28. Magnetic Measurements on Maple and Sequoia Trees

Magnetic measurements of soil and tree bark adjacent to a busy highway revealed a significant variation in the concentration of magnetic particles with distance from the highway. Furthermore, forest-facing tree-bark contains significantly more magnetic particles than road-facing tree-bark. Magnetic particles were detected both on the bark of the maple trees and in the first centimeter of the soil cover (O/A horizon). Stability of the Saturation Isothermal Magnetization (SIRM) and the hysteresis parameters of the soil indicates the presence of Single-Domain/Pseudo-Single-Domain (SD/PSD) magnetic carriers. Measurements of the tree bark hysteresis parameters and SIRM detect a significantly lower coercivity component that we interpret to be an indication of more abundant PSD-type magnetic grains. Magnetic measurements around the perimeters of eight tree trunks reveal magnetic carriers whose distribution is antipodal to the source direction (highway). We interpret our observation by adopting an air circulation model, where suspended PSD/SD particles are carried in the air stream. The air stream from the heavy traffic lowers the amount of moisture on the tree trunk surfaces facing the highway and thus reduces an adhesive potential on this side. Therefore, more particles can stay on the moist side of the trunk protected from the direct airflow. A magnetic signature of tree rings was tested as a potential paleo-climatic indicator. We have examined wood from sequoia tree, located in Mountain Home State Forest, California, whose tree ring record spans over the period 600–1700 A.D. We have measured low and high-field magnetic susceptibility, the Natural Remanent Magnetization (NRM), Saturation Isothermal Remanent Magnetization (SIRM), and stability against thermal and Alternating Field (AF) demagnetization. Magnetic investigation of the 200 mm long sequoia material suggests that the magnetic efficiency of natural remanence (=natural remanent magnetization normalized by saturation remanence) may be a sensitive paleoclimate indicator because it is substantially higher (in average > 0.01=1%) during the Medieval Warm Epoch (700–1300 A.D.) than during the Little Ice Age (1300–1850 A.D.) where it is < 0.01=1%. Diamagnetic behavior has been noted to be prevalent in regions with higher tree ring density. The mineralogical nature of the remanence carrier was not directly detected but maghemite is suggested due to low coercivity and absence of Verwey transition. Tree ring density, along with the wood’s magnetic remanence efficiency, records the Little Ice Age (LIA), which is well documented in Europe and elsewhere. Magnetic analysis of the thermal stability reveals the blocking temperatures near 200°C. This phenomenon suggests that the remanent component in this tree may be thermal in origin and was controlled by local thermal conditions.