Joshua M. Feinberg

FORCinel: An improved algorithm for calculating first‐order reversal curve distributions using locally weighted regression smoothing

Figure 8. Smoothing effects on LOESS smoothed FORC distributions for a natural sample of pure hematite. In Figure 8a, optimum smoothing range is shown in green. Figures 8b, 8c, and 8d correspond to smoothing factors 2, 6, and 10, respectively. Undersmoothing produces a distribution that is dominated by noise (Figure 8b). Oversmoothing produces a distribution that obscures the presence of noninteracting grains along Hu = 0 (Figure 8d). Using the optimization routine described in Figure 5, the ideal FORC distribution is SF = 6.

The Buttermilk Creek Complex and the Origins of Clovis at the Debra L. Friedkin Site, Texas

A large artifact assemblage dating to 15,000 years ago lies beneath a Clovis assemblage in central Texas. Compelling archaeological evidence of an occupation older than Clovis (~12.8 to 13.1 thousand years ago) in North America is present at only a few sites, and the stone tool assemblages from these sites are small and varied. The Debra L. Friedkin site, Texas, contains an assemblage of 15,528 artifacts that define the Buttermilk Creek Complex, which stratigraphically underlies a Clovis assemblage and dates between ~13.2 and 15.5 thousand years ago. The Buttermilk Creek Complex confirms the emerging view that people occupied the Americas before Clovis and provides a large artifact assemblage to explore Clovis origins.

The magnetism of mantle xenoliths and potential implications for sub‐Moho magnetic sources

Mantle xenoliths provide our clearest look at the magnetic mineral assemblages below the Earths crust. Previous investigations of mantle xenoliths suggested the absence of magnetite and metals, and proposed that even if such minerals were present, they would be above their Curie temperatures at mantle conditions. Here we use magnetic measurements to examine four exceptionally fresh suites of xenoliths, and show that magnetite occurs systematically, albeit in variable amounts depending on the tectonic setting. Specimens from low geotherm regions hold the largest magnetic remanence. Petrographic evidence shows that this magnetite did not form through serpentinization or other alteration processes. Magnetite, which is generally stable at the P-T-fO2 conditions in the uppermost mantle, had to have formed either in the mantle or, less likely, in the volcanic conduit. In some cases, the source of the xenoliths was at temperatures <600 C, which may have allowed this portion of the lithospheric mantle to carry a magnetic remanence. Whether such magnetite carries a remanent magnetization or is simply the source of a strong induced magnetization, these new results suggest that the concept of the Moho as a major magnetic boundary needs to be revisited.

Pre-Clovis occupation 14,550 years ago at the Page-Ladson site, Florida, and the peopling of the Americas

Page-Ladson, Florida, provides evidence of the oldest human occupation in the North American Gulf Coastal Plain at 14,550 B.P. Stone tools and mastodon bones occur in an undisturbed geological context at the Page-Ladson site, Florida. Seventy-one radiocarbon ages show that ~14,550 calendar years ago (cal yr B.P.), people butchered or scavenged a mastodon next to a pond in a bedrock sinkhole within the Aucilla River. This occupation surface was buried by ~4 m of sediment during the late Pleistocene marine transgression, which also left the site submerged. Sporormiella and other proxy evidence from the sediments indicate that hunter-gatherers along the Gulf Coastal Plain coexisted with and utilized megafauna for ~2000 years before these animals became extinct at ~12,600 cal yr B.P. Page-Ladson expands our understanding of the earliest colonizers of the Americas and human-megafauna interaction before extinction.

Low-altitude magnetic field measurements by MESSENGER reveal Mercury’s ancient crustal field

Old minerals expose an ancient field Mercury is the only terrestrial planet other than Earth with an active, internally generated magnetic field. Results from the MESSENGER spacecraft indicate that the field is almost as old as the planet. Johnson et al. took advantage of close flybys to extract evidence of an ancient magnetic field. Certain minerals are able to “lock in” the signature of a field at the time they crystallize. This remnant magnetization was found in a region on Mercury believed to be 3.8 billion years old. Science, this issue p. 892 Magnetic field measurements of Mercury’s crust uncover a global magnetic field operating at least 3.8 billion years ago. Magnetized rocks can record the history of the magnetic field of a planet, a key constraint for understanding its evolution. From orbital vector magnetic field measurements of Mercury taken by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft at altitudes below 150 kilometers, we have detected remanent magnetization in Mercury’s crust. We infer a lower bound on the average age of magnetization of 3.7 to 3.9 billion years. Our findings indicate that a global magnetic field driven by dynamo processes in the fluid outer core operated early in Mercury’s history. Ancient field strengths that range from those similar to Mercury’s present dipole field to Earth-like values are consistent with the magnetic field observations and with the low iron content of Mercury’s crust inferred from MESSENGER elemental composition data.

Low‐temperature domain wall pinning in titanomagnetite: Quantitative modeling of multidomain first‐order reversal curve diagrams and AC susceptibility

Domain wall pinning in titanomagnetite has been investigated at low temperatures using first-order reversal curve (FORC) diagrams, AC magnetic susceptibility, and Lorentz transmission electron microscopy. A discontinuous transition from a low-coercivity extrinsic pinning regime to a high-coercivity intrinsic pinning regime is evident in low-temperature FORC diagrams on cooling from 100 to 50 K. Intrinsic pinning is characterized by a “crescent moon” FORC distribution with narrow coercivity distribution centered on 10–20 mT. This crescent-shaped FORC distribution is reproduced using a modification of Neels (1955) one-dimensional theory of domain wall pinning in a random field. The pinning transition coincides with a thermally activated relaxation process (activation energy 0.13 ± 0.01 eV), attributed to electron hopping. The relaxation and intrinsic pinning are explained as a magnetoelastic aftereffect caused by enhancement of magnetocrystalline anisotropy due to rearrangement and localization of Fe2+−Fe3+ cations within the domain walls. This study provides experimental verification that Neels theory is an appropriate quantitative framework for the analysis of FORC diagrams in multidomain titanomagnetite and suggests a potential method for the quantitative unmixing of multidomain signals from FORC diagrams in rock and environmental magnetic studies.

Ferrimagnetic/ferroelastic domain interactions in magnetite below the Verwey transition. Part I: electron holography and Lorentz microscopy

The crystallographic and magnetic microstructure of magnetite (Fe3O4) below the Verwey transition (∼120 K) is studied using transmission electron microscopy. The low temperature phase is found to have a monoclinic C-centered lattice with a c-glide plane perpendicular to the b-axis, which allows twin-related crystal orientations to be distinguished. Off-axis electron holography and Lorentz electron microscopy are used to show that magnetic domains present at room temperature become subdivided into sub-micron-sized magnetic domains below the Verwey transition, with the magnetization direction in each magnetic domain oriented along the monoclinic [001] axis. The nature of the interactions between the magnetic domain walls and the ferroelastic twin walls is investigated. Cooling and warming cycles through the transition temperature are used to show that a memory effect is likely to exist between the magnetic states that form above and below the transition. Our results suggest that ferroelastic twin walls have a strong influence on the low temperature magnetic properties of magnetite.

Magnetic and microscopic characterization of magnetite nanoparticles adhered to clay surfaces

Abstract When suspended in solution, clay platelets coated with nanometer-scale magnetite particles behave as magnetorheologic fluids that are important to a variety of industrial applications. Such dual-phase assemblages are also similar to natural aggregates that record the direction and intensity of the Earth’s magnetic field in lake and marine depositional environments. This study characterizes the mineralogical structure and magnetic behavior of montmorillonite platelets coated with aggregates of nanometerscale magnetite crystals. The distribution of magnetite crystal sizes in three different clay-magnetite assemblages was directly measured using conventional transmission electron microscopy and agrees within error with estimates derived from magnetic hysteresis measurements. Magnetic hysteresis and low field susceptibility measurements combined with electron holography experiments indicate that all three samples behave superparamagnetically at room temperature, and show increasing levels of single domain behavior as the samples are cooled to liquid nitrogen temperatures. At such low temperatures, magnetostatic interactions are observed to partially stabilize otherwise superparamagnetic grains in flux closure structures.

Decadal-scale variations in geomagnetic field intensity from ancient Cypriot slag mounds

Geomagnetic models based on direct observations since the 1830s show that the averaged relative change in field intensity on Earths surface over the past 170 years is less than 4.8% per decade. It is unknown if these rates represent the typical behavior of secular variations due to insufficient temporal resolution of archaeomagnetic records from earlier periods. To address this question, we investigate two ancient slag mounds in Cyprus—Skouriotissa Vouppes (SU1, fourth to fifth centuries CE, 21 m in height), and Mitsero Kokkinoyia (MK1, seventh to fifth centuries BCE, 8 m in height). The mounds are multilayered sequences of slag and charcoals that accumulated near ancient copper production sites. We modeled the age-height relation of the mounds using radiocarbon dates, and estimated paleointensities using Thellier-type IZZI experiments with additional anisotropy, cooling rate, and nonlinear TRM assessments. To screen out ambiguous paleointensity interpretations, we applied strict selection criteria at the specimen/sample levels. To ensure objectivity, consistency, and robust error estimation, we employed an automatic interpretation technique and put the data available in the MagIC database. The analyses yielded two independent subcentury-scale paleointensity time series. The MK1 data indicate relatively stable field at the time the mound accumulated. In contrast, the SU1 data demonstrate changes that are comparable in magnitude to the fastest changes inferred from geomagnetic models. We suggest that fast changes observed in the published archaeomagnetic data from the Levant are driven by two longitudinally paired regions, the Middle East and South Africa, that show unusual activity in geomagnetic models.

MAX UnMix

It is common in the fields of rock and environmental magnetism to unmix magnetic mineral components using statistical methods that decompose various types of magnetization curves (e.g., acquisition, demagnetization, or backfield). A number of programs have been developed over the past decade that are frequently used by the rock magnetic community, however many of these programs are either outdated or have obstacles inhibiting their usability. MAX UnMix is a web application (available online at http://www.irm.umn.edu/maxunmix), built using the shiny package for R studio, that can be used for unmixing coercivity distributions derived from magnetization curves. Here, we describe in detail the statistical model underpinning the MAX UnMix web application and discuss the programs functionality. MAX UnMix is an improvement over previous unmixing programs in that it is designed to be user friendly, runs as an independent website, and is platform independent. HighlightsOnline application for unmixing magnetic coercivity distributions.User-friendly interface increases accessibility.Results from MAX UnMix are comparable with existing methods.