Yusuke Suganuma

No extreme bipolar glaciation during the main Eocene calcite compensation shift

Major ice sheets were permanently established on Antarctica approximately 34 million years ago, close to the Eocene/Oligocene boundary, at the same time as a permanent deepening of the calcite compensation depth in the world’s oceans. Until recently, it was thought that Northern Hemisphere glaciation began much later, between 11 and 5 million years ago. This view has been challenged, however, by records of ice rafting at high northern latitudes during the Eocene epoch and by estimates of global ice volume that exceed the storage capacity of Antarctica at the same time as a temporary deepening of the calcite compensation depth ∼41.6 million years ago. Here we test the hypothesis that large ice sheets were present in both hemispheres ∼41.6 million years ago using marine sediment records of oxygen and carbon isotope values and of calcium carbonate content from the equatorial Atlantic Ocean. These records allow, at most, an ice budget that can easily be accommodated on Antarctica, indicating that large ice sheets were not present in the Northern Hemisphere. The records also reveal a brief interval shortly before the temporary deepening of the calcite compensation depth during which the calcite compensation depth shoaled, ocean temperatures increased and carbon isotope values decreased in the equatorial Atlantic. The nature of these changes around 41.6 million years ago implies common links, in terms of carbon cycling, with events at the Eocene/Oligocene boundary and with the ‘hyperthermals’ of the Early Eocene climate optimum. Our findings help to resolve the apparent discrepancy between the geological records of Northern Hemisphere glaciation and model results that indicate that the threshold for continental glaciation was crossed earlier in the Southern Hemisphere than in the Northern Hemisphere.

Age of Matuyama-Brunhes boundary constrained by U-Pb zircon dating of a widespread tephra

ABSTRACTThe youngest geomagnetic polarity reversal, the Matuyama-Brunhes boundary (MBB), provides an important datum plane for sediments, ice cores, and lavas. Its frequently cited age of 780 ka is based on orbital tuning of marine sedimentary records, and is supported by 40 Ar/ 39 Ar dating of Hawaiian lavas using recent age calibrations. Challenging this age, how-ever, are reports of younger astrochronological ages based on oxygen isotope stratigraphy of high-sedimentation-rate marine records, and cosmogenic nuclides in marine sediments and an Antarctic ice core. Here, we present a U-Pb zircon age of 772.7 ± 7.2 ka from a marine-deposited tephra just below the MBB in a forearc basin in Japan. U-Pb dating has a distinct advantage over 40 Ar/ 39 Ar dating in that it is relatively free from assumptions regarding stan-dardization and decay constants. This U-Pb zircon age, coupled with a newly obtained oxygen isotope chronology, yields an MBB age of 770.2 ± 7.3 ka. Our MBB age is consistent with those based on the latest orbitally tuned marine sediment records and on an Antarctic ice core. We provide the first direct comparison between orbital tuning, U-Pb dating, and magnetostratig-raphy for the MBB, fulfilling a key requirement in calibrating the geological time scale.INTRODUCTION

A Reassessment of the Matuyama–Brunhes Boundary Age Based on the Post-depositional Remanent Magnetization (PDRM) Lock-In Effect for Marine Sediments

The age of the Matuyama–Brunhes (M–B) boundary has been estimated from the astronomical ages of marine sediments and the 40Ar/39Ar ages of volcanic rocks. Although the accepted age for the M–B boundary is 780 ka, recent studies have questioned conventional estimates of the boundary age. In this paper, I present clear evidence for the existence of errors in palaeomagnetic dating due to the effect of the post-depositional remanent magnetization (PDRM) lock-in depth, based on a comparison between previously published marine isotope ages for the M–B boundary and sedimentation rates. These findings indicate that the age of the M–B boundary should be revised to ca. 770–773 ka and that the boundary most likely lies in the late Marine Isotope Stage (MIS) 19 rather than in the middle of MIS 19. This new age for the M–B boundary is consistent with that obtained from the EPICA Dome C ice core using an EDC3 age model. In contrast, an age offset for the M–B boundary is recognized between marine sediments and 40Ar/39Ar ages. To resolve this discrepancy, additional data are required from marine sediments, volcanic rocks, and ice cores.

지난 70만 년 동안 동남극 Lützow-Holm만 주변 해역의 생물기원 퇴적물 함량 변화

Contents of biogenic components [opal, CaCO₃, TOC (total organic carbon)] were measured in Core LHB-3PC sediments collected off Lutzow-Holm Bay, in order to understand glacial-interglacial cyclic variation of the high-latitude surface-water paleoproductivity, in the Indian Sector of the Southern Ocean. An age model was established from the correlation of ARM/IRM ratios of Core LHB-3PC with LR04 stack benthic δ¹?O records, in complement with radiocarbon isotope ages and biostratigraphic Last Appearance Datum (LAD). The core-bottom age was estimated to be about 700 ka. Although the CaCO₃ content is very low less than 1.0% throughout the core, the opal and TOC contents show clear glacial-interglacial cyclic variation such that they are high during the interglacial periods (7.2-50.3% and 0.05-1.00%, respectively) and low during the glacial periods (5.2-25.2% and 0.01-0.68%, respectively). According to the spectral analysis, the variation of opal content is controlled mainly by eccentricity forcing and subsequently by obliquity forcing during the last 700 kyrs. The opal contents of Core LHB-3PC also represent the apparent Mid-Pleistocene Transition (MPT)-related climatic variation in the glacial-interglacial cycles. In particular, the orbital variation of the opal contents shows increasing amplitudes since marine isotope stage (MIS) 11, which defines one of the important paleoclimatic events during the late Quaternary, called the “Mid-Brunhes Event”. Based on the variation of the opal contents in Core LHB-3PC, we suggest that the surfacewater paleoproductivity in the Indian Sector of the Southern Ocean followed the orbital (glacial-interglacial) cycles, and was controlled mainly by the extent of sea ice distribution during the last 700 kyrs.