New Geology Articles Published Online Ahead of Print in August
Boulder, Colo., USA: Article topics include the mid-Atlantic coastal plain;
the Archean Pilbara Craton of Western Australia; the “Great
pre-Unconformity”; spontaneous reheating of crystallizing lava; the Russian
northern Barents Sea; the Moby-Dick gas hydrate system in the Gulf of
Mexico; and a fjord network in Namibia. These Geology articles are
online at
https://geology.geoscienceworld.org/content/early/recent
.
A molecular biomarker for end-Permian plant extinction in South China
Chunjiang Wang; Henk Visscher
Abstract:
To help resolve current controversies surrounding the fundamental question
of synchrony between end-Permian mass extinction on land and in the sea, we
examined the marine Permian–Triassic reference section at Meishan
(southeastern China) for land-derived molecular degradation products of
pentacyclic triterpenoids with oleanane carbon skeletons, diagnostic for
the Permian plant genus Gigantopteris. We identified a continuous
quantitative record of mono-aromatic des-A-oleanane, which
abruptly ends in the main marine extinction interval just below the
Permian-Triassic boundary. This taxon-specific molecular biomarker,
therefore, reveals in unmatched detail the timing and tempo of the demise
of one of the most distinctive Permian plants and provides evidence of
synchronous extinction among continental and marine organisms. Parallel
reduction in the relative abundance of lignin phenols confirms that
aridity-driven extinction was not restricted to Gigantopteris but
likely affected the entire wetland flora of the equatorial South China
microcontinent.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49123.1/607262/A-molecular-biomarker-for-end-Permian-plant
Fjord network in Namibia: A snapshot into the dynamics of the late
Paleozoic glaciation
Pierre Dietrich; Neil P. Griffis; Daniel P. Le Heron; Isabel P. Montañez;
Christoph Kettler ...
Abstract:
Fjords are glacially carved estuaries that profoundly influence ice-sheet
stability by draining and ablating ice. Although abundant on modern
high-latitude continental shelves, fjord-network morphologies have never
been identified in Earth’s pre-Cenozoic glacial epochs, hindering our
ability to constrain ancient ice-sheet dynamics. We show that U-shaped
valleys in northwestern Namibia cut during the late Paleozoic ice age
(LPIA, ca. 300 Ma), Earth’s penultimate icehouse, represent intact
fjord-network morphologies. This preserved glacial morphology and its
sedimentary fill permit a reconstruction of paleo-ice thicknesses, glacial
dynamics, and resulting glacio-isostatic adjustment. Glaciation in this
region was initially characterized by an acme phase, which saw an extensive
ice sheet (1.7 km thick) covering the region, followed by a waning phase
characterized by 100-m-thick, topographically constrained outlet glaciers
that shrank, leading to glacial demise. Our findings demonstrate that both
a large ice sheet and highland glaciers existed over northwestern Namibia
at different times during the LPIA. The fjords likely played a pivotal role
in glacier dynamics and climate regulation, serving as hotspots for organic
carbon sequestration. Aside from the present-day arid climate, northwestern
Namibia exhibits a geomorphology virtually unchanged since the LPIA,
permitting unique insight into this icehouse.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49067.1/607263/Fjord-network-in-Namibia-A-snapshot-into-the
River sinuosity describes a continuum between randomness and ordered
growth
Ajay B. Limaye; Eli D. Lazarus; Yuan Li; Jon Schwenk
Abstract:
River channels are among the most common landscape features on Earth. An
essential characteristic of channels is sinuosity: their tendency to take a
circuitous path, which is quantified as along-stream length divided by
straight-line length. River sinuosity is interpreted as a characteristic
that either forms randomly at channel inception or develops over time as
meander bends migrate. Studies tend to assume the latter and thus have used
river sinuosity as a proxy for both modern and ancient environmental
factors including climate, tectonics, vegetation, and geologic structure.
But no quantitative criterion for planform expression has distinguished
between random, initial sinuosity and that developed by ordered growth
through channel migration. This ambiguity calls into question the utility
of river sinuosity for understanding Earth’s history. We propose a
quantitative framework to reconcile these competing explanations for river
sinuosity. Using a coupled analysis of modeled and natural channels, we
show that while a majority of observed sinuosity is consistent with
randomness and limited channel migration, rivers with sinuosity ≥1.5 likely
formed their geometry through sustained, ordered growth due to channel
migration. This criterion frames a null hypothesis for river sinuosity that
can be applied to evaluate the significance of environmental
interpretations in landscapes shaped by rivers. The quantitative link
between sinuosity and channel migration further informs strategies for
preservation and restoration of riparian habitat and guides predictions of
fluvial deposits in the rock record and in remotely sensed environments
from the seafloor to planetary surfaces.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49153.1/607264/River-sinuosity-describes-a-continuum-between
Early Pleistocene climate-induced erosion of the Alaska Range formed
the Nenana Gravel
Rachel N. Sortor; Brent M. Goehring; Sean P. Bemis; Chester A. Ruleman;
Marc W. Caffee ...
Abstract:
The Pliocene-Pleistocene transition resulted in extensive global cooling
and glaciation, but isolating this climate signal within erosion and
exhumation responses in tectonically active regimes can be difficult. The
Nenana Gravel is a foreland basin deposit in the northern foothills of the
Alaska Range (USA) that has long been linked to unroofing of the Alaska
Range starting ca. 6 Ma. Using 26Al/10Be cosmogenic
nuclide burial dating, we determined the timing of deposition of the Nenana
Gravel and an overlying remnant of the first glacial advance into the
northern foothills. Our results indicate that initial deposition of the
Nenana Gravel occurred at the onset of the Pleistocene ca. 2.34 Ma and
continued until at least ca. 1.7 Ma. The timing of initial deposition is
correlative with expansion of the Cordilleran ice sheet, suggesting that
the deposit formed due to increased glacial erosion in the Alaska Range.
Abandonment of Nenana Gravel deposition occurred prior to the first
glaciation extending into the northern foothills. This glaciation was
hypothesized to have occurred ca. 1.5 Ma, but we found that it occurred ca.
0.39 Ma. A Pleistocene age for the Nenana Gravel and marine oxygen isotope
stage 10 age for the oldest glaciation of the foothills necessitate
reanalysis of incision and tectonic rates in the northern foothills of the
Alaska Range, in addition to a shift in perspective on how these deposits
fit into the climatic and tectonic history of the region.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49094.1/607265/Early-Pleistocene-climate-induced-erosion-of-the
Contribution of orbital forcing and Deccan volcanism to global climatic
and biotic changes across the Cretaceous-Paleogene boundary at Zumaia,
Spain
Vicente Gilabert; Sietske J. Batenburg; Ignacio Arenillas; José A. Arz
Abstract:
Untangling the timing of the environmental effects of Deccan volcanism with
respect to the Chicxulub impact is instrumental to fully assessing the
contributions of both to climate change over the Cretaceous-Paleogene
boundary (KPB) interval. Despite recent improvements in radiometric age
calibrations, the accuracy of age constraints and correlations is
insufficient to resolve the exact mechanisms leading to environmental and
climate change in the 1 m.y. across the KPB. We present new high-resolution
planktic foraminiferal, geochemical, and geophysical data from the Zumaia
section (Spain), calibrated to an updated orbitally tuned age model. We
provide a revised chronology for the major carbon isotope excursions (CIEs)
and planktic foraminiferal events and test temporal relationships with
different models of the eruptive phases of the Deccan Traps. Our data show
that the major CIEs near the KPB, i.e., the late Maastrichtian warming
event (66.25–66.10 Ma) and the Dan-C2 event (65.8–65.7 Ma), are synchronous
with the last and the first 405 k.y. eccentricity maximum of the
Maastrichtian and the Danian, respectively, and that the minor Lower C29n
event (65.48–65.41 Ma) is well constrained to a short eccentricity maximum.
Conversely, we obtained evidence of abrupt environmental change likely
related to Deccan volcanism at ca. 65.9 Ma, based on a bloom of
opportunistic triserial guembelitriids (Chiloguembelitria). The
orbital, isotopic, and paleobiological temporal relationships with Deccan
volcanism established here provide new insights into the role of Deccan
volcanism in climate and environmental change in the 1 m.y. across the KPB.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49214.1/607267/Contribution-of-orbital-forcing-and-Deccan
Cryogenian glaciostatic and eustatic fluctuations and massive
Marinoan-related deposition of Fe and Mn in the Urucum District, Brazil
B.T. Freitas; I.D. Rudnitzki; L. Morais; M.D.R. Campos; R.P. Almeida ...
Abstract:
Global Neoproterozoic glaciations are related to extreme environmental
changes and the reprise of iron formation in the rock record. However, the
lack of narrow age constraints on Cryogenian successions bearing
iron-formation deposits prevents correlation and understanding of these
deposits on a global scale. Our new multiproxy data reveal a long
Cryogenian record for the Jacadigo Group (Urucum District, Brazil) spanning
the Sturtian and Marinoan ice ages. Deposition of the basal sequence of the
Urucum Formation was influenced by Sturtian continental glaciation and was
followed by a transgressive interglacial record of >600 m of carbonates
that terminates in a glacioeustatic unconformity. Overlying this, there are
up to 500 m of shale and sandstone interpreted as coeval to global Marinoan
glacial advance. Glacial outwash delta deposits at the top of the formation
correlate with diamictite-filled paleovalleys and are covered by massive Fe
and Mn deposits of the Santa Cruz Formation and local carbonate. This
second transgression is related to Marinoan deglaciation. Detrital zircon
provenance supports glaciostatic control on Cryogenian sedimentary yield at
the margins of the Amazon craton. These findings reveal the sedimentary
response to two marked events of glacioeustatic incision and transgression,
culminating in massive banded iron deposition during the Marinoan
cryochron.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49134.1/607268/Cryogenian-glaciostatic-and-eustatic-fluctuations
Active faulting controls bedform development on a deep-water fan
Vittorio Maselli; Aaron Micallef; Alexandre Normandeau; Davide Oppo; David
Iacopini ...
Abstract:
Tectonically controlled topography influences deep-water sedimentary
systems. Using 3-D seismic reflection data from the Levant Basin, eastern
Mediterranean Sea, we investigate the spatial and temporal evolution of
bedforms on a deep-water fan cut by an active normal fault. In the
footwall, the fan comprises cyclic steps and antidunes along its axial and
external portions, respectively, which we interpret to result from the
spatial variation in flow velocity due to the loss of confinement at the
canyon mouth. Conversely, in the hanging wall, the seafloor is nearly
featureless at seismic scale. Numerical modeling of turbidity currents
shows that the fault triggers a hydraulic jump that suppresses the flow
velocity downstream, which thus explains the lack of visible bedforms
basinward. This study shows that the topography generated by active normal
faulting controls the downslope evolution of turbidity currents and the
associated bedforms and that seafloor geomorphology can be used to evince
syn-tectonic deposition.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49206.1/607269/Active-faulting-controls-bedform-development-on-a
Near-constant retreat rate of a terrestrial margin of the Laurentide
Ice Sheet during the last deglaciation
Thomas V. Lowell; Meredith A. Kelly; Jennifer A. Howley; Timothy G. Fisher;
Peter J. Barnett ...
Abstract:
The Laurentide Ice Sheet (LIS) was the largest ice sheet during the last
glacial period. An accurate representation of its behavior during the last
deglaciation is critical to understanding its influence on and response to
a changing climate. We use 10Be dating and Bayesian modeling to
track the recession of the southwest sector of the Labrador Dome of the LIS
along an ~500-km-long transect west of Lake Superior during the last
deglaciation. This transect reflects terrestrial ice-margin retreat and
crosses multiple moraine sets, with the southwestern part of the transect
deglaciated by ca. 19 ka and the northeastern part deglaciated by ca. 10
ka. The predominant behavior of the ice margin during this interval is
near-constant retreat with retreat rates varying between ~59 m/a and 38
m/a. The moraine sets mark standstills and/or readvances that in total
constitute only ~17% of the retreat interval. The spatial and temporal
pattern of ice-margin retreat tracked here differs from existing
reconstructions that are based on using isochrons to define ice-margin
positions. Acknowledging the uncertainties associated with the modeled ages
of ice-margin retreat, we suggest that the overall retreat pattern is
consistent with forcing by a gradual increase in Northern Hemisphere,
high-latitude summer insolation. The pattern of ice-margin retreat is
inconsistent with Greenland ice-core temperature records, and thus these
records may not be suitable to drive models of the LIS.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49081.1/607270/Near-constant-retreat-rate-of-a-terrestrial-margin
Clear as mud: Clinoform progradation and expanded records of the
Paleocene-Eocene Thermal Maximum
Luca G. Podrecca; Maria Makarova; Kenneth G. Miller; James V. Browning;
James D. Wright
Abstract:
The mid-Atlantic coastal plain (eastern United States) preserves
high-resolution records of the Paleocene-Eocene Thermal Maximum (PETM) and
attendant carbon isotope excursion (CIE), though preservation is highly
variable from site to site. Here, we use a dip transect of expanded (as
much as 15 m thick) PETM sections from the New Jersey coastal plain to
build a cross-shelf PETM depositional model that explains the variability
of these records. We invoke enhanced delivery of fine-grained sediments,
due to the rapid environmental changes associated with this hyperthermal
event, to explain relatively thick PETM deposits. We utilize δ13
Cbulk, percent CaCO3, and percent coarse fraction
(>63 μm) data, supported by biostratigraphic records, to correlate sites
along a paleoslope dip transect. Updip cores from Medford, New Jersey,
preserve expanded sections of the initiation of the PETM and the earliest
portion of the CIE. Medial sites (Wilson Lake, Millville) preserve an
expanded CIE body, and downdip Bass River records the CIE recovery. We
interpret this pattern to reflect the progradation of clinoform foresets
across the paleoshelf via fluid mud, similar to modern high-sediment-supply
rivers and adjacent muddy shelves (e.g., the Amazon, Mahakam [Indonesia],
and Ayeyarwady [Myanmar] Rivers). Our subaqueous-clinoform delta model
explains the pattern of the CIE records and provides a framework for future
PETM studies in the region.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49061.1/606719/Clear-as-mud-Clinoform-progradation-and-expanded
The impact of postdepositional alteration on iron- and molybdenum-based
redox proxies
Suemeyya Eroglu; Florian Scholz; Renato Salvatteci; Christopher Siebert;
Ralph Schneider ...
Abstract:
Ratios of (un)reactive iron species, authigenic molybdenum contents (Moauth), and molybdenum isotope compositions (δ98Mo auth) in sedimentary rocks are geochemical proxies that are
widely used to reconstruct past marine redox states, which have been
calibrated in modern marine settings covering oxic to euxinic conditions.
However, syn- and postdepositional processes can result in alterations and
ambiguities of proxy-derived redox signals that can challenge the validity
of paleoreconstructions. We present new data from modern organic-rich
sediments of two oxygen minimum zone settings in the Gulf of California and
the Peruvian margin. The results show that Mo is fully immobilized shortly
after deposition by reaction with hydrogen sulfide (H2S)
produced during organoclastic sulfate reduction. Thus, any H2S
produced deeper in the sediment (e.g., by sulfate reduction coupled to
anaerobic methane oxidation) leaves the initially deposited Mo
concentrations and δ98Mo signatures unaltered, which supports
the robustness of Mo-based redox proxies. In contrast, the Fe speciation
data reveal continued pyritization due to constant exposure of Fe minerals
to H2S. Importantly, both Fe bound to oxides and carbonates
(highly reactive Fe) and also poorly reactive Fe (e.g., sheet silicates)
undergo pyritization during early diagenesis. This process generates
Fe-based proxy signatures that falsely imply ferruginous or euxinic
conditions.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49291.1/606720/The-impact-of-postdepositional-alteration-on-iron
Rhabdophane Th-Pb ages indicate reactivation of Mesoarchean structures
in west Pilbara Craton during breakup of Greater India and
Australia-Antarctica
Birger Rasmussen; Jian-Wei Zi; Janet R. Muhling
Abstract:
Uranium-Th-Pb dating of phosphate minerals in very low-grade
metasedimentary rocks from the Archean Pilbara Craton, Western Australia,
has revealed a long history of deformation and fluid flow during the
Paleoproterozoic. However, this technique has not detected evidence for
fluid flow along craton margins during Phanerozoic rifting and breakup. We
report the use of in situ Th-Pb geochronology of rhabdophane, a
hydrous light rare earth element phosphate, to date fluid flow in shale
from the 2.76 Ga Mount Roe Basalt from drill hole number 6 of the Archean
Biosphere Drilling Program (ABDP6), northwestern Pilbara Craton. Thorium-Pb
dating of rhabdophane in carbonaceous shale yields three main populations
with weighted mean 208Pb/232Th ages of 152 ± 6 Ma,
132 ± 4 Ma, and 119 ± 4 Ma, which indicates phosphate growth up to 2.64
b.y. after deposition. The rhabdophane ages are coeval with three major
breakup events in eastern Gondwana: separation of Southwest Borneo and
Argoland from Australia (ca. 156–152 Ma), breakup of Greater India from
Australia (ca. 140–135 Ma), and separation of Greater India/India from
Antarctica (ca. 123 Ma). The proximity of drill hole ABDP6 to major
Mesoarchean faults and shear zones on the craton margin, which are parallel
to rift propagation and basin development, points to episodic reactivation
of ancient crustal structures >2.8 b.y. after their formation. Our
results also highlight the potential of rhabdophane as a U-Th-Pb
geochronometer for dating low-temperature (<200 °C) fluid flow and
hydrous alteration. The migration of Mesozoic fluids through Archean shales
adds weight to questions about the origin of geochemical signals in ancient
altered rocks and how to extract information about the early environment
and biosphere.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49250.1/606721/Rhabdophane-Th-Pb-ages-indicate-reactivation-of
Zircon (U-Th)/He thermochronology reveals pre-Great Unconformity
paleotopography in the Grand Canyon region, USA
B.A. Peak; R.M. Flowers; F.A. Macdonald; J.M. Cottle
Abstract:
The Great Unconformity is an iconic geologic feature that coincides with an
enigmatic period of Earth’s history that spans the assembly and breakup of
the supercontinent Rodinia and the Snowball Earth glaciations. We use
zircon (U-Th)/He thermochronology (ZHe) to explore the erosion history
below the Great Unconformity at its classic Grand Canyon locality in
Arizona, United States. ZHe dates are as old as 809 ± 25 Ma with data
patterns that differ across both long (~100 km) and short (tens of
kilometers) spatial wavelengths. The spatially variable thermal histories
implied by these data are best explained by Proterozoic syndepositional
normal faulting that induced differences in exhumation and burial across
the region. The data, geologic relationships, and thermal history models
suggest Neoproterozoic rock exhumation and the presence of a basement paleo
high at the present-day Lower Granite Gorge synchronous with Grand Canyon
Supergroup deposition at the present-day Upper Granite Gorge. The paleo
high created a topographic barrier that may have limited deposition to
restricted marine or nonmarine conditions. This paleotopographic evolution
reflects protracted, multiphase tectonic activity during Rodinia assembly
and breakup that induced multiple events that formed unconformities over
hundreds of millions of years, all with claim to the title of a “Great
Unconformity.”
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49116.1/606722/Zircon-U-Th-He-thermochronology-reveals-pre-Great
Spontaneous reheating of crystallizing lava
Alan G. Whittington; Alexander Sehlke
Abstract:
We show that recalescence, or spontaneous reheating of a cooling material
due to rapid release of latent heat, can occur during disequilibrium
crystallization of depolymerized Mg-rich melts. This can only happen at
fast cooling rates, where the melt becomes undercooled by tens to hundreds
of degrees before crystallization begins. Using a forward-looking infrared
(FLIR) camera, we documented recalescence in pyroxene (Fe, Mg)SiO3 and komatiite lavas that initially cooled at 25–50 °C s –1. Local heating at the crystallization front exceeds 150 °C
for the pyroxene and 10 °C for komatiite and lasts for several seconds as
the crystallization front migrates through. We determined the latent heat
release by differential scanning calorimetry to be 440 J g–1 for
pyroxene and 275 J g–1 for komatiite with a brief power output
of ~100 W g–1 or ~300 MW m–3. Recalescence may be a
widespread process in the solar system, particularly in lava fountains, and
cooling histories of mafic pyroclasts should not be assumed a priori to be
monotonic.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49148.1/606723/Spontaneous-reheating-of-crystallizing-lava
Curved orogenic belts, back-arc basins, and obduction as consequences
of collision at irregular continental margins
Nicholas Schliffke; Jeroen van Hunen; Frédéric Gueydan; Valentina Magni;
Mark B. Allen
Abstract:
Continental collisions commonly involve highly curved passive plate
margins, leading to diachronous continental subduction during trench
rollback. Such systems may feature back-arc extension and ophiolite
obduction postdating initial collision. Modern examples include the Alboran
and Banda arcs. Ancient systems include the Newfoundland and Norwegian
Caledonides. While external forces or preexisting weaknesses are often
invoked, we suggest that ophiolite obduction can equally be caused by
internal stress buildup during collision. Here, we modeled collision with
an irregular subducting continental margin in three-dimensional (3-D)
thermo-mechanical models and used the generated stress field evolution to
understand resulting geologic processes. Results show how tensional
stresses are localized in the overriding plate during the diachronous onset
of collision. These stresses thin the overriding plate and may open a
back-arc spreading center. Collision along the entire trench follows
rapidly, with inversion of this spreading center, ophiolite obduction, and
compression in the overriding plate. The models show how subduction of an
irregular continental margin can form a highly curved orogenic belt. With
this mechanism, obduction of back-arc oceanic lithosphere naturally evolves
from a given initial margin geometry during continental collision.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48919.1/606724/Curved-orogenic-belts-back-arc-basins-and
Pulsed volcanism and rapid oceanic deoxygenation during Oceanic Anoxic
Event 1a
Kohen W. Bauer; Cinzia Bottini; Robert Frei; Dan Asael; Noah J. Planavsky
...
Abstract:
Widespread oceanic anoxia, biological crises, and volcanic activity are
associated with the onset of Early Aptian (ca. 120 Ma) Oceanic Anoxic Event
1a (OAE1a). Reconstructions of oceanic deoxygenation and its links to
broadly contemporaneous volcanism, however, remain poorly resolved. We use
geochemical data, including δ53Cr ratios and rare Earth element
abundances, to define the timing and tempo of submarine volcanism and
global oceanic deoxygenation across this event. Pacific Ocean sediments
deposited in the run up to OAE1a record multiple phases of marine volcanism
associated with the emplacement of Ontong Java Plateau lavas. Rapid oceanic
deoxygenation followed the initial phases of volcanism and a
biocalcification crisis. Large swaths of the oceans likely became anoxic
from the Tethys to the Pacific Oceans in <30 k.y. Oceanic anoxia
persisted for almost one million years after this and was likely sustained
through intensified continental and submarine weathering. These results
paint a new picture of OAE1a in which volcanism, biological crisis, and
oceanic deoxygenation are separated in time and linked through Earth system
responses that operate on time scales of tens of thousands of years.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49065.1/606725/Pulsed-volcanism-and-rapid-oceanic-deoxygenation
Late Weichselian ice-sheet flow directions in the Russian northern
Barents Sea from high-resolution imagery of submarine glacial landforms
Julian A. Dowdeswell; Aleksandr Montelli; Grigorii Akhmanov; Marina
Solovyeva; Yana Terekhina ...
Abstract:
The locations and orientations of more than 1000 late Quaternary subglacial
and ice-marginal landforms, including streamlined sedimentary bed forms,
glacitectonic hill-hole pairs, meltwater channels, and eskers, were mapped
from blocks of multibeam data (area of 4861 km2) in the
little-known Russian Barents Sea. Between Sentralbanken and Admiralty Bank,
at ~75°N, there is evidence for southward ice flow. Ice-flow indicators
between Franz Josef Land and Novaya Zemlya show northeast flow into the
head of St. Anna Trough. There is also evidence of southeast flow off the
bank to the south of Franz Josef Land, and of flow convergence with
northeast-flowing ice in Sedov Trough. Northeast flow of ice between Novaya
Zemlya and Franz Josef Land suggests that the latter archipelago was not
overrun by ice flowing north from the Barents Sea and, therefore, that a
subsidiary ice dome was likely on Franz Josef Land. A major ice divide was
also present at ~76°N –77°N in the Russian Barents Sea.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49252.1/606726/Late-Weichselian-ice-sheet-flow-directions-in-the
Diverse gas composition controls
the Moby-Dick gas hydrate system in the Gulf of Mexico
Alexey Portnov; A.E. Cook; S. Vadakkepuliyambatta
Abstract:
In marine basins, gas hydrate systems are usually identified by a bottom
simulating reflection (BSR) that parallels the seafloor and coincides with
the base of the gas hydrate stability zone (GHSZ). We present a newly
discovered gas hydrate system, Moby-Dick, located in the Ship Basin in the
northern Gulf of Mexico. In the seismic data, we observe a channel-levee
complex with a consistent phase reversal and a BSR extending over an area
of ~14.2 km2, strongly suggesting the presence of gas hydrate.
In contrast to classical observations, the Moby-Dick BSR abnormally shoals
150 m toward the seafloor from west to east, which contradicts the
northward-shallowing seafloor. We argue that the likely cause of the
shoaling BSR is a gradually changing gas mix across the basin, with gas
containing heavier hydrocarbons in the west transitioning to methane gas in
the east. Our study indicates that such abnormal BSRs can be controlled by
gradual changes in the gas mix influencing the shape of the GHSZ over
kilometers on a basin scale.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49310.1/606727/Diverse-gas-composition-controls-the-Moby-Dick-gas
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