New Geology Articles Published Online Ahead of Print
Boulder, Colo., USA: Article topics and locations include detrital glass in
a Bering Sea sediment core; evidence for microbial mediation of
silicification in trilobites; sponge-rich sediment recycling;
pre-agricultural soil erosion rates in the midwestern United States; and
pacing of the East Asian summer monsoon over the past five glacial cycles
inferred from land snails. These Geology articles are online at
https://geology.geoscienceworld.org/content/early/recent.
Paleozoic vegetation increased fine sediment in fluvial and tidal
channels: Evidence from secular changes to the mudrock content of
ancient point bars
William J. McMahon; Neil S. Davies; Maarten G. Kleinhans; Ria L. Mitchell
The amount of mudrock preserved globally in alluvium increased in
stratigraphic synchrony with the Paleozoic evolution of land plants. This
observation has been explained by vegetation promoting both the retention
of mud through baffling, stabilization, and flocculation, and the
production of mud through chemical weathering. However, the latter
explanation has been challenged on the basis that it is perceived to
require imbalance in the long-term global carbon cycle. We present a
compendium of empirical evidence that is supportive of increased global
fine sediment supply, and thus the contention that land plants did, in
fact, promote the production of mud on the continents. We refine previous
broad-brush analyses of Paleozoic mudrock content by specifically tracking
shifts in the mudrock content of regions of alluvial and tidal landscapes
that remained locally unvegetated even after the greening of the
continents, namely inclined heterolithic stratification (IHS) that records
submerged in-channel bars. We show that the Paleozoic mudrock increase was
pronounced even within these areas, away from any biomechanical binding and
baffling effects of plants. Precambrian and Cambrian IHS are composed
almost exclusively of sandstone, whereas Silurian through to Carboniferous
examples show a steady increase in total mudrock content. This progressive
rise in the mudrock component of channel bars cannot alone be explained by
physical retention of mud by vegetation and requires heightened fine
sediment concentrations from the hinterland, which suggests that plants
increased the volume of mud available at source. The muddying of Earth’s
preserved IHS serves as a proxy that suggests evolving Paleozoic land
plants triggered a global increase in the production and supply of
fine-grained sediment.
Late Miocene to recent tectonic evolution of the Macquarie Triple
Junction
Luca Gasperini; Marco Ligi; Daniela Accettella; Alessandro Bosman; Marco
Cuffaro ...
The Pacific, Antarctic, and Macquarie lithospheric plates diverge from the
Macquarie Triple Junction (MTJ) in the southwestern Pacific Ocean, south of
Macquarie Island. Morphobathymetric, magnetic, and gravity data have been
used to understand the evolution of the three accretionary/transform
boundaries that meet at the MTJ. Plate velocities, estimated near the MTJ
and averaged over the past 3 m.y., indicate an unstable ridge–fault–fault
triple junction. The long life (>6 m.y.) of this configuration can be
attributed to a rapid increase in spreading asymmetry along the Southeast
Indian Ridge segment as it approaches the MTJ, and to transtension along
the southernmost strand of the Macquarie–Pacific transform boundary. A
major change in plate motion triggered the development of the Macquarie
plate at ca. 6 Ma and makes clear the recent evolution of the MTJ,
including (1) shortening of the Southeast Indian Ridge segment; (2)
formation of the westernmost Pacific-Antarctic Ridge, which increased its
length over time; and (3) lengthening of the two transform boundaries
converging in the MTJ. The clockwise change of the Pacific-Antarctic motion
(ca. 12–10 Ma) led to complex geodynamic evolution of the plate boundary to
the east of the triple junction, with fragmentation of the long-offset
Emerald transform fault and its replacement over a short time interval (1–2
m.y.) with closely spaced, highly variable transform offsets that were
joined by short ridge segments with time-varying asymmetries in the
spreading rates.
Deep magma mobilization years before the 2021 CE Fagradalsfjall
eruption, Iceland
M. Kahl; E.J.F. Mutch; J. Maclennan; D.J. Morgan; F. Couperthwaite ...
The deep roots of volcanic systems play a key role in the priming,
initiation, and duration of eruptions. Causative links between initial
magmatic unrest at depth and eruption triggering remain poorly constrained.
The 2021 CE eruption at Fagradalsfjall in southwestern Iceland, the first
deep-sourced eruption on a spreading-ridge system monitored with modern
instrumentation, presents an ideal opportunity for comparing geophysical
and petrological data sets to explore processes of deep magma mobilization.
We used diffusion chronometry to show that deep magmatic unrest in the
roots of volcanic systems can precede apparent geophysical eruption
precursors by years, suggesting that early phases of magma accumulation and
reorganization can occur in the absence of significant increases in shallow
seismicity (<7 km depth) or rapid geodetic changes. Closer correlation
between geophysical and diffusion age records in the months and days prior
to eruption signals the transition from a state of priming to full-scale
mobilization in which magma begins to traverse the crust. Our findings
provide new insights into the dynamics of near-Moho magma storage and
mobilization. Monitoring approaches optimized to detect early phases of
magmatic unrest in the lower crust, such as identification and location of
deep seismicity, could improve our response to future eruptive crises.
Shale mobility: From salt-like shale flow to fluid mobilization in
gravity-driven deformation, the late Albian–Turonian White Pointer
Delta (Ceduna Subbasin, Great Bight, Australia)
Gulce Dinc; Jean-Paul Callot; Jean-Claude Ringenbach
Large offshore depocenters above a weak detachment level (either salt or
shale) can undergo gravity spreading and/or gliding. The gravitational
systems (e.g., gliding deltas) are classically composed of an updip domain
affected by extensional listric normal faults and a downdip domain affected
by toe thrusts. While the role of salt in such systems is a classic
tectonic process, the role and mechanical behavior of mobile shale levels
in shale-prone gravity-driven systems are increasingly questioned. A
three-dimensional seismic data set in the Ceduna Subbasin (Australia)
displays the late Albian–Turonian White Pointer Delta (WPD) as having an
unusual diversity of shale-cored structures. The early flow of shale
resulted in depocenters showing wedges, internal unconformities, and shale
diapirs and ridges, while fluidization of shales underneath a significant
burial resulted in mud volcanism, secondary radial fault sets, and collapse
features beneath the Campanian–Maastrichtian Hammerhead Delta, which lies
above the WPD. Massive shale mobilization, together with downdip shortening
and distal margin uplift, localized a major thrust in the core of the
basin, ending the downward-propagating failure of the WPD. Mobilization of
thick shale intervals, either as salt-like flow or mud volcanism, appears
to have been a key process in the deformation, which should be considered
at large scale for worldwide gravity-driven deformation systems.
Subsurface heat and salts cause exceptionally limited methane
hydrate stability in the Mediterranean Basin
A. Camerlenghi; C. Corradin; U. Tinivella; M. Giustiniani; C. Bertoni
Knowledge of the global reservoir of submarine gas hydrates is of great
relevance for understanding global climate dynamics, submarine geohazards,
and unconventional hydrocarbon energy resources. Despite the expected
presence of gas hydrates from modeling studies, the land-locked
Mediterranean Basin displays a lack of evidence of extensive gas hydrate
presence from samples and seismic data. We modeled the theoretical
Mediterranean distribution of methane hydrate below the seafloor and in the
water column using available geological information provided by 44 Deep Sea
Drilling Project (DSDP) and Ocean Drilling Program (ODP) boreholes,
measured geothermal gradients, and thermohaline characteristics of the
water masses. We find that the pervasive presence of high-salinity waters
in sediments, coupled with the unique warm and salty water column, limit
the thickness of the theoretical methane hydrate stability zone in the
subsurface and deepen its top surface to 1163–1391 m water depth. The
theoretical distribution of methane hydrates coincides well with the
distribution of shallow, low-permeability Messinian salt deposits, further
limiting the formation of pervasive gas hydrate fronts and controlling
their or distribution due to the prevention of upward hydrocarbon gas
migration. We conclude that the Mediterranean Basin, hosting the youngest
salt giant on Earth, is not prone to the widespread formation and
preservation of gas hydrates in the subsurface and that the gas hydrate
potential of salt-bearing rifted continental margins may be considerably
decreased by the presence of subsurface brines.
100 k.y. pacing of the East Asian summer monsoon over the past five
glacial cycles inferred from land snails
Rui Bao; Xuefen Sheng; Xianqiang Meng; Tao Li; Chenglong Li ...
The effects of orbital forcing on the East Asian summer monsoon (EASM)
after the mid-Pleistocene transition are controversial. Chinese cave δ 18O records only show low-latitude ~20 k.y. cycles, while
pedogenic proxy records from Chinese loess are dominated by high-latitude
100 k.y. cycles. This discrepancy may result from the multicomponent origin
of proxies, particularly for pedogenic signals in loess deposits, where the
primary climatic signals are modified by pedogenic smoothing, leaching, and
changes in sedimentation rate, and the latter are also being forced by 100
k.y. cycles. We present an EASM record spanning the past 470 k.y. from the
central Chinese Loess Plateau based on the δ13C values of land
snail shells (δ13Cshell), which eliminates the
influence of the above processes and exclusively records the local past
EASM precipitation. The δ13Cshell record is dominated
by the 100 k.y. cycle, with more depleted values during interglacials
compared to glacials. At the end of marine isotope stage (MIS) 11, δ 13Cshell-based precipitation remained at an
interglacial level following the MIS 11 super-interglacial climate in the
Northern Hemisphere, although a glacial period had commenced on a global
basis. Overall, our δ13Cshell record is highly
coupled with high-northern-latitude ice-volume variations, possibly
supporting the high-latitude forcing of the EASM.
Origin of the Sierras Pampeanas, Argentina: Flat-slab subduction
and inherited structures
Xiaowen Liu; Claire A. Currie
The Sierras Pampeanas (27°–33°S) in South America are characterized by
basement-cored uplifts and shortening that occurs >500 km from the
nearest convergent margin. The deformation correlates spatially and
temporally with an area of flat-slab subduction of the Nazca plate in the
last 10 m.y. We use two-dimensional thermal-mechanical models to study the
dynamics of Pampean flat-slab subduction and the origin of the Sierras
Pampeanas. Models examine a geological time from ca. 12 Ma to present day,
during which time the Juan Fernández Ridge subducted beneath South America.
Models show that the buoyant ridge triggers slab flattening, resulting in
regional continental compression through end loading at the plate margin.
Deformation in the continental interior depends on the inherited structure
of the continent, where surface uplifts and shortening are concentrated at
preexisting weak zones. The inboard migration of deformation is controlled
by surface topography caused by the buoyant ridge rather than basal shear
from the growing flat slab. Deformation occurs prior to the passage of the
ridge and is inhibited when the ridge is beneath the region owing to
dynamic uplift.
Eustatic change modulates exhumation in the Japanese Alps
Georgina E. King; Floriane Ahadi; Shigeru Sueoka; Frédéric Herman; Leif
Anderson ...
The exhumation of bedrock is controlled by the interplay between tectonics,
surface processes, and climate. The highest exhumation rates of centimeters
per year are recorded in zones of highly active tectonic convergence such
as the Southern Alps of New Zealand or the Himalayan syntaxes, where high
rock uplift rates combine with very active surface processes. Using a
combination of different thermochronometric systems including
trapped-charge thermochronometry, we show that such rates also occur in the
Hida Mountain Range, Japanese Alps. Our results imply that centimeter per
year rates of exhumation are more common than previously thought. Our
thermochronometry data allow the development of time series of exhumation
rate changes at the time scale of glacial-interglacial cycles, which show a
four-fold increase in baseline rates to rates of ~10 mm/yr within the past
~65 k.y. This increase in exhumation rate is likely explained by knickpoint
propagation due to a combination of very high precipitation rates, climatic
change, sea-level fall, range-front faulting, and moderate rock uplift. Our
data resolve centimeter-scale sub-Quaternary exhumation rate changes, which
show that in regions with horizontal convergence, coupling between climate,
surface processes, and tectonics can exert a significant and rapid effect
on rates of exhumation.
Europium anomalies in detrital zircons record major transitions in
Earth geodynamics at 2.5 Ga and 0.9 Ga
A. Triantafyllou; M.N. Ducea; G. Jepson; J.D. Hernández-Montenegro; A.
Bisch ...
Trace elements in zircon are a promising proxy with which to quantitatively
study Earth’s long-term lithospheric processes and its geodynamic regimes.
The zircon Eu anomaly reflects the crystallization environment of its
felsic or intermediate parental magma. In particular, it provides insight
into the water content, magmatic redox conditions, and the extent of
plagioclase fractionation in the source rock or its occurrence as a
cogenetic crystallizing phase from the magma. We performed a statistical
analysis of Eu anomalies from a compilation of detrital zircons over
geologic time and found a major decrease in Eu anomaly ca. 2.5 Ga and an
important increase ca. 0.9 Ga. Coupled with thermodynamic modeling, we
suggest that these variations could be due to long-term change in the
chemical system of the mafic source from which the intermediate to felsic
melt and derived zircons were produced. The 2.5 Ga drop was likely
associated with an enrichment in incompatible elements in the mafic source,
which extended the pressure-temperature field of plagioclase stability as a
cogenetic melt phase. We interpret the 0.9 Ga rise to record increasing
hydration of magmagenetic sites due to the general development of cold
subduction systems, which would delay and/or suppress the saturation of
plagioclase in hydrous magmagenetic sites.
Fault permeability from stochastic modeling of clay smears
Lluís Saló-Salgado; J. Steven Davis; Ruben Juanes
In normally consolidated, shallow (depth <~3 km) siliciclastic
sequences, faults develop clay smears. Existing models include the
dependence of permeability on the clay fraction, but improved predictions
of fault permeability should account for uncertainty and anisotropy. We
introduce PREDICT, a methodology that computes probability distributions
for the directional components (dip-normal, strike-parallel, and
dip-parallel) of the fault permeability tensor from statistical samples for
a set of geological variables. These variables, which include geometrical,
compositional, and mechanical properties, allow multiple discretizations of
the fault core to be populated with sand and clay smears, which can be used
to upscale the permeability to a coarser scale (e.g., suitable for
reservoir modeling). We validated our implementation with experimental data
and applied PREDICT to several stratigraphic sequences. We show that fault
permeability is controlled by the clay smear configuration and, crucially,
that it typically exhibits multimodal probability distributions due to the
existence of holes. The latter is a unique feature of our algorithm, which
can be used to build fault permeability scenarios to manage and mitigate
risk in subsurface applications.
Trends and rhythms in carbonatites and kimberlites reflect
thermo-tectonic evolution of Earth
Shuang-Liang Liu; Lin Ma; Xinyu Zou; Linru Fang; Ben Qin ...
Earth’s thermo-tectonic evolution determines the way the planet’s interior
and surface interact and shows temporal changes in both trends and periodic
rhythms. By sampling the subcontinental lithospheric mantle that represents
the interface between the convecting mantle and the crust, carbonatite and
kimberlite should be ideal rock types for documenting this evolution. The
first-order secular rise of kimberlites over time has been noted by
researchers, but there is much debate over how to interpret this trend, and
their second-order variability has received less attention. We compiled a
comprehensive global carbonatite database and compared it with an existing
kimberlite one. We find that the numbers of carbonatites and kimberlites
have similar increasing secular trends, with accelerated growth after ca. 1
Ga, and show the same periodic rhythms that have been synchronized to the
supercontinent cycle since ca. 2.1 Ga. We link these trends and rhythms to
the long-term change of Earth and the supercontinent cycle, both of which
have altered the temperature of, and the subduction-recycled volatile flux
into, the subcontinental lithosphere. Such consistent records in
carbonatite and kimberlite behavior provide critical evidence for the
synchronous thermo-tectonic evolution of the entire subcontinental
lithosphere.
Diagenetic priming of submarine landslides in ooze-rich substrates
Nan Wu; Christopher A.-L. Jackson; Michael A. Clare; David M. Hodgson;
Harya D. Nugraha ...
Oozes are the most widespread deep-sea sediment in the global ocean, but
very little is known about how changes in their physical properties during
burial impact slope stability and related geohazards. We used
three-dimensional seismic reflection, geochemical, and petrophysical data
acquired both within and adjacent to 13 large (in total ~6330 km 2) submarine slides on the Exmouth Plateau, North West Shelf,
Australia, to investigate how the pre-slide physical properties of oozes
control slope failure and emplacement processes. Our integrated data set
allows potential slide surfaces to be detected within ooze successions, a
crucial advance for improved submarine geohazard assessment. Moreover, we
demonstrate that the interplay of tectonics, ocean current activity, and
silica diagenesis can prime multiple slides on very low-gradient slopes in
tropical oceanic basins. Therefore, the diagenetic state of silica-rich
sediments should be considered in future studies to improve slope stability
assessments.
Fluid inclusion evidence for overpressure-induced shale oil
accumulation
Yingqi Wang; Jian Cao; Wenxuan Hu; Dongming Zhi; Yong Tang ...
Shale oil is becoming increasingly important in the global energy market,
but its accumulation mechanism is not fully understood. We present novel
and direct fluid inclusion data from the Lower Permian Fengcheng Formation,
Mahu Sag, Junggar Basin, northwest China. Shortite veins in this source
rock contain abundant two-phase gas-liquid hydrocarbon inclusions and
coeval aqueous inclusions. The inclusions have highly variable degrees of
bubble filling (5–80 vol% vapor) and homogenization temperature differences
between oil and aqueous inclusions (~50 °C), which demonstrate that fluid
(oil-gas-water) immiscibility occurred at high pressures. The hydrocarbon
inclusions record different levels of fluid overpressure (32.9–43.0 MPa),
with a paleopressure coefficient of 1.3–1.7. Episodic fluid overpressure
release resulted in shale oil accumulation, with faults/fractures acting as
important migration pathways. Oil from deeper and more mature source rocks
within the Fengcheng Formation was expelled upward to the shale oil
reservoir. These processes are common and important in shale oil systems.
These results show that the accumulation of unconventional hydrocarbons
occurs pervasively within the reservoirs, and fluid displacement is
critical in exploration and exploitation.
Post-subduction porphyry Cu magmas in the Sanjiang region of
southwestern China formed by fractionation of lithospheric
mantle–derived mafic magmas
Jia Chang; Andreas Audétat
For porphyry Cu deposits that formed during oceanic slab subduction, there
is a general consensus that the ore-forming magmas evolved through
fractionation of mafic magmas that were generated by slab fluid
(±melt)–fluxed melting of the asthenospheric mantle wedge. This model,
however, is not applicable to post-subduction porphyry Cu deposits because
they formed distinctly after cessation of oceanic slab subduction. A
popular model proposes that post-subduction porphyry Cu magmas were
produced by partial melting of lower-crustal, sulfide-rich arc cumulates,
with or without minor contributions from potassic mafic magmas. To
reappraise the crustal melting model, we focused on one of the largest
post-subduction porphyry Cu belts on Earth, which formed during the
India-Asia collision in the Sanjiang region of southwestern China. Detailed
petrographic studies and new Nd-Sr isotopic data from non-metasomatized
versus metasomatized lower-crustal xenoliths suggest that previous models
based on crustal melting rest upon wrong radiogenic isotope constraints due
to pervasive metasomatism of the xenoliths. Based on trace-element
quantitative modeling and regional-scale geochemical trends of magmatic
rocks, we demonstrate that the Sanjiang post-subduction porphyry Cu magmas
were produced by fractionation of potassic mafic magmas derived from
lithospheric mantle. This study highlights that post-subduction porphyry Cu
magmas attain their K-rich composition, and all the ore-forming
ingredients, from subduction-modified lithospheric mantle, the existence of
which may be a prerequisite for the formation of porphyry Cu deposits in
post-subduction settings.
Detrital glass in a Bering Sea sediment core yields a ca. 160 ka
Marine Isotope Stage 6 age for Old Crow tephra
Alberto V. Reyes; Britta J.L. Jensen; Shaun H. Woudstra; Matthew S.M.
Bolton; Serhiy D. Buryak ...
For decades, the Old Crow tephra has been a prominent stratigraphic marker
for the onset of Marine Isotope Stage (MIS) 5e, the last interglaciation,
in subarctic northwest North America. However, new zircon U-Pb dates for
the tephra suggest that the tephra was deposited ca. 207 ka during MIS 7,
with wide-ranging implications for chronologies of glaciation,
paleoclimate, relict permafrost, and phylogeography. We analyzed ~1900
detrital glass shards from 28 samples collected at Integrated Ocean
Drilling Program Site U1345 in the Bering Sea, which has a well-constrained
age model from benthic foraminiferal δ18O. Except for one
possibly contaminant shard dated at 165 ka, Old Crow tephra was absent from
all samples spanning 220–160 ka. Old Crow tephra appeared abruptly at 157
ka, comprising >40% of detrital shards between 157 and 142 ka. This
abrupt increase in the concentration of detrital Old Crow tephra, its
absence in earlier intervals, and its presence at low concentrations in all
samples between 134 and 15 ka collectively indicate that the tephra was
deposited during the middle of MIS 6 with a likely age of 159 ± 8 ka. As a
result, the late Quaternary chronostratigraphic framework for unglaciated
northwest North America remains intact, and the timing of key events in the
region (e.g., bison entry into North America; interglacial paleoclimate;
permafrost history; the penultimate glaciation) does not require wholesale
revision.
"Excess Ar" by laboratory alteration of biotite
Igor M. Villa; Giulia Bosio
Many biotite phenocrysts from marine tephra layers have substoichiometric
potassium concentrations and alkali occupation <<2.0 atoms per
formula unit. Diagenetic alteration is an expected effect of exposure of
fresh magmatic minerals to interstitial water and brine intrusions after
the deposition and burial of sediments. To test the effect of diagenetic
alteration on potassium-argon ages, we irradiated and step heated untreated
Fish Canyon biotite (t = 28.2 Ma) and several aliquots leached to
various extents in strong and weak acids. Laboratory alteration caused loss
of K, age spectrum discordance, high step ages and total gas ages, Ar
release at lower furnace temperature, higher Cl/K and Ca/K, and a slight
decrease in 36Ar concentration. Potassium loss was always higher
than 40Ar* loss. Electron microprobe element maps document that
acids preferentially penetrated in phyllosilicate interlayers, removing K
(and Na). Because Ar* is removed to a lesser extent than K, we propose that
natural 40K decay partly implants radiogenic Ar* into the
tetrahedral-octahedral-tetrahedral (T-O-T) phyllosilicate layer, where Ar
is shielded from interlayer leaching. The recoiled 39Ar, which
was produced by irradiation after the leaching, also partitioned between
T-O-T and the interlayer; age spectrum discordance was probably enhanced by
the heterogeneous partition of 39Ar and 40Ar* in
leached samples.
Chromium isotopes track redox fluctuations in Proterozoic
successions of the Chapada Diamantina, São Francisco craton, Brazil
Fabrício A. Caxito; Robert Frei; Alcides N. Sial; Gabriel J. Uhlein;
Willian Alexandre Lima Moura ...
The Chapada Diamantina region in the São Francisco craton of eastern Brazil
is composed of sedimentary successions containing both Mesoproterozoic and
Neoproterozoic carbonate levels, making it a key natural laboratory for
understanding the fluctuations of Earth’s biogeochemical cycles during its
middle age. The ca. 1.4–1.2 Ga Caboclo Formation stromatolites yielded
unfractionated δ53Crauth (authigenic) (~–0.54‰ to
+0.08‰). Ediacaran cap carbonates and phosphatic stromatolites of the
Salitre Formation, on the other hand, yielded fractionated δ53Crauth reaching as high as +0.51‰, suggesting the input of 53Cr-rich Cr(VI), first delivered through meltwater-induced
post–snowball Earth fluctuating redox conditions and then through
weathering and mobilization under a fully oxygenated environment. The
acquired data set highlights the very distinct redox conditions throughout
the Proterozoic and reinforces the suggestion that after the Cryogenian
global glaciations, Earth’s atmosphere and hydrosphere became progressively
oxygenated during the Ediacaran-Cambrian transition.
Silicification of trilobites and biofilm from the Cambrian Weeks
Formation, Utah: Evidence for microbial mediation of silicification
Leslie A. Melim; Sebastien R. Mure-Ravaud; Thomas A. Hegna; Brian J.
Bellott; Rudy Lerosey-Aubril
We report on silicified trilobite sclerites with associated silicified
biofilm from the Cambrian Weeks Formation, Utah (USA), that support a role
for microbial biofilms in silicification. Silicified sclerites are
typically small (<3 mm) and incompletely preserved. All studied
specimens are partly coated in 5–20 μm (rarely >500 μm) of
silica-cemented matrix. High-resolution scanning electron microscope (SEM)
study reveals the presence of two different forms of carbon-rich threads,
ribbons and mats, coating both sclerites and silica-cemented matrix.
Crystalline-looking biofilm threads and ribbons composed of Si, O, and C
are interpreted as silicified biofilm associated with the trilobite
silicification. Rippled to smooth biofilm mats composed of more C, less Si
and O, and a trace of N are post-silicification. Embedded in the silica of
the sclerites and matrix are molds of framboids that we interpret as
originally framboidal pyrite that was engulfed by silica. These data
indicate that silica precipitation continued into the surrounding matrix
following the propagation of sulfate-reducing bacteria feeding on the
organic matter present in the sclerite and the neighboring sediment. This
strongly supports the model that bacterially mediated decay is key to the
silicification of carbonate bioclasts and provides the first direct
evidence of a microbial community (biofilm). A literature review reveals
that silica extends past the fossil more frequently than is recognized,
suggesting that silicified biofilm might be common but overlooked.
Eocene magmatism in the Himalaya: Response to lithospheric flexure
during early Indian collision?
Lin Ma; Qiang Wang; Andrew C. Kerr; Zheng-Xiang Li; Wei Dan ...
Eocene mafic magmatism in the Himalaya provides a crucial window for
probing the evolution of crustal anatexis processes within the lower plate
in a collisional orogen. We report geochemical data from the earliest
postcollision ocean-island basalt–like mafic dikes intruding the Tethyan
Himalaya near the northern edge of the colliding Indian plate. These dikes
occurred coeval, and spatially overlap, with Eocene granitoids in the cores
of gneiss domes and were likely derived from interaction between melts from
the lithosphere-asthenosphere boundary and the Indian continental
lithosphere. We propose that these mafic magmas were emplaced along
lithospheric fractures in response to lithospheric flexure during initial
subduction of the Indian continent and that the underplating of such mafic
magmas resulted in orogen-parallel crustal anatexis within the Indian
continent. This mechanism can explain the formation of coeval magmatism and
the geologic evolution of a collisional orogen on both sides of the suture
zone.
Sponge-rich sediment recycling in a Paleozoic continental arc
driven by mélange melting
Huichuan Liu; Sune G. Nielsen; Guangyou Zhu
Slab material transfer processes in continental arcs can be challenging to
decipher because magmas are often characterized by significant
contributions from continental material. In this study, we identified a
Prototethyan continental arc (419–418 Ma) that is now located in the
Dazhonghe area of the southeast Tibetan Plateau, which, based on
Sr-Nd-Hf-O-Si isotope relationships, implies no detectable continental
material contributions. The Dazhonghe arc rocks display much lower δ 30Si values than modern arc rocks and average mantle; this is
best explained by subduction of sponge-rich marine sediments, which are
thought to have been the dominant marine organisms during the
Neoproterozoic and early Paleozoic. Our mixing calculations reveal that
only bulk mixing among sponge-rich sediments, altered oceanic crust (AOC),
and the depleted mantle would be capable of accounting for all the
Sr-Nd-Hf-O-Si isotope compositions. This finding implies that the Dazhonghe
arc magmas were generated by melting of a mélange that formed at the
slab-mantle interface. The Dazhonghe continental arc is the first for which
mélange melting has been confirmed.
Preferential preservation of low-elevation biotas in the nonmarine
fossil record
Steven M. Holland; Katharine M. Loughney; Marjean Cone
Modern coastal sedimentary basins typically lie at low elevations (<600
m), whereas inland basins commonly occupy elevations as high as 4000–5000
m. Individual basins of all types typically preserve a narrower span of
elevations, generally <1000 m, and typically near 200–300 m in coastal
basins. As a result, the nonmarine fossil record is expected to preserve
mainly low-elevation habitats and a relatively narrow range of elevations.
Because many of the basins that preserve high elevations are likely to
undergo subsequent destruction via erosion or continental collision, the
dominance of low-elevation habitats is likely to become stronger into deep
time. This selective preservation of nonmarine communities from sedimentary
basins, and specifically from low elevations, suggests that much or even
most of ancient nonmarine biodiversity is not preserved. Given the
occurrence of many modern biodiversity hotspots in regions of high
elevation, long ghost lineages are likely common in the nonmarine fossil
record, and divergence times estimated from the nonmarine fossil record may
be systematically far too short. The spans of elevations that are preserved
in sedimentary basins suggest that the fossil record may preserve gradients
in community composition that are correlated with elevation, yet they have
been largely undetected.
Pre-agricultural soil erosion rates in the midwestern United States
Caroline L. Quarrier; Jeffrey S. Kwang; Brendon J. Quirk; Evan A. Thaler;
Isaac J. Larsen
Erosion degrades soils and undermines agricultural productivity. For
agriculture to be sustainable, soil erosion rates must be low enough to
maintain fertile soil. Hence, quantifying both pre-agricultural and
agricultural erosion rates is vital for determining whether farming
practices are sustainable. However, there have been few measurements of
pre-agricultural erosion rates in major farming areas where soils form from
Pleistocene deposits. We quantified pre-agricultural erosion rates in the
midwestern United States, one of the world’s most productive agricultural
regions. We sampled soil profiles from 14 native prairies and used in situ–produced 10Be and geochemical mass balance to
calculate physical erosion rates. The median pre-agricultural erosion rate
of 0.04 mm yr–1 is orders of magnitude lower than agricultural
values previously measured in adjacent fields, as is a site-averaged
diffusion coefficient (0.005 m2 yr–1) calculated from erosion
rate and topographic curvature data. The long-term erosion rates are also
one to four orders of magnitude lower than the assumed 1 mm yr–1
soil loss tolerance value assigned to these locations by the U.S.
Department of Agriculture. Hence, quantifying long-term erosion rates using
cosmogenic nuclides provides a means for more robustly defining rates of
tolerable erosion and for developing management guidelines that promote
soil sustainability.
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