Geomagnetic intensity as a tool for high resolution stratigraphy for marine sediments and ice cores

第3回極域科学シンポジウム　横断セッション「海・陸・氷床から探る後期新生代の南極寒冷圏環境変動」11月26日（月） 国立国語研究所 ２階講

Refining chronology for the Brunhes-Matuyama geomagnetic boundary

The geomagnetic polarity reversals, including the Matuyama–Brunhes boundary (MBB), provides an invariant datum plane for sediments and lavas. However, geomagnetic synchronization has complications despite its potential. Its popular age of 780 ka for the MBB is based on astrochronologically-tuned marine sedimentary records, and is supported by ^Ar/^Ar ages of 781–784 ka from Hawaiian lavas using a recent age calibration. Challenging this age, however, younger astrochronological ages using oxygen isotope stratigraphy of highsedimentation-rate marine records, and records of cosmogenic nuclides in marine sediments and an Antarctic ice core have been reported. Moreover, a high-precision U-Pb zircon age of 772.7 ± 7.2 ka is reported from a marine-deposited tephra near the MBB in the Chiba composite section in the Kokumoto Formation, Kazusa Group. U-Pb dating has a distinct advantage over ^Ar/^Ar dating in that it is relatively free from assumptions regarding standardization and decay constants. In addition, a high-resolution oxygen isotope chronology is also obtained in this section through the MBB. Therefore, the Chiba composite section will provide a basis for the first direct comparison between astrochronology, U-Pb dating, and magnetostratigraphy for the MBB, fulfilling a key requirement for calibrating the geological timescale. In this paper, I report these recent achievements according to the MBB

Glacial history of Sør Rondane Mountains in Dronning Maud Land, East Antarctica introduced by geomorphology and surface exposure ages

第3回極域科学シンポジウム　横断セッション「海・陸・氷床から探る後期新生代の南極寒冷圏環境変動」11月26日（月）、27日（火） ２階ラウン

Magnetostratigraphy and Rock Magnetism Study of Hole U1524A from IODP Expedition 374

The Tenth Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Wed. 4 Dec. / 3F Seminar room, National Institute of Polar Researc

Ice Retreat History in Pine Island Bay, Revealed by Sedimentary Be-10 Records

Reconstruction of the past dynamics of the Antarctic ice sheets by studying records from their margin is essential to evaluate their stability and their contribution to future sea level rise. Recently, the first direct evidence for a paleo-subglacial lake on the Antarctic continental shelf was reported from a sediment core from a small bedrock basin in Pine Island Bay (PIB), West Antarctica. Here we report further evidence for this paleosubglacial lake based on down-core changes in Be-10 concentrations in the sediments. Very low Be-10 concentration in the lower part of the core indicate limited input of meteoric Be-10, suggesting deposition of the corresponding sediments in isolation from the open ocean. The Be-10 concentration shows a drop within a sand, silt and mud interval in the middle part of the core that was interpreted to result from deposition during the transition from the subglacial lake to a sub-ice shelf cavern caused by grounding line retreat in PIB around 11 kyrs B.P.. The Be-10 concentration increases significantly toward the top of the core, indicating the establishment of an open marine setting later during the Holocene. In addition, we report new Be-10 data from marine sediment cores in other parts of PIB. Our results demonstrate that Be-10 concentration changes in marine sediments from glaciated margins are a valuable recorder of ice sheet - ice shelf transitions

Applying the Burr Type XII Distribution to Decompose Remanent Magnetization Curves

Discriminating magnetic minerals of different origins in natural samples is useful to reveal their associated geological and environmental processes, which can be achieved by the analysis of remanent magnetization curves. The analysis relies on the choice of the model distribution to unmix magnetic components. Three model distributions were proposed in past studies, namely, the lognormal, skew normal, and skewed generalized Gaussian distributions, which are related to the normal distribution. In this study, the Burr type XII distribution is tested and compared with existing model distributions. An automated protocol is proposed to assign parameters necessary to initiate the component analysis, which improves the efficiency and objectivity. Results show that the new model distribution exhibits similar flexibility to the skew normal and skewed generalized Gaussian distributions in approximating skewed coercivity distributions and can fit end‐member components better than the commonly used lognormal distribution. We demonstrate that the component analysis is sensitive to model distribution as well as measurement noise. As a consequence, the decomposition is subject to bias that is hard to identify due to the lack of ground‐truth data. It is therefore recommended to compare results derived from various model distributions to identify spurious components.This work was supported by NIPR through an Advanced Project (KP7 and KP301) and JSPS KAKENHI grants (15K13581, 16H04068, 17H06321, and 18K13638). This study was also performed under the cooperative research program of the Center for Advanced Marine Core Research, Kochi University (14A037, 14B034, 15A047, and 15B042). Z. J. acknowledges Natural Science Foundation of China (grant 41504055)

Reconstruction of the East Antarctic ice sheet variability during the last 3 Ma: A perspective for future JARE expeditions at the central & eastern Droning Maud Land

第6回極域科学シンポジウム分野横断セッション：[IG] 全球環境変動を駆動する南大洋・南極氷床11月17日（火）　国立極地研究所１階交流アトリウ

Ice sheet history in the southern part of the Soya Coast, East Antarctica revealed by glacial landforms and surface exposure dating

The Tenth Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Wed. 4 Dec. / 3F Seminar room, National Institute of Polar Researc

Antarctic Circumpolar Current Fluctuation in the Late Neogene: constraint from sediment wave on the Conrad Rise, Indian Sector of the Southern Ocean

第3回極域科学シンポジウム　横断セッション「海・陸・氷床から探る後期新生代の南極寒冷圏環境変動」11月26日（月） 国立国語研究所 ２階講