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At least 1,297 records · Page 72Linked to original sources
USGS2012

The Steens Mountain (Oregon) geomagnetic polarity transition: 1. Directional history, duration of episodes, and rock magnetism

The thick sequence of Miocene lava flows exposed on Steens Mountain in southeastern Oregon is well known for containing a detailed record of a reversed‐to‐normal geomagnetic polarity transition. Paleomagnetic samples were obtained from the sequence for a combined study of the directional and intensity variations recorded; the paleointensity study is reported in a companion paper. This effort has resulted in the first detailed history of total geomagnetic field behavior during a reversal of polarity. A comparison of the directional variation history of the reversed and normal polarity intervals on either side of the transition with the Holocene record has allowed an estimate of the duration of these periods to be made. These time estimates were then used to calculate accumulation rates for the volcanic sequence and thereby provide a means for estimating time periods within the transition itself. The polarity transition was found to consist of two phases, each with quite different characteristics. At the onset of the first phase, a one‐third decrease in magnetic field intensity may have preceded the first intermediate field directions by about 600 years. Changes in field direction were confined near the local north‐south vertical plane when the actual reversal in direction occurred and normal polarity directions may have been attained within 550±150 years. The end of the first phase of the transition was marked by a brief (possibly 100–300 years) period with normal polarity and a pretransitional intensity which suggests a quasi‐normal dipole field structure existed during this interval. The second phase of the transition was characterized by a return to very low field intensities with the changes in direction describing a long counterclockwise loop in contrast to the earlier narrowly constrained changes. This second phase lasted 2900±300 years, and both normal directions and intensities were recovered at the same time. Both directional and intensity data document very erratic geomagnetic field behavior during the polarity transition. Changes in magnetic field direction were variable and occurred either (1) in a regular, progressive manner, (2) with sudden, extremely rapid angular changes (58°±21°/year), or (3) with little or no movement for periods of the order of 600±200 years. Changes in magnetic intensity occurred in a like manner and were sometimes correlated with changes in direction, but during other periods both directional and intensity changes occurred independently. Directional changes following the polarity transition occurred in a seemingly normal manner, although intensity fluctuations attest to some instability of the newly reestablished dipole.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Fitting degradation of shoreline scarps by a nonlinear diffusion model

The diffusion model of degradation of topographic features is a promising means by which vertical offsets on Holocene faults might be dated. In order to calibrate the method, we have examined present-day profiles of wave-cut shoreline scarps of late Pleistocene lakes Bonneville and Lahontan. It may be assumed that these scarps were initially at least as steep as the angle of repose. Offsets range from 1 to 12 m, and present slope angles range from 9° to 29°. A parameter called apparent diffusion age, defined as half the mean square horizontal extent of the slope function of each profile, is plotted as a function of scarp offset. The points show a clear trend of apparent age increasing nearly linearly with offset. If linear diffusion held and scarps were initially vertical, apparent diffusion age would be the same for all the shoreline profiles. The increasing trend can only partly be explained by nonvertical initial scarp slope, and therefore the rate of transport of material downslope must increase significantly faster than a linear law in the range of slopes spanned by the data. The transport law must become linear at small slope to reduce scatter between profiles with varying ambient slopes. The transport law adopted for the purpose of dating is k 0 (1 + 5 s 2 ), where s is local slope. The transport coefficient k 0 is correlated inversely with fan slope, suggesting that there is a dependence on the particle size distribution. A table is included that allows easy application of the model to scarps with simple initial shape.

Journal of Geophysical Research Solid Earth↗
USGS2012

The hydrothermal system at Newberry Volcano, Oregon

Results of recent geological and geophysical studies at Newberry Volcano have been incorporated into conceptual and numerical models of a magma-based hydrothermal system. Numerical simulations begin with emplacement of a small magma body, the presumed source of silicic eruptions at Newberry that began about 10,000 B.P., into a thermal regime representing 100,000 years of cooling of a large underlying intrusion. Simulated flow patterns and thermal histories for three sets of hypothetical permeability values are compatible with data from four geothermal drill holes on the volcano. Meteoric recharge cools the caldera-fill deposits, but thermal water moving up a central conduit representing a permeable volcanic vent produces temperatures close to those observed in drill holes within the caldera. Meteoric recharge from the caldera moves down the flanks and creates a near-isothermal zone that extends several hundred meters below the water table, producing temperature profiles similar to those observed in drill holes on the flanks. The temperatures observed in drill holes on the flanks are not influenced by the postulated Holocene magma body. The elevated temperature gradients measured in the lower portions of these holes may be related to the cumulative effect of older intrusions. The models also indicate that meteoric recharge to the deep hydrothermal system probably originates within or near the caldera. Relatively low fluid velocities at depth suggest that at least a significant fraction of the thermal fluid may be very old.

Journal of Geophysical Research Solid Earth↗
USGS2012

Deuterium in interstitial water from deep-sea cores

As part of the Joint Oceanographic Institutions Deep Earth Sampling project, the interstitial waters of cores from 69 holes were sampled for deuterium analysis. Sixteen of the cores penetrated sediments as old as Eocene, and several sampled Cretaceous sediments, which allowed us to examine changes in the deuterium content of the oceans with time. Deuterium is shown to be a conservative constituent of the pore water. Its abundance in the pore fluids can be changed by diffusion, but the rate is slow, and corrections for this effect have been made. Changes in the abundance of deuterium can be related to changes in the amount of ice stored in continental glaciers, inasmuch as precipitation in the form of snow is highly depleted in deuterium compared with the oceans. Many of the cores show a change in isotopic composition of samples from early to late Miocene that can be ascribed to the buildup of the Antarctic ice sheets. After correcting for the role of diffusion in reducing the isotopic contrast between samples from a single core, we estimate an increase of 10 per mil (‰) δD (corresponding to a° δ 18 O change of about 1.2‰) between the early and late Miocene. A similiar analysis of Pleistocene to Holocene changes indicates a δD rise of 8‰ during the time of maximum continental ice, which corresponds to a δ 18 O increase of about 1.0‰. On the basis of limited data, we find no δD change in the oceans from Cretaceous to Miocene.

Journal of Geophysical Research Solid Earth↗
USGS2010

Changes in redox conditions in deep‐sea sediments of the subarctic North Pacific Ocean: Possible evidence for the presence of North Pacific Deep Water

Cores of upper Quaternary and Holocene sediment from the subarctic North Pacific north of about 48°N contain one or more layers of oxidized brown sediment interbedded within predominantly reduced green sediment. The brown layers are enriched in several trace elements, especially Mn, Mo, Ni, and Co, relative to the green layers. Where multiple oxidized layers are present, the intensity of the brown coloration and the magnitude of trace element enrichment often decrease with depth, suggesting that the oxidized layers are unstable and are being chemically reduced at depth. The oxidized layers represent a change in redox conditions between the North Pacific red clay province and the subarctic biosiliceous green clay province. The redox change may have been caused by an increase in supply of dissolved oxygen to bottom waters during glacial‐interglacial transitions as the result of the periodic formation of a seasonal bottom water mass in the northeastern Pacific Ocean.

Alaska↗
USGS1970

Geological significance of coccoliths in fine-grained carbonate bands of postglacial Black Sea sediments

The origin of fine carbonate muds in deep parts of the Black Sea has been explained in various ways, but details of how the carbonate was formed are poorly understood. We have studied samples containing fine carbonate from cores obtained during the cruise of Atlantis II (Woods Hole Oceanographic Institute) to the Black Sea in April and May 1969. Examination of the light-coloured bands and darker bands by scanning electron micrographs and light microscopy shows that Holocene coccoliths are the overwhelmingly dominant constituents in the light bands, whereas Cretaceous and Eocene coccoliths are important components of the carbonate in the darker matrix.

Nature↗
USGS2011

Extended megadroughts in the southwestern United States during Pleistocene interglacials

The potential for increased drought frequency and severity linked to anthropogenic climate change in the semi-arid regions of the southwestern United States (US) is a serious concern. Multi-year droughts during the instrumental period and decadal-length droughts of the past two millennia were shorter and climatically different from the future permanent, ‘dust-bowl-like’ megadrought conditions, lasting decades to a century, that are predicted as a consequence of warming. So far, it has been unclear whether or not such megadroughts occurred in the southwestern US, and, if so, with what regularity and intensity. Here we show that periods of aridity lasting centuries to millennia occurred in the southwestern US during mid-Pleistocene interglacials. Using molecular palaeotemperature proxies to reconstruct the mean annual temperature (MAT) in mid-Pleistocene lacustrine sediment from the Valles Caldera, New Mexico, we found that the driest conditions occurred during the warmest phases of interglacials, when the MAT was comparable to or higher than the modern MAT. A collapse of drought-tolerant C 4 plant communities during these warm, dry intervals indicates a significant reduction in summer precipitation, possibly in response to a poleward migration of the subtropical dry zone. Three MAT cycles ∼ 2 °C in amplitude occurred within Marine Isotope Stage (MIS) 11 and seem to correspond to the muted precessional cycles within this interglacial. In comparison with MIS 11, MIS 13 experienced higher precessional-cycle amplitudes, larger variations in MAT (4–6 °C) and a longer period of extended warmth, suggesting that local insolation variations were important to interglacial climatic variability in the southwestern US. Comparison of the early MIS 11 climate record with the Holocene record shows many similarities and implies that, in the absence of anthropogenic forcing, the region should be entering a cooler and wetter phase.

Nature↗
USGS2015

North Pacific deglacial hypoxic events linked to abrupt ocean warming

Marine sediments from the North Pacific document two episodes of expansion and strengthening of the subsurface oxygen minimum zone (OMZ) accompanied by seafloor hypoxia during the last deglacial transition 1 , 2 , 3 , 4 . The mechanisms driving this hypoxia remain under debate 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 . We present a new high-resolution alkenone palaeotemperature reconstruction from the Gulf of Alaska that reveals two abrupt warming events of 4–5 degrees Celsius at the onset of the Bølling and Holocene intervals that coincide with sudden shifts to hypoxia at intermediate depths. The presence of diatomaceous laminations and hypoxia-tolerant benthic foraminiferal species, peaks in redox-sensitive trace metals 12 , 13 , and enhanced 15 N/ 14 N ratio of organic matter 13 , collectively suggest association with high export production. A decrease in 18 O/ 16 O values of benthic foraminifera accompanying the most severe deoxygenation event indicates subsurface warming of up to about 2 degrees Celsius. We infer that abrupt warming triggered expansion of the North Pacific OMZ through reduced oxygen solubility and increased marine productivity via physiological effects; following initiation of hypoxia, remobilization of iron from hypoxic sediments could have provided a positive feedback on ocean deoxygenation through increased nutrient utilization and carbon export. Such a biogeochemical amplification process implies high sensitivity of OMZ expansion to warming.

Nature↗
USGS2011

Loading of the San Andreas fault by flood-induced rupture of faults beneath the Salton Sea

The southern San Andreas fault has not experienced a large earthquake for approximately 300 years, yet the previous five earthquakes occurred at ~180-year intervals. Large strike-slip faults are often segmented by lateral stepover zones. Movement on smaller faults within a stepover zone could perturb the main fault segments and potentially trigger a large earthquake. The southern San Andreas fault terminates in an extensional stepover zone beneath the Salton Sea—a lake that has experienced periodic flooding and desiccation since the late Holocene. Here we reconstruct the magnitude and timing of fault activity beneath the Salton Sea over several earthquake cycles. We observe coincident timing between flooding events, stepover fault displacement and ruptures on the San Andreas fault. Using Coulomb stress models, we show that the combined effect of lake loading, stepover fault movement and increased pore pressure could increase stress on the southern San Andreas fault to levels sufficient to induce failure. We conclude that rupture of the stepover faults, caused by periodic flooding of the palaeo-Salton Sea and by tectonic forcing, had the potential to trigger earthquake rupture on the southern San Andreas fault. Extensional stepover zones are highly susceptible to rapid stress loading and thus the Salton Sea may be a nucleation point for large ruptures on the southern San Andreas fault.

California↗
USGS2008

Increasing eolian dust deposition in the western United States linked to human activity

Mineral aerosols from dust are an important influence on climate and on marine and terrestrial biogeochemical cycles. These aerosols are generated from wind erosion of surface soils. The amount of dust emission can therefore be affected by human activities that alter surface sediments. However, changes in regional- and global-scale dust fluxes following the rapid expansion of human populations and settlements over the past two centuries are not well understood. Here we determine the accumulation rates and geochemical properties of alpine lake sediments from the western interior United States for the past 5,000 years. We find that dust load levels increased by 500% above the late Holocene average following the increased western settlement of the United States during the nineteenth century. We suggest that the increased dust deposition is caused by the expansion of livestock grazing in the early twentieth century. The larger dust flux, which persists into the early twenty-first century, results in a more than fivefold increase in inputs of K, Mg, Ca, N and P to the alpine ecosystems, with implications for surface-water alkalinity, aquatic productivity and terrestrial nutrient cycling.

Colorado↗
USGS2010

Complex rupture during the 12 January 2010 Haiti earthquake

Initially, the devastating Mw 7.0, 12 January 2010 Haiti earthquake seemed to involve straightforward accommodation of oblique relative motion between the Caribbean and North American plates along the Enriquillog-Plantain Garden fault zone. Here, we combine seismological observations, geologic field data and space geodetic measurements to show that, instead, the rupture process may have involved slip on multiple faults. Primary surface deformation was driven by rupture on blind thrust faults with only minor, deep, lateral slip along or near the main Enriquillog-Plantain Garden fault zone; thus the event only partially relieved centuries of accumulated left-lateral strain on a small part of the plate-boundary system. Together with the predominance of shallow off-fault thrusting, the lack of surface deformation implies that remaining shallow shear strain will be released in future surface-rupturing earthquakes on the Enriquillog-Plantain Garden fault zone, as occurred in inferred Holocene and probable historic events. We suggest that the geological signature of this earthquakeg-broad warping and coastal deformation rather than surface rupture along the main fault zoneg-will not be easily recognized by standard palaeoseismic studies. We conclude that similarly complex earthquakes in tectonic environments that accommodate both translation and convergenceg-such as the San Andreas fault through the Transverse Ranges of Californiag-may be missing from the prehistoric earthquake record. ?? 2010 Macmillan Publishers Limited. All rights reserved.

Nature Geoscience↗
USGS2019

Deglacial water-table decline in Southern California recorded by noble gas isotopes

Constraining the magnitude of past hydrological change may improve understanding and predictions of future shifts in water availability. Here we demonstrate that water-table depth, a sensitive indicator of hydroclimate, can be quantitatively reconstructed using Kr and Xe isotopes in groundwater. We present the first-ever measurements of these dissolved noble gas isotopes in groundwater at high precision (≤0.005‰ amu −1 ; 1σ), which reveal depth-proportional signals set by gravitational settling in soil air at the time of recharge. Analyses of California groundwater successfully reproduce modern groundwater levels and indicate a 17.9 ± 1.3 m (±1 SE) decline in water-table depth in Southern California during the last deglaciation. This hydroclimatic transition from the wetter glacial period to more arid Holocene accompanies a surface warming of 6.2 ± 0.6 °C (±1 SE). This new hydroclimate proxy builds upon an existing paleo-temperature application of noble gases and may identify regions prone to future hydrological change.

California↗
USGS2020

Expert assessment of future vulnerability of the global peatland carbon sink

The carbon balance of peatlands is predicted to shift from a sink to a source this century. However, peatland ecosystems are still omitted from the main Earth system models that are used for future climate change projections, and they are not considered in integrated assessment models that are used in impact and mitigation studies. By using evidence synthesized from the literature and an expert elicitation, we define and quantify the leading drivers of change that have impacted peatland carbon stocks during the Holocene and predict their effect during this century and in the far future. We also identify uncertainties and knowledge gaps in the scientific community and provide insight towards better integration of peatlands into modelling frameworks. Given the importance of the contribution by peatlands to the global carbon cycle, this study shows that peatland science is a critical research area and that we still have a long way to go to fully understand the peatland–carbon–climate nexus.

Nature Climate Change↗
USGS2021

Arctic Ocean stratification set by sea level and freshwater inputs since the last ice age

Salinity-driven density stratification of the upper Arctic Ocean isolates sea-ice cover and cold, nutrient-poor surface waters from underlying warmer, nutrient-rich waters. Recently, stratification has strengthened in the western Arctic but has weakened in the eastern Arctic; it is unknown if these trends will continue. Here we present foraminifera-bound nitrogen isotopes from Arctic Ocean sediments since 35,000 years ago to reconstruct past changes in nutrient sources and the degree of nutrient consumption in surface waters, the latter reflecting stratification. During the last ice age and early deglaciation, the Arctic was dominated by Atlantic-sourced nitrate and incomplete nitrate consumption, indicating weaker stratification. Starting at 11,000 years ago in the western Arctic, there is a clear isotopic signal of Pacific-sourced nitrate and complete nitrate consumption associated with the flooding of the Bering Strait. These changes reveal that the strong stratification of the western Arctic relies on low-salinity inflow through the Bering Strait. In the central Arctic, nitrate consumption was complete during the early Holocene, then declined after 5,000 years ago as summer insolation decreased. This sequence suggests that precipitation and riverine freshwater fluxes control the stratification of the central Arctic Ocean. Based on these findings, ongoing warming will cause strong stratification to expand into the central Arctic, slowing the nutrient supply to surface waters and thus limiting future phytoplankton productivity.

Nature Geoscience↗
USGS2025

Quantitative subsurface characterization illuminates the origin of the Quaternary Mississippi River Valley alluvial aquifer

The Mississippi River Valley alluvial aquifer (MRVA) is vital to U.S. food security and global agricultural supply. However, quantitative understanding of its Quaternary origin, architecture, and hydrologic function remains incomplete. Here we develop a three-dimensional hydrostratigraphic model to characterize the deposition of clay and silt, fine-medium sands, and graveliferous sands using lithologic data from 75,000 boreholes compiled across the Lower Mississippi Valley and a geostatistical method—interval kriging. We find that cyclic glacial entrenchments, evidenced by remnants of pre-Wisconsinan postglacial sediments, alongside geodynamic activities shaped the MRVA basal configuration. Stratal weakening from faulting and salt diapirism enhanced glacial incision and thereby produced abrupt aquifer thickening. We demarcate the top of graveliferous sands as the regional marker of the Pleistocene-Holocene transition. The MRVA hydrostratigraphy reveals hydrologic function and geologic controls on groundwater storage and quality, advancing the assessment of aquifer sustainability under a changing climate, with implications for alluvial aquifers globally.

Arkansas, Illinois, Kentucky, Louisiana, Mississip↗
USGS2017

Evidence for coseismic subsidence events in a southern California coastal saltmarsh

Paleoenvironmental records from a southern California coastal saltmarsh reveal evidence for repeated late Holocene coseismic subsidence events. Field analysis of sediment gouge cores established discrete lithostratigraphic units extend across the wetland. Detailed sediment analyses reveal abrupt changes in lithology, percent total organic matter, grain size, and magnetic susceptibility. Microfossil analyses indicate that predominantly freshwater deposits bury relic intertidal deposits at three distinct depths. Radiocarbon dating indicates that the three burial events occurred in the last 2000 calendar years. Two of the three events are contemporaneous with large-magnitude paleoearthquakes along the Newport-Inglewood/Rose Canyon fault system. From these data, we infer that during large magnitude earthquakes a step-over along the fault zone results in the vertical displacement of an approximately 5-km 2 area that is consistent with the footprint of an estuary identified in pre-development maps. These findings provide insight on the evolution of the saltmarsh, coseismic deformation and earthquake recurrence in a wide area of southern California, and sensitive habitat already threatened by eustatic sea level rise.

California↗
USGS2002

Interactions between onshore bedrock-channel incision and nearshore wave-base erosion forced by eustasy and tectonics

We explore the response of bedrock streams to eustatic and tectonically induced fluctuations in base level. A numerical model coupling onshore fluvial erosion with offshore wave‐base erosion is developed. The results of a series of simulations for simple transgressions with constant rate of sea‐level change ( SLR ) show that response depends on the relative rates of rock uplift ( U ) and wave‐base erosion (ɛ w ). Simple regression runs highlight the importance of nearshore bathymetry. Shoreline position during sea‐level fall is set by the relative rate of base‐level fall ( U‐SLR ) and ɛ w , and is constant horizontally when these two quantities are equal. The results of models forced by a realistic Late Quaternary sea‐level curve are presented. These runs show that a stable shoreline position cannot be obtained if offshore uplift rates exceed ɛ w . Only in the presence of a relatively stable shoreline position, fluvial profiles can begin to approximate a steady‐state condition, with U balanced by fluvial erosion rate (ɛ f ). In the presence of a rapid offshore decrease in rock‐uplift rate ( U ), short (∼5 km) fluvial channels respond to significant changes in rock‐uplift rate in just a few eustatic cycles. The results of the model are compared to real stream‐profile data from the Mendocino triple junction region of northern California. The late Holocene sea‐level stillstand response exhibited by the simulated channels is similar to the low‐gradient mouths seen in the California streams.

California↗
USGS2003

Gaussian statistics for palaeomagnetic vectors

With the aim of treating the statistics of palaeomagnetic directions and intensities jointly and consistently, we represent the mean and the variance of palaeomagnetic vectors, at a particular site and of a particular polarity, by a probability density function in a Cartesian three-space of orthogonal magnetic-field components consisting of a single (unimoda) non-zero mean, spherically-symmetrical (isotropic) Gaussian function. For palaeomagnetic data of mixed polarities, we consider a bimodal distribution consisting of a pair of such symmetrical Gaussian functions, with equal, but opposite, means and equal variances. For both the Gaussian and bi-Gaussian distributions, and in the spherical three-space of intensity, inclination, and declination, we obtain analytical expressions for the marginal density functions, the cumulative distributions, and the expected values and variances for each spherical coordinate (including the angle with respect to the axis of symmetry of the distributions). The mathematical expressions for the intensity and off-axis angle are closed-form and especially manageable, with the intensity distribution being Rayleigh-Rician. In the limit of small relative vectorial dispersion, the Gaussian (bi-Gaussian) directional distribution approaches a Fisher (Bingham) distribution and the intensity distribution approaches a normal distribution. In the opposite limit of large relative vectorial dispersion, the directional distributions approach a spherically-uniform distribution and the intensity distribution approaches a Maxwell distribution. We quantify biases in estimating the properties of the vector field resulting from the use of simple arithmetic averages, such as estimates of the intensity or the inclination of the mean vector, or the variances of these quantities. With the statistical framework developed here and using the maximum-likelihood method, which gives unbiased estimates in the limit of large data numbers, we demonstrate how to formulate the inverse problem, and how to estimate the mean and variance of the magnetic vector field, even when the data consist of mixed combinations of directions and intensities. We examine palaeomagnetic secular-variation data from Hawaii and Re??union, and although these two sites are on almost opposite latitudes, we find significant differences in the mean vector and differences in the local vectorial variances, with the Hawaiian data being particularly anisotropic. These observations are inconsistent with a description of the mean field as being a simple geocentric axial dipole and with secular variation being statistically symmetrical with respect to reflection through the equatorial plane. Finally, our analysis of palaeomagnetic acquisition data from the 1960 Kilauea flow in Hawaii and the Holocene Xitle flow in Mexico, is consistent with the widely held suspicion that directional data are more accurate than intensity data.

Geophysical Journal International↗