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The Geology Society of America's latest issue of
GEOLOGY

Excerpts from the  January 21st, 2017 issue

Tidal rhythmites in the southern Bouse Formation as evidence for post-Miocene uplift of the lower Colorado River corridor

Uncertainty over depositional paleoenvironments of the upper Miocene to lower Pliocene Bouse Formation obscures our understanding of the timing and magnitude of regional uplift as well as the conditions and processes that were active during integration and early evolution of the Colorado River (western United States). This paper presents new sedimentologic and quantitative evidence for a strong tidal influence on deposition of the basal carbonate member of the southern Bouse Formation. Lithofacies with tidal sedimentary structures include lime mudstone with reed and grass imprints; heterolithic facies with horizontal, flaser, wavy, and lenticular bedding; and cross-bedded grainstone with sigmoidal bundles, reactivation surfaces, and complex bedding. Systematic alternation of relatively thick and thin layers records a predominantly semidiurnal mixed-tidal depositional environment, and Fourier analysis highlights the cyclic, non-random nature of layer-thickness trends. These observations provide evidence that the southern Bouse Formation accumulated in a pre–Colorado River tide-dominated marine setting at the north end of the Gulf of California. Deposition at sea level requires ~330 m of uplift in the lower Colorado River and western Colorado Plateau region during the past ~5.0 m.y.


Tracing crustal evolution by U-Th-Pb, Sm-Nd, and Lu-Hf isotopes in detrital monazite and zircon from modern rivers

Detrital zircon U-Pb age and Hf isotope studies are useful for identifying the chemical evolution of the continental crust. Zircon, however, is typically a magmatic mineral and thus often fails to document the timing of low-grade metamorphism, and its survival through multiple sedimentary cycles potentially biases the crustal evolution record toward older events. In contrast, monazite typically records metamorphic events and is less likely to survive sedimentary recycling processes, thus providing information not available by zircon. Here, we demonstrate that monazite apparently faithfully records the Sm-Nd isotope composition of the bulk rock and can therefore track the record of crustal evolution and growth, similar to Hf isotopes in zircon. We examine the utility of detrital zircon and monazite for studies of crustal evolution through a comparison of age and tracer isotope information using sediments from two large rivers draining the South China block (SCB). Monazite and zircon grains yield mostly Mesozoic and Paleozoic U-Pb ages and depleted mantle model age peaks at ca. 1900–1300 Ma, indicating that both minerals preserve similar, yet critical, information on the crustal evolution of the catchment area. In contrast, zircon yields abundant Neoproterozoic and older U-Pb ages with a very large spread of model ages, preserving a history strongly skewed to older ages. Based on the lack of known rocks of this age in the catchments, ancient zircon was likely sourced from sedimentary rocks within the catchment area. This combined data set presents a more complete history of crustal evolution and growth in the SCB and demonstrates the advantages of an integrated approach that includes both detrital monazite and zircon.


Overbank sedimentation from the historic A.D. 2011 flood along the Lower Mississippi River, USA

In this study, we document sedimentary characteristics of overbank flood deposits associated with the epic A.D. 2011 flood along the Lower Mississippi River (southern USA) and directly compare the findings to sedimentation from a comparable flood event in 1973, with the general purpose of understanding how extreme floods contribute to floodplain depositional patterns and accretion rates of embanked fluvial systems. The thicknesses of the 2011 flood deposits averaged 138 mm along natural levee crests, 9 mm on meander scrolls, and 3 mm in backswamps. These thicknesses are considerably less than those documented for the 1973 flood, sampled at the same locations. We contend that less sedimentation in 2011 occurred because the flood was not supplied with much upstream sediment from the Missouri River. Further, the 2011 sediments are coarser than in 1973, indicating that the higher 2011 flood levels were associated with more energetic overbank flows that flushed fine-grained sediments downstream within the narrow embanked floodplain corridor. The largest recorded flood in North American history is only marginally preserved in the embanked floodplain stratigraphy of the alluvial valley of Earth’s third largest fluvial system.


Regional variability in the frequency and magnitude of large explosive volcanic eruptions

Quantifying the frequency at which volcanic eruptions of different size occurs is important for hazard assessment. Volcanic records can be used to estimate the recurrence rate of large-magnitude eruptions (magnitude ≥4), but recording biases that impact data completeness complicate analysis. To overcome these biases, we conceptualize the volcanic record as a series of individual and unique time series associated by a common behavior. Thus, we approach issues of completeness on a volcano-by-volcano basis and use a hierarchical Bayesian approach to characterize the common frequency-magnitude (f-M) behavior for different groups of volcanoes. We identify variations in the f-M relationship between different volcano types and between different volcanic arcs. By accounting for systematic under-recording in the volcanic record, we also calculate the global recurrence rates for large-magnitude eruptions during the Holocene, which are similar to previous estimates. However, higher recurrence rates for smaller-magnitude events are observed, which is a result of our adjustments for data completeness. Quantifying how the f-M relationship varies between different groups of volcanoes provides an opportunity to understand how the tectonic setting influences f-M behavior, which is important to quantify long-term regional volcanic hazard.


Silicified glendonites in the Ediacaran Doushantuo Formation (South China) and their potential paleoclimatic implications

The Ediacaran Period is punctuated by the ca. 580 Ma Gaskiers glaciation in Newfoundland. However, paleoclimatic data are scarce in Ediacaran successions in South China, where abundant geochemical and paleobiological data are shaping current understanding of Ediacaran evolutionary and environmental history. Here, we report the occurrence of silicified glendonites in the Ediacaran Doushantuo Formation deposited in an inner-shelf environment on the South China block. Petrographic evidence suggests that these silicified glendonites are pseudomorphs after syndepositional or early authigenic ikaites formed at near-freezing temperatures. The glendonite-bearing stratigraphic interval is characterized by positive 13C values. It predates both the negative 13C excursion EN3 (widely believed to be an equivalent of the Shuram negative excursion) and excursion EN2. Although alternative interpretations may be possible, these glendonites may be related to and correlated with the Gaskiers glaciation. If confirmed, this correlation suggests that the Shuram event postdates the Gaskiers glaciation, thus having important implications for Ediacaran climate changes, carbon cycles, and biological evolution.


An urban collection of modern-day large micrometeorites: Evidence for variations in the extraterrestrial dust flux through the Quaternary

We report the discovery of significant numbers (500) of large micrometeorites (>100 μm) from rooftops in urban areas. The identification of particles as micrometeorites is achieved on the basis of their compositions, mineralogies, and textures. All particles are silicate-dominated (S type) cosmic spherules with subspherical shapes that form by melting during atmospheric entry and consist of quench crystals of magnesian olivine, relict crystals of forsterite, and iron-bearing olivine within glass. Four particles also contain Ni-rich metal-sulfide beads. Bulk compositions are chondritic apart from depletions in the volatile, moderately volatile, and siderophile elements, as observed in micrometeorites from other sources. The reported particles are likely to have fallen on Earth in the past 6 yr and thus represent the youngest large micrometeorites collected to date. The relative abundance ratio of barred olivine to cryptocrystalline spherule types in the urban particles of 1.45 is shown to be higher than a Quaternary average of ~0.9, suggesting variations in the extraterrestrial dust flux over the past 800 k.y. Changes in the entry velocities of dust caused by quasi-periodic gravitational perturbation during transport to Earth are suggested to be responsible. Variations in cosmic spherule abundance within the geologic column are thus unavoidable and can be a consequence of dust transport as well as major dust production events.


Barrier island migration dominates ecogeomorphic feedbacks and drives salt marsh loss along the Virginia Atlantic Coast, USA

Coupling between barrier islands and their associated backbarrier environments (salt marsh, tidal flats) leads to complex ecogeomorphic feedbacks that are proposed to control the response of barrier island systems to relative sea-level rise. This study tests the applicability of these still-theoretical concepts through investigation of the Virginia barrier islands (eastern United States), which are located in an area of accelerated sea-level rise. Using historical maps and photographs from A.D. 1851 to 2010, we determine that rapid landward island migration (1–6 m yr–1) is leading to backbarrier area reduction and large-scale salt marsh loss (63 km2 or 19%) at a rate of 0.45 km2 yr–1. Landward barrier island migration far outpaces upland marsh migration and is responsible for 51% of marsh loss; the remainder is due to backbarrier processes (e.g., edge erosion). In direct contrast to proposed ecogeomorphic feedbacks linking barrier island and backbarrier environments, shoreline retreat rates were not related to changes in backbarrier marsh, open-water areas, or tidal prism. Rather, these results indicate that, for barrier island systems already undergoing migration, the primary barrier-backbarrier coupling is the loss of marsh and tidal-flat area because of barrier island migration.


The dynamics of gold in regolith change with differing environmental conditions over time

Significant Au discoveries are becoming less common because the remaining prospective, underexplored areas are obscured by transported cover. At Moolart Well (Western Australia), secondary Au deposits hosted in transported pisolitic ferricrete and saprolite are overlain by younger transported cover. Here, we show how Au has been, and is being, dispersed and concentrated in these deposits and the overlying younger transported cover and biota during the evolution of the landscape. We identified coarse (>400 µm), Ag-rich, primary, angular Au accumulated residually along with some precipitated, Ag-poor (<0.5% Ag) secondary Au in saprolite. Gold enrichment in the pisolitic ferricrete is mostly secondary nanometer- to micron-sized spheres, chains, triangles, and wires in precipitates of organic carbon (C), goethite, kaolinite, and amorphous Si within cortices, cracks, and cavities in pisoliths. Spectacular secondary Au as clumps and larger clusters, not previously reported in ferricrete, occurs in organic C-rich zones of the cortices and cavities of pisoliths, implying a role for organic matter in their formation. To our knowledge, the organic C–Au relationship in pisolitic ferricrete described in this study has not been documented previously. We propose that minor Au was recycled from the pisolitic ferricrete formed during humid conditions (Miocene) into an overlying silicified subsoil that was formed in an arid climate of the mid-Miocene to the present. The presence of Au in vegetation (Acacia aneura) and termite mounds indicates active dispersion.


A new attraction-detachment model for explaining flow sliding in clay-rich tephras

Altered pyroclastic (tephra) deposits are highly susceptible to landsliding, leading to fatalities and property damage every year. Halloysite, a low-activity clay mineral, is commonly associated with landslide-prone layers within altered tephra successions, especially in deposits with high sensitivity, which describes the post-failure strength loss. However, the precise role of halloysite in the development of sensitivity, and thus in sudden and unpredictable landsliding, is unknown. Here we show that an abundance of mushroom cap–shaped (MCS) spheroidal halloysite governs the development of sensitivity, and hence proneness to landsliding, in altered rhyolitic tephras, North Island, New Zealand. We found that a highly sensitive layer, which was involved in a flow slide, has a remarkably high content of aggregated MCS spheroids with substantial openings on one side. We suggest that short-range electrostatic and van der Waals interactions enabled the MCS spheroids to form interconnected aggregates by attraction between the edges of numerous paired silanol and aluminol sheets that are exposed in the openings and the convex silanol faces on the exterior surfaces of adjacent MCS spheroids. If these weak attractions are overcome during slope failure, multiple, weakly attracted MCS spheroids can be separated from one another, and the prevailing repulsion between exterior MCS surfaces results in a low remolded shear strength, a high sensitivity, and a high propensity for flow sliding. The evidence indicates that the attraction-detachment model explains the high sensitivity and contributes to an improved understanding of the mechanisms of flow sliding in sensitive, altered tephras rich in spheroidal halloysite.


A probabilistic analysis of meteorically altered {delta}13C chemostratigraphy from late Paleozoic ice age carbonate platforms

The stratigraphic expression of meteoric diagenesis in carbonates is a glimpse into the weathering, fluid transport, and biological productivity of the ancient near-surface terrestrial environment. To infer this environmental information, we use a probabilistic approach to merge an isotope-based reactive transport model with chemostratigraphic data from carbonates that were subaerially exposed during an expansion of the late Paleozoic ice age in the middle Carboniferous. The rate of carbon flowing through the carbonate platform relative to the rate of carbon reacting between mineral and fluid phases controls the length scale, curvature, and magnitude of the diagenetic carbon isotope profiles. The ratio of advection to reaction is determined for seven stratigraphic sections, and the advection rate is used to estimate the minimum carbonate weathering associated with each profile. These carbonate weathering rates extrapolated over the expansive shallow carbonate platforms of the middle Carboniferous indicate that glacioeustatic fall may have caused a 20%–50% increase in the dissolved CaCO3 flux to the ocean. The complex feedbacks among carbonate weathering and accumulation, atmospheric pCO2, glacioeustasy, and passive margin subsidence may have played an unexplored role in the glacial-interglacial climate dynamics of the late Paleozoic ice age.


Foreland exhumation controlled by crustal thickening in the Western Alps

In alpine-type collision belts, deformation of the foreland may occur as a result of forward propagation of thrusting and is generally associated with thin-skinned deformation mobilizing the sedimentary cover in fold-and-thrust belts. Locally, foreland deformation can involve crustal-scale thrusting and produce large-scale exhumation of crystalline basement resulting in significant relief generation. In this study, we investigate the burial and exhumation history of Tertiary flexural basins located in the Western Alpine foreland, at the front of the Digne thrust sheet (southeast France), using low-temperature apatite fission-track and (U-Th)/He thermochronology. Based on the occurrence of partially to totally reset ages, we document 3.3–4.0 km of burial of these basin remnants between ca. 12 Ma and 6 Ma, related to thin-skinned thrust-sheet emplacement without major relief generation. The onset of exhumation is dated at ca. 6 Ma and is linked to erosion associated with significant relief development. This evolution does not appear to have been controlled by major climate changes (Messinian crisis) or by European slab breakoff. Rather, we propose that the erosional history of the Digne thrust sheet corresponds to basement involvement in foreland deformation, leading to crustal thickening. Our study highlights the control of deep-crustal tectonic processes on foreland relief development and its erosional response at mountain fronts.


Evidence of an axial magma chamber beneath the ultraslow-spreading Southwest Indian Ridge

Ultraslow-spreading ridges are a novel class of spreading centers symbolized by amagmatic crustal accretion, exposing vast amounts of mantle-derived peridotites on the seafloor. However, distinct magmatic centers with high topographies and thick crusts are also observed within the deep axial valleys. This suggests that despite the low overall melt supply, the magmatic process interacting with the tectonic process should play an important role in crustal accretion; however, this has been obscured due to the lack of seismic images of magma chambers. Using a combination of seismic tomography and full waveform inversion of ocean bottom seismometer data from the Southwest Indian Ridge at 50°28'E, we report the presence of a large low-velocity anomaly (LVA) ~4–9 km below the seafloor, representing an axial magma chamber (AMC) in the lower crust. This suggests that the 9.5-km-thick crust here is mainly formed by a magmatic process. The LVA is overlain by a high-velocity layer, possibly forming the roof of the AMC and defining the base of hydrothermal circulation. The steep velocity gradient just below the high-velocity layer is explained by the ponding of magma at the top of the AMC; this could provide the overpressure for lateral dike propagation along the ridge axis, leading to a complex interaction between magma emplacement, tectonic, and hydrothermal processes, and creating a diversity of seafloor morphology and extremely heterogeneous crust.


Contrasting magmatic cannibalism forms evolved phonolitic magmas in the Canary Islands

Volcanic sequences on ocean islands record the temporal evolution of underlying magmatic systems and provide insights into how silicic crust is produced away from convergent margins. Assimilation has often been suspected to contribute, but the detection of such a process and its evolving maturity during migration across a mantle plume is less well documented. Here we present new major and trace element and Sr-Nd-Pb-U-Th-Ra-Pa isotope data that facilitate comparison of basanite to phonolite evolution on Tenerife (Canary Islands) with that shown by published data from La Palma. On both islands, (230Th/238U) ratios decrease with differentiation from parental magmas with 230Th excess toward different, silicic contaminants in secular equilibrium. On La Palma, this is inferred to reflect assimilation of small amounts of mafic wall rock. On Tenerife, both (230Th/238U) and (231Pa/235U) ratios decrease toward 1 with increasing differentiation, and this is accompanied by a subtle increase in Pb isotope ratios. At the same time, (226Ra/230Th) ratios change from >1 to <1 (a hitherto unreported magnitude). The Tenerife assimilant is thus constrained to be a partial melt of syenite formed in equilibrium with residual feldspar. The differences reflect a primarily deeper, more mafic magma system beneath La Palma during its late shield-building stage, whereas recent magmatic evolution at Tenerife occurs primarily at lower temperatures in small, shallower magma systems formed during its post–basaltic shield stage. Differentiation takes millennia or less.


Storage filters upland suspended sediment signals delivered from watersheds

Climate change, tectonics, and humans create long- and short-term temporal variations in the supply of suspended sediment to rivers. These signals, generated in upland erosional areas, are filtered by alluvial storage before reaching the basin outlet. We quantified this filter using a random walk model driven by sediment budget data, a power-law distributed probability density function (PDF) to determine how long sediment remains stored, and a constant downstream drift velocity during transport of 157 km/yr. For 25 km of transport, few particles are stored, and the median travel time is 0.2 yr. For 1000 km of transport, nearly all particles are stored, and the median travel time is 2.5 m.y. Both travel-time distributions are power laws. The 1000 km travel-time distribution was then used to filter sinusoidal input signals with periods of 10 yr and 104 yr. The 10 yr signal is delayed by 12.5 times its input period, damped by a factor of 380, and is output as a power law. The 104 yr signal is delayed by 0.15 times its input period, damped by a factor of 3, and the output signal retains its sinusoidal input form (but with a power-law "tail"). Delivery time scales for these two signals are controlled by storage; in-channel transport time is insignificant, and low-frequency signals are transmitted with greater fidelity than high-frequency signals. These signal modifications are essential to consider when evaluating watershed restoration schemes designed to control sediment loading, and where source-area geomorphic processes are inferred from the geologic record.


Mantle earthquakes, crustal structure, and gravitational instability beneath western North Island, New Zealand

A cluster of 30–52-km-deep earthquakes, and a 7–10 km step in the Moho beneath western North Island, New Zealand, are both interpreted as manifestations of active delamination of the continental lower crust and mantle lithosphere. These phenomena occur in the back-arc region beneath the east-west–oriented Taranaki-Ruapehu (TR) line, which strikes at a high angle to the present-day plate boundary through New Zealand. Across the line, there is an abrupt change in crustal and mantle lid thickness, and in upper-mantle seismic attenuation (Qp–1), showing that the mantle lithosphere has been highly thinned on the north side. We show from a receiver function profile that nearly all of the deep earthquakes reside in the uppermost mantle on the northern side of the TR line. A sum of earthquake moment tensors suggests strike-slip motion either parallel, or orthogonal, to the TR line, resulting in northwest-southeast–oriented horizontal extension. Active normal faults, oriented northwest-southeast and north-south, are seen at the surface on each side of the TR line, but the surface is uplifting here at ~0.4 mm/yr. This requires the mantle lithosphere to be thinning at a higher rate than the overlying crust, consistent with a delamination process.


Large subglacial meltwater features in the central Barents Sea

During the last glacial period large parts of the Arctic, including the Barents Sea, north of Norway and Russia, were covered by ice sheets. Despite several studies indicating that melting occurred beneath much of the Barents Sea ice sheet, very few meltwater-related landforms have been identified. We document ~200 seafloor valleys in the central Barents Sea and interpret them to be tunnel valleys formed by meltwater erosion beneath an ice sheet. This is the first account of widespread networks of tunnel valleys in the Barents Sea, and confirms previous predictions that large parts of the ice sheet were warm based. The tunnel valleys are interpreted to be formed through a combination of steady-state drainage and outburst floods close to the ice margin, as a result of increased melting within a period of rapid climate warming during late deglaciation. This is the first study documenting widespread tunnel valley formation at the northern reaches of a Northern Hemisphere paleo–ice sheet, during advanced deglaciation and beneath a much reduced ice sheet. This indicates that suitable conditions for tunnel valley formation may have occurred more widely than previously reported, and emphasizes the need to properly incorporate hydrological processes in current efforts to model ice sheet response to climate warming. This study provides valuable empirical data, to which modeling results can be compared.


Global-ocean redox variation during the middle-late Permian through Early Triassic based on uranium isotope and Th/U trends of marine carbonates

Uranium isotopes (238U/235U) in carbonates, a proxy for global-ocean redox conditions owing to their redox sensitivity and long residence time in seawater, exhibit substantial variability in the Daxiakou section of south China from the upper-middle Permian through the mid-lower Triassic (~9 m.y.). Middle and late Permian ocean redox conditions were similar to that of the modern ocean and were characterized by improving oxygenation in the ~2 m.y. prior to the latest Permian mass extinction (LPME), countering earlier interpretations of sustained or gradually expanding anoxia during this interval. The LPME coincided with an abrupt negative shift of >0.5 in 238U that signifies a rapid expansion of oceanic anoxia. Intensely anoxic conditions persisted for at least ~700 k.y. (Griesbachian), lessening somewhat during the Dienerian. Th/U concentration ratios vary inversely with 238U during the Early Triassic, with higher ratios reflecting reduced U concentrations in global seawater as a consequence of large-scale removal to anoxic facies. Modeling suggests that 70%–100% of marine U was removed to anoxic sinks during the Early Triassic, resulting in seawater U concentrations of <5% that of the modern ocean. Rapid intensification of anoxia concurrent with the LPME implies that ocean redox changes played an important role in the largest mass extinction event in Earth history.


Resolving the role of carbonaceous material in gold precipitation in metasediment-hosted orogenic gold deposits

Carbonaceous material (CM) is commonly associated with gold and sulfides in metasediment-hosted orogenic gold deposits. The role of CM in Au deposition is controversial; CM has been proposed to contribute to gold deposition by reducing Au bisulfide complexes, or by facilitating sulfidation, which destabilizes Au in bisulfide complexes with resultant Au deposition. Integration of petrographic observations, thermodynamic models, and geochemical data from metasediment-hosted orogenic gold deposits in New Zealand, Australia, Canada, and West Africa reveals genetic links between sulfides, CM, and mineralization. The results are consistent with the coexistence of CM and pyrite as a consequence of their codeposition from ore fluids, with a minor proportion of CM originally in situ in the host rocks. Au is deposited when pyrite and CM deposition decreases H2S concentration in ore fluids, destabilizing Au(HS)2 complexes. Most CM in gold deposits is deposited from CO2 and CH4 in ore fluids. These findings are applicable to similar deposits worldwide.


Combining radiocarbon and cosmogenic ages to constrain the timing of the last glacial-interglacial transition in the Uinta Mountains, Utah, USA

Twenty lake sediment cores extracted upstream from Last Glacial Maximum terminal moraines constrain the timing of the glacial-interglacial transition in the Uinta Mountains, Utah (USA). The stratigraphy observed in the cores, and accelerator mass spectrometry 14C dating of inorganic silty clay beneath gyttja, temporally constrain local deglaciation. The majority of basal ages fall within the Bølling-Allerød interval, with strong overlap at ca. 12.7 cal. (calibrated) kyr B.P. This convergence matches regional evidence of rising temperatures, increasing aridity, falling pluvial lake levels, and glacial retreat near the end of the last glacial-interglacial transition. Normalized estimates of glacier terminus retreat and elevation rise in the Uinta Mountains and elsewhere in the Rocky Mountains derived from consideration of cosmogenic ages on terminal moraines consistently average ~10%/k.y. between ca. 20 and 13 ka, implying a regionally uniform climate forcing during deglaciation. Minor variations between these rates likely reflect hypsometric effects during deglaciation. Seven lakes dammed by cirque-floor moraines have basal ages within, or slightly younger than, the Younger Dryas interval, suggesting advances of favorably located cirque glaciers before the glacial-interglacial transition was complete.


Explosive eruption of El Chichon volcano (Mexico) disrupted 6th century Maya civilization and contributed to global cooling

A remarkably long period of Northern Hemispheric cooling in the 6th century CE, which disrupted human societies across large parts of the globe, has been attributed to volcanic forcing of climate. A major tropical eruption in 540 CE is thought to have played a key role, but there is no consensus about the source volcano to date. Here, we present evidence for El Chichón in southern Mexico as the most likely candidate, based on a refined reconstruction of the volcano’s eruption history. A new chronological framework, derived from distal tephra deposits and the world’s largest Holocene beach ridge plain along the Gulf of Mexico, enabled us to positively link a major explosive event to a prominent volcanic sulfur spike in bipolar ice core records, dated at 540 CE. We speculate that voluminous tephra fall from the eruption had a severe environmental impact on Maya societies, leading to temporary cultural decline, site abandonment, and migration within the core area of Maya civilization.


Late Quaternary climatic control of Lake Baikal (Russia) turbidite systems: Implications for turbidite systems worldwide

Lake Baikal (Russia) contains a variety of turbidite systems in different tectonic and depositional settings that provide tests for the role of Quaternary climatic change on turbidite system growth. During Pleistocene glacial climates, all types of systems exhibit increased sediment supply (high sedimentation rates, high net sand percent, thick sand turbidites) and progradation. During Holocene interglacial climate, all systems exhibit reduced sediment supply and retreat. Seismic profiles from the large Selenga Fan and small Tompuda Fan show (1) maximum fan growth during the late Pleistocene glacial melt time, where lobes and large channels reached the distal outer fan, and (2) fan retreat during the transition to the fully developed Holocene interglacial climate. For example, the Selenga Fan surface lobes backstepped ~55 km from the distal outer fan to the distal inner fan, and the large outer fan surface channel (~750 m wide, ~20 m levee relief) evolved to a smaller surface channel (~450 m wide, ~13 m levee relief) that extended only to the end of the inner fan. These results show that Quaternary climate controls the growth of the Lake Baikal turbidite systems in a setting where there are no significant water-level changes, which often are cited as the main control on turbidite system growth. The Lake Baikal and other marine turbidite systems suggest that climatic control of sediment supply, unrelated to sea-level lowering and tectonic effects, may have been a much more important control for turbidite systems than previously believed, not only during the Pleistocene, but perhaps also for ancient systems.


Nonequilibrium degassing, regassing, and vapor fluxing in magmatic feeder systems

Magma degassing models typically invoke volatile depletion of a single parental melt, with permeable loss of exsolved gas having served for many years as the paradigm for the transition from volatile-rich, explosive eruptions to volatile-depleted lava flows. These degassing models are guided by measurements of H2O, CO2, and hydrogen isotope variations retained in melt that quenched to glass, but the existing models are not uniquely constrained by the data. There also remains uncertainty surrounding the origin and significance of volcanic glass fragments. We show that individual obsidian pyroclasts from Mono Craters, California (USA), are heterogeneous in dissolved H2O and CO2, suggesting that clasts are assembled from juvenile melt and rewelded ash during magma ascent. This is in contrast to the conventional view that clasts are chemically homogeneous and sample the chilled, glassy margins of conduit walls. The new measurements of dissolved H2O and CO2 help reconcile existing open-system degassing models used to explain elevated CO2/H2O ratios, provide time scales based on diffusion modeling for pyroclast formation, and show that magma does not necessarily lose volatiles monotonically during ascent-driven decompression.


Early Cenozoic drainage reorganization of the United States Western Interior-Gulf of Mexico sediment routing system

Continental-scale drainages host the world’s largest rivers and offshore sediment accumulations, many of which contain significant petroleum reserves. Rate of sediment supply in these settings may be a signal of external controls (e.g., tectonics) on landscape evolution, yet deciphering these controls remains a major challenge in interpreting the ancient stratigraphic record. Integration of new and published detrital zircon U-Pb ages from the United States Rocky Mountain region and Gulf of Mexico (GOM) sedimentary basin demonstrates profound changes in the U.S. continental drainage divide that controlled the rate of sediment delivery to the northern GOM during Paleocene–Eocene time. Sedimentation rate increased dramatically during deposition of the lower Wilcox Group, reaching approximately three times the Cenozoic average, accompanied by pronounced shoreline regression and delivery of a large volume of sand to the basin floor. We hypothesize that this increase in sediment delivery to the GOM resulted from drainage capture of a significant portion of the Sevier-Laramide structural province (~900,000 km2) that included the headwaters of the California and Idaho Rivers. Capture of the California River drainage may have occurred in the vicinity of the Hanna Basin of eastern Wyoming that previously emptied northward into a shallow seaway, but was subsequently diverted southward to the Rockdale delta, which accumulated within the Houston embayment during the time of deposition of the lower Wilcox Group. Detrital zircon U-Pb ages from Wilcox samples within the Rockdale delta show a remarkable similarity with contemporaneous Laramide synorogenic units, including enrichment in detritus derived from the Cordilleran arc and basement terranes of western North America relative to older and younger units in the Houston embayment. A subsequent order of magnitude decline in sedimentation rate to the GOM can be partly attributed to well-documented drainage closure (~800,000 km2) that accompanied lake formation in interior Laramide basins (ca. 53–51.8 Ma). Our results demonstrate that tectonically induced drainage migration in the high-relief segments of continental-scale drainages can have a pronounced effect on the rate of sediment transfer to continental margins.


Tracking large volcanic eruptions and their regional variability


Exceptional preservation of soft-bodied Ediacara Biota promoted by silica-rich oceans: COMMENT


Exceptional preservation of soft-bodied Ediacara Biota promoted by silica-rich oceans: REPLY


Extensive Noachian fluvial systems in Arabia Terra: Implications for early Martian climate: COMMENT


The Great Mars Climate Paradox Redux: REPLY