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At least 19 recordsLinked to original sources
USGS2000

Recognition of primary and diagenetic magnetizations to determine the magnetic polarity record and timing of deposition of the moat-fill rocks of the Oligocene Creede Caldera, Colorado

Sedimentary and volcaniclastic rocks of the Oligocene Creede Formation fill the moat of the Creede caldera, which formed at about 26.9 Ma during the eruption of the Snowshoe Mountain Tuff. Paleomagnetic and rock magnetic studies of two cores (418 and 703 m long) that penetrated the lower half of the Creede Formation, in addition to paleomagnetic and isotopic dating studies of stratigraphically bracketing volcanic units, provide information on the age and the time span of sedimentation of the caldera fill. Normal polarity magnetization are found in Snowshoe Mountain Tuff beneath the moat sediments; in detrital-magnetite-bearing graded tuffs near the bottom of the moat fill; in an ash-fall deposit about 200 m stratigraphically about the top of core 2; and in postcaldera lava flows of the Fisher Dacite that overlie the Creede Formation. Normal polarity also characterizes detrital-magnetite-bearing tuff and sandstone unites within the caldera moat rocks that did not undergo severe sulfidic alteration. The combination of initially low magnitude of remanent magnetization and the destructive effects of subsequent diagenetic sulfidization on detrital iron oxides results in a poor paleomagnetic record for the fine-grained sedimentary rocks of the Creede Formation. these fine-grained rocks have either normal or revered polarity magnetizations that are carried by magnetite and/or maghemite. Many more apparent reversals are found that can be accommodated by any geomagnetic polarity time scale over the interval spanned by the ages of the bracketing extrusive rocks. Moreover, opposite polarity magnetization are found in specimens separated by only a few centimeters, without intervening hiatuses, and by specimens in several tuff beds, each of which represents a single depositional event. These polarity changes cannot, therefore, be attributed to detrital remanent magnetization. Many polarity changes are apparently related to chemical remanent magnetizations carried by postdepositional magnetite and maghemite that formed in rocks in which most or all detrital megnetic iron oxide was destroyed. Incipient oxidation of early diagenetic pyrite may have normal polarity Snowshoe Mountain Tuff (26.89 ± 0.0 Ma, 1 δ) and on the normal polarity postcaldera Fisher lava flows (as young as 26.23 ± 0.05 Ma, 1 δ) indicate that deposition of the Creede Formation spanned about 340-660 k.y. The intermittently defined normal polarity magnetization for the caldera-fill sequence, compared with different versions of the geomagnetic polarity time scale, is consistent with the shorter time span.

Colorado↗
USGS2011

Evidence from lava flows for complex polarity transitions: The new composite Steens Mountain reversal record

Geomagnetic polarity transitions may be significantly more complex than are currently depicted in many sedimentary and lava-flow records. By splicing together paleomagnetic results from earlier studies at Steens Mountain with those from three newly studied sections of Oregon Plateau flood basalts at Catlow Peak and Poker Jim Ridge 70–90 km to the southeast and west, respectively, we provide support for this interpretation with the most detailed account of a magnetic field reversal yet observed in volcanic rocks. Forty-five new distinguishable transitional (T) directions together with 30 earlier ones reveal a much more complex and detailed record of the 16.7 Ma reversed (R)-to-normal (N) polarity transition that marks the end of Chron C5Cr. Compared to the earlier R-T-N-T-N reversal record, the new record can be described as R-T-N-T-N-T-R-T-N. The composite record confirms earlier features, adds new west and up directions and an entire large N-T-R-T segment to the path, and fills in directions on the path between earlier directional jumps. Persistent virtual geomagnetic pole (VGP) clusters and separate VGPs have a preference for previously described longitudinal bands from transition study compilations, which suggests the presence of features at the core–mantle boundary that influence the flow of core fluid and distribution of magnetic flux. Overall the record is consistent with the generalization that VGP paths vary greatly from reversal to reversal and depend on the location of the observer. Rates of secular variation confirm that the flows comprising these sections were erupted rapidly, with maximum rates estimated to be 85–120 m ka −1 at Catlow and 130–195 m ka −1 at Poker Jim South. Paleomagnetic poles from other studies are combined with 32 non-transitional poles found here to give a clockwise rotation of the Oregon Plateau of 11.4°± 5.6° with respect to the younger Columbia River Basalt Group flows to the north and 14.5°± 4.6° with respect to cratonic North America (95 per cent confidence interval).

Geophysical Journal International↗
USGS2011

Position of the Triassic-Jurassic boundary and timing of the end-Triassic extinctions on land: Data from the Moenave Formation on the southern Colorado Plateau, USA

Strata of the Moenave Formation on and adjacent to the southern Colorado Plateau in Utah–Arizona, U.S.A., represent one of the best known and most stratigraphically continuous, complete and fossiliferous terrestrial sections across the Triassic–Jurassic boundary. We present a synthesis of new biostratigraphic and magnetostratigraphic data collected from across the Moenave Formation outcrop belt, which extends from the St. George area in southwestern Utah to the Tuba City area in northern Arizona. These data include palynomorphs, conchostracans and vertebrate fossils (including footprints) and a composite polarity record based on four overlapping magnetostratigraphic sections. Placement of the Triassic–Jurassic boundary in strata of the Moenave Formation has long been imprecise and debatable, but these new data (especially the conchostracans) allow us to place the Triassic–Jurassic boundary relatively precisely in the middle part of the Whitmore Point Member of the Moenave Formation, stratigraphically well above the highest occurrence of crurotarsan body fossils or footprints. Correlation to marine sections based on this placement indicates that major terrestrial vertebrate extinctions preceded marine extinctions across the Triassic–Jurassic boundary and therefore were likely unrelated to the Central Atlantic Magmatic Province (CAMP) volcanism.

Arizona, Utah↗
USGS2012

Reading the climate record of the martian polar layered deposits

The martian polar regions have layered deposits of ice and dust. The stratigraphy of these deposits is exposed within scarps and trough walls and is thought to have formed due to climate variations in the past. Insolation has varied significantly over time and caused dramatic changes in climate, but it has remained unclear whether insolation variations could be linked to the stratigraphic record. We present a model of layer formation based on physical processes that expresses polar deposition rates of ice and dust in terms of insolation. In this model, layer formation is controlled by the insolation record, and dust-rich layers form by two mechanisms: (1) increased summer sublimation during high obliquity, and (2) variations in the polar deposition of dust modulated by obliquity variations. The model is simple, yet physically plausible, and allows for investigations of the climate control of the polar layered deposits (PLD). We compare the model to a stratigraphic column obtained from the north polar layered deposits (NPLD) (Fishbaugh, K.E., Hvidberg, C.S., Byrne, S., Russel, P.S., Herkenhoff, K.E., Winstrup, M., Kirk, R. [2010a]. Geophys. Res. Lett., 37, L07201) and show that the model can be tuned to reproduce complex layer sequences. The comparison with observations cannot uniquely constrain the PLD chronology, and it is limited by our interpretation of the observed stratigraphic column as a proxy for NPLD composition. We identified, however, a set of parameters that provides a chronology of the NPLD tied to the insolation record and consistently explains layer formation in accordance with observations of NPLD stratigraphy. This model dates the top 500 m of the NPLD back to ∼1 million years with an average net deposition rate of ice and dust of 0.55 mm a−1. The model stratigraphy contains a quasi-periodic ∼30 m cycle, similar to a previously suggested cycle in brightness profiles from the NPLD (Laskar, J., Levrard, B., Mustard, F. [2002]. Nature, 419, 375–377; Milkovich, S., Head, J.W. [2005]. J. Geophys. Res. 110), but here related to half of the obliquity cycles of 120 and 99 kyr and resulting from a combination of the two layer formation mechanisms. Further investigations of the non-linear insolation control of PLD formation should consider data from other geographical locations and include radar data and other stratigraphic datasets that can constrain the composition and stratigraphy of the NPLD layers.

Icarus↗
USGS2012

Evidence for a new geomagnetic reversal from lava flows in Idaho: Discussion of short polarity reversals in the Brunhes and late Matuyama polarity chrons

K-Ar ages and paleomagnetic data for basalt samples from a new core hole (site E) at the Idaho National Engineering Laboratory (INEL) indicate that the age of the reversed polarity event recorded in Snake River Plain lavas is older than 465±50 ka (1000 years before present) reported previously by Champion et al. (1981). Nine basalt flows, eight with normal polarity and one with reversed polarity, were recognized in the site E core hole. The flows above and below the reversed flow have ages of 491±80 ka and 580±93 ka, respectively. The inclination of the paleomagnetic field direction of the reversed flow at site E agrees with the inclination of reversed flows elsewhere at INEL which have an age of 565±14 ka. These reversed flows were previously thought to be correlative with the Emperor event. We suggest that this polarity event is an older event which we name the Big Lost Reversed Polarity Subchronozone and Subchron. A review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the Brunhes and two besides the Jaramillo in the late Matuyama. These 10 short subchrons begin to indicate the many short events that Cox (1968) hypothesized must exist if polarity interval lengths have a Poisson distribution. These events are true subchrons, not excursions, and may or may not have low associated paleointensities, although low field strengths might explain why the reversal process aborts. The mean sustained polarity interval length since late Matuyama Chron time is 90,000 years. The similarity of this number with the 10 5 -year period of the Earth's orbital eccentricity suggests anew that linkage between geomagnetic, paleoclimatic, and possible underlying Earth orbital parameters should be evaluated.

Journal of Geophysical Research Solid Earth↗
USGS2012

The Steens Mountain (Oregon) geomagnetic polarity transition, 2. Field intensity variations and discussion of reversal models

We carried out an extensive paleointensity study of the 15.5±0.3 m.y. Miocene reversed‐to‐normal polarity transition recorded in lava flows from Steens Mountain (south central Oregon). One hundred eighty‐five samples from the collection whose paleodirectional study is reported by Mankinen et al. (this issue) were chosen for paleointensity investigations because of their low viscosity index, high Curie point and reversibility, or near reversibility, of the strong field magnetization curve versus temperature. Application of the Thellier stepwise double heating method was very successful, yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows. After grouping successive lava flows that did not differ significantly in direction and intensity, we obtained 51 distinguishable, complete field vectors of which 10 are reversed, 28 are transitional, and 13 are normal. The record is complex, quite unlike that predicted by simple flooding or standing nondipole field models. It begins with an estimated several thousand years of reversed polarity with an average intensity of 31.5±8.5 μT, about one third lower than the expected Miocene intensity. This difference is interpreted as a long‐term reduction of the dipole moment prior to the reversal. When site directions and intensities are considered, truly transitional directions and intensities appear almost at the same time at the beginning of the transition, and they disappear simultaneously at the end of the reversal. Large deviations in declination occur during this approximately 4500±1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site. The transitional intensity is generally low, with an average of 10.9±4.9 μT for directions more than 45° away from the dipole field and a minimum of about 5 μT. The root‐mean‐square of the three field components X , Y , and Z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude. However, a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions, which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field. Changes in the field vector are progressive but jerky, with at least two, and possibly three, large swings at astonishingly high rates. Each of those transitional geomagnetic impulses occurs when the field intensity is low (less than 10 μT) and is followed by an interval of directional stasis during which the magnitude of the field increases greatly. For the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700±2700 nT yr −1 , which is about 15–50 times larger than the maximum rate of change of the nondipole field observed during the last centuries. The occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions. The record closes with an estimated several thousand years of normal polarity with an average intensity of 46.7±20.1 μT, agreeing with the expected Miocene value. However, the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable.

Journal of Geophysical Research B: Solid Earth↗
USGS2017

Using tri-axial accelerometers to identify wild polar bear behaviors

Tri-axial accelerometers have been used to remotely identify the behaviors of a wide range of taxa. Assigning behaviors to accelerometer data often involves the use of captive animals or surrogate species, as their accelerometer signatures are generally assumed to be similar to those of their wild counterparts. However, this has rarely been tested. Validated accelerometer data are needed for polar bears Ursus maritimus to understand how habitat conditions may influence behavior and energy demands. We used accelerometer and water conductivity data to remotely distinguish 10 polar bear behaviors. We calibrated accelerometer and conductivity data collected from collars with behaviors observed from video-recorded captive polar bears and brown bears U. arctos , and with video from camera collars deployed on free-ranging polar bears on sea ice and on land. We used random forest models to predict behaviors and found strong ability to discriminate the most common wild polar bear behaviors using a combination of accelerometer and conductivity sensor data from captive or wild polar bears. In contrast, models using data from captive brown bears failed to reliably distinguish most active behaviors in wild polar bears. Our ability to discriminate behavior was greatest when species- and habitat-specific data from wild individuals were used to train models. Data from captive individuals may be suitable for calibrating accelerometers, but may provide reduced ability to discriminate some behaviors. The accelerometer calibrations developed here provide a method to quantify polar bear behaviors to evaluate the impacts of declines in Arctic sea ice.

Endangered Species Research↗
USGS2001

Atmospheric regime of dust and salt through 75,000 years of Taylor Dome ice core: Refinement by measurement of major, minor, and trace metal suites

Measurement of absolute and relative amounts of dust and salt deposited in the polar ice record is central to several fields of study, including nutrient delivery, atmospheric deposition of trace elements, past wind strengths, dust provenance, and other aspects of climate and geochemical history. We present a method intended to give a more accurate picture than has been possible before of the total amounts and relative proportions of the dust and salt deposited by the atmosphere into polar ice. It also permits us to distinguish different compositional types of dust in the ice. The method is based on precise measurement of a suite of several metals whose proportions contrast strongly between dust and salt and vary substantially between dust types. We apply the method to a small suite of ice samples from the Taylor Dome core in coastal West Antarctica. In full glacial times, when total impurities were high and dust dominated over salt, wind strength in the West Antarctic region was apparently high, and extensive sea-ice cover prevented incorporation of salt into the atmospheric load. At the termination of the glacial period, increased salt in the dust-salt mixture indicates that sea ice diminished, but wind strength continued high, and unchanged dust composition indicates unchanged source areas. At about 10,000-11,000 y.B.P., sea-ice cover appears to have briefly returned to glacial conditions, but wind conditions remained in the milder postglacial condition. Soon after, sea ice retreated, and an abrupt change in dust composition indicates changed source materials or terranes. If extended by analysis of more samples from more sites, such information on salt and dust could provide firm constraints on past wind strengths, extent of sea-ice cover, deposition fluxes of salt and dust, and changing continental source areas of dust, for both polar regions of the Earth.

Journal of Geophysical Research D: Atmospheres↗
USGS2009

Characteristics of vegetation phenology over the Alaskan landscape using AVHRR time-series data

Advanced Very High Resolution Radiometer (AVHRR) satellite data were acquired and composited into twice-a-month periods from 1 May 1991 to 15 October 1991 in order to map vegetation characteristics of the Alaskan landscape. Unique spatial and temporal qualities of the AVHRR data provide information that leads to a better understanding of regional biophysical characteristics of vegetation communities and patterns. These data provided synoptic views of the landscape and depicted phenological diversity, temporal vegetation phenology (green-up, peak of green, and senescence), photosynthetic activity, and regional landscape patterns. Products generated from the data included a phenological class map, phenological composite maps (onset, peak, and duration), and photosynthetic activity maps (mean and maximum greenness). The time-series data provide opportunities to study phenological processes at small landscape scales over time periods of weeks, months, and years. Regional patterns identified on some of the maps are unique to specific areas; others correspond to biophysical or ecoregional boundaries. The data provide new insights to landscape processes, ecology, and landscape physiognomy that allow scientists to look at landscapes in ways that were previously difficult to achieve.

Alaska↗
USGS2009

Antarctic icebergs as a fresh water resource

In a recent study (Weeks and Campbell, in press), we made a rather complete examination of the idea that icebergs could advantageously be used as a source of fresh water. The origin of the general idea of transporting icebergs is obscure; it may date from the 1850's when ice was transported from Alaska to California as a commercial venture. The current revival of the idea, in particular as applied to the water problems of southern California, can be credited to John Issacs of the Scripps Institute of Oceanography (Burt, 1956). However no critical evaluation was ever published. We initially undertook the present study because our intuition “told” us that the idea was untenable and we thought that we could easily prove that this was the case, thereby laying the idea to rest. Our intuition proved to be wrong and we now believe that the idea is, indeed, highly attractive when applied to certain locations in the Southern Hemisphere. This note briefly outlines our approach to analyzing the problem and our results.

Polar Record↗
USGS2013

The complexity of reversals

This chapter contains sections titled: Introduction Some Illustrative Polarity Transition Records The Case for Complexity Conclusions

Geophysical Monograph Series↗
USGS2019

Energetic costs of aquatic locomotion in a subadult polar bear

Most marine mammals rely on swimming as their primary form of locomotion. These animals have evolved specialized morphologies, physiologies, and behaviors that have enabled them to efficiently move through an aquatic environment (Williams 1999 ). Such adaptations include body streamlining, modified plantar surfaces for propulsion, and abilities to remain submerged for extended durations (Williams 1989 ). As a result of these adaptations, many marine mammal species exhibit minimal increases in metabolism at routine swim speeds relative to resting rates (Williams et al . 1992 , 2017 ; Yazdi et al . 1999 ). Contrary to most marine mammals, polar bears ( Ursus maritimus ) rely on walking as their primary form of locomotion. As a consequence, they exhibit little evidence of body streamlining or abilities to remain submerged for extended durations. The longest dive recorded for a polar bear is 3 min and 10 s (Stirling and van Meurs 2015 ), a relatively brief period compared to other marine mammals (Ponganis 2015 ). Nevertheless, polar bears do exhibit large forepaws (DeMaster and Stirling 1981 ), lower and flatter heads (Slater et al . 2010 ), and more dense forelimb bones (Wall 1983 ) relative to other bear species, potentially as adaptations for swimming. Polar bears further exhibit some distinct physiological and behavioral adaptations from other bear species likely as a consequence of their marine existence (Pagano et al . 2018 a ).

Marine Mammal Science↗
USGS1985

How the geomagnetic field vector reverses polarity

A highly detailed record of both the direction and intensity of the Earth's magnetic field as it reverses has been obtained from a Miocene volcanic sequence. The transitional field is low in intensity and is typically non-axisymmetric. Geomagnetic impulses corresponding to astonishingly high rates of change of the field sometimes occur, suggesting that liquid velocity within the Earth's core increases during geomagnetic reversals. ?? 1985 Nature Publishing Group.

Nature↗
USGS2018

The widespread influence of Great Lakes microseisms across the United States revealed by the 2014 polar vortex

During the winter of 2014, a weak polar vortex brought record cold temperatures to the north‐central (“Midwest”) United States, and the Great Lakes reached the highest extent of ice coverage (92.5%) since 1979. This event shut down the generation of seismic signals caused by wind‐driven wave action within the lakes (termed “lake microseisms”), giving an unprecedented opportunity to isolate and characterize these novel signals through comparison with nonfrozen time periods. Using seismic records at 72 broadband stations, we observe Great Lakes microseism signals at distances >300 km from the lakes. In contrast to conventional oceanic microseisms, there is no clear relationship between the frequency content of the seismic signals (observed from ~0.5–5‐s period) and the dominant swell period or resonance periods of the lakes based on their bathymetric profiles. Thus, the exact generation mechanism is not readily explained by conventional microseism theory and warrants further investigation.

Geophysical Research Letters↗
USGS1976

Paleomagnetism of the Plio-Pleistocene sediments of Lake Tecopa, California, and East Rudolf, Kenya; magnetic stratigraphy and polarity transitions

High resolution paleomagnetic records were obtained from Plio-Pleistocene lake deposits in California and Kenya. The objectives of this research were correlation of the stratigraphic sections with the geomagnetic reversal time scale and measurement of the magnetic changes which occur during polarity transitions. In addition, the properties of reliable and unreliable paleomagnetic sediments were investigated. The Brunhes-Matuyama reversal boundary (700,000 years B.P.) was located in the deposits of Lake Tecopa. A detailed study of this polarity transition gave the .following results: 1). .During the transitions the intensity of the field in the southwestern U.S. dropped to a minimum of 10 percent of the intensity after the transition. 2). Intervals of low field intensity preceded and followed the interval during which the field direction reversed, the latter being shorter than the interval of low intensity by a factor of at least 2.5. 3). Pole paths for the Brunhes-Matuyama transition observed in Japan and California are completely different, indicating that the dipole field decayed. The transition field appears to be essentially the non-dipole field. 4). The transitional field may be a stationary non-dipole field, but one which is different during different transitions.. The small amount of drift during polarity transitions may reflect loss of core mantle induction coupling, which is usually provided by the dipole field. Paleomagnetic stratigraphy was determined from the archaeological sites at Lake Rudolf; however, the dating of these deposits remains ambiguous. Two interpretations have been proposed. In the first . xi interpretation the age of the lower member of the Koobi Fora Formation is 2..6 - 3.7 m.y.; whereas, in the second interpretation the age is 1.6 - 3.1 m.y. Studies of rock magnetism showed that in the deposits of both lakes the magnetic remanence is carried by grains of oxidized titano-magnetite, most with diameters ranging between 1 and 100 microns. At Lake Tecopa the least reliable paleomagnetic results were found in sediments which had undergone diagenesis in a highly oxidizing environment. In a comparison of magnetically stable and unstable specimens from Lake Rudolf, it was found that stability depended on grain size. Stable magnetic behavior was observed in sediments in which the bulk of the magnetic grains had diameters of less than 20 microns. Specimen: which contained large proportions of grains with diameters greater than 50 microns tended to be unstable.

California↗
USGS1992

Calibration of the latest Eocene-Oligocene geomagnetic polarity time scale using 40 Ar/ 39 Ar dated ignimbrites

A discontinuous record of late Eocene-Oligocene geomagnetic polarity has been determined using high-precision (±<0.15 m.y.) 40 Ar/ 39 Ar sanidine dating and a paleomagnetic study of 37-27 Ma ignimbrites in New Mexico, Colorado, and Texas. This record provides age control for several geomagnetic polarity reversals that occurred during three periods of intense ignimbrite volcanism: 36.8-33.5 Ma, 32.7-31.4 Ma, 29.1-26.9 Ma. The relative timing of these polarity reversals permits four possible correlations with the geomagnetic polarity time scale (GPTS). The preferred correlation yields calibration ages for Chron C10R (28.0-29.0 Ma) and Chron C13R (34.4-33.1 Ma) that indicate an Eocene-Oligocene boundary age near 33.4 Ma, some 0.3-0.6 m.y. younger than boundary ages indicated by other recently proposed GPTS calibrations based on terrestrial and marine sedimentary sequences.

Geology↗