New Geology Articles Published Online Ahead of Print in March
Boulder, Colo., USA: Twenty-eight new articles were published online ahead
of print for Geology in March. Topics include “the age of dust,”
volcanic carbon cycling at Newberry Volcano, Archean geodynamics, the North
China Craton, anaerobic oxidation of methane, and the small-world dynamics
that drove the divergence of burrowing behaviors. These Geology
articles are online at
https://geology.geoscienceworld.org/content/early/recent
.
Uplifted marine terraces on Santa Catalina Island, California, USA
Margarita McInnis; Nicholas Pinter
Abstract:
Marine terraces are widespread along California’s coastline, including on
all of the Channel Islands, with the possible exception of Santa Catalina.
For over a century, the origins of subhorizontal surfaces and gravel
deposits on Santa Catalina have been debated, with recent suggestions that
Santa Catalina has no marine terraces and is subsiding. We mapped,
measured, and described terrace deposits on Santa Catalina Island,
including both in situ deposits and distributed gravel float.
Rounded gravels and cobbles, locally pholad-bored, are present as float
across low-relief surfaces in the Little Harbor area. We also mapped and
described the Eagles Nest Gravels, an ~8-m-thick package overlying a broad
bedrock-cut platform at ~200 m elevation and dipping 3.2° northward. The
Eagles Nest Gravels contain rounded cobbles and boulders, many of which
contain pholad borings. Two other platforms are inferred from concordant
gravels with similar orientations but at lower elevations. Terrace deposits
on Santa Catalina truncate underlying lithological units, including a
narrow band of fossiliferous Miocene to Pliocene sands. Terrace deposits
and gravel lag on Santa Catalina closely resemble older terrace deposits on
other California Channel Islands. The terraces on Santa Catalina Island
remain undated but document at least 200 m of net uplift, similar to the
elevations of undated terraces on the other Channel Islands. While the
timing of uplift of Santa Catalina is unclear, analysis of terrace deposits
in the Little Harbor area confirms their marine origin and settles the
debate regarding the presence of marine terraces on Santa Catalina Island.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48219.1/595727/Uplifted-marine-terraces-on-Santa-Catalina-Island
New chronology of the Chinese loess-paleosol sequence by leaf wax δD
records during the past 800 k.y.
Zheng Wang; Weiguo Liu; Hong Wang; Yunning Cao; Jing Hu ...
Abstract:
The Chinese loess-paleosol sequences provide important archives for
studying paleoenvironmental changes. However, the lack of independent and
accurate time scales hinders the study between loess and other records.
Asian stalagmite δ18O records indicate synchronous patterns of
paleoprecipitation δ18O over large geographic regions. The
record of hydrogen isotopic composition of plant wax (δDwax) in
Chinese loess is also controlled by rainwater δD. Both share a common
origin. The linear relationship between rainfall δ18O and δD
variance provides the basis to tie together chronologies of the same
climate event in different records. Here, we show a new loess chronology by
correlating chronologies of marker boundaries of the prominent climate
chronozones in stalagmite δ18O and summer insolation to the
equivalent climate stratigraphy in the loess δDwax sequence. We
first developed and tested this novel methodology with data since the last
interglacial on a millennial scale, and then applied this approach to the
loess δDwax sequence for the past 800 k.y. to improve the
traditional chronology based on magnetic susceptibility and grain size. The
new δDwax time series provides not only an improved chronology
for studying paleoclimate changes during interglacial intervals, it also
represents a unique database with which to better understand the links
between the Asian monsoon changes in the Chinese loess and other global
climate events, especially for the periods prior to 640 ka, for which
stalagmite records are not available.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48833.1/595728/New-chronology-of-the-Chinese-loess-paleosol
Re-Os geochronology highlights widespread latest Mesoproterozoic (ca.
1090–1050 Ma) cratonic basin development on northern Laurentia
J. Wilder Greenman; Alan D. Rooney; Mollie Patzke; Alessandro Ielpi; Galen
P. Halverson
Abstract:
The terminal Mesoproterozoic was a period of widespread tectonic
convergence globally, culminating in the amalgamation of the Rodinia
supercontinent. However, in Laurentia, long-lived orogenesis on its eastern
margin was punctuated by short-lived extension that generated the
Midcontinent Rift ca. 1110–1090 Ma. Whereas this cratonic rift basin is
typically considered an isolated occurrence, a series of new depositional
ages demonstrate that multiple cratonic basins in northern Laurentia
originated around this time. We present a Re-Os isochron date of 1087.1 ±
5.9 Ma from organic-rich shales of the Agu Bay Formation of the Fury and
Hecla Basin, which is one of four closely spaced cratonic basins spanning
from northeastern Canada to northwestern Greenland known as the Bylot
basins. This age is identical, within uncertainty, to ages from the
Midcontinent Rift and the Amundsen Basin in northwestern Canada. These ages
imply that the late Mesoproterozoic extensional episode in Laurentia was
widespread and likely linked to a common origin. We propose that
significant thermal anomalies and mantle upwelling related to
supercontinent assembly centered around the Midcontinent Rift influenced
the reactivation of crustal weaknesses in Arctic Laurentia beginning ca.
1090 Ma, triggering the formation of a series of cratonic basins.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48521.1/595729/Re-Os-geochronology-highlights-widespread-latest
Decoupling of Au and As during rapid pyrite crystallization
Ya-Fei Wu; Katy Evans; Si-Yu Hu; Denis Fougerouse; Mei-Fu Zhou ...
Abstract:
Gold (Au) is largely hosted by pyrite in a variety of hydrothermal systems,
but the incorporation of Au into pyrite under disequilibrium conditions
remains poorly understood. We integrate synchrotron X-ray fluorescence
microscopy, electron backscatter diffraction, nanoscale secondary ion mass
spectrometry, and laser ablation–multicollector–inductively coupled
plasma–mass spectrometry to constrain the processes that sequester Au into
zoned pyrite in the hydrothermal cement of breccia ores from the
world-class Daqiao orogenic Au deposit, central China. Euhedral pyrite
cores with oscillatory and sector zoning, variable δ34S values,
and lower Au-As contents than the mantles are attributed to crystallization
during oxidation of metal-depleted ore fluids with local variation in fluid
conditions. The isotopically uniform colloform mantles are formed by pyrite
crystallites separated by low-angle boundaries and are characterized by
unusual decoupling of Au and As. Mantle formation is attributed to rapid
disequilibrium precipitation from a metal-rich FeS2
-supersaturated fluid. Incorporation of Au into the pyrite mantles was
facilitated by abundant lattice defects produced by rapid nucleation.
Gold-As–poor pyrite rims were deposited from an evolved ore fluid or other
metal-depleted fluids. These results show that chemical variations recorded
by fine layering within minerals can provide valuable insights into
disequilibrium mass transfer and ore formation. The decoupling between Au
and As in pyrite mantles indicates that As is not always a reliable proxy
for Au enrichment in rapidly crystallized porous pyrite.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48443.1/595730/Decoupling-of-Au-and-As-during-rapid-pyrite
Isotopically “heavy” pyrite in marine sediments due to high
sedimentation rates and non-steady-state deposition
Jiarui Liu; Gilad Antler; André Pellerin; Gareth Izon; Ingrid Dohrmann ...
Abstract:
Sedimentary pyrite formation links the global biogeochemical cycles of
carbon, sulfur, and iron, which, in turn, modulate the redox state of the
planet’s surficial environment over geological time scales. Accordingly,
the sulfur isotopic composition (δ34S) of pyrite has been widely
employed as a geochemical tool to probe the evolution of ocean chemistry.
Characteristics of the depositional environment and post-depositional
processes, however, can modify the δ34S signal that is captured
in sedimentary pyrite and ultimately preserved in the geological record.
Exploring sulfur and iron diagenesis within the Bornholm Basin, Baltic Sea,
we find that higher sedimentation rates limit the near-surface
sulfidization of reactive iron, facilitating its burial and hence the
subsurface availability of reactive iron for continued and progressively
more 34S-enriched sediment-hosted pyrite formation (δ34S ≈ –5‰). Using a diagenetic model, we show that the amount of
pyrite formed at the sediment-water interface has increased over the past
few centuries in response to expansion of water-column hypoxia, which also
impacts the sulfur isotopic signature of pyrite at depth. This contribution
highlights the critical role of reactive iron in pyrite formation and
questions to what degree pyrite δ34S values truly reflect past
global ocean chemistry and biogeochemical processes. This work strengthens
our ability to extract local paleoenvironmental information from pyrite δ34S signatures.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48415.1/595731/Isotopically-heavy-pyrite-in-marine-sediments-due
Archean geodynamics: Ephemeral supercontinents or long-lived
supercratons
Yebo Liu; Ross N. Mitchell; Zheng-Xiang Li; Uwe Kirscher; Sergei A.
Pisarevsky ...
Abstract:
Many Archean cratons exhibit Paleoproterozoic rifted margins, implying they
were pieces of some ancestral landmass(es). The idea that such an ancient
continental assembly represents an Archean supercontinent has been proposed
but remains to be justified. Starkly contrasting geological records between
different clans of cratons have inspired an alternative hypothesis where
cratons were clustered in multiple, separate “supercratons.” A new ca. 2.62
Ga paleomagnetic pole from the Yilgarn craton of Australia is compatible
with either two successive but ephemeral supercontinents or two long-lived
supercratons across the Archean-Proterozoic transition. Neither
interpretation supports the existence of a single, long-lived
supercontinent, suggesting that Archean geodynamics were fundamentally
different from subsequent times (Proterozoic to present), which were
influenced largely by supercontinent cycles.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48575.1/595625/Archean-geodynamics-Ephemeral-supercontinents-or
North China craton: The conjugate margin for northwestern Laurentia in
Rodinia
Jikai Ding; Shihong Zhang; David A.D. Evans; Tianshui Yang; Haiyan Li ...
Abstract:
In the Rodinia supercontinent, Laurentia is placed at the center because it
was flanked by late Neoproterozoic rifted margins; however, the conjugate
margin for western Laurentia is still enigmatic. In this study, new
paleomagnetic results have been obtained from 15 ca. 775 Ma mafic dikes in
eastern Hebei Province, North China craton (NCC). Stepwise thermal
demagnetization revealed a high-temperature component, directed northeast
or southwest with shallow inclinations, with unblocking temperatures of as
high as 580 °C. Rock magnetism suggests the component is carried by
single-domain and pseudo-single-domain magnetite grains. Its primary origin
is supported by a positive reversal test and regional remanence direction
correlation test, and the paleomagnetic pole (29.0°S, 64.7°E, A 95 = 5.4°) is not similar to any published younger poles of the
NCC. Matching the late Mesoproterozoic to early Neoproterozoic (ca.
1110–775 Ma) apparent polar wander paths of the NCC and Laurentia suggests
that the NCC could have been the conjugate margin for northwestern
Laurentia in Rodinia, rather than sitting off the northeast coast of the
main Rodinian landmass. Geological data indicate that breakup of the NCC
and Laurentia occurred between ca. 775 and 720 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48483.1/595626/North-China-craton-The-conjugate-margin-for
Ice-sheet melt drove methane emissions in the Arctic during the last
two interglacials
P.-A. Dessandier; J. Knies; A. Plaza-Faverola; C. Labrousse; M. Renoult ...
Abstract:
Circum-Arctic glacial ice is melting in an unprecedented mode, and release
of currently trapped geological methane may act as a positive feedback on
ice-sheet retreat during global warming. Evidence for methane release
during the penultimate (Eemian, ca. 125 ka) interglacial, a period with
less glacial sea ice and higher temperatures than today, is currently
absent. Here, we argue that based on foraminiferal isotope studies on drill
holes from offshore Svalbard, Norway, methane leakage occurred upon the
abrupt Eurasian ice-sheet wastage during terminations of the last
(Weichselian) and penultimate (Saalian) glaciations. Progressive increase
of methane emissions seems to be first recorded by depleted benthic
foraminiferal δ13C. This is quickly followed by the
precipitation of methane-derived authigenic carbonate as overgrowth inside
and outside foraminiferal shells, characterized by heavy δ18O
and depleted δ13C of both benthic and planktonic foraminifera.
The similarities between the events observed over both terminations
advocate for a common driver for the episodic release of geological methane
stocks. Our favored model is recurrent leakage of shallow gas reservoirs
below the gas hydrate stability zone along the margin of western Svalbard
that can be reactivated upon initial instability of the grounded,
marine-based ice sheets. Analogous to this model, with the current
acceleration of the Greenland ice melt, instabilities of existing methane
reservoirs below and nearby the ice sheet are likely.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48580.1/595627/Ice-sheet-melt-drove-methane-emissions-in-the
Anaerobic oxidation of methane by Mn oxides in sulfate-poor
environments
Chunfang Cai; Kaikai Li; Dawei Liu; Cedric M. John; Daowei Wang ...
Abstract:
Strongly 13C-depleted authigenic carbonates (e.g., δ 13CVPDB <−30‰; VPDB—Vienna Peedee belemnite) in
nature are generally believed to form by sulfate-dependent anaerobic
oxidation of methane (AOM). However, we demonstrate using geochemical data
and thermodynamic calculation that such calcites are most likely derived
from biogenic oxidation of methane in sulfate-poor, nonmarine environments
during early diagenesis, as observed in the Triassic sandy conglomerates
from the Junggar Basin, northwestern China. This process operated through
preferential oxidation of 13C-depleted methane by Mn oxides in
closed conditions, producing calcites with higher Mn contents and δ 13C values in association with more 13C-enriched
residual methane as a result of kinetic isotope fractionation. Thus, the
Mn-rich and 13C-depleted carbonates are proposed as tracers of
Mn-dependent AOM, which should have served as an important sink of
greenhouse methane in low-sulfate early Earth’s oceans.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48553.1/595628/Anaerobic-oxidation-of-methane-by-Mn-oxides-in
Spatio-temporal patterns of Pyrenean exhumation revealed by inverse
thermo-kinematic modeling of a large thermochronologic data set
Magdalena Ellis Curry; Peter van der Beek; Ritske S. Huismans; Sebastian G.
Wolf; Charlotte Fillon ...
Abstract:
Large thermochronologic data sets enable orogen-scale investigations into
spatio-temporal patterns of erosion and deformation. We present the results
of a thermo-kinematic modeling study that examines large-scale controls on
spatio-temporal variations in exhumation as recorded by multiple
low-temperature thermochronometers in the Pyrenees mountains
(France/Spain). Using 264 compiled cooling ages spanning ~200 km of the
orogen, a recent model for its topographic evolution, and the
thermo-kinematic modeling code Pecube, we evaluated two models for Axial
Zone (AZ) exhumation: (1) thrust sheet–controlled (north-south) exhumation,
and (2) along-strike (east-west) variable exhumation. We also measured the
degree to which spatially variable post-orogenic erosion influenced the
cooling ages. We found the best fit for a model of along-strike variable
exhumation. In the eastern AZ, rock uplift rates peak at ≥1 mm/yr between
40 and 30 Ma, whereas in the western AZ, they peak between 30 and 20 Ma.
The amount of post-orogenic (<20 Ma) erosion increases from <1.0 km
in the eastern Pyrenees to >2.5 km in the west. The data reveal a
pattern of exhumation that is primarily controlled by structural
inheritance, with ancillary patterns reflecting growth and erosion of the
antiformal stack and post-orogenic surface processes.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48687.1/595629/Spatio-temporal-patterns-of-Pyrenean-exhumation
Small-world dynamics drove Phanerozoic divergence of burrowing
behaviors
Andrea Baucon; Carlos Neto de Carvalho; Fabrizio Felletti; Gabriele
Tosadori; Alexandre Antonelli
Abstract:
Species of burrowing animals have changed substantially over evolutionary
time scales, but, surprisingly, burrows display persisting morphological
patterns throughout the Phanerozoic. Deep-sea burrows are geometrically
patterned, whereas shallow-marine burrows display simpler morphologies.
This divergence between burrow associations is one of the central
conundrums of paleontology, but it has never been quantitatively
demonstrated, and the organizing principles responsible for this structure
remain unknown. We show that the divergence of burrow associations has been
shaped by small-world dynamics, which is proposed as a major
macroevolutionary force in marine environments. Using network analysis, our
study reveals that the association patterns between burrow morphotypes in
45 paleontological sites span ~500 m.y. Strong statistical support is
demonstrated for a surprising association pattern, according to which the
data set is optimally partitioned into two subgroups of tightly associated
burrow types. These groups correspond to shallow- and deep-marine biomes.
Our analysis demonstrates that across the Phanerozoic Eon, burrows did not
assemble randomly nor regularly, following instead small-world assembly
rules remarkably similar to those that shape human social networks. As
such, small-world dynamics deeply influenced gene flow and natural
variation in heritable behavior across evolutionary time.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48523.1/595630/Small-world-dynamics-drove-Phanerozoic-divergence
Maturation experiments reveal bias in the chemistry of fossil
melanosomes
Valentina Rossi; Samuel M. Webb; Maria McNamara
Abstract:
Fossil melanosomes are a major focus of paleobiological research because
they can inform on the original coloration, phylogenetic affinities, and
internal anatomy of ancient animals. Recent studies of vertebrate
melanosomes revealed tissue-specific trends in melanosome-metal
associations that can persist in fossils. In some fossil vertebrates,
however, melanosomes from all body regions are enriched only in Cu,
suggesting diagenetic overprinting of original chemistry. We tested this
hypothesis using laboratory experiments on melanosomes from skin and liver
of the African clawed frog Xenopus laevis. After maturation in
Cu-rich media, the metal chemistry of melanosomes from these tissues
converged toward a common composition, and original differences in Cu
oxidation state were lost. Elevated Cu concentrations and a pervasive
Cu(II) signal are likely indicators of diagenetically altered melanosomes.
These results provide a robust experimental basis for interpretating the
chemistry of fossil melanosomes.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48696.1/595631/Maturation-experiments-reveal-bias-in-the
New zircon radiometric U-Pb ages and Lu-Hf isotopic data from the
ultramafic-mafic sequences of Ranau and Telupid (Sabah, eastern
Malaysia): Time to reconsider the geological evolution of Southeast
Asia?
Basilios Tsikouras; Chun-Kit Lai; Elena Ifandi; Nur’Aqidah Norazme;
Chee-Hui Teo ...
Abstract:
New zircon U-Pb geochronology from a peridotite suite near Ranau and the
Telupid ophiolite in Sabah, eastern Malaysia, contradict previous studies,
which assumed that the Sabah mafic-ultramafic rocks are largely ophiolitic
and Jurassic–Cretaceous in age. We show that these rocks formed during a
magmatic episode in the Miocene (9.2–10.5 Ma), which is interpreted to
reflect infiltration of melts and melt-rock reaction in the Ranau
subcontinental peridotites during extension, and concurrent seafloor
spreading forming the Telupid ophiolite further south. Older zircons from
the Ranau peridotites have Cretaceous, Devonian, and Neoproterozoic ages.
Zircon Lu-Hf isotopic data suggest their derivation from a depleted mantle.
However, significant proportions of crustal components have been
incorporated in their genesis, as evidenced by their less-radiogenic Hf
signature compared to a pristine mantle reservoir. The involvement of a
crustal component is consistent with our interpreted continental setting
for the Ranau peridotite and formation in a narrow backarc basin for the
Telupid ophiolite. We infer that the Sulu Sea, which was expanding
throughout much of the Miocene, may have extended to the southwest into
central Sabah. The Telupid oceanic strand formed during the split,
collapse, and rollback of the Sulu arc due to the subduction of the Celebes
Sea beneath Sabah. Incorporation of the Sulu arc in the evolving Miocene
oceanic basin is a potential source to explain the involvement of crustal
material in the zircon evolution of the Telupid ophiolite.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48126.1/595632/New-zircon-radiometric-U-Pb-ages-and-Lu-Hf
New multicellular marine macroalgae from the early Tonian of
northwestern Canada
Katie M. Maloney; Galen P. Halverson; James D. Schiffbauer; Shuhai Xiao;
Timothy M. Gibson ...
Abstract:
Molecular phylogenetic data suggest that photosynthetic eukaryotes first
evolved in freshwater environments in the early Proterozoic and diversified
into marine environments by the Tonian Period, but early algal evolution is
poorly reflected in the fossil record. Here, we report newly discovered,
millimeter- to centimeter-scale macrofossils from outershelf marine facies
of the ca. 950–900 Ma (Re-Os minimum age constraint = 898 ± 68 Ma) Dolores
Creek Formation in the Wernecke Mountains, northwestern Canada. These
fossils, variably preserved by iron oxides and clay minerals, represent two
size classes. The larger forms feature unbranching thalli with uniform
cells, differentiated cell walls, longitudinal striations, and probable
holdfasts, whereas the smaller specimens display branching but no other
diagnostic features. While the smaller population remains unresolved
phylogenetically and may represent cyanobacteria, we interpret the larger
fossils as multicellular eukaryotic macroalgae with a plausible green algal
affinity based on their large size and presence of rib-like wall
ornamentation. Considered as such, the latter are among the few green algae
and some of the largest macroscopic eukaryotes yet recognized in the early
Neoproterozoic. Together with other Tonian fossils, the Dolores Creek
fossils indicate that eukaryotic algae, including green algae, colonized
marine environments by the early Neoproterozoic Era.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48508.1/595633/New-multicellular-marine-macroalgae-from-the-early
The age of dust—A new hydrological indicator in arid environments?
Simon Turner; Heather Handley; Paul Hesse; Bruce Schaefer; Anthony Dosseto
Abstract:
Dust plays important roles in the environment, and there has been much
interest in the formation, provenance, and age of the world’s dust
deposits. Ongoing debates are concerned with the importance of glacial
grinding versus eolian abrasion and fluvial transport in the formation of
silt-sized particles. Short-lived uranium-series isotopes afford new
insights because they can be used both for provenance fingerprinting and
for constraining the integrated age of chemical and physical weathering and
subsequent transport and storage of sediments. Here we present trace
element and Sr, Nd, and U-series isotope analyses from a number of
Australian dusts and suspended river sediments remobilized during floods a
year later. The inferred ages of the Australian dust appear to be linked to
aridification and the loss of inland megalakes ~30–120 k.y. ago. This
provides preliminary evidence that the age of dust may provide a new
hydrological indicator in arid environments.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48592.1/595183/The-age-of-dust-A-new-hydrological-indicator-in
Mid-Miocene volcanic migration in the westernmost Sunda arc induced by
India-Eurasia collision
Yu-Ming Lai; Sun-Lin Chung; Azman A. Ghani; Sayed Murtadha; Hao-Yang Lee
...
Abstract: The migration of arc magmatism that is a fundamental aspect of
plate tectonics may reflect the complex interaction between subduction zone
processes and regional tectonics. Here we report new observations on
volcanic migration from northwestern Sumatra, in the westernmost Sunda arc,
characterized by an oblique convergent boundary between the Indo-Australian
and Eurasian plates. Our study indicates that in northwestern Sumatra,
volcanism ceased at 15–10 Ma on the southern coast and reignited to form a
suite of active volcanoes that erupt exclusively to the north of the
trench-parallel Sumatran fault. Younger volcanic rocks from the north are
markedly more enriched in K2O and other highly incompatible
elements, delineating a geochemical variation over space and time similar
to that in Java and reflecting an increase in the Benioff zone depth. We
relate this mid-Miocene volcanic migration in northwestern Sumatra to the
far-field effect of propagating extrusion tectonics driven by the
India-Eurasia collision. The extrusion caused regional deformation
southward through Myanmar to northwestern Sumatra and thus transformed the
oblique subduction into a dextral motion–governed plate boundary. This
tectonic transformation, associated with opening of the Andaman Sea, is
suggested to be responsible for the volcanic migration in northwestern
Sumatra.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48568.1/595184/Mid-Miocene-volcanic-migration-in-the-westernmost
Volcanic carbon cycling in East Lake, Newberry Volcano, Oregon, USA
H.D. Brumberg; L. Capece; C.N. Cauley; P. Tartell; C. Smith
Abstract:
The carbon cycle in East Lake (Newberry Volcano, Oregon, USA) is fueled by
volcanic CO2 inputs with traces of Hg and H2S. The CO 2 dissolves in deep lake waters and is removed in shallow waters
through largely diffusive surface degassing and photosynthesis. Escaping
gas and photosynthate have low δ13C values, leading to δ 13C(DIC) (DIC—dissolved inorganic carbon) as high as +5.7‰ in
surface waters, well above the common global lake range. A steep δ 13C depth gradient is further established by respiration and
absorption of light volcanic CO2 in bottom waters. The seasonal
CO2 degassing starts at >100 t CO2/day after ice
melting in the spring and declines to ~40 t/day in late summer, degassing
~11,700 t CO2/yr. Thus, volcano monitoring through gas fluxes
from crater lakes should consider lacustrine processes that modulate the
volcanic gas output over time. The flux contribution of a bubbling CO2 “hotspot” increased from 20% to >90% of the lake-wide CO 2 flux from 2015 to 2019 CE, followed by a “toxic gas alert” in
July 2020. East Lake is an active volcanic lake with a “geogenic” ecosystem
driven primarily by hydrothermal inputs.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48388.1/595185/Volcanic-carbon-cycling-in-East-Lake-Newberry
Magnetostratigraphy of U-Pb–dated boreholes in Svalbard, Norway,
implies that magnetochron M0r (a proposed Barremian-Aptian boundary
marker) begins at 121.2 ± 0.4 Ma
Yang Zhang; James G. Ogg; Daniel Minguez; Mark W. Hounslow; Snorre Olaussen
...
Abstract:
The age of the beginning of magnetic polarity Chron M0r, a proposed marker
for the base of the Aptian Stage, is disputed due to a divergence of
published radioisotopic dates and ambiguities in stratigraphic correlation
of sections. Our magnetostratigraphy of core DH1 from Svalbard, Norway,
calibrates a bentonite bed, dated by U-Pb methods to 123.1 ± 0.3 Ma, to the
uppermost part of magnetozone M1r, which is ~1.9 m.y. before the beginning
of Chron M0r. This is the first direct calibration of any high-precision
radioisotopic date to a polarity chron of the M sequence. The interpolated
age of 121.2 ± 0.4 Ma for the beginning of Chron M0r is younger by ~5 m.y.
than its estimated age used in the Geologic Time Scale 2012, which had been
extrapolated from radioisotopic dates on oceanic basalts and from Aptian
cyclostratigraphy. The adjusted age model implies a commensurate faster
average global oceanic spreading rate of ~12% during the Aptian–Santonian
interval. Future radioisotopic dating and high-resolution cyclostratigraphy
are needed to investigate where to expand the mid-Jurassic to earliest
Cretaceous interval by the required ~4 m.y.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48591.1/595186/Magnetostratigraphy-of-U-Pb-dated-boreholes-in
Calcium isotope composition of Morozovella over the late
Paleocene–early Eocene
Gabriella D. Kitch; Andrew D. Jacobson; Dustin T. Harper; Matthew T.
Hurtgen; Bradley B. Sageman ...
Abstract:
Ocean acidification (OA) during the Paleocene-Eocene thermal maximum (PETM)
likely caused a biocalcification crisis. The calcium isotope composition (δ 44/40Ca) of primary carbonate producers may be sensitive to OA.
To test this hypothesis, we constructed the first high-resolution,
high-precision planktic foraminiferal δ44/40Ca records before
and across the PETM. The records employ specimens of Morozovella
spp. collected from Ocean Drilling Program Sites 1209 (Shatsky Rise,
Pacific Ocean) and 1263 (Walvis Ridge, Atlantic Ocean). At Site 1209, δ 44/40Ca values start at –1.33‰ during the Upper Paleocene and
increase to a peak of –1.15‰ immediately before the negative carbon isotope
excursion (CIE) that marks the PETM onset. Values remain elevated through
the PETM interval and decrease into the earliest Eocene. A shorter-term
record for Site 1263 shows a similar trend, although δ44/40Ca
values are on average 0.22‰ lower and decrease shortly after the CIE onset.
The trends support neither diagenetic overprinting, authigenic carbonate
additions, nor changes in the δ44/40Ca value of seawater.
Rather, they are consistent with a kinetic isotope effect, whereby calcite
δ44/40Ca values inversely correlate with precipitation rate.
Geologically rapid Ca isotope shifts appear to reflect the response of Morozovella to globally forced changes in the local carbonate
geochemistry of seawater. All data combined suggest that the PETM-OA event
occurred near the peak of a gradual reduction in seawater carbonate ion
concentrations during a time of elevated atmospheric pCO 2, potentially driven by North Atlantic igneous province
emplacement.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48619.1/595187/Calcium-isotope-composition-of-Morozovella-over
Archean geodynamics: Ephemeral supercontinents or long-lived
supercratons
Yebo Liu; Ross N. Mitchell; Zheng-Xiang Li; Uwe Kirscher; Sergei A.
Pisarevsky ...
Abstract:
Many Archean cratons exhibit Paleoproterozoic rifted margins, implying they
were pieces of some ancestral landmass(es). The idea that such an ancient
continental assembly represents an Archean supercontinent has been proposed
but remains to be justified. Starkly contrasting geological records between
different clans of cratons have inspired an alternative hypothesis where
cratons were clustered in multiple, separate “supercratons.” A new ca. 2.62
Ga paleomagnetic pole from the Yilgarn craton of Australia is compatible
with either two successive but ephemeral supercontinents or two long-lived
supercratons across the Archean-Proterozoic transition. Neither
interpretation supports the existence of a single, long-lived
supercontinent, suggesting that Archean geodynamics were fundamentally
different from subsequent times (Proterozoic to present), which were
influenced largely by supercontinent cycles.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48575.1/595625/Archean-geodynamics-Ephemeral-supercontinents-or
North China craton: The conjugate margin for northwestern Laurentia in
Rodinia
Jikai Ding; Shihong Zhang; David A.D. Evans; Tianshui Yang; Haiyan Li ...
Abstract:
In the Rodinia supercontinent, Laurentia is placed at the center because it
was flanked by late Neoproterozoic rifted margins; however, the conjugate
margin for western Laurentia is still enigmatic. In this study, new
paleomagnetic results have been obtained from 15 ca. 775 Ma mafic dikes in
eastern Hebei Province, North China craton (NCC). Stepwise thermal
demagnetization revealed a high-temperature component, directed northeast
or southwest with shallow inclinations, with unblocking temperatures of as
high as 580 °C. Rock magnetism suggests the component is carried by
single-domain and pseudo-single-domain magnetite grains. Its primary origin
is supported by a positive reversal test and regional remanence direction
correlation test, and the paleomagnetic pole (29.0°S, 64.7°E, A 95 = 5.4°) is not similar to any published younger poles of the
NCC. Matching the late Mesoproterozoic to early Neoproterozoic (ca.
1110–775 Ma) apparent polar wander paths of the NCC and Laurentia suggests
that the NCC could have been the conjugate margin for northwestern
Laurentia in Rodinia, rather than sitting off the northeast coast of the
main Rodinian landmass. Geological data indicate that breakup of the NCC
and Laurentia occurred between ca. 775 and 720 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48483.1/595626/North-China-craton-The-conjugate-margin-for
Ice-sheet melt drove methane emissions in the Arctic during the last
two interglacials
P.-A. Dessandier; J. Knies; A. Plaza-Faverola; C. Labrousse; M. Renoult ...
Abstract:
Circum-Arctic glacial ice is melting in an unprecedented mode, and release
of currently trapped geological methane may act as a positive feedback on
ice-sheet retreat during global warming. Evidence for methane release
during the penultimate (Eemian, ca. 125 ka) interglacial, a period with
less glacial sea ice and higher temperatures than today, is currently
absent. Here, we argue that based on foraminiferal isotope studies on drill
holes from offshore Svalbard, Norway, methane leakage occurred upon the
abrupt Eurasian ice-sheet wastage during terminations of the last
(Weichselian) and penultimate (Saalian) glaciations. Progressive increase
of methane emissions seems to be first recorded by depleted benthic
foraminiferal δ13C. This is quickly followed by the
precipitation of methane-derived authigenic carbonate as overgrowth inside
and outside foraminiferal shells, characterized by heavy δ18O
and depleted δ13C of both benthic and planktonic foraminifera.
The similarities between the events observed over both terminations
advocate for a common driver for the episodic release of geological methane
stocks. Our favored model is recurrent leakage of shallow gas reservoirs
below the gas hydrate stability zone along the margin of western Svalbard
that can be reactivated upon initial instability of the grounded,
marine-based ice sheets. Analogous to this model, with the current
acceleration of the Greenland ice melt, instabilities of existing methane
reservoirs below and nearby the ice sheet are likely.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48580.1/595627/Ice-sheet-melt-drove-methane-emissions-in-the
Anaerobic oxidation of methane by Mn oxides in sulfate-poor
environments
Chunfang Cai; Kaikai Li; Dawei Liu; Cedric M. John; Daowei Wang ...
Abstract:
Strongly 13C-depleted authigenic carbonates (e.g., δ 13CVPDB <−30‰; VPDB—Vienna Peedee belemnite) in
nature are generally believed to form by sulfate-dependent anaerobic
oxidation of methane (AOM). However, we demonstrate using geochemical data
and thermodynamic calculation that such calcites are most likely derived
from biogenic oxidation of methane in sulfate-poor, nonmarine environments
during early diagenesis, as observed in the Triassic sandy conglomerates
from the Junggar Basin, northwestern China. This process operated through
preferential oxidation of 13C-depleted methane by Mn oxides in
closed conditions, producing calcites with higher Mn contents and δ 13C values in association with more 13C-enriched
residual methane as a result of kinetic isotope fractionation. Thus, the
Mn-rich and 13C-depleted carbonates are proposed as tracers of
Mn-dependent AOM, which should have served as an important sink of
greenhouse methane in low-sulfate early Earth’s oceans.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48553.1/595628/Anaerobic-oxidation-of-methane-by-Mn-oxides-in
Spatio-temporal patterns of Pyrenean exhumation revealed by inverse
thermo-kinematic modeling of a large thermochronologic data set
Magdalena Ellis Curry; Peter van der Beek; Ritske S. Huismans; Sebastian G.
Wolf; Charlotte Fillon ...
Abstract:
Large thermochronologic data sets enable orogen-scale investigations into
spatio-temporal patterns of erosion and deformation. We present the results
of a thermo-kinematic modeling study that examines large-scale controls on
spatio-temporal variations in exhumation as recorded by multiple
low-temperature thermochronometers in the Pyrenees mountains
(France/Spain). Using 264 compiled cooling ages spanning ~200 km of the
orogen, a recent model for its topographic evolution, and the
thermo-kinematic modeling code Pecube, we evaluated two models for Axial
Zone (AZ) exhumation: (1) thrust sheet–controlled (north-south) exhumation,
and (2) along-strike (east-west) variable exhumation. We also measured the
degree to which spatially variable post-orogenic erosion influenced the
cooling ages. We found the best fit for a model of along-strike variable
exhumation. In the eastern AZ, rock uplift rates peak at ≥1 mm/yr between
40 and 30 Ma, whereas in the western AZ, they peak between 30 and 20 Ma.
The amount of post-orogenic (<20 Ma) erosion increases from <1.0 km
in the eastern Pyrenees to >2.5 km in the west. The data reveal a
pattern of exhumation that is primarily controlled by structural
inheritance, with ancillary patterns reflecting growth and erosion of the
antiformal stack and post-orogenic surface processes.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48687.1/595629/Spatio-temporal-patterns-of-Pyrenean-exhumation
Small-world dynamics drove Phanerozoic divergence of burrowing
behaviors
Andrea Baucon; Carlos Neto de Carvalho; Fabrizio Felletti; Gabriele
Tosadori; Alexandre Antonelli
Abstract:
Species of burrowing animals have changed substantially over evolutionary
time scales, but, surprisingly, burrows display persisting morphological
patterns throughout the Phanerozoic. Deep-sea burrows are geometrically
patterned, whereas shallow-marine burrows display simpler morphologies.
This divergence between burrow associations is one of the central
conundrums of paleontology, but it has never been quantitatively
demonstrated, and the organizing principles responsible for this structure
remain unknown. We show that the divergence of burrow associations has been
shaped by small-world dynamics, which is proposed as a major
macroevolutionary force in marine environments. Using network analysis, our
study reveals that the association patterns between burrow morphotypes in
45 paleontological sites span ~500 m.y. Strong statistical support is
demonstrated for a surprising association pattern, according to which the
data set is optimally partitioned into two subgroups of tightly associated
burrow types. These groups correspond to shallow- and deep-marine biomes.
Our analysis demonstrates that across the Phanerozoic Eon, burrows did not
assemble randomly nor regularly, following instead small-world assembly
rules remarkably similar to those that shape human social networks. As
such, small-world dynamics deeply influenced gene flow and natural
variation in heritable behavior across evolutionary time.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48523.1/595630/Small-world-dynamics-drove-Phanerozoic-divergence
Maturation experiments reveal bias in the chemistry of fossil
melanosomes
Valentina Rossi; Samuel M. Webb; Maria McNamara
Abstract:
Fossil melanosomes are a major focus of paleobiological research because
they can inform on the original coloration, phylogenetic affinities, and
internal anatomy of ancient animals. Recent studies of vertebrate
melanosomes revealed tissue-specific trends in melanosome-metal
associations that can persist in fossils. In some fossil vertebrates,
however, melanosomes from all body regions are enriched only in Cu,
suggesting diagenetic overprinting of original chemistry. We tested this
hypothesis using laboratory experiments on melanosomes from skin and liver
of the African clawed frog Xenopus laevis. After maturation in
Cu-rich media, the metal chemistry of melanosomes from these tissues
converged toward a common composition, and original differences in Cu
oxidation state were lost. Elevated Cu concentrations and a pervasive
Cu(II) signal are likely indicators of diagenetically altered melanosomes.
These results provide a robust experimental basis for interpretating the
chemistry of fossil melanosomes.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48696.1/595631/Maturation-experiments-reveal-bias-in-the
New zircon radiometric U-Pb ages and Lu-Hf isotopic data from the
ultramafic-mafic sequences of Ranau and Telupid (Sabah, eastern
Malaysia): Time to reconsider the geological evolution of Southeast
Asia?
Basilios Tsikouras; Chun-Kit Lai; Elena Ifandi; Nur’Aqidah Norazme;
Chee-Hui Teo ...
Abstract:
New zircon U-Pb geochronology from a peridotite suite near Ranau and the
Telupid ophiolite in Sabah, eastern Malaysia, contradict previous studies,
which assumed that the Sabah mafic-ultramafic rocks are largely ophiolitic
and Jurassic–Cretaceous in age. We show that these rocks formed during a
magmatic episode in the Miocene (9.2–10.5 Ma), which is interpreted to
reflect infiltration of melts and melt-rock reaction in the Ranau
subcontinental peridotites during extension, and concurrent seafloor
spreading forming the Telupid ophiolite further south. Older zircons from
the Ranau peridotites have Cretaceous, Devonian, and Neoproterozoic ages.
Zircon Lu-Hf isotopic data suggest their derivation from a depleted mantle.
However, significant proportions of crustal components have been
incorporated in their genesis, as evidenced by their less-radiogenic Hf
signature compared to a pristine mantle reservoir. The involvement of a
crustal component is consistent with our interpreted continental setting
for the Ranau peridotite and formation in a narrow backarc basin for the
Telupid ophiolite. We infer that the Sulu Sea, which was expanding
throughout much of the Miocene, may have extended to the southwest into
central Sabah. The Telupid oceanic strand formed during the split,
collapse, and rollback of the Sulu arc due to the subduction of the Celebes
Sea beneath Sabah. Incorporation of the Sulu arc in the evolving Miocene
oceanic basin is a potential source to explain the involvement of crustal
material in the zircon evolution of the Telupid ophiolite.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48126.1/595632/New-zircon-radiometric-U-Pb-ages-and-Lu-Hf
New multicellular marine macroalgae from the early Tonian of
northwestern Canada
Katie M. Maloney; Galen P. Halverson; James D. Schiffbauer; Shuhai Xiao;
Timothy M. Gibson ...
Abstract:
Molecular phylogenetic data suggest that photosynthetic eukaryotes first
evolved in freshwater environments in the early Proterozoic and diversified
into marine environments by the Tonian Period, but early algal evolution is
poorly reflected in the fossil record. Here, we report newly discovered,
millimeter- to centimeter-scale macrofossils from outershelf marine facies
of the ca. 950–900 Ma (Re-Os minimum age constraint = 898 ± 68 Ma) Dolores
Creek Formation in the Wernecke Mountains, northwestern Canada. These
fossils, variably preserved by iron oxides and clay minerals, represent two
size classes. The larger forms feature unbranching thalli with uniform
cells, differentiated cell walls, longitudinal striations, and probable
holdfasts, whereas the smaller specimens display branching but no other
diagnostic features. While the smaller population remains unresolved
phylogenetically and may represent cyanobacteria, we interpret the larger
fossils as multicellular eukaryotic macroalgae with a plausible green algal
affinity based on their large size and presence of rib-like wall
ornamentation. Considered as such, the latter are among the few green algae
and some of the largest macroscopic eukaryotes yet recognized in the early
Neoproterozoic. Together with other Tonian fossils, the Dolores Creek
fossils indicate that eukaryotic algae, including green algae, colonized
marine environments by the early Neoproterozoic Era.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48508.1/595633/New-multicellular-marine-macroalgae-from-the-early
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