Peter A. Selkin

Long-term variations in palaeointensity

We compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass. The basaltic glass data more than double the number of reliable (Thellier method with pTRM checks) palaeointensity estimates available. Although the new data dramatically improve both spatial and temporal coverage, there is still a strong bias toward the most recent past. The last 0.3 Ma claim over half of the data in our combined database. We therefore divide the data into two groups, the densely sampled last 0.3 Myr and the more sparsely sampled period of time comprising roughly half of the data from 0.3 to 300 Ma. Separating them in this way, it is clear that the dipole moment of the Earth over the past 0.3 Myr (ca.8 × 1022 A m2) is dramatically higher than the average dipole moment over the preceding 300 Myr (ca.5 × 1022 A m2). Inclusion of poor–quality results leads to an overestimate of the average dipole moment. Interestingly, no other significant changes in the distribution of dipole moments are evident over the 300 million year span of the data.

The effect of remanence anisotropy on paleointensity estimates: a case study from the Archean Stillwater Complex

Paleomagnetism of Archean rocks potentially provides information about the early development of the Earth and of the geodynamo. Precambrian layered intrusive rocks are good candidates for paleomagnetic studies: such complexes are commonly relatively unaltered and may contain some single-domain magnetite ‘armored’ by silicate mineral grains. However, layered intrusives often have a strong petrofabric that may result in a strong remanence anisotropy. Magnetic anisotropy can have particularly disastrous consequences for paleointensity experiments if the anisotropy is unrecognized and if its effects remain uncorrected. Here we examine the magnetic anisotropy of an anorthosite sample with a well-developed magmatic foliation. The effect of the sample’s remanence fabric on paleointensity determinations is significant: paleointensities estimated by the method of Thellier and Thellier range from 17 to 55 μT for specimens magnetized in a field of 25 μT. We describe a technique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a paleointensity for the Stillwater Complex (MT, USA) of ∼32 μT (adjusted for the effects of slow cooling).

Noise in the Quiet Zone

Abstract We have carried out a detailed paleomagnetic investigation of two stratigraphically overlapping sections from the Scaglia Bianca Formation (∼85–89.5 Ma) in the Umbria–Marche area in central Italy. Sampling was conducted over 32 m and 7 m intervals at La Roccaccia and Furlo respectively. After AF cleaning the majority of specimens show the expected normal magnetic field orientation, however a number of specimens are directionally anomalous. Some of these deviant specimens are accompanied by apparent spikes or dips in normalized intensity. A detailed investigation of rock magnetics shows that most of these deviations are not a sign of excursionary geomagnetic field behavior, but rather correspond to specimens with distinct rock magnetic characteristics and are therefore rock magnetic ‘noise’. Such specimens should not be interpreted as records of the geomagnetic field. Our experience suggests that detailed rock magnetic and magnetic fabric analysis should be done on all anomalous directions prior to interpreting them as geomagnetic field behavior. After elimination of rock magnetic noise in the Scaglia Bianca data sets, there is a high degree of agreement in direction and to a lesser extent relative intensity between correlative portions of the two sections. We therefore offer this data set as a robust record of geomagnetic field behavior during the 4.5 Myr interval represented by the La Roccaccia section. A statistical analysis of the relative intensity observations suggests that this period of the Cretaceous Normal Superchron is characterized by a normalized variability in paleointensity (standard deviation about 28% of the mean value) that is significantly lower than seen during the Oligocene over intervals in which reversals or tiny wiggles occur (typically about 50%). The directional stability results in virtual geomagnetic pole dispersion compatible with that found in volcanic rocks from around the same latitude and ranging in age from 80 to 110 Ma.

Climate, dust, and fire across the Eocene-Oligocene transition, Patagonia

The Eocene-Oligocene transition (EOT) is typically interpreted as a time of drastic global cooling and drying associated with massive growth of a glacial icecap in Antarctica and the shift to an “icehouse” climate. The effects of this transition on the terrestrial environments, floras, and faunas of the Southern Hemisphere, however, have been unclear. Here we document simultaneous changes in fire regime and plant community in Patagonia, Argentina. Decreases in the concentration of magnetite in loessites from the Eocene-Oligocene Vera Member of the Sarmiento Formation correlate with decreases in the fraction of burnt palm phytoliths as well as more consistently palm-dominated phytolith assemblages. Association of magnetite and burnt palm phytoliths suggests intense wildfires, which appear to have been suppressed for ∼200 k.y. shortly after the EOT. The disappearance of fire-related characteristics near the EOT is possible if changes in regional wind patterns—consistent with observed changes in sediment particle sizes—caused changes in seasonal precipitation. These results imply a more important role for fire in structuring Eocene-Oligocene landscapes than previously thought.

Evaluation of student learning, self-efficacy, and perception of the value of geologic monitoring from Living on the Edge, an InTeGrate curriculum module

ABSTRACT The InTeGrate curriculum, Living on the Edge: Building Resilient Societies on Active Plate Margins (LOE), explores geologic hazards and related societal risks at plate boundaries. The curriculum incorporates research-based pedagogical practices and the use of geologic data for students to analyze and interpret. Materials were developed for broad use and have been implemented at 3 institutions in a range of undergraduate geoscience classroom settings. Here we present an analysis of student learning based on new data collected with a pre- and post-LOE curriculum instrument. Results indicate increased student learning in all types of classrooms examined. Learning gains occur in large classes (50–140 students) that spent the least amount of time on each unit (≤ 50 minutes), but students achieve even larger gains in small classes (< 15 students) that spent more time on curriculum (≥ 75 minutes per curricular unit). Learning gains occured in all class types in activities that required application of information as well as in items that required only simple recall. Students’ self-efficacy in their ability to accurately identify factors that determine hazards and risks at plate boundaries increased by at least 1 point on a 5-point scale following their participation in the curriculum. Most students agree (or strongly agree) that geologic monitoring is likely to be valuable to them and to society, which makes LOE activities relevant to them and correlates with their learning. Significant student learning also occurred in courses taught by an instructor who was not an LOE author, which supports the goal that InTeGrate materials can be widely used.