ...
Abstract: The Paleocene-Eocene thermal maximum (PETM) was the most extreme example of an abrupt global warming event in the Cenozoic, and it is widely discussed as a past analog for contemporary climate change. Anomalous accumulation of terrigenous mud in marginal shelf environments and concentration of sand in terrestrial deposits during the PETM have both been inferred to represent an increase in fluvial sediment flux. A corresponding increase in water discharge or river slope would have been required to transport this additional sediment. However, in many locations, evidence for changes in fluvial slope is weak, and geochemical proxies and climate models indicate that while runoff variability may have increased, mean annual precipitation was unaffected or potentially decreased. Here, we explored whether changes in river morphodynamics under variable-discharge conditions could have contributed to increased fluvial sand concentration during the PETM. Using field observations, we reconstructed channel paleohydraulics, mobility, and avulsion behavior for the Wasatch Formation (Piceance Basin, Colorado, USA). Our data provide no evidence for changes in fluvial slope during the PETM, and thus no evidence for enhanced sediment discharge. However, our data do show evidence of increased fluvial bar reworking and advection of sediment to floodplains during channel avulsion, consistent with experimental studies of alluvial systems subjected to variable discharge. High discharge variability increases channel mobility and floodplain reworking, which retains coarse sediment while remobilizing and exporting fine sediment through the alluvial system. This mechanism can explain anomalous fine sediment accumulation on continental shelves without invoking sustained increases in fluvial sediment and water discharge.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49149.1/608224/Evidence-for-enhanced-fluvial-channel-mobility-and
The role of phyllosilicate partial melting in segregating tungsten and tin deposits in W-Sn metallogenic provinces
Panlao Zhao; Xu Chu; Anthony E. Williams-Jones; Jingwen Mao; Shunda Yuan
Abstract: Most tungsten (W) and tin (Sn) deposits are associated with highly evolved granites derived from the anatexis of metasedimentary rocks. They are commonly separated in both space and time, and in the rare cases where the W and Sn mineralization are part of a single deposit, the two metals are temporally separate. The factors controlling this behavior, however, are not well understood. Our compilation of whole-rock geochemical data for W- and Sn-related granites in major W-Sn metallogenic belts shows that the Sn-related granites are generally the products of higher-temperature partial melting (~800 °C) than the W-related granites (~750 °C). Thermodynamic modeling of partial melting and metal partitioning shows that W is incorporated into the magma formed during low-temperature muscovite-dehydration melting, whereas most of the Sn is released into the magma at a higher temperature during biotite-dehydration melting; the Sn of the magma may be increased significantly if melt is extracted prior to biotite melting. At the same degree of partial melting, the concentrations of the two metals in the partial melt are controlled by their concentration in the protolith. Thus, the nature of the protolith and the melting temperature and subsequent evolution of the magma all influence the metallogenic potential of a magma and, in combination, helped control the spatial and temporal segregation of W and Sn deposits in all major W-Sn metallogenic belts.
View article: https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49248.1/608225/The-role-of-phyllosilicate-partial-melting-in
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