Rock magnetic chronostratigraphy of the Shuram carbon isotope excursion: Wonoka Formation, Australia

The Shuram carbon isotope excursion represents a late Precambrian perturbation to Earth’s carbon cycle. Several mechanisms have been proposed, including global ocean oxygenation, methane hydrate release, and diagenesis. The plurality of hypotheses in part stems from a lack of chronostratigraphic constraints that are needed to provide the boundary conditions for geochemical models. In this study we use magnetostratigraphy and astrochronology to establish a chronostratigraphic framework for the excursion. Paleomagnetic and isotopic results from the Flinders Ranges, South Australia, and from Death Valley, California (USA), demonstrate that the nadir of the excursion is coincident with a correlative polarity transition at each locality, suggesting global synchroneity. Rock magnetic cyclostratigraphy yields an astrochronologic estimate of ~8 m.y. for the excursion’s duration at both locations. Based on these observations, and the chronologic requirements of each proposed mechanism, we rule out diagenesis and methane hydrate release as sole causes for the excursion, and suggest reexamination of the ocean oxidation hypothesis.

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A common origin of carbonatite magmas

The more than 500 fossil Ca-carbonatite occurrences on Earth are at odds with the only active East African Rift carbonatite volcano, Oldoinyo Lengai (Tanzania), which produces Na-carbonatite magmas. The volcano’s recent major explosive eruptions yielded a mix of nephelinitic and carbonatite melts, supporting the hypothesis that carbonatites and spatially associated peralkaline silicate lavas are related through liquid immiscibility. Nevertheless, previous eruption temperatures of Na-carbonatites were 490–595 °C, which is 250–450 °C lower than for any suitable conjugate silicate liquid. This study demonstrates experimentally that moderately alkaline Ca-carbonatite melts evolve to Na-carbonatites through crystal fractionation. The thermal barrier of the synthetic Na-Ca-carbonate system, held to preclude an evolution from Ca-carbonatites to Na-carbonatites, vanishes in the natural system, where continuous fractionation of calcite + apatite leads to Na-carbonatites, as observed at Oldoinyo Lengai. Furthermore, saturating the Na-carbonatite with minerals present in possible conjugate nephelinites yields a parent carbonatite with total alkali contents of 8–9 wt%, i.e., concentrations that are realistic for immiscible separation from nephelinitic liquids at 1000–1050 °C. Modeling the liquid line of descent along the calcite surface requires a total fractionation of ~48% calcite, ~12% apatite, and ~2 wt% clinopyroxene. SiO2 solubility only increases from 0.2 to 2.9 wt% at 750–1200 °C, leaving little leeway for crystallization of silicates. The experimental results suggest a moderately alkaline parent to the Oldoinyo Lengai carbonatites and therefore a common origin for carbonatites related to alkaline magmatism.

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Evidence for hydrothermal alteration and source regions for the Kiruna iron oxide-apatite ore (northern Sweden) from zircon Hf and O isotopes

Zircon grains from the Kiruna iron oxide–apatite (IOA) ore bodies in northern Sweden are distinct in their hafnium and oxygen isotopic ratios compared to zircon grains from adjacent metavolcanic host rocks and related intrusions. Here, we combine these two isotopic systems on previously dated zircon grains to improve our understanding of these ore deposits with a long-debated origin. Contrasting theories for the formation of the Kiruna iron ores suggest either (1) emplacement through immiscible silicate–iron oxide melts or (2) transportation and deposition of iron by hydrothermal fluids. Zircon from the metavolcanic host rocks and intrusions have oxygen isotopic ratios (18O ~3) that lie below typical magmatic compositions, which is evidence that roof rocks altered by meteoric water were digested into the magma. In contrast, the ores show an influence of a fluid that is higher in 18O (~7). Based on these findings, we propose the involvement of episodic magmatic-hydrothermal fluids in the ore genesis of the Kiruna iron ore deposits: (1) the first episode related to a deep-seated magmatism and to regional-scale metasomatic alteration, and (2) a later fluid event related to shallow intrusions and responsible for the ore formation. Distinct differences in the Hf isotopic ratios for host rocks and intrusions (Hfi = –6 to –10, Archean crust) and ore (Hfi = –5 to +3, depleted mantle) further allow us to screen possible fluid sources for their involvement in the ore process.

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Modification of river meandering by tropical deforestation

Tropical forests are the only forest biome to have experienced increased rates of forest loss during the past decade because of global demands for food and biofuels. The implications of such extensive forest clearing on the dynamics of tropical river systems remain relatively unknown, despite significant progress in our understanding of the role of trees in riverbank stability. Here, we document rates of deforestation and corresponding average annual rates of riverbank erosion along the freely meandering Kinabatangan River in Sabah, Malaysia, from Landsat satellite imagery spanning A.D. 1989–2014. We estimate that deforestation removed over half of the river’s floodplain forest and up to 30% of its riparian cover, which increased rates of riverbank erosion by >23% within our study reaches. Further, the correlation between the magnitude of planform curvature and rates of riverbank erosion only became strongly positive and significant following deforestation, suggesting an important role of forests in the evolution of meandering rivers, even when riverbank heights exceed the depth of root penetration.

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Cascadia subduction tremor muted by crustal faults

Deep, episodic slow slip on the Cascadia subduction megathrust of western North America is accompanied by low-frequency tremor in a zone of high fluid pressure between 30 and 40 km depth. Tremor density (tremor epicenters per square kilometer) varies along strike, and lower tremor density statistically correlates with upper plate faults that accommodate northward motion and rotation of forearc blocks. Upper plate earthquakes occur to 35 km depth beneath the faults. We suggest that the faults extend to the overpressured megathrust, where they provide fracture pathways for fluid escape into the upper plate. This locally reduces megathrust fluid pressure and tremor occurrence beneath the faults. Damping of tremor and related slow slip caused by fluid escape could affect fault properties of the megathrust, possibly influencing the behavior of great earthquakes.

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Mantle melt production during the 1.4 Ga Laurentian magmatic event: Isotopic constraints from Colorado Plateau mantle xenoliths

Plutons associated with a 1.4 Ga magmatic event intrude across southwestern Laurentia. The tectonic setting of this major magmatic province is poorly understood. Proposed melting models include anorogenic heating from the mantle, continental arc or transpressive orogeny, and anatexis from radiogenic heat buildup in thickened crust. Re-Os analyses of refractory mantle xenoliths from the Navajo volcanic field (NVF; central Colorado Plateau) yield Re depletion ages of 2.1–1.7 Ga, consistent with the age of the overlying Yavapai and Mazatzal crust. However, new Sm-Nd isotope data from clinopyroxene in peridotite xenoliths from NVF diatremes show a subset of xenoliths that plot on a ca. 1.4 Ga isochron, which likely reflects mantle melt production and isotopic resetting at 1.4 Ga. This suggests that Paleoproterozoic subcontinental lithospheric mantle was involved in the 1.4 Ga magmatic event. Our constraints support a subduction model for the generation of the 1.4 Ga granites but are inconsistent with rifting and anorogenic anatexis models, both of which would require removal of ancient lithosphere.

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Modelling satellite-derived magma discharge to explain caldera collapse

Many effusive eruptions are characterized by effusion rates that decay exponentially with time, a trend which is generally ascribed to elastic relaxation of a deep magma chamber. Thermal emissions, detected by satellite during the A.D. 2014–2015 Bárðarbunga-Holuhraun eruption (Iceland), indicate that the volume of the erupted magma and effusion rates followed an overall exponential trend that fits the observed major subsidence of the Bárðarbunga caldera floor. This trend continued until a critical flow rate was reached. Hence, the subsidence slowed down and the eruption rapidly ceased, reflecting the ultimate closure of the magma path. We present a model of inelastic magma withdrawal that very closely reproduces all the observed phenomena and provides new insights into the caldera collapses and the driving pressure of other effusive eruptions.

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Timing of initial seafloor spreading in the Newfoundland-Iberia rift

Broad areas of subcontinental lithospheric mantle are commonly exposed along ocean-continent transition zones in magma-poor rifts and are thought to be exhumed along lithospheric-scale detachment faults during the final stages of rifting. However, the nature of the transition from final rifting to seafloor spreading is controversial. We present the first high-precision U-Pb zircon geochronologic and Hf isotopic data from gabbros that intrude exhumed mantle at Ocean Drilling Program (ODP) Sites 1070 and 1277 in the Newfoundland-Iberia rift (North Atlantic). The sites are conjugate to one another within crust that is commonly considered to have been emplaced during early seafloor spreading. Magnetic data suggest that crustal accretion occurred at both sites during magnetic polarity chrons M3–M0 (130–126 Ma). However, our data indicate that asthenospheric melts were emplaced over brief intervals (≤1 m.y.) prior to or coeval with mantle exhumation at 124 Ma at ODP Site 1070 and 115 Ma at ODP Site 1277. We suggest that this discrepancy is the result of continued mantle exhumation along large, west-dipping detachment faults until lithospheric breakup. The breakup location is likely coincident with the large-amplitude magnetic J anomaly, and our 115 Ma date for magmatism within this anomaly provides the best available age constraint for breakup along the studied transect.

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Paleozoic echinoderm hangovers: Waking up in the Triassic

Echinoderms are among the marine invertebrates that underwent the most severe losses at the end-Permian extinction. The prevailing paradigm claims an extreme bottleneck with only very few, if not single, holdovers (“hangovers” herein) sparking the post-Paleozoic radiation. Here we identify previously overlooked Triassic echinoids, ophiuroids, and asteroids as unambiguous members of Paleozoic stem groups. These echinoderm hangovers occurred almost worldwide and had spread into a wide range of paleoenvironments by the Late Triassic. Our discovery challenges fundamentals of echinoderm evolution with respect to end-Permian survival and sheds new light on the early evolution of the modern clades, in particular on Triassic ghost lineages (i.e., inferred but undocumented fossil record) of the crown-group look-alikes of the Paleozoic hangovers.

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Tectonic melange records the Silurian-Devonian subduction-metamorphic process of the southern Dunhuang terrane, southernmost Central Asian Orogenic Belt

The Hongliuxia tectonic mélange of the southern Dunhuang terrane, northwestern China, southernmost Central Asian Orogenic Belt (CAOB), consists of eclogite, mafic granulite, and amphibolite as puddingstones within a matrix of metapelitic gneiss and marble; these rocks are interpreted to be part of an ancient subduction zone setting. Secondary ion mass spectrometry U-Pb dating of metamorphic zircons obtained from the puddingstones and matrix metapelite suggests that the metamorphism occurred at ca. 428–391 Ma. The metamorphic rocks all record similar clockwise metamorphic pressure-temperature-time (PTt) paths of the western Alpine type. However, remarkable differences between metamorphic peak PT conditions ranging from 830 °C and 24.2 kbar for the eclogite puddingstone to 700 °C and 10.2 kbar for the metapelite matrix were found in the mélange rocks. This indicates the mixing of rocks from significantly different depths to create a tectonic mélange in a subduction channel, possibly juxtaposed during the uplift stage. These data suggest that the southernmost CAOB underwent subduction and subsequent exhumation caused by subduction of the Paleozoic Hongliuxia ocean during the middle Silurian to middle Devonian.

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Structure and dynamics of surface uplift induced by incremental sill emplacement

Shallow-level sill emplacement can uplift Earth’s surface via forced folding, providing insight into the location and size of potential volcanic eruptions. Linking the structure and dynamics of ground deformation to sill intrusion is thus critical in volcanic hazard assessment. This is challenging, however, because (1) active intrusions cannot be directly observed, meaning that we rely on transient host-rock deformation patterns to model their structure; and (2) where ancient sill-fold structure can be observed, magmatism and deformation has long since ceased. To address this problem, we combine structural and dynamic analyses of the Alu dome, Ethiopia, a 3.5-km-long, 346-m-high, elliptical dome of outward-dipping, tilted lava flows cross-cut by a series of normal faults. Vents distributed around Alu feed lava flows of different ages that radiate out from or deflect around its periphery. These observations, coupled with the absence of bounding faults or a central vent, imply that Alu is not a horst or a volcano, as previously thought, but is instead a forced fold. Interferometric synthetic aperture radar data captured a dynamic growth phase of Alu during a nearby eruption in A.D. 2008, with periods of uplift and subsidence previously attributed to intrusion of a tabular sill at 1 km depth. To localize volcanism beyond its periphery, we contend that Alu is the first forced fold to be recognized to be developing above an incrementally emplaced saucer-shaped sill, as opposed to a tabular sill or laccolith.

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Millennial-scale hydroclimate variations in southwest China linked to tropical Indian Ocean since the Last Glacial Maximum

Indian summer monsoon (ISM) variations have been linked to the orbital-scale boreal summer insolation and millennial-scale North Atlantic climates. Recent studies show the critical role of Indian Ocean sea-surface temperatures (SSTs) in affecting deglacial millennial-scale monsoon oscillations. However, it is unclear whether SSTs can affect monsoon rainfall and terrestrial hydroclimate during the Holocene. Here we report multiproxy evidence of hydroclimate changes in southwest China since the Last Glacial Maximum. Similar to the often-documented gradual decrease in Holocene monsoon rainfall with superimposed millennial-scale variations, our records particularly show pronounced hydroclimate fluctuations including wet conditions at ~5000-4000 yr ago, and perhaps over the past 1000 yr. We also find coherent variations between our records and sea-surface salinities in the eastern Indian Ocean, suggesting that terrestrial hydroclimate and resultant continental drainage have affected surface ocean conditions. These fluctuations are likely linked to changes in SSTs downstream of the monsoon source in the tropical western Indian Ocean, i.e., a warmer ocean and more monsoon rainfall. We conclude that the influence of both insolation and tropical SSTs on the ISM has persisted from the last deglaciation into Holocene.

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A Laurentian record of the earliest fossil eukaryotes

The oldest evidence of eukaryotes in the fossil record comes from a recurrent assemblage of morphologically differentiated late Paleoproterozoic to early Mesoproterozoic microfossils. Although widely distributed, the principal constituents of this TappaniaDictyosphaeraValeria assemblage have not hitherto been recognized on Laurentia. We have recovered all three taxa from a shallow-water shale succession in the early Mesoproterozoic Greyson Formation (Belt Supergroup, Montana, USA). An exceptionally preserved population of Tappania substantially expands the morphological range of this developmentally complex organism, suggesting phylogenetic placement within, or immediately adjacent to, crown-group eukaryotes. Correspondence with Tappania-bearing biotas from China, India, Australia, and Siberia demonstrates an open-ocean connection to the intracratonic Belt Basin and, along with broadly co-occurring macrofossils Grypania and Horodyskia, supports the recognition of a globally expressed biozone. The Greyson Formation, along with contiguous strata in Glacier National Park, is unique in preserving all currently confirmed taxa of early eukaryotic and macroscopic fossils.

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Evolution of seaward-dipping reflectors at the onset of oceanic crust formation at volcanic passive margins: Insights from the South Atlantic

Seaward-dipping reflectors (SDRs) have long been recognized as a ubiquitous feature of volcanic passive margins, yet their evolution is much debated, and even the subject of the nature of the underlying crust is contentious. This uncertainty significantly restricts our understanding of continental breakup and ocean basin–forming processes. Using high-fidelity reflection data from offshore Argentina, we observe that the crust containing the SDRs has similarities to oceanic crust, albeit with a larger proportion of extrusive volcanics, variably interbedded with sediments. Densities derived from gravity modeling are compatible with the presence of magmatic crust beneath the outer SDRs. When these SDR packages are restored to synemplacement geometry we observe that they thicken into the basin axis with a nonfaulted, diffuse termination, which we associate with dikes intruding into initially horizontal volcanics. Our model for SDR formation invokes progressive rotation of these horizontal volcanics by subsidence driven by isostasy in the center of the evolving SDR depocenter as continental lithosphere is replaced by more dense oceanic lithosphere. The entire system records the migration of >10-km-thick new magmatic crust away from a rapidly subsiding but subaerial incipient spreading center at rates typical of slow oceanic spreading processes. Our model for new magmatic crust can explain SDR formation on magma-rich margins globally, but the estimated crustal thickness requires elevated mantle temperatures for their formation.

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The basement revealed: Tectonic insight from a digital elevation model of the Great Unconformity, USA cratonic platform

Across much of North America, the contact between Precambrian basement and Paleozoic strata is the Great Unconformity, a surface that represents a >0.4 b.y.-long hiatus. A digital elevation model (DEM) of this surface visually highlights regional-scale variability in the character of basement topography across the United States cratonic platform. Specifically, it delineates Phanerozoic tectonic domains, each characterized by a distinct structural wavelength (horizontal distance between adjacent highs) and/or structural amplitude (vertical distance between adjacent lows and highs). The largest domain, the Midcontinent domain, includes long-wavelength epeirogenic basins and domes, as well as fault-controlled steps. The pronounced change in land-surface elevation at the Rocky Mountain Front coincides with the western edge of the Midcontinent domain on the basement DEM. In the Rocky Mountain and Colorado Plateau domains, west of the Rocky Mountain Front, structural wavelength is significantly shorter and structural amplitude significantly higher than in the Midcontinent domain. The Bordering Basins domain outlines the southern and eastern edges of the Midcontinent domain. As emphasized by the basement DEM, several kilometers of structural relief occur across the boundary between these two domains, even though this boundary does not stand out on ground-surface topography. A plot of epicenters on the basement DEM supports models associating intraplate seismicity with the Midcontinent domain edge. Notably, certain changes in crustal thickness also coincide with distinct changes in basement depth.

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The role of F-clinohumite in volatile recycling processes in subduction zones

It is not well understood how much water and other volatiles are really contained in different mantle rocks or minerals, and how these volatiles are transported down into the deeper mantle in subduction zones. Here we present new experimental data showing that a common mineral found in subduction zone rocks, the hydrated Mg-silicate clinohumite, is much more stable in the mantle than previously anticipated. We show that even small amounts of F substituting for OH are sufficient to stabilize clinohumite to temperatures well above the normal mantle geotherm. Based on this finding we propose that in subduction zones, clinohumite effectively transports water and other volatiles from shallow depths to the transition zone of the mantle. This can drastically increase the amount of fluorine and water recycled into the deep mantle.

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Extinction selectivity among marine fishes during multistressor global change in the end-Permian and end-Triassic crises

Ancient mass extinction events such as the end-Permian and end-Triassic crises provide analogues for multistressor global change of ocean warming, pH reduction, and deoxygenation. Organism physiology is hypothesized to be a key trait influencing vulnerability to these stressors, but it is not certain how physiology predicts survival over evolutionary time scales and when organisms are faced with opposing or synergistic stressors. Fishes (bony fishes and chondrichthyan fishes) are active organisms with high aerobic scope for thermal tolerance and well-developed acid-base regulation, traits that should confer resilience to global change. To test this, we compiled a database of fossil marine fish occurrences to quantify extinction rates during background and mass extinctions from the Permian through Early Jurassic, using maximum likelihood estimation to compare extinction trajectories with marine invertebrates. Our results show that fewer chondrichthyan fishes underwent extinction than marine invertebrates during the end-Permian crisis. End-Triassic chondrichthyan extinction rates also were not elevated above background levels. In contrast, bony fishes underwent an end-Triassic extinction comparable to that of marine invertebrates. The differing responses of these two groups imply that a more active physiology can be advantageous during global change, although not uniformly. Permian–Triassic chondrichthyan fishes may have had broader environmental tolerances, facilitating survival. Alternatively, the larger offspring size of chondrichthyan fishes may provide greater energy reserves to offset the demands of warming and acidification. Although more active organisms have adult adaptations for thermal tolerance and pH regulation, some may nevertheless be susceptible to global change during early life stages.

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The transitional climate of the late Miocene Arctic: Winter-dominated precipitation with high seasonal variability

The late Miocene (11.6–5.3 Ma) was an important transitional period following the greenhouse conditions of the Eocene. In order to gain insight into the Arctic paleoclimate of the time, we performed high-resolution intraring 13C analyses on fossil wood collected from the late Miocene Khapchansky sediments of northeastern Siberia (~69°N). From these data we quantified the ratio of summer to winter precipitation (Ps/Pw) and compared it to current values for the region determined from modern wood samples and instrumental records. We observed much greater frequency of winter-dominated precipitation (Ps/Pw < 1) and much greater variability in Ps/Pw during the Miocene than today. Specifically, years with Ps/Pw < 1 occurred three times more often, and years with at least three times as much precipitation in summer or winter (0.33 < Ps/Pw < 3.0) occurred approximately twice as often during the Miocene than today. We attribute the high interannual variability in precipitation to an inconsistent moisture source associated with the relatively unstable and incomplete ice cover in the Arctic Ocean during the late Miocene. Our result highlights the potential for enhanced variability in Arctic precipitation in response to Arctic sea ice decline caused by anthropogenic, CO2-induced warming.

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Evidence from paleosols for low to moderate elevation of the India-Asia suture zone during mid-Cenozoic time

The post-collisional uplift history of the India-Asia suture zone in southern Tibet is important for understanding the geodynamic conditions of the India-Asia collision and how it may have modified regional and global climate. Here we use stable isotope and major element analyses of paleosols preserved in the Liuqu Conglomerate to reconstruct India-Asia suture zone paleoclimate and paleoelevation. Paleosol carbonate 13C (Vienna Peedee belemnite) average values of –9.4 ± 1.1 indicate that Liuqu paleosols were well vegetated by C3 plants, unlike thinly vegetated and arid modern Tibet. Major element weathering indices show that these soils experienced significant collapse and loss of cations due to weathering of similar intensity to that in the wet, low elevations of the Neogene Himalayan foreland. Age estimates for the Liuqu vary, but current evidence points to an early Miocene age. Our evidence requires that the India-Asia suture zone experienced wet, well-vegetated conditions during Liuqu Conglomerate deposition. Geodynamically this implies that the suture zone was at relatively low elevation and was topographically open to monsoon moisture as late as 40 m.y. after the start of the India-Asian collision. This challenges the idea that uplift of the India-Asia suture zone was monotonic and tied directly to crustal thickening.

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Seismic imaging of magma sills beneath an ultramafic-hosted hydrothermal system

Hydrothermal circulation at mid-ocean ridge volcanic segments extracts heat from crustal magma bodies. However, the heat source driving hydrothermal circulation in ultramafic outcrops, where mantle rocks are exhumed in low-magma-supply environments, has remained enigmatic. Here we use a three-dimensional P-wave velocity model derived from active-source wide-angle refraction-reflection ocean bottom seismometer data and pre-stack depth-migrated images derived from multichannel seismic reflection data to investigate the internal structure of the Rainbow ultramafic massif, which is located in a non-transform discontinuity of the Mid-Atlantic Ridge. Seismic imaging reveals that the ultramafic rocks composing the Rainbow massif have been intruded by a large number of magmatic sills, distributed throughout the massif at depths of ~2–10 km. These sills, which appear to be at varying stages of crystallization, can supply the heat needed to drive high-temperature hydrothermal circulation, and thus provide an explanation for the hydrothermal discharge observed in this ultramafic setting. Our results demonstrate that high-temperature hydrothermal systems can be driven by heat from deep-sourced magma even in exhumed ultramafic lithosphere with very low magma supply.

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Widespread persistence of expanded East Antarctic glaciers in the southwest Ross Sea during the last deglaciation

It has been suggested that the grounding line of the Last Glacial Maximum (LGM) ice sheet in the Ross Sea, Antarctica, receded in an approximately north-to-south pattern during the Holocene. An implication of this hypothesis is that geological evidence from the southwestern Ross Sea has been used widely to interpret retreat histories of the West Antarctic Ice Sheet (WAIS) across the wider Ross Sea embayment. Accurately constraining the timing and pattern of marine-based ice sheet retreat in this embayment is critical to understanding the drivers that may have triggered this event, and its contribution to rapid sea-level rise events. Here, we present new multibeam swath bathymetry data that identifies well-preserved glacial features indicating that thick (>700 m) marine-based ice derived from the East Antarctic Ice Sheet coastal outlet glaciers dominated the ice sheet input into the southwestern Ross Sea during the last phases of glaciation. Subglacial geomorphic features indicate that ice derived from present outlet glacier valleys in South Victoria Land flowed southeastward. This is more consistent with flowlines from model-based interpretations of an earlier retreat of the WAIS in the central Ross Sea than with previous land-based geological reconstructions. This implies that coastal records of deglaciation along the Transantarctic Mountains front record only the final phases of glacial retreat in the Ross Sea. Therefore, chronological data from the central embayment are required to accurately constrain the timing of large-scale glacial retreat in the Ross Sea and to identify the mechanisms that drove it.

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Ina pit crater on the Moon: Extrusion of waning-stage lava lake magmatic foam results in extremely young crater retention ages

The enigmatic Ina feature on the Moon was recently interpreted to represent extrusive basaltic volcanic activity within the past 100 m.y. of lunar history, an extremely young age for volcanism on the Moon. Ina is a 2 x 3 km D-shaped depression that consists of a host of unusual bleb-like mounds surrounded by a relatively optically fresh hummocky and blocky floor. Documentation of magmatic-volcanic processes from shield volcano summit pit craters in Hawai’i and new insights into shield-building and dike evolution processes on the Moon provide important perspectives on the origin of Ina. We show that the size, location, morphology, topography, and optical maturity of Ina are consistent with an origin as a subsided summit pit crater lava lake on top of a broad ~22-km-diameter, ~3.5-b.y.-old shield volcano. New theoretical treatments of lunar shield-building magmatic dike events predict that waning-stage summit activity was characterized by the production of magmatic foam in the dike and lake; the final stages of dike stress relaxation and closure cause the magmatic foam to extrude to the surface through cracks in the lava lake crust to produce the mounds. The high porosity of the extruded foams (>75%) altered the nature of subsequent impact craters (the aerogel effect), causing them to be significantly smaller in diameter, which could bias the crater-derived model ages. Accounting for this effect allows for significantly older model ages, to ~3.5 b.y., contemporaneous with the underlying shield volcano. Thus extremely young volcanic eruptions are not required to explain the unusual nature of Ina.

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