New Geology Articles Published Online Ahead of Print in June
Boulder, Colo., USA: Article topics and locations include natural levees in
Texas; internal breadcrust surfaces; a submarine volcano near Tokyo Bay,
Japan; what controlled the thickness of continental crust in the Archean?;
and Silurian wildfire proxies and atmospheric oxygen. These Geology articles are online at
https://geology.geoscienceworld.org/content/early/recent
.
Halogen ratios in crustal fluids through time—Proxies for the emergence
of aerobic life?
Tobias Fusswinkel; Paula Niinikoski-Fusswinkel; Thomas Wagner
Abstract:
Halogens (Cl, Br, I) are exceptional provenance tracers in crustal fluids
because their ratios are not strongly altered during most fluid-rock
interaction processes. The halogen systematics of metamorphic fluids are of
particular interest because such fluids are key drivers of crustal-scale
element fluxes and ore formation in orogenic belts, but they remain poorly
studied due to analytical challenges. We present novel triple-halogen laser
ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS)
fluid-inclusion data from metamorphic systems ranging in age from Archean
to Phanerozoic. Our results show that the halogen signatures in Phanerozoic
metamorphic fluids are controlled by variable degrees of organic-matter
interaction in their source rocks, leading to increased I/Cl and decreased
Br/I ratios relative to seawater. By contrast, Archean metamorphic fluids
from organic matter–rich source rocks have low I/Cl and very high Br/I
ratios, distinctly different from any known fluid source signature. We
propose that these signatures nevertheless are consistent with
organic-matter interaction because dominantly prokaryotic Archean lifeforms
did not yet produce iodine-bearing metabolites. This prevented
biosequestration and accumulation of iodine-rich organic matter in
sediments and imposed halogen signatures onto Archean metamorphic fluids
entirely unlike those in younger fluids.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50182.1/614875/Halogen-ratios-in-crustal-fluids-through-time
Past eruptions of a newly discovered active, shallow, silicic submarine
volcano near Tokyo Bay, Japan
Iona M. McIntosh; Kenichiro Tani; Alexander R.L. Nichols; Qing Chang;
Jun-Ichi Kimura
Abstract:
Oomurodashi is a newly discovered active, shallow, silicic submarine
volcano only 60 km from Tokyo Bay. We reveal its past eruptive activity,
and potential future hazards, by examining volatile contents of its
subaerial and submarine pumice and lava deposits. These novel data for
shallow silicic submarine eruption products were obtained using new Fourier
transform infrared spectroscopy (FTIR) analytical techniques for vesicular
and hydrated glasses. All matrix glasses have H2O species data
consistent with low-temperature hydration following eruption. We therefore
used unaltered OH data to investigate past eruptions. Geochemistry
confirmed that Oomurodashi was the source of a ca. 13.5 ka subaerial tephra
deposit on nearby inhabited islands. We infer from pumice OH contents and
tephra characteristics that this deposit was formed by explosive submarine
phreatomagmatic activity that produced the shallow crater in the submarine
edifice. OH contents of in-place submarine lavas are lower than expected
for their current water depth; comparison with past sea level implies that
these lavas erupted at ca. 7–10 ka and ca. 14 ka when sea level was lower.
Oomurodashi has also erupted submarine pumice with different densities,
quench depths, and dispersal histories; however, any pumice sufficiently
buoyant to produce floating pumice rafts will have been lost from the local
geological record, so pumice rafts remain a potential future hazard.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50148.1/614876/Past-eruptions-of-a-newly-discovered-active
Evidence for a lithospheric step and pervasive lithospheric thinning
beneath southern New England, northeastern USA
Gillian B. Goldhagen; Heather A. Ford; Maureen D. Long
Abstract:
In this study, we use data from the SEISConn seismic experiment to
calculate Sp receiver functions in order to characterize the geometry of
upper-mantle structure beneath southern New England (northeastern United
States). We image robust negative-velocity-gradient discontinuities beneath
southern New England that we interpret as corresponding to the
lithosphere-asthenosphere boundary (LAB) and identify a well-defined step
of 15 km in LAB depth at a longitude of 73°W, which we interpret to be the
boundary between Laurentian and Appalachian lithosphere, although the
offset may be larger if the putative LAB phase is reinterpreted to be a
mid-lithospheric discontinuity. We infer that the lithosphere throughout
the region is substantially thinner than elsewhere in the continental
interior, consistent with regional tomographic studies and previously
published Sp receiver function results. The presence of thinned lithosphere
suggests that the low-velocity Northern Appalachian Anomaly (NAA) in the
upper mantle may extend as far south as coastal Connecticut. The presence
of regionally thinned lithosphere and a step in lithospheric thickness
suggests that inherited structure may be preserved in present-day
lithosphere, even in the presence of more recent dynamic processes
associated with the NAA.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50133.1/614551/Evidence-for-a-lithospheric-step-and-pervasive
Eruptive tempo of Emeishan large igneous province, southwestern China
and northern Vietnam: Relations to biotic crises and paleoclimate
changes around the Guadalupian-Lopingian boundary
Hu Huang; Magdalena H. Huyskens; Qing-Zhu Yin; Peter A. Cawood; Mingcai Hou
...
Abstract:
The Emeishan large igneous province (LIP), southwestern China and northern
Vietnam, is thought to have been a potential driver for the biotic crises
and paleoclimate changes around the Guadalupian-Lopingian boundary (GLB;
Permian), but the lack of high-precision radiometric dates to constrain the
duration and eruption rates of the volcanism has limited the assessment of
their relationship. We present new chemical abrasion–isotope
dilution–isotope ratio mass spectrometry U-Pb zircon geochronology of three
samples from the lowermost and uppermost parts of the volcanic succession
in the central portion of the Emeishan LIP. The results demonstrate that
Emeishan volcanism began by 260.55 ± 0.07 Ma and persisted until at least
257.22 ± 0.37 Ma. Combined with a previously published age of 259.1 ± 0.5
Ma for silicic ignimbrites, we estimate that ~85% of Emeishan LIP volume
erupted within 1.45 ± 0.50 m.y. Our new results confirm that the Emeishan
volcanism began slightly prior to the initiation of the associated mass
extinction event and was contemporaneous with the associated warming
interval. The new data support the hypothesis that the Emeishan LIP likely
triggered the biotic crises and paleoclimate changes around the GLB.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50183.1/614552/Eruptive-tempo-of-Emeishan-large-igneous-province
What controlled the thickness of continental crust in the Archean?
Vuong V. Mai; Jun Korenaga
Abstract:
Exposed continents are one of Earth’s major characteristics. Recent studies
on ancient ocean volume and exposed landmasses suggest, however, that early
Earth was possibly a water world, where any significant landmass was
unlikely to have risen above sea level. On modern Earth, the thickness of
continental crust seems to be controlled by sea level and the buoyancy of
continental crust. Simply applying this concept to the Archean would not
explain the absence of exposed continents, and we suggest that a third
element that is currently insignificant was important during early Earth:
the strength of continental upper crust. Based on the pressure imbalance
expected at continent-ocean boundaries, we quantified the conditions under
which rock strength controls the thickness of continental crust. With the
level of radiogenic heat production expected for early Earth, continents
may have been too weak to have maintained their thickness against a deep
ocean.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50350.1/614553/What-controlled-the-thickness-of-continental-crust
Crustal transpressional fault geometry influenced by viscous lower
crustal flow
Haibin Yang; Louis N. Moresi; Mark Quigley; Metin Kahraman; Doğan Kalafat
Abstract:
The San Andreas fault (California, USA) is near vertical at shallow (<
10 km) depth. Geophysical surveys along the San Andreas fault reveal that,
at depths of 10–20 km, it dips ~50–70° to the southwest near the Western
Transverse Ranges and dips northeast in the San Gorgonio region. We
investigate the possible origin of along-strike geometric variations of the
fault using a three-dimensional thermomechanical model. For two blocks
separated by transpressional faults, our model shows that viscous lower
crustal material moves from the high-viscosity block into the low-viscosity
block. Fault plane-normal flow in the viscous lower crust rotates the fault
plane due to the simple shear flow at the brittle-ductile transition depth.
This occurs irrespective of initial fault dip direction. Rheological
variations used to model the lower crust of Southern California are
verified by independent observations. Block extrusion due to lower crustal
viscosity variation facilitates the formation of the Garlock Fault and
sustains the geometric complexity of the fault.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50061.1/614554/Crustal-transpressional-fault-geometry-influenced
Quantitative relationships between river and channel-belt planform
patterns
Tian Y. Dong; Timothy A. Goudge
Abstract:
Channel planform patterns arise from internal dynamics of sediment
transport and fluid flow in rivers and are affected by external controls
such as valley confinement. Understanding whether these channel patterns
are preserved in the rock record has critical implications for our ability
to constrain past environmental conditions. Rivers are preserved as channel
belts, which are one of the most ubiquitous and accessible parts of the
sedimentary record, yet the relationship between river and channel-belt
planform patterns remains unquantified. We analyzed planform patterns of
rivers and channel belts from 30 systems globally. Channel patterns were
classified using a graph theory-based metric, the Entropic Braided Index
(eBI), which quantifies the number of river channels by considering the
partitioning of water and sediment discharge. We find that, after
normalizing by river size, channel-belt width and wavelength, amplitude,
and curvature of the belt edges decrease with increasing river channel
number (eBI). Active flow in single-channel rivers occupies as little as 1%
of the channel belt, while in multichannel rivers it can occupy >50% of
the channel belt. Moreover, we find that channel patterns lie along a
continuum of channel numbers. Our findings have implications for studies on
river and floodplain interaction, storage timescales of floodplain
sediment, and paleoenvironmental reconstruction.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49935.1/614344/Quantitative-relationships-between-river-and
Natural levees increase in prevalence in the backwater zone: Coastal
Trinity River, Texas, USA
Hima J. Hassenruck-Gudipati; Paola Passalacqua; David Mohrig
Abstract:
Flood dynamics in low-relief landscapes control the lateral exchange of
water and sediment between a river and its floodplain. Locations where
these exchanges occur for any given river discharge depend on local bank
elevations, which in turn depend on the type of landform present
immediately adjacent to the river channel. Our analysis separated landforms
bordering a river into six categories: levee, scroll bar, counter point
bar, channel-bend cutoff, erosional surface, and inactive surface. Each
landform is associated with a different range of elevations. Levees are the
highest, and counter point bars and cutoffs are the lowest. Using a
combination of lidar-derived measurements of topography and water-surface
profiles derived from U.S. Geological Survey gauge data, we show that
landforms at the margins of the river change with downstream position on
the coastal reach of the Trinity River in the southern United States. The
fractions of counter point bars and cutoffs decrease downstream, while the
fraction and continuity of levees increase to nearly 100%. This spatial
change correlates with downstream reductions in channel-bend migration and
deformation, and the measured range in river stage. As a result, the
greatest range in bank elevations occurs upstream where variation in river
stage is also highest. Meanwhile, the smallest range in bank elevation and
river stage exists in the coastal backwater zone. Our analysis indicates
that essentially all levees within the backwater zone are overtopped by
flow associated with a single river discharge. Moving upriver, the
discharge associated with levee-overtopping flow systematically increases.
This study highlights the morphodynamic control on coastal river flooding.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50011.1/614345/Natural-levees-increase-in-prevalence-in-the
Mid-loaf crisis: Internal breadcrust surfaces in rhyolitic pyroclasts
reveal dehydration quenching
Hugh Tuffen; Jamie I. Farquharson; Fabian B. Wadsworth; Cameron Webb;
Jacqueline Owen ...
Abstract:
Breadcrust bombs are pyroclasts displaying fractured, dense surfaces
enveloping expanded interiors, and are associated with Vulcanian
explosions. We document pyroclasts from the 2008–2009 CE eruption of
Chaitén (Chile) that are internally as well as externally breadcrusted. The
pyroclasts are cut by intersecting micrometer- to millimeter-thick
tuffisites with dense glassy walls, which grade into strongly inflated
pumiceous material. We find H2O diffusion gradients proximal to
the breadcrusted surfaces, such that H2O is depleted from
far-field magma (0.68 ± 0.04 wt%) into dense, fractured vein walls (0.2–0.3
wt%), indicating a spatial association between H2O mass transfer
within the pyroclast interior and both suppressed vesiculation and
breadcrusting. We experimentally confirm that diffusive H2O
depletion suppresses bubble growth at shallow conduit conditions.
Therefore, we interpret the breadcrust formation to be induced by H 2O diffusion and the associated rise in viscosity rather than by
cooling in the classical breadcrust-formation models. We posit that a
“dehydration quench” is important as degassing continues to very low H 2O contents in shallow-conduit magma that continues to
vesiculate.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49959.1/614346/Mid-loaf-crisis-Internal-breadcrust-surfaces-in
Epidote U-Pb geochronology and H isotope geochemistry trace
pre-orogenic hydration of midcrustal granitoids
Veronica Peverelli; Alfons Berger; Andreas Mulch; Thomas Pettke; Francesca
Piccoli ...
Abstract:
Hydrothermal veins and altered feldspar are evidence for fluid circulation
in granitic rocks in the continental crust. The hydrothermal alteration of
feldspar affects the deformation behavior of granitoids, especially if it
occurs before orogeny. Geochronology can establish the timing of fluid
circulation to determine if this fluid-driven alteration plays a role in
crustal deformation. Although existing dating techniques cannot be applied
to feldspar alteration directly, absolute ages of fluid circulation can be
obtained from hydrothermal veins. We combined U-Pb geochronology and
hydrogen isotope data (δD) from epidote [Ca2Al2(Al,Fe 3+)Si3O12(OH)] to unravel the hydration of
post-Variscan granitoids in the Alpine orogen. The recent protocol for
epidote U-Pb dating proves for the first time that fluids of meteoric
origin infiltrated the granitoids in Permian times by exploiting synrift
faults, consistent with the δD values of the epidote-forming fluids. This
hydration event caused at least some degree of feldspar hydration and
weakening of the granitic rocks ~260 m.y. before their pervasive structural
overprint by the Alpine orogeny. The preservation of Permian U-Pb ages
despite Alpine orogenic processes confirms epidote as a powerful tool with
which to unveil pre-orogenic hydration events in metagranitoids. Our
analytical approach broadens insights into the water cycle in the middle
continental crust in orogens.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50028.1/614347/Epidote-U-Pb-geochronology-and-H-isotope
Silurian wildfire proxies and atmospheric oxygen
Ian J. Glasspool; Robert A. Gastaldo
Abstract:
The earliest evidence of wildfire is documented from two localities: the
early mid-Silurian Pen-y-lan Mudstone, Rumney, Wales (UK), and the late
Silurian Winnica Formation, Winnica, Poland. Nematophytes dominate both
charcoal assemblages. Reflectance data indicate low-temperature fires with
localized intense conditions. Fire temperatures are greater in the older
and less evolved assemblage. These charcoal assemblages and others, new and
previously documented, from the Silurian and earliest Devonian are compared
to box models of atmospheric oxygen concentration (pO2
). Based on modern charring experiments, these data indicate pO 2 is divergent from the broad trends predicted by the
COPSE-revisited and GEOCARBSULFOR models. Sustained burns require a minimum pO2 threshold of 16%, or ~0.75 present atmospheric
level. This threshold was first met and, our charcoal data indicate, was
exceeded in the mid-Silurian and then, later in the Silurian, attained
again repeatedly.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50193.1/614348/Silurian-wildfire-proxies-and-atmospheric-oxygen
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