Geomagnetic record in trees affected by fires or lightning?

<p>International Ocean Drilling Program (IODP) Expedition 341 in the Gulf of Alaska recovered a 112-meter-long sedimentary record from the continental slope at Site U1419. At this site, an exceptionally expanded late Pleistocene sequence (sedimentation rates >100 cm/kyr) combined with a high-resolution radiocarbon chronology (Walczak et al., 2020), provide an opportunity to study Paleomagnetic Secular Variations (PSV) on centennial to millennial timescales over the past ~43,000 years.<br>Natural and laboratory-induced magnetic remanence were measured on u-channels using the stepwise AF demagnetization procedure. In addition to continuous magnetic susceptibility measurements, hysteresis parameters were obtained on 95 discrete samples, and IRM acquisition curves on 9 discrete samples to obtain additional information on the magnetic mineralogy of the sediment. Due to the influence of lithology, magnetic mineralogy, depositional and post-depositional processes, Site U1419 is not suitable for paleointensity studies. However, with removal of intervals influenced by the environmental signal and/or coring deformation, the high sedimentation rates at this site have helped to preserve a reliable record of inclination. Because of signal to noise issues, inclination as measured after the 20 mT AF demagnetization step provides the most accurate estimate. This is demonstrated by comparing the U1419 inclination to a stack of the shipboard inclination at Site U1418 on a new age model developed from 19 radiocarbon dates on U1418 and 18 magnetic susceptibility-based tie-points to site survey core EW0408-87JC (Praetorius et al., 2015). This independently replicated inclination record verifies centennial to millennial scale variations in the Gulf of Alaska that can now be compared with other northeast Pacific and western North American records to begin deciphering geomagnetic variability and provide a new stratigraphic correlation tool for 15 and 30 cal kyr BP interval in this region.</p>

The quality of a scanning electron micrograph is determined by both the resolution and the contrast: the size of the features that can be resolved and the type of information that the micrograph gives about them. Recent advances in SEM have improved both, for example using magnetic immersion or electrostatic lenses for ultra‐high resolution, or by employing in‐lens, angle‐sensitive detection for tunable contrast. This paper introduces a further improvement in resolution and in contrast using a new FEI SEM equipped with a compound electrostatic‐magnetic final lens. The compound final lens SEM combines a magnetic final lens in the pole piece, a magnetic immersion final lens and an electrostatic lens formed by the potential at the bottom of the column. This new final lens design provides a resolution equal to 1.0 nm at 1 kV acceleration voltage. Figure 1 demonstrates the ultra‐high resolution performance by imaging the SBA‐15 sample at 500 V landing energy. The contrast performance is based on the independent in‐lens detection of secondary (SE) and backscattered (BSE) electrons. Secondary electrons are further separated and independently detected by in‐lens (T2, higher SE energies) and in‐column (T3, lowest SE energies) detectors. Separate collection of the lowest energy SEs delivers extremely surface sensitive imaging. Simultaneous detection of high energy SEs adds more freedom for imaging of non‐conductive samples – high energy SEs are less sensitive to sample charging while still clearly showing the topography – see the high energy SE image of a hydroxyapatite nano‐sheets grown on bioactive glass fiber [1] in Figure 2. The backscattered electrons are detected by the T1 detector which is positioned at the very bottom of the pole piece. Thanks to that position it receives a high signal intensity which enables ultra‐low beam current BSE imaging. This is important for beam sensitive samples, such as polymers, porous materials or other fragile samples, which require the maximum amount of signal to be acquired in the shortest amount of time with the smallest dose possible. With the T1 detector BSE imaging is possible during TV‐rate navigation, so that material contrast is always available. Combined with the new compound final lens, it is possible to further energy filter the backscattered electrons detected on the T1 detector. When high‐loss (low energy) BSEs are filtered out, T1 provides strong material contrast images formed by low‐loss (high energy) BSEs only – see surface of the Pt activated carbon particle in Figure 3. The acquisition of topographical information is ensured by the SEs simultaneously collected by the T2 and T3 detectors. The T1 detector keeps providing low noise images even at probe currents down to few tens of pA. The low probe current operation together with the energy selective BSE detection enables charge‐free material contrast imaging of non‐conductive samples. The SE and BSE filtering combined with excellent low dose performance (low kV and low current) and “smart” scanning strategies allow for high vacuum imaging of uncoated insulators that are otherwise susceptible to charging. The capabilities on these samples are completed with a low vacuum mode with chamber pressure up to 500 Pa, which is essential for analytical measurements where high acceleration voltages and probe currents are required. In conclusion, the new SEM equipped with the compound final lens and the in‐lens and in‐column detectors improves both the imaging resolution and contrast performance. It allows researchers to capture the maximum amount of information from conductive as well as insulating samples, with the right detail and with the least amount of compromises.

Chapter Three Magnetic Stratigraphy in Paleoceanography: Reversals, Excursions, Paleointensity, and Secular Variation

Early Aphebian Gowganda sedimentary rocks and intruding Nipissing diabase sills were sampled for paleomagnetic study at 88 sites (~500 cores, ~1000 specimens) along two ~42 km long profiles extending north from the Grenville Front into the Cobalt Plate of the Southern Structural Province in the River Valley – Lake Temagami area of Ontario. After AF demagnetization a postfolding pre-Nipissing ~2200 Ma remanence was found in eight of the 37 Gowganda sediment sites that were > 2 km north of the front, giving a pole at 109°W, 63°N (dp = 10°, dm = 19°). The Nippissing diabase from > 2 km north of the front retains a stable antiparallel prefolding N1 remanence direction in 22 of 40 sites, giving a pole position of 85°W, 17°S (dp = 6°, dm = 10°). These "south and down" remanence directions found in the southern portion of the plate contrast with the antiparallel "north and up" directions found in the northern portion, thereby indicating the occurrence of either two nearly cogenetic Nipissing intrusive events or the sequential emplacement of the Nipissing during an Earth's magnetic field reversal across the plate. At two sites a Nipissing remagnetized remanence was found in Gowganda sediments with a pole of 115°W, 18°S. Also three "Nipissing" sites give a pole at 164°W, 3°N, which is close to the known pole for the later ~1.25 Ga Sudbury olivine diabase dikes. One site is adjacent to a large dike and two were found on thin-section examination to be olivine diabase. The eight sites in Gowganda sediment matrix and conglomerate clasts and in Nipissing diabase from within < 2 km from the front were found to have a postfolding metamorphic remanence with a Grenville orogenic pole at 45°W, 51°N (dp = 19°, dm = 21°). Finally, the results lead to a suggested revision in the APW path for the ~2300–~1650 Ma interval for North America.

Being a global phenomenon, SRs have numerous applications in lightning research. Background SR records can serve as a convenient and a low-cost tool for global lightning activity monitoring. The SR can provide a global geoelectric index for monitoring climate changes. It provides one of the few tools that through variations in global lightning activity, can provide continuous and long-term monitoring of such important global climate change parameters as tropical land surface temperature and tropical UTWV.SR transients (Q-bursts) can be used to geolocate intense lightning strikes anywhere on the planet. These large-amplitude pulses are apparently related to the occurrence of sprites and elves above thunderstorms, and therefore TLEs can be studied using SR observations. An additional application of SR is extraterrestrial lightning research. SRs may be used to detect and, if necessary, monitor lightning activity on the planets and moons of the Solar System.There are still many open questions in SR research: importance of the day-night variation in the ionosphere conductivity profile [178,179], influence of the latitudinal changes in the Earth magnetic field, impacts of cosmic, solar and geomagnetic disturbances [180,181], polar cap absorption, accuracy of source geolocation and the determination of the spatial lightning distribution from the background records [43,89,182]. Despite these open problems, SR is one of the most promising tools in a variety of fields related to lightning electromagnetics.In summary, the knowledge of the Schumann resonances in the atmosphere, their origins, propagation, variability and changes, may have implications for monitoring global climate changes, upper atmospheric TLEs, for space missions to other planets, for space weather research, and possibly even in the field of medicine and agriculture.