New Geology Articles Published Online Ahead of Print in October
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 Δ 199Hg 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 CO 2 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: CH 4 + 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 δ 2H 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 19 th 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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