A triple S-shaped compositional profile in a Karoo dolerite sill–Evidence of concurrent multiple fractionation processes

Thick dolerite sills show a range of vertical geochemical variation trends attributable to various processes during slow crystallization. We have identified chemical parameters in a 169-m-thick sill from the Karoo igneous province in South Africa that define three different lower crossover levels (maximum or minimum concentrations) creating S-shaped variation trends. The crossover level for whole-rock MgO is at 20 m height (due to mechanical sorting of olivine); the anorthite content of plagioclase is at 52 m (due to addition of primitive magma); and that of the incompatible trace elements is at 75 m (due to different proportions of early formed grains to trapped liquid). Each process can operate independently and concurrently, leading to their maximum effects occurring at different levels in the intrusion. The independence of these processes and the triple S-shaped geochemical profiles have not been recognized before in any mafic-ultramafic sills.

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Precarious ephemeral refugia during the earliest Triassic

The term refuge describes, in both ecology and paleoecology, an ecosystem that acts as a sanctuary during times of environmental stress. This study tests the concept by examining the fate of a single community that lived ~50 k.y. after the end-Permian mass extinction (EPME). An assemblage of trace fossils, bivalves, and echinoids, living on a microbial mat in a slope environment, is preserved on a single bedding plane in the Shangsi section, south China. The microbial community was vital to the success of the refuge, acting as a stable substrate, food source, and oxygen supply. Shallow-water microbial communities have been interpreted as refugia, but this deeper site may have been critical to organisms with temperature sensitivities. Published paleotemperature calculations suggest sublethal surface temperatures of 34 °C at Shangsi. A species of cidaroid echinoid likely migrated to cooler deep waters to optimize development, suggesting that the success of this shallow-water clade is attributable to such refugia, when survival was most precarious after the EPME. The ecosystem was short lived, depending on low productivity and slow sedimentation. When conditions became suboptimal due to ash input and increasing productivity, the ecosystem quickly collapsed, allowing for colonization by opportunistic taxa including Claraia and microconchids. Elsewhere the ecosystem may have remained unchanged. Earliest Triassic refugia may have been restricted to these ephemeral environmental settings until organisms adapted to continuing harsh conditions.

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Biotic effects on oxygen consumption during weathering: Implications for the second rise of oxygen

The oxygen concentration of the atmosphere likely increased substantially in the late Neoproterozoic. Although several studies have presented compelling geochemical evidence for this stepwise oxygenation, few have addressed the mechanisms behind it. Recently it was hypothesized that the advent of eukaryotic life on land, and the associated increase in soil respiration, led to a transient reduction in the supply of oxygen for rock weathering, temporarily reducing oxidative weathering rates, allowing atmospheric oxygen levels to rise to restore the oxygen supply. To evaluate this hypothesis quantitatively, we developed a simple one-dimensional diffusion-reaction soil model that reduces the many oxygen weathering sinks to one, pyrite, given that it is the dominant sink at low oxygen concentrations. In simulations with no biological respiration, pyrite weathering rates become oxygen independent at an atmospheric oxygen concentration between 10–6x the present-day atmospheric level (PAL) and 1 PAL. On the other hand, when biological respiration is considered, pyrite weathering remains oxygen dependent even at modern oxygen levels. Constrained by modern weathering profiles and soil respiration rates, we find that the atmospheric oxygen level may have increased by up to two orders of magnitude as biotic soil respiration increased. This may be sufficient to explain the second rise in atmospheric oxygen inferred for the Neoproterozoic.

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Persistence of a freshwater surface ocean after a snowball Earth

Geochemical data from cap carbonates deposited above Cryogenian glacial deposits have been widely used to infer the conditions that prevailed in the aftermath of snowball Earth. However, the time scale over which these carbonates were deposited and the degree to which they record the chemistry of a globally well-mixed ocean have remained poorly constrained. During deglaciation, a large volume of meltwater entered the ocean, creating two distinct layers: the fresh, hot, and light upper layer, and the salty, cold, and dense lower layer. Here we estimate the ocean mixing time scale based on energetic constraints. We find that the mixing time scale is 104–105 yr, with a best estimate of ~5 x 104 yr, or up to 100 times longer than that of the modern ocean. Mixing of the surface temperature anomaly implies a delayed sea-level rise of 40–50 m associated with pure thermal expansion. This result reconciles geological, geochemical, and paleomagnetic data from basal Ediacaran cap carbonates with physical oceanographic theory. In particular, our model suggests that (1) the cap dolostones formed predominantly in a freshwater environment; (2) the waters in which the dolostones formed were not well mixed with saline deep water, allowing for large geochemical differences between the cap dolostones and the deep ocean; and (3) the cap carbonate sequences formed in a two-phase transgression that lasted >104 yr, which is consistent with both local sea-level records and the preservation of magnetic excursions and reversals.

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Middle Eocene CO2 and climate reconstructed from the sediment fill of a subarctic kimberlite maar

Eocene paleoclimate reconstructions are rarely accompanied by parallel estimates of CO2 from the same locality, complicating assessment of the equilibrium climate response to elevated CO2. We reconstruct temperature, precipitation, and CO2 from latest middle Eocene (ca. 38 Ma) terrestrial sediments in the posteruptive sediment fill of the Giraffe kimberlite in subarctic Canada. Mutual climatic range and oxygen isotope analyses of botanical fossils reveal a humid-temperate forest ecosystem with mean annual temperatures (MATs) more than 17 °C warmer than present and mean annual precipitation ~4x present. Metasequoia stomatal indices and gas-exchange modeling produce median CO2 concentrations of ~630 and ~430 ppm, respectively, with a combined median estimate of ~490 ppm. Reconstructed MATs are more than 6 °C warmer than those produced by Eocene climate models forced at 560 ppm CO2. Estimates of regional climate sensitivity, expressed as MAT per CO2 doubling above preindustrial levels, converge on a value of ~13 °C, underscoring the capacity for exceptional polar amplification of warming and hydrological intensification under modest CO2 concentrations once both fast and slow feedbacks become expressed.

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Zircon petrochronological evidence for a plutonic-volcanic connection in porphyry copper deposits

Bridging the gap between the plutonic and volcanic realms is essential for understanding a variety of magmatic processes from caldera-forming eruptions to the formation of magmatic-hydrothermal ore deposits. Porphyry copper deposits are commonly associated with large and long-lived volcanic centers, but the temporal and dynamic link between mineralized intrusions and volcanic eruptions has remained controversial. Based on the combination of (1) high-precision zircon U-Pb geochronology and trace element geochemistry with (2) plagioclase textures, we discovered an intimate connection between an ignimbrite eruption and a nearby world-class porphyry deposit (Bajo de la Alumbrera in the late Miocene Farallón Negro Volcanic Complex of Argentina). Our results indicate that the magmatic-hydrothermal deposit and explosive volcanism were derived from a common magma reservoir that evolved over a minimum duration of 217 ± 25 k.y. before the final eruption. We show that the volcanic pile represents the inverted magma reservoir, recording systematic differences in plagioclase textures and juvenile clast content from bottom to top. This tight temporal and geochemical link suggests that deposit formation and volcanic eruption were both triggered by the same injection of a volatile-saturated primitive magma into the base of the magma chamber. A time gap of 19 ± 12 k.y. between porphyry mineralization and the onset of explosive volcanism indicates a minimum duration of magma reservoir rejuvenation that led to the explosive eruptive event. Catastrophic loss of volatiles by explosive volcanism terminated the ore-forming capacity of the upper-crustal magma chamber, as evidenced by the intrusion of a syn-eruptive barren quartz-feldspar porphyry. Our results demonstrate that porphyry copper deposits provide critical information to understand how volatiles control the fate of hydrous magmas between pluton formation and explosive volcanism.

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Small profile concavity of a fine-bed alluvial channel

Although most natural streams have concave-upward longitudinal profiles, there are conflicting perspectives about whether an alluvial channel is less concave than a bedrock channel. Alluvial channels can be classified as coarse-bed and fine-bed channels, depending on the bed grain size. Both are transport limited, but the threshold of motion differs greatly. Whereas a coarse-bed alluvial channel can be claimed to be as concave as a bedrock channel, we claim that a fine-bed channel is distinguishably less concave. We derive the concavity index of a fine-bed alluvial channel using the power-law relationships emergent at a steady-state river network. For known ranges of the scaling parameters, our formulation informs a range of concavity index as 0.07 ± 0.09 for a fine-bed alluvial channel. Our analyses of previous laboratory experiments and real fine-bed alluvial channels in the midwestern U.S. and northern Europe also support our conclusions, i.e., small profile concavity of steady-state fine-bed alluvial channels.

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A volcanic trigger for the Late Ordovician mass extinction? Mercury data from south China and Laurentia

The Late Ordovician mass extinction (LOME), one of the five largest Phanerozoic biodiversity depletions, occurred in two pulses associated with the expansion and contraction of ice sheets on Gondwana during the Hirnantian Age. It is widely recognized that environmental disruptions associated with changing glacial conditions contributed to the extinctions, but neither the kill mechanisms nor the causes of glacial expansion are well understood. Here we report anomalously high Hg concentrations in marine strata from south China and Laurentia deposited immediately before, during, and after the Hirnantian glacial maximum that we interpret to reflect the emplacement of a large igneous province (LIP). An initial Hg enrichment occurs in the late Katian Age, while a second enrichment occurs immediately below the Katian-Hirnantian boundary, which marks the first pulse of extinction. Further Hg enrichment occurs in strata deposited during glacioeustatic sea-level fall and the glacial maximum. We propose that these Hg enrichments are products of multiple phases of LIP volcanism. While elevated Hg concentrations have been linked to LIP emplacement coincident with other Phanerozoic mass extinctions, the climate response during the LOME may have been unique owing to different climatic boundary conditions, including preexisting ice sheets. Our observations support a volcanic trigger for the LOME and further point to LIP volcanism as a primary driver of environmental changes that caused mass extinctions.

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High levels of radioactive material migrating down into soil around Fukushima

High levels of radioactive cesium remain in the soil near the Fukushima Daiichi nuclear power plant and these radionuclides have migrated at least 5 centimeters down into the ground at several areas since the nuclear accident five years ago, according to preliminary results of a massive sampling project being presented at the JpGU-AGU joint meeting in Chiba, Japan.

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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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Microfossil measures of rapid sea-level rise: Timing of response of two microfossil groups to a sudden tidal-flooding experiment in Cascadia

Comparisons of pre-earthquake and post-earthquake microfossils in tidal sequences are accurate means to measure coastal subsidence during past subduction earthquakes, but the amount of subsidence is uncertain, because the response times of fossil taxa to coseismic relative sea-level (RSL) rise are unknown. We measured the response of diatoms and foraminifera to restoration of a salt marsh in southern Oregon, USA. Tidal flooding following dike removal caused an RSL rise of ~1 m, as might occur by coseismic subsidence during momentum magnitude (Mw) 8.1–8.8 earthquakes on this section of the Cascadia subduction zone. Less than two weeks after dike removal, diatoms colonized low marsh and tidal flats in large numbers, showing that they can record seismically induced subsidence soon after earthquakes. In contrast, low-marsh foraminifera took at least 11 months to appear in sizeable numbers. Where subsidence measured with diatoms and foraminifera differs, their different response times may provide an estimate of postseismic vertical deformation in the months following past megathrust earthquakes.

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Subtropical climate conditions and mangrove growth in Arctic Siberia during the early Eocene

The early Eocene (ca. 56–47.8 Ma) was an interval of exceptional warmth with reduced pole-to-equator temperature gradients. Climate proxies indicate mean annual air temperatures (MATs) and sea-surface temperatures (SSTs) exceeding 8–18 °C and frost-free, mild winters in polar areas, features that have proven difficult to reproduce with the most elaborate climate models. A full appraisal of the early Eocene polar climate has been, however, limited by possible seasonal biases associated with geochemical proxies and the lack of data from the vast Eurasian Arctic. Here we present multiproxy data from lower-middle Eocene coastal plain sediments of the New Siberian Islands (Russia) showing that taxodioid Cupressaceae, palms, and the mangrove Avicennia grew in Arctic Siberia above 72°N under air temperatures averaging 16–21 °C annually and 5.5–14 °C in winter. Kaolinite contents are exceptionally high (up to 60% of clay assemblages) and comparable to those found in present-day subtropical soils formed under high mean annual precipitation (MAP >1000 mm) and warm (MAT >15 °C) conditions. The Avicennia pollen records the northernmost mangrove growth ever documented and indicates early Eocene SSTs exceeding 13 °C in winter and 18 °C in summer. Considering the high MAP estimated for Arctic Siberia and other pan-Arctic landmasses, we propose that the heat from warm river waters draining into the Arctic might have amplified early Eocene polar warmth. Our results provide the first climate constraints for the early Eocene of Arctic Siberia and support the view that most climate models underestimate polar warming in greenhouse conditions.

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Warming and increased aridity during the earliest Triassic in the Karoo Basin, South Africa

The Permian-Triassic (P-T) mass extinction is the largest extinction event of the Phanerozoic and has been causally related to eruption of the Siberian Traps (Siberia, Russia) through climatic and ecological effects of volcanically forced greenhouse gas emissions. Given anthropogenic changes to the atmosphere, documenting details of links between greenhouse warming and catastrophic biological consequences has significance beyond better understanding of the P-T boundary. Earliest Triassic warming is supported by modeling and isotopic studies; however, the empirical data are limited. In this paper we show that 18O values of phosphate in therapsid tusks from the interior of Pangea (paleolatitude 60°S) were relatively constant through the latest Permian but increased by ~2 during the earliest Triassic. Over the same interval, 18O values of carbonate soil nodules increased by ~5. The increases in both measurements and the disproportionately large shift among nodules indicate that the interior of Pangea warmed and became markedly more arid immediately after the P-T boundary.

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New age constraints on Aptian evaporites and carbonates from the South Atlantic: Implications for Oceanic Anoxic Event 1a

High-resolution carbon isotope (13C) profiles from shallow- and deep-water carbonates in the South Atlantic (Campos and Santos Basins) are here correlated to stratigraphically well calibrated Tethyan sections, constraining the end of major evaporite deposition in the South Atlantic to the early Aptian Oceanic Anoxic Event (OAE) 1a interval. The unusually extensive evaporite deposition would have reduced the global dissolved sulfate inventory, possibly increasing global preservation of organic matter by decreasing sulfate reduction; this could explain the coincidence in timing between OAE 1a and the dramatic negative sulfur isotope excursion over this interval. Therefore, in addition to the coeval eruption of the Ontong Java Plateau, the opening of the South Atlantic may have played an important role in the genesis and character of OAE 1a.

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The rise and fall of stromatolites in shallow marine environments

Stromatolites are abundant in shallow marine sediments deposited before the evolution of animals, but in the modern ocean they are restricted to locations where the activity of animals is limited. Overall decline in the abundance of stromatolites has, therefore, been attributed to the evolution of substrate-modifying metazoans, with Phanerozoic stromatolite resurgences attributed to the aftermaths of mass extinctions. Here we use a comprehensive stratigraphic database, the published literature, and a machine reading system to show that the rock record–normalized occurrence of stromatolites in marine environments in North America exhibits three phases: an initial Paleoproterozoic (ca. 2500 Ma) increase, a sustained interval of dominance during the Proterozoic (2500–800 Ma), and a late Neoproterozoic (700–541 Ma) decline to lower mean prevalence during the Phanerozoic (541–0 Ma). Stromatolites continued to exhibit large changes in prevalence after the evolution of metazoans, and they transiently achieved Proterozoic-like prevalence during the Paleozoic. The aftermaths of major mass extinctions are not well correlated with stromatolite resurgence. Instead, stromatolite occurrence is well predicted by the prevalence of dolomite, a shift in carbonate mineralogy that is sensitive to changes in water-column and pore-water chemistry occurring during continent-scale marine transgressive-regressive cycles.

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Recent retreat of Columbia Glacier, Alaska: Millennial context

Columbia Glacier in Prince William Sound, Alaska, has retreated ~20 km in the past three decades. We use marine sediment records to document the Columbia Glacier advance and retreat history over the past 1.6 k.y. in an effort to place its recent retreat in the context of the Common Era (C.E.). A change in magnetic mineralogy coincided with a shift in sediment geochemistry ca. 0.9 ka. This provenance change documents the advance of Columbia Glacier across a fault, resulting in glacial erosion of mafic rocks near the coast; this agrees with the timing of ice advance reconstructed using dendrochronology. Our marine provenance records show that Columbia Glacier remained advanced south of this fault into the 21st century. Columbia Glacier has now retreated north of this fault, making its recent retreat unprecedented since before ca. 0.9 ka. Southern Alaska temperatures have now warmed to pre–0.9 ka levels, based on tree-ring and reanalysis data. We show with glacier model simulations that the warming between C.E. 1910 and 1980, that includes anthropogenic forcing, was sufficient to trigger the recent retreat of Columbia Glacier from its extended position of the past 0.9 k.y., consistent with our data-driven assessment of the relationship between regional climate change and glacier extent. We conclude that the recent retreat of Columbia Glacier is a response to climate change rather than part of a natural internal tidewater-glacier oscillation.

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A 65 k.y. time series from sediment-hosted glasses reveals rapid transitions in ocean ridge magmas

Studies of ocean ridge magmatism have been hampered by the difficulty in constructing time-series data over more than a few thousand years. Sediment rapidly covers newly formed ocean crust, and older rocks, even when recovered from fault scarps, cannot be dated accurately. Ridge eruptions, however, disperse pyroclastic glass over distances as far as 5 km, and these glasses have been shown to persist for thousands of years in on-ridge sediment push cores. Here we present data on such glasses from a piston core that impacted basement in much older (600 ka) sediment. The age of deposition was determined using established stratigraphic methods to date the host sediment, yielding an average sample resolution of a few thousand years and a continuous 65 k.y. time series. The new time-series data show systematic temporal variations in magma compositions related to a change to the dynamics of crustal storage, which led to greater extents of pre-eruptive differentiation. Shortly thereafter was a small but discernable shift toward more enriched primary melt compositions. These events coincide with the onset of enhanced crustal production, previously identified using seismic data and interpreted to reflect the capture of a hotspot by the ridge. These results show the long-term preservation of pyroclastic glasses and suggest that the construction of high-resolution volcanic stratigraphy over a million years or more may be possible at ocean ridges, using multiple piston cores that impact basement. Sediment-hosted glasses have the potential to transform ocean ridges from the volcanic setting with the worst time-series data to that with the best.

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Paleofluvial and subglacial channel networks beneath Humboldt Glacier, Greenland

The identification of subglacial drainage systems can inform our understanding of past and present hydrological processes, landscape evolution, and ice dynamics. Here, we present evidence from satellite imagery, digital elevation models, and radio-echo sounding data for a series of channelized networks with contrasting paleofluvial and subglacial origins beneath Humboldt Glacier, northern Greenland. A >250-km-long, dendritic paleofluvial channel network beneath the northern portion of Humboldt is interpreted as a preglacial feature. Roughly linear channels beneath the southern portion of Humboldt, which display a similar distribution to tunnel valleys found on the beds of former ice sheets, are likely to have been eroded by subglacial meltwater routed along the ice-sheet bed. We suggest that basal meltwater is actively being routed down both the paleofluvial and subglacially formed channel networks to the coast. Inheritance of the preglacial channel network may have influenced the present-day location and dynamics of Humboldt Glacier and enhanced selective erosion at its down-glacier end.

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Synconvergent exhumation of metamorphic core complexes in the northern North American Cordillera

Continental metamorphic core complexes of the northern North American Cordillera were exhumed during the early Paleogene while the Farallon–North American plate convergence rate remained high. Such convergent boundary conditions can excite localized mid-crustal exhumation in numerical simulations of collisions of softer accreted terranes with a rigid, irregular craton margin. Resulting simulated temperature-time (Tt) paths match Tt paths interpreted from observed footwall exposures in several core complexes of the northern North American Cordillera, including variability of maximum temperatures, duration of exhumation, and exhumation velocities among individual complexes.

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