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At least 1,477 records · Page 82Linked to original sources

USGS2021

Kīlauea’s 2008–2018 summit lava lake—Chronology and eruption insights

The first eruption at Kīlauea’s summit in 25 years began on March 19, 2008, and persisted for 10 years. The onset of the eruption marked the first explosive activity at the summit since 1924, forming the new “Overlook crater” (as the 2008 summit eruption crater has been informally named) within the existing crater of Halemaʻumaʻu. The first year consisted of sporadic lava activity deep within the Overlook crater. Occasional small explosions deposited spatter and small wall-rock lithic pieces around the Halemaʻumaʻu rim. After a month-long pause at the end of 2008, deep sporadic lava lake activity returned in 2009. Continuous lava lake activity began in February 2010. The lake rose significantly in late 2010 and early 2011, before subsequently draining briefly in March 2011. This disruption of the summit eruption was triggered by eruptive activity on the East Rift Zone. Rising lake levels through 2012 established a more stable, larger lake in 2013, with continued enlargement over the subsequent 5 years. Lava reached the Overlook crater rim and overflowed on the Halemaʻumaʻu floor in brief episodes in 2015, 2016, and 2018, but the lake level was more commonly 20–60 meters below the rim during 2014–18. The lake was approximately 280×200 meters (~42,000 square meters) by early 2018 and formed one of the two largest lava lakes on Earth. A new eruption began in the lower East Rift Zone on May 3, 2018, causing magma to drain from the summit reservoir complex. The lava in Halemaʻumaʻu had drained below the crater floor by May 10, followed by collapse of the Overlook and Halemaʻumaʻu craters. The collapse region expanded as much of the broader summit caldera floor subsided incrementally during June and July. By early August 2018, the collapse sequence had ended, and the summit was quiet. The historical changes in May–August 2018 brought a dramatic end to the decade of sustained activity at Kīlauea’s summit. The unique accessibility of the 2008–18 lava lake provided new observations of lava lake behavior and open-vent basaltic outgassing. Data indicated that explosions were triggered by rockfalls from the crater walls, that the lake consisted of a low-density foamy lava, that cycles of gas pistoning were rooted at shallow depths in the lake, and that lake level fluctuations were closely tied to the pressure of the summit magma reservoir. Lava chemistry added further support for an efficient hydraulic connection between the summit and East Rift Zone. Notwithstanding the benefits to scientific understanding, the eruption presented a persistent hazard of volcanic air pollution (vog) that commonly extended far from Kīlauea’s summit.

Hawaii↗

USGS2006

Continental crustal evolution observations

How has the continental crust evolved? What are the primary processes responsible for its composition, structure, and mode of deformation? What role do fluids play in deep crustal processes? In the last dozen years, geophysicists have obtained images of the deep continental crust that can be used to examine these questions and refine geologic models of crustal evolution. In this report we summarize recent progress in geophysical studies of the deep continental crust and highlight some of the more important implications of deep crustal processes.

Eos, Earth and Space Science News↗

USGS2017

The concurrent use of novel soil surface microclimate measurements to evaluate CO2 pulses in biocrusted interspaces in a cool desert ecosystem

Carbon cycling associated with biological soil crusts, which occupy interspaces between vascular plants in drylands globally, may be an important part of the coupled climate-carbon cycle of the Earth system. A major challenge to understanding CO 2 fluxes in these systems is that much of the biotic and biogeochemical activity occurs in the upper few mm of the soil surface layer (i.e., the ‘mantle of fertility’), which exhibits highly dynamic and difficult to measure temperature and moisture fluctuations. Here, we report a multi-sensor approach to simultaneously measuring temperature and moisture of this biocrust surface layer (0–2 mm), and the deeper soil profile, concurrent with automated measurement of surface soil CO 2 effluxes. Our results illuminate robust relationships between biocrust water content and field CO 2 pulses that have previously been difficult to detect and explain. All observed CO 2 pulses over the measurement period corresponded to surface wetting events, including when the wetting events did not penetrate into the soil below the biocrust layer (0–2 mm). The variability of temperature and moisture of the biocrust surface layer was much greater than even in the 0–5 cm layer of the soil beneath the biocrust, or deeper in the soil profile. We therefore suggest that coupling surface measurements of biocrust moisture and temperature to automated CO 2 flux measurements may greatly improve our understanding of the climatic sensitivity of carbon cycling in biocrusted interspaces in our study region, and that this method may be globally relevant and applicable.

Biogeochemistry↗

USGS2024

Seasonality of retreat rate of a wave-exposed marsh edge

Wave-driven erosion of marsh boundaries is a major cause of marsh loss, but little research has captured the effect of seasonal differences on marsh-edge retreat rates to illuminate temporal patterns of when the majority of this erosion is occurring. Using five surface models captured over a study year of a marsh with a steep escarped boundary in South San Francisco Bay, we find a pronounced seasonal signal, where rapid marsh retreat in the spring and summer is driven by a strong sea breeze but little change is found in the marsh-edge position in the fall and winter. We found accretion in the mudflat transition region close to the marsh boundary in the calmer seasons however, suggesting intertwined morphodynamics of mudflats and the eroding marsh-scarp. We observed large spatial heterogeneity in retreat rates within seasons, but less on longer (annual and decadal) timescales. The relationship between marsh-edge retreat rates and properties of the wave field nearby is explored and contextualized against extant relationships, but our results speak to the difficulty in addressing spatial erosion/accretion variability on short (seasonal) timescales with simple models.

JGR Earth Surface↗

USGS2016

Barrier displacement on a neutral landscape: Towards a theory of continental biogeography

Macroevolutionary theory posits three processes leading to lineage diversification and the formation of regional biotas: dispersal (species geographic range expansion), speciation (species lineage splitting), and extinction (species lineage termination). The Theory of Island Biogeography (TIB) predicts species richness values using just two of these processes; dispersal and extinction. Yet most species on Earth live on continents or continental shelves, and the dynamics of evolutionary diversification at regional and continental scales are qualitatively different from those that govern the formation of species richness on biogeographic islands. Certain geomorphological processes operating perennially on continental platforms displace barriers to gene flow and organismal dispersal, and affect all three terms of macroevolutionary diversification. For example, uplift of a dissected landscape and river capture both merge and separate portions of adjacent areas, allowing dispersal and larger geographic ranges, vicariant speciation and smaller geographic ranges, and extinction when range sizes are subdivided below a minimum persistence threshold. The TIB also does not predict many biogeographic and phylogenetic patterns widely observed in continentally distributed taxa, including: 1, power function-like species-area relationships; 2, log-normal distribution of species geographic range sizes, in which most species have restricted ranges (are endemic) and few species have broad ranges (are cosmopolitan); 3, mid-domain effects with more species towards the geographic center, and more early-branching, species-poor clades towards the geographic periphery; 4, exponential rates of net diversification with log-linear accumulation of lineages through geological time; and 5, power function-like relationships between species-richness and clade diversity, in which most clades are species-poor and few clades are species-rich. Current theory does not provide a robust mechanistic framework to connect these seemingly disparate patterns. Here we present SEAMLESS (Spatially-Explicit Area Model of Landscape Evolution by SimulationS) that generates clade diversification by moving geographic barriers on a continuous, neutral landscape. SEAMLESS is a neutral Landscape Evolution Model (LEM) that treats species and barriers as functionally equivalent with respect to model parameters. SEAMLESS differs from other model-based biogeographic methods (e.g. Lagrange, GeoSSE, BayArea, BioGeoBEARS) by modeling properties of dispersal barriers rather than areas, and by modeling the evolution of species lineages on a continuous landscape, rather than the evolution of geographic ranges along branches of a phylogeny. SEAMLESS shows how dispersal is required to maintain species richness and avoid clade-wide extinction, demonstrates that ancestral range size does not predict species richness, and provides a unified explanation for the suite of commonly observed biogeographic and phylogenetic patterns listed above. SEAMLESS explains how a simple barrier-displacement mechanism affects lineage diversification under neutral conditions, and is advanced here towards the formulation of a general theory of continental biogeography.

Systematic Biology↗

USGS1988

Measuring contemporary crustal motions; NASA’s Crustal Dynamics Project

In the early and middle 1970's, two new space-based geodetic techniques became available that offered unprecedented accuracy in the measurement of distances over long baselines. As described below, Satellite Laser Ranging (SLR) and Very Long Baseline Interferometry (VLBI) provided a capability to determine in relatively short periods of time the inter-site distance between two observing stations to a level of several centimeters, even if they were separated by thousands or tens of thousands of kilometers. During the 1980's the two techniques have evolved to the point where baselines can now be routinely measured to a level below one centimeter. This is a tenfold improvement in about ten years. Perhaps more important than providing the measurement capability are the consequences of this capability when applied over a few years: the motion of any one site with respect to another can be monitored at the level of better than a cm/yr. This has made possible, for the first time, direct measurement of the motion of the Earth's tectonic plates and of the deformation of its crust in active plate boundary regions. Because such measurements have the potential for providing important information on the mechanisms that drive plates and cause them to slip during earthquakes, a coordinated federal program for the application of this space technology to crustal dynamics and earthquake research was established in 1979. The participating agencies were the National Aeronautics and Space Administration (NASA), the National Oceanic and Atmospheric Administration (NOAA), the United States Geological Survey (USGS), the National Science Foundation (NSF), and the Defense Mapping Agency. NASA formed the Crustal Dynamics Project to further develop the SLR and VLBI techniques, to implement global networks of stations with the cooperation of many different countries, and to conduct measurements of plate motions and regional deformations. this project has grown into an international effort to collect information directly relevant to understanding the threat of the earthquake hazard as well as fundamental research about the crust and upper mantle. In this article we describe briefly the two space geodetic techniques and how they are used by the Crustal Dynamics Project, show some of the very exciting results that have emerged at the halfway point in the project's life, describe the availability and utilization of the data being collected, and consider what the future may hold when measurement accuracies eventually exceed even those now available and when other international groups become more heavily involved.

Earthquakes & Volcanoes (USGS)↗

USGS1988

Experiments with microcomputer-based artificial intelligence environments

The U.S. Geological Survey (USGS) has been experimenting with the use of relatively inexpensive microcomputers as artificial intelligence (AI) development environments. Several AI languages are available that perform fairly well on desk-top personal computers, as are low-to-medium cost expert system packages. Although performance of these systems is respectable, their speed and capacity limitations are questionable for serious earth science applications foreseen by the USGS. The most capable artificial intelligence applications currently are concentrated on what is known as the "artificial intelligence computer," and include Xerox D-series, Tektronix 4400 series, Symbolics 3600, VAX, LMI, and Texas Instruments Explorer. The artificial intelligence computer runs expert system shells and Lisp, Prolog, and Smalltalk programming languages. However, these AI environments are expensive. Recently, inexpensive 32-bit hardware has become available for the IBM/AT microcomputer. USGS has acquired and recently completed Beta-testing of the Gold Hill Systems 80386 Hummingboard, which runs Common Lisp on an IBM/AT microcomputer. Hummingboard appears to have the potential to overcome many of the speed/capacity limitations observed with AI-applications on standard personal computers. USGS is a Beta-test site for the Gold Hill Systems GoldWorks expert system. GoldWorks combines some high-end expert system shell capabilities in a medium-cost package. This shell is developed in Common Lisp, runs on the 80386 Hummingboard, and provides some expert system features formerly available only on AI-computers including frame and rule-based reasoning, on-line tutorial, multiple inheritance, and object-programming. ?? 1988 International Association for Mathematical Geology.

Mathematical Geology↗

USGS2011

The indication of Martian gully formation processes by slope-area analysis

The formation process of recent gullies on Mars is currently under debate. This study aims to discriminate between the proposed formation processes - pure water flow, debris flow and dry mass wasting - through the application of geomorphological indices commonly used in terrestrial geomorphology. High-resolution digital elevation models (DEMs) of Earth and Mars were used to evaluate the drainage characteristics of small slope sections. Data from Earth were used to validate the hillslope, debris-flow and alluvial process domains previously found for large fluvial catchments on Earth, and these domains were applied to gullied and ungullied slopes on Mars. In accordance with other studies, our results indicate that debris flow is one of the main processes forming the Martian gullies that were being examined. The source of the water is predominantly distributed surface melting, not an underground aquifer. Evidence is also presented indicating that other processes may have shaped Martian crater slopes, such as ice-assisted creep and solifluction, in agreement with the proposed recent Martian glacial and periglacial climate. Our results suggest that, within impact craters, different processes are acting on differently oriented slopes, but further work is needed to investigate the potential link between these observations and changes in Martian climate.

Geological Society Special Publication↗

USGS2017

A fault‐based model for crustal deformation in the western United States based on a combined inversion of GPS and geologic inputs

We develop a crustal deformation model to determine fault‐slip rates for the western United States (WUS) using the Zeng and Shen (2014) method that is based on a combined inversion of Global Positioning System (GPS) velocities and geological slip‐rate constraints. The model consists of six blocks with boundaries aligned along major faults in California and the Cascadia subduction zone, which are represented as buried dislocations in the Earth. Faults distributed within blocks have their geometrical structure and locking depths specified by the Uniform California Earthquake Rupture Forecast, version 3 (UCERF3) and the 2008 U.S. Geological Survey National Seismic Hazard Map Project model. Faults slip beneath a predefined locking depth, except for a few segments where shallow creep is allowed. The slip rates are estimated using a least‐squares inversion. The model resolution analysis shows that the resulting model is influenced heavily by geologic input, which fits the UCERF3 geologic bounds on California B faults and ±one‐half of the geologic slip rates for most other WUS faults. The modeled slip rates for the WUS faults are consistent with the observed GPS velocity field. Our fit to these velocities is measured in terms of a normalized chi‐square, which is 6.5. This updated model fits the data better than most other geodetic‐based inversion models. Major discrepancies between well‐resolved GPS inversion rates and geologic‐consensus rates occur along some of the northern California A faults, the Mojave to San Bernardino segments of the San Andreas fault, the western Garlock fault, the southern segment of the Wasatch fault, and other faults. Off‐fault strain‐rate distributions are consistent with regional tectonics, with a total off‐fault moment rate of 7.2 × 10 18 "> 7.2 × 10 18 and 8.5 × 10 18    N · m / year "> 8.5 × 10 18 N ⋅ m / year for California and the WUS outside California, respectively.

Bulletin of the Seismological Society of America↗

USGS2005

Ground water to surface water: Chemistry of thermal outflows in Yellowstone National Park

Geothermal waters in the earth’s subsurface boil with steam separation and may mix with dilute ground waters (that may or may not contain sulfuric acid from sulfur oxidation), resulting in a wide range of compositions when they discharge and emerge at the surface. As they discharge onto the ground surface they undergo evaporative cooling, degassing, oxidation, and mineral precipitation. Within this aquatic environment of rapidly changing physical and chemical parameters, numerous microbial communities develop—some of which affect oxidation and mineral precipitation. Microbes are responsible for rapid oxidation of iron and arsenic in thermal outflows, and for catalyzing the production of sulfuric acid from the oxidation of elemental sulfur. The attractive visual display of colors observed in Yellowstone’s geothermal waters reflects this interplay of physical, chemical, and biological phenomena. Oxidation of dissolved sulfide to thiosulfate occurs abiotically, and thiosulfate can be found in many of Yellowstone’s thermal waters—at any pH, temperature, and composition. Polythionates, on the other hand, are rarely found in Yellowstone waters but are associated with sulfur hydrolysis in Cinder Pool. Oxidation rates of iron and arsenic in overflows have been estimated at 1-3 mM/h and 0.04-0.1 mM/h, respectively—orders of magnitude faster than the abiotic rate. The abiotic production of thiosulfate from oxidation of dissolved sulfi de at Angel Terrace and Ojo Caliente is about 3-30 µM/min, faster by 2-3 orders of magnitude than the laboratory rate at 25°C. The partitioning of dissolved sulfide between that volatilized to the air and that oxidized to thiosulfate has been estimated at Angel Terrace and at Ojo Caliente. For the pH range of 6-8 and the temperature range of 50-93°C, 67-86% of the dissolved sulfide is lost to the atmosphere and 10-33% is oxidized to thiosulfate. Only a very small percentage, if any, forms elemental sulfur under these conditions.

Wyoming↗

USGS1999

Gravel sediment routing from widespread, low-intensity landscape disturbance, Current River basin, Missouri

During the last 160 years, land-use changes in the Ozarks have had the potential to cause widespread, low-intensity delivery of excess amounts of gravel-sized sediment to stream channels. Previous studies have indicated that this excess gravel bedload is moving in wave-like forms through Ozarks drainage basins. The longitudinal, areal distribution of gravel bars along 160 km of the Current River, Missouri, was evaluated to determine the relative effects of valley-scale controls, tributary basin characteristics, and lagged sediment transport in creating areas of gravel accumulations. The longitudinal distribution of gravel-bar area shows a broad scale wave-like form with increases in gravel-bar area weakly associated with tributary junctions. Secondary peaks of gravel area with 1·8–4·1 km spacing (disturbance reaches) are superimposed on the broad form. Variations in valley width explain some, but not all, of the short-spacing variation in gravel-bar area. Among variables describing tributary drainage basin morphometry, present-day land use and geologic characteristics, only drainage area and road density relate even weakly to gravel-bar areal inventories. A simple, channel network-based sediment routing model shows that many of the features of the observed longitudinal gravel distribution can be replicated by uniform transport of sediment from widespread disturbances through a channel network. These results indicate that lagged sediment transport may have a dominant effect on the synoptic spatial distribution of gravel in Ozarks streams; present-day land uses are only weakly associated with present-day gravel inventories; and valley-scale characteristics have secondary controls on gravel accumulations in disturbance reaches.

Earth Surface Processes and Landforms↗

USGS2014

The 17 May 2012 M4.8 earthquake near Timpson, East Texas: An event possibly triggered by fluid injection

This study summarizes our investigation of the 17 May 2012 MW-RMT4.8 earthquake near Timpson, Texas, the largest earthquake recorded historically in eastern Texas. To identify preshocks and aftershocks of the 17 May event we examined the arrivals recorded at Nacogdoches (NATX) 30 km from the 17 May epicenter, at nearby USArray Transportable Array stations, and at eight temporary stations deployed between 26 May 2012 and mid-2013. At NATX we identified seven preshocks, the earliest occurring in April 2008. Reliably located aftershocks recorded by the temporary stations lie along a 6 km long NW-SE linear trend corresponding to a previously mapped basement fault that extends across the highest-intensity (MMI VII) region of the 17 May main shock. Earthquakes in this sequence are relatively shallow—with focal depths ranging from 1.6 to 4.6 km. Evidence supporting these depths include: hypocentral locations of exceptionally well-recorded aftershocks, S-P intervals at the nearest stations, and comparisons of synthetics and observed seismograms. Within 3 km of the linear trend of aftershock activity there are two Class II injection disposal wells injecting at 1.9 km depth beginning in August 2006 and February 2007, with injection rates averaging 42,750 m3/mo and 15,600 m3/mo, respectively. Several observations support the hypothesis that fluid injection triggered the Timpson sequence: well-located epicenters are situated near a mapped basement fault and near high-volume injection wells, focal depths are at or below the depths of injection, and the earliest preshock (April 2008) occurred after the onset of injection in 2006.

Texas↗

USGS2021

Dating fault damage along the eastern Denali fault zone with hematite (U-Th)/He thermochronometry

Unraveling complex slip histories in fault damage zones to understand relations among deformation, hydrothermal alteration, and surface uplift remains a challenge. The dextral eastern Denali fault zone (EDFZ; southwest Yukon, Canada) bounds the Kluane Ranges and hosts a variety of fault-related rocks, including hematite fault surfaces, which have been exhumed through the brittle regime over a protracted period of geologic time. Scanning electron microscopy-based microtextural observations and hematite (U-Th)/He (hematite He) thermochronometry from these surfaces indicate multiple generations of foliated, high-aspect ratio hematite plates. Single-aliquot hematite He dates ( n = 38 "> n=38 ) from 11 samples range from 11.5 ± 3.2 Ma (2 σ ) to 3.4 ± 2.1 Ma and exhibit moderate inter- and intrasample dispersion. A subset of dates is 15-20 Myr younger than previously published apatite (U-Th)/He dates from collocated host rocks, despite similar closure temperatures, precluding a simple ambient cooling interpretation for our hematite He data. Mixture modeling defines hematite He date populations at ∼8 Ma, ∼6 Ma, and ∼4 Ma, and when combined with microtextural observations, supports episodes of hydrothermal alteration and fault reactivation at aseismic to subseismic slip rates. There is no evidence that hematite experienced deformation- or hydrothermal fluid-related He loss. Hematite He dates overlap previously documented Kluane Ranges surface uplift and shifting dynamics of the Yakutat microplate, pinpointing fault networks and deformation processes that accommodate regional deformation in response to far-field plate boundary processes.

Alaska↗

USGS2009

Three‐dimensional model of Hellenic Arc deformation and origin of the Cretan uplift

[1] The Hellenic Arc of Greece is the most seismically active part of Europe, but little is know about its mechanics. We modeled deformation along the arc using a finite element model. The model was intended to capture large‐scale 3‐D structure of Nubian plate subduction beneath the Aegean block and its deformational consequences. The shape of the interface was developed using mapped traces at the surface and earthquake hypocenters at depth. Model block motions were constrained by recent compilations of GPS velocity vectors. We simulated a 10 ka period of convergence between Nubia and the Aegean and calculated the strain field in the overriding plate as well as the spatial distribution and orientation of differential stress (∣ σ 1 − σ 3 ∣). From these calculations we derived testable quantities such as the expected seismic moment rate on the interplate contact, uplift pattern, and distribution of strain modes. Our relatively simple model broadly reproduced observed uplift patterns, earthquake activity, and loci of extension and contraction. The model showed a localization of uplift near the island of Crete, where the fastest Aegean uplift rates are well documented. Comparison of calculated expected seismic moment and observed earthquake catalogs implies a nearly fully coupled interplate contact. On the basis of our modeling results, we suggest that south Aegean deformation is driven primarily by the fast moving (∼33 mm a −1 ) Aegean upper plate overriding a nearly stalled (∼5 mm a −1 ) Nubian lower plate. This tectonic setting thus more closely resembles a continental thrust than it does a typical oceanic subduction zone.

Aegean Sea, Hellenic Arc↗

USGS2026

Challenges and opportunities for national-scale projections of future coastal landscape change

Local to global scale projections of future coastal landscape change are essential to improve land and resource management decisions that aim to prepare for and reduce risk exposure to impending coastal hazards. However, the availability of actionable knowledge is often limited due to the complexity of drivers of change, their consequences, and uncertainties that span disciplines. Varying spatial and temporal scales of physical processes that occur on landscapes with differing levels of coastal development and modification complicate decision-making. This paper reviews advances in observational approaches, modeling, and quantification of uncertainty to support the next generation of coastal change projections. We also highlight transdisciplinary opportunities for making this information more accessible and useful to support decision-making in the coastal zone. This includes consideration of different levels of user engagement to address specific needs, as well as resources and mechanisms that can support knowledge transfer broadly. A central recommendation is the development of a coordinated framework to support research integration including three components: accounting for relevant drivers, processes, and feedbacks; working with users to identify their information needs; and generating meaningful projections and uncertainty. Actions that would support such frameworks include the evaluation of models through coordinated research approaches and multi-model comparisons; ongoing investments in observations and data-driven approaches necessary to understand this complex environment; expansion of modeling capabilities using novel techniques; prioritization of co-development efforts between researchers and users; and continued training and support for transdisciplinary research.

Alaska, California, Florida, New York↗

USGS2012

Interseismic deformation at the Nankai Trough, Japan, subduction zone

Uplift along the coast of southwestern Japan following the 1944 Tonankai ( M s = 8.0) and 1946 Nankaido ( M s = 8.2) earthquakes has been inferred from the 1950–1985 tide gage records. Although uplift rates vary with geographic position, the temporal dependence at each site can be described as the superposition of an exponentially decaying (time constant ∼5 years) transient, significant only in the first decade postseismic, and a steady interseismic rate. The steady interseismic uplift rate is defined only over the 1956–1985 interval, but the linearity in time ofthat uplift record is such that uplift over the remainder of the interseismic cycle (∼80 years) might reasonably be expected to occur at the same rate. Moreover, the spatial pattern and magnitude of the steady uplift rate are reasonably close to those predicted by a simple dislocation model of subduction. However, the currently observed steady uplift rate is not consistent with the uplift rate inferred from geodetic leveling for the final half of the previous earthquake cycle. Thus, either the hypothesis of a steady interseismic uplift rate after the first decade postseismic is wrong or the strain accumulation pattern for one earthquake need not be the same as for the preceding earthquake on the same segment.

Journal of Geophysical Research B: Solid Earth↗

USGS2024

An integrated perspective of paleoenvironmental change in the Western Interior Seaway before and during OAE-2 reveals how organic-rich mudstones form in dynamic environments

Organic and inorganic geochemistry was integrated with sedimentology from the Eagle Ford Group in the U.S. Geological Survey Gulf Coast-3 drill core to investigate why organic matter enrichment and anoxia predate the Cenomanian–Turonian oceanic anoxic event (OAE-2) at the southern Western Interior Seaway (WIS). The relationship between the degree of organic matter sulfurization and preservation in the southern WIS is comparable to OAE-2 sites outside of the WIS, but enhanced organic matter sulfurization and preservation predated OAE-2, distinguishing the southern WIS from other OAE-2 localities. Persistent euxinia and organic matter sulfurization before and during the mid-Cenomanian event (MCE) facilitated maximum organic matter enrichment and preservation to coincide with extensive lateral sediment transport. The new perspective detailed here reconciles geochemical evidence of depositional euxinia with sedimentological evidence of sediment transport, which informs how organic-rich mudstones form in dynamic environments. After the MCE but before OAE-2, reducing conditions weakened as relative sea-level continued to rise, resulting in an oxygen-depleted environment that was prone to transient euxinia. The extent of organic matter sulfurization, preservation, and enrichment declined as euxinia became intermittent. As maximum sea-level was established during OAE-2, boreal water masses flowed into the southern WIS and unrestricted, oxygenated marine conditions developed, which resulted in degradation of marine organic matter and less organic enrichment. The highest relative contribution of marine bacterial organic matter occurred during the persistently euxinic interval before and during the MCE. The relative input of algal organic matter increased as the depositional conditions became less reducing after the MCE but before OAE-2. Prasinophyte green algae contributed to the consistent dominance of C 28 steranes preceding OAE-2, which is becoming a widely observed feature across the southern and central WIS. A sharp drop in the C 28 sterane relative abundance reflects a decline in the abundance of prasinophytes as oxic depositional conditions developed during OAE-2. Relative abundances of 2-methylhopanes increased during OAE-2, which is a common signature of OAEs, suggesting a modified nitrogen cycle in the offshore southern WIS despite oxic depositional conditions during OAE-2.

Arkansas, Louisiana, Oklahoma, Texas↗

USGS1997

Continents as lithological icebergs: The importance of buoyant lithospheric roots

An understanding of the formation of new continental crust provides an important guide to locating the oldest terrestrial rocks and minerals. We evaluated the crustal thicknesses of the thinnest stable continental crust and of an unsubductable oceanic plateau and used the resulting data to estimate the amount of mantle melting which produces permanent continental crust. The lithospheric mantle is sufficiently depleted to produce permanent buoyancy (i.e., the crust is unsubductable) at crustal thicknesses greater than 25–27 km. These unsubductable oceanic plateaus and hotspot island chains are important sources of new continental crust. The newest continental crust (e.g., the Ontong Java plateau) has a basaltic composition, not a granitic one. The observed structure and geochemistry of continents are the result of convergent margin magmatism and metamorphism which modify the nascent basaltic crust into a lowermost basaltic layer overlain by a more silicic upper crust. The definition of a continent should imply only that the lithosphere is unsubductable over ≥ 0.25 Ga time periods. Therefore, the search for the oldest crustal rocks should include rocks from lower to mid-crustal levels.

Earth and Planetary Science Letters↗