5:15 PM - 7:15 PM
[SEM16-P02] Magnetic properties of meteorite impact layers and individual impact spherules in deep-sea clay
Impact spherules often contain dendritic magnetite (s.l.). They are considered to be the cause of the positive magnetic susceptibility peaks seen in impact-related sedimentary layers. However, it is still unclear whether they can fully explain the magnetic anomalies, because there are only a few reports of magnetic properties of single impact spherules. Besides, the internal texture and chemical composition of spherules may vary among different impacts, but it has been difficult to exclude the influence of diagenesis. Since magnetism is sensitive to slight changes in composition and particle shape, there may also be variations among different impacts. Because sediments over the deep Pacific Ocean are in an oxidizing environment, the original mineralogy may be relatively well preserved. Thus, measurements of single spherules from deep-sea sediments will allow us to examine the contribution of impact spherules to the magnetic anomalies, and to assess the presence of differences among different impact events. Here we report magnetic measurements of sediment collected around Minamitorishima, including the K-Pg boundary and Miocene impact-related sedimentary layers identified from platinum group element concentrations, and individual spherules within the K-Pg boundary. The saturation magnetization of the K-Pg boundary was 18.8 × 10-2 Am2/kg (471 A/m), while the layers above the impact layer averaged 6.1 × 10-2 Am2/kg (151 A/m), and the layers below the impact layer averaged 11.1 × 10-2 Am2/kg (281 A/m). On the other hand, the saturation magnetization of the Miocene impact-related layer was 13.0 × 10-2 Am2/kg (310 A/m), that of the layer above the impact layer was 7.6 × 10-2 Am2/kg (189 A/m), and that of the layer below the impact layer was 8.0 × 10-2 Am2/kg (201 A/m). The spherules were classified into three types based on their appearance: black, brown, and light yellow. The black particles account for 11.1 %, the brown particles 42.4 %, and the pale-yellow particles 46.5 %. The saturation magnetization was 4281 to 11971 A/m for the black, 9529 to 37219 A/m for the brown, and below the measurement limit of the instrument for the pure pale-yellow. To investigate whether the spherules can explain the magnetic anomalies in the sediments and whether the spherules associated with different impacts have different magnetic properties, we performed a model calculation that numerically sums the magnetic properties of the spherules and the sediments. The hysteresis loop of the K-Pg boundary sediments is well reproduced by adding 1.7 vol% spherules to the sediments just below the boundary. For the Miocene impact-related layer, the hysteresis loop was fitted by the least-squares method under the assumption that the spherules have the same magnetic properties as those in the K-Pg layer. The result shows that the magnetic susceptibility would be lower and the coercivity would be higher than the measured values with those assumptions. Therefore, we expect that the Miocene impact-related layer contains spherules with lower magnetic susceptibility and higher coercivity than those in the K-Pg boundary.