Closing Pandora's Box: Additional Insights on Inclination Bias Using a Random Walk Approach

Abstract A fundamental working assumption in paleomagnetic studies is that the Earth's magnetic field averages to a geocentric axial dipole (GAD) when sufficiently sampled. One of the main tools for evaluating the GAD hypothesis in pre-Cenozoic times is based on the distribution of inclination values. Recent studies of inclination-only data show a bias towards low inclination and a number of alternative explanations were forwarded to explain this bias. The inclination only analysis relies on the fact that the planet has been adequately sampled in a spatially and/or temporally random manner. A recent paper argued that the inclination only studies might misrepresent the field because the extant global paleomagnetic database does not provide an adequate sampling of the field. In this study, we examine other sources of bias in the database. We find that the apparent contributions of quadrupolar and octupolar fields may depend upon the binning procedure used. For example, the Cenozoic database can be favorably compared to GAD when assigned to temporal bins based on geologic periods, but is decidedly non-GAD when averaged on a finer temporal scale. We also demonstrate that the Paleozoic inclination distribution may result from a regional sampling bias and we quantitatively assess the probability that the Precambrian global paleomagnetic dataset sufficiently integrates the time-averaged Earth's magnetic field. Our analysis suggests that the extant inclination database contains myriad forms of bias and may not represent the Earth's magnetic field. Unfortunately, the analysis cannot rule out the existence of persistent nondipolar fields. The global paleomagnetic database does indeed show a rather consistent bias towards low-inclination values (median inclination is 40° versus 49° for the GAD). Models of the earth's magnetic field and the thermal evolution of the planet may yield additional clues regarding its GAD or non-GAD nature

Heat flow at the Platanares, Honduras, geothermal site

Three boreholes, PLTG-1, PLTG-2 and PLTG-3, were drilled in the Platanares, Honduras geothermal system to evaluate the geothermal energy potential of the site. The maximum reservoir temperature was previously estimated at 225-240[deg]C using various types of chemical and isotopic geothermometry. Geothermal gradients of 139-239[deg]C/km, calculated from two segments of the temperature-depth profile for borehole PLTG-2, were used to project a minimum depth to the geothermal reservoir of 1.2-1.7 km. Borehole PLTG-1 exhibited an erratic temperature distribution attributed to fluid movement through a series of isolated horizontal and subhorizontal fractures. The maximum measured temperature in borehole PLTG-1 was 150.4[deg]C, and in PLTG-2 the maximum measured temperature was 104.3[deg]C. PLTG-3 was drilled after this study and the maximum recorded temperature of 165[deg]C is similar to the temperature encountered in PLTG-1.Heat flow values of 392 mWm-2 and 266 mWm-2 represent the first directly-measured heat flow values for Honduras and northen Central America. Radioactive heat generation, based on gamma-ray analyses of uranium, thorium and potassium in five core samples, is less than 2.0 [mu]Wm-3 and does not appear to be a major source of the high heat flow. Several authors have proposed a variety of extensional tectonic environments for western Honduras and these heat flow values, along with published estimates of heat flow, are supportive of this type of tectonic regime.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29424/1/0000503.pd

Late Proterozoic paleomagnetism and tectonic models: a critical appraisal

Interpretations of Proterozoic orogenic belts in terms of plate tectonic processes have been widely divergent. Published models range between the extremes of no relative motions between continental nuclei (implying ensialic orogenic processes) on the one hand, to large-scale relative motions with oceans opening and closing (resulting in continent-continent collisions) on the other hand. Paleomagnetic data can, in theory, contribute significantly to this debate; however, as shown in this paper, several tectonic interpretations on the basis of paleomagnetic data have been premature. A critical continent in many of the previous models is Africa. In order to test hypotheses, for instance, for the late Proterozoic-Cambrian Pan African orogeny, a compilation of paleopoles has been made for Africa, with age ranges falling fully or partially within the interval of 1150 to 500 Ma. A quantitative comparison of the quality of this African dataset with the Phanerozoic poles for North American and Europe shows that the late Proterozoic paleopoles of Africa generally have very low reliability. It appears that the data from other Gondwana continents are equally unreliable and even less abundant. This means that currently the dataset of Gondwanaland cannot be used with confidence for the testing of tectonic models such as the Precambrian supercontinent, at least for the time after 1150 Ma. Well-dated late Proterozoic paleopoles from the three cratonic nuclei within Africa (Congo, Kalahari, West Africa) define relatively short apparent polar wander path segments, but each with different age ranges. This implies that they cannot be compared with each other to test relative motions between the cratonic nuclei and that a choice between ensialic and ensimatic models for the Pan African orogenic belts cannot yet be based on paleomagnetic data. While this does not imply that the tectonic models (e.g. those of Piper and McWilliams) are wrong, it does mean that substantial paleomagnetic support for them will have to wait more and higher-quality paleopole determinations with better dating precision.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29100/1/0000136.pd

Paleomagnetism of the Late Archean Nyanzian System, western Kenya

The Nyanzian System lavas of western Kenya are believed to be the oldest rocks of the Tanzanian Craton. Intrusive age relationships suggest an age [ges]2850 Ma although direct attempts at dating the Nyanzian have produced disparate results. Our study involves a suite of samples collected from the Nyanzian basalts, pillow basalts, andesites and rhyolites from sixteen sites in western Kenya. These rocks yield a tilt-corrected paleomagnetic pole at 14[deg]N, 150[deg]E (K=59, dp=5[deg], dm=7[deg]). This pole is constrained to be older than the first (D1) deformation (>2472+/-30 Ma) by positive fold, conglomerate and reversal tests. Analysis of the paleomagnetic data base for three African cratonic nuclei (Tanzanian, Kaapvaal/Zimbabwe and West Africa) for the time period from 2.0 Ga to 3.0 Ga demonstrates a paucity of well-dated poles, although there are several poles from the Kaapvaal/Zimbabwe and Tanzanian Cratons which allow "spot-readings" of their relative positions. We demonstrate, based on these data, that the Kaapvaal/Zimbabwe and Tanzanian Cratons were drifting independently at ~ 2875 Ma, ~ 2700 Ma and ~ 2450 Ma. This independent motion of the Tanzanian and Kaapvaal/Zimbabwe Cratons indicates that previously proposed models involving African cratonic coherence can no longer be considered valid for the time period from 2850 to 2500 Ma.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31278/1/0000184.pd

The magnificent seven : A proposal for modest revision of the Van der Voo (1990) quality index

Thirty years ago, Rob Van der Voo proposed an elegant and simple system for evaluating the quality of paleomagnetic data. As a second-year Ph.D. student, the lead author remembers Rob waxing philosophical about the need to have an appropriate, but not overly rigid evaluation system. The end result was a 7-point system that assigned a (1) or (0) for any paleomagnetic result based on objective criteria. The goal was never to reject or blindly accept any particular result, but merely to indicate the degree of quality for any paleomagnetic pole. At the time, the global paleomagnetic database was burgeoning and it was deemed useful to rank older paleo magnetic results with the newer data being developed in modern laboratories. Van der Voo's, 1990 paper launched a silent revolution in paleomagnetism. Researchers began to evaluate their data against those seven criteria with the anticipation that reviewers would be similarly critical. Today, paleomagnetism is a mature science. Our methods, analyses, and results are more sophisticated than they were 30 years ago. Therefore, we feel it is appropriate to revisit the Van der Voo (1990) criteria in light of those developments. We hope to honor the intention of the original paper by keeping the criteria simple and easy to evaluate while also acknowledging the advances in science. This paper aims to update the criteria and modernize the process. We base our changes on advances in paleomagnetism and geochronology with a faithful adherence to the simplicity of the original publication. We offer the "Reliability" or "R" index as the next generation of the Van der Voo "Quality" or "Q" index. The new R-criteria evaluate seven different information items for each paleomagnetic pole including age, statistical requirements, identification of magnetic carriers, field tests, structural integrity, presence of reversals and an evaluation for possible remagnetization.Peer reviewe

Reconstructions of the continents around the North Atlantic at about the 60th parallel

Abstract Late Carboniferous^Early Tertiary apparent polar wander (APW) paths (300^40 Ma) for North America and Europe have been tested in various reconstructions. These paths demonstrate that the 500 fathom Bullard et al. fit is excellent from Late Carboniferous to Late Triassic times, but the continental configuration in northern Pangea changed systematically between the Late Triassic (ca. 214 Ma) and the Mid-Jurassic (ca. 170 Ma) due to pre-drift extension. Best fit North Atlantic reconstructions minimize differences in the Late Carboniferous^Early Jurassic and Late CretaceousT ertiary segments of the APW paths, but an enigmatic difference exists in the paths for most of the Jurassic, whereas for the Early Cretaceous the data from Europe are nearly non-existent. Greenland's position is problematic in a Bullard et al. fit, because of a Late Triassic^Early Jurassic regime of compression ( s 300 km) that would be inherently required for the Norwegian Shelf and the Barents Sea, but which is geologically not defensible. We suggest a radically new fit for Greenland in between Europe and North America in the Early Mesozoic. This fit keeps Greenland`locked' to Europe for the Late Paleozoic^Early Mesozoic and maintains a reconstruction that better complies with the offshore geological history of the Norwegian Shelf and the Barents Sea. Pre-drift (A24) extension amounted to approximately 450 km on the Mid-Norwegian Shelf but with peak extension in the Late Cretaceous. ß 2001 Published by Elsevier Science B.V

LSST Science Book, Version 2.0

A survey that can cover the sky in optical bands over wide fields to faint magnitudes with a fast cadence will enable many of the exciting science opportunities of the next decade. The Large Synoptic Survey Telescope (LSST) will have an effective aperture of 6.7 meters and an imaging camera with field of view of 9.6 deg^2, and will be devoted to a ten-year imaging survey over 20,000 deg^2 south of +15 deg. Each pointing will be imaged 2000 times with fifteen second exposures in six broad bands from 0.35 to 1.1 microns, to a total point-source depth of r~27.5. The LSST Science Book describes the basic parameters of the LSST hardware, software, and observing plans. The book discusses educational and outreach opportunities, then goes on to describe a broad range of science that LSST will revolutionize: mapping the inner and outer Solar System, stellar populations in the Milky Way and nearby galaxies, the structure of the Milky Way disk and halo and other objects in the Local Volume, transient and variable objects both at low and high redshift, and the properties of normal and active galaxies at low and high redshift. It then turns to far-field cosmological topics, exploring properties of supernovae to z~1, strong and weak lensing, the large-scale distribution of galaxies and baryon oscillations, and how these different probes may be combined to constrain cosmological models and the physics of dark energy.Comment: 596 pages. Also available at full resolution at http://www.lsst.org/lsst/sciboo

Outcomes and characteristics of cardiac arrest in children with pulmonary hypertension: A secondary analysis of the ICU-RESUS clinical trial.

BACKGROUND: Previous studies have identified pulmonary hypertension (PH) as a relatively common diagnosis in children with in-hospital cardiac arrest (IHCA), and preclinical laboratory studies have found poor outcomes and low systemic blood pressures during CPR for PH-associated cardiac arrest. The objective of this study was to determine the prevalence of PH among children with IHCA and the association between PH diagnosis and intra-arrest physiology and survival outcomes. METHODS: This was a prospectively designed secondary analysis of patients enrolled in the ICU-RESUS clinical trial (NCT02837497). The primary exposure was a pre-arrest diagnosis of PH. The primary survival outcome was survival to hospital discharge with favorable neurologic outcome (Pediatric Cerebral Performance Category score 1-3 or unchanged from baseline). The primary physiologic outcome was event-level average diastolic blood pressure (DBP) during CPR. RESULTS: Of 1276 patients with IHCAs during the study period, 1129 index IHCAs were enrolled; 184 (16.3%) had PH and 101/184 (54.9%) were receiving inhaled nitric oxide at the time of IHCA. Survival with favorable neurologic outcome was similar between patients with and without PH on univariate (48.9% vs. 54.4%; p = 0.17) and multivariate analyses (aOR 0.82 [95%CI: 0.56, 1.20]; p = 0.32). There were no significant differences in CPR event outcome or survival to hospital discharge. Average DBP, systolic BP, and end-tidal carbon dioxide during CPR were similar between groups. CONCLUSIONS: In this prospective study of pediatric IHCA, pre-existing PH was present in 16% of children. Pre-arrest PH diagnosis was not associated with statistically significant differences in survival outcomes or intra-arrest physiologic measures