Oxygen isotope stratigraphy and calcareous nannofossil biostratigraphy of the Lower Pleistocene in the Boso Peninsula, central Japan

ABSTRACT: Oxygen isotope stratigraphy and calcareous nannofossil biostratigraphy in the upper part of the Kiwada Formation, which is part of the Kazusa Group in the Boso Peninsula in the central part of the Pacific coast of Japan, were examined to establish a high-resolution age model and estimate the age of the lower part of Pleistocene nannofossil biohorizons in the northwestern Pacific region. The new age model indicates that the upper part of the Kiwada Formation corresponds to Marine Isotope Stage (MIS) 41 through MIS 36. Two nannofossil biohorizons, the last occurrences of large forms of Gephyrocapsa (>5.5 micrometers) and Helicosphaera sellii,were recorded in the examined section. The LO of large Gephyrocapsa spp. coincides with the MIS 37/38 boundary which is 1241.2 plus or minus 0.4 ka. The LO of H. sellii is located in late MIS 40 and has a calculated age of 1291.4 plus or minus 1.4 ka. These biohorizons are traceable even though the LO of H. sellii is a diachronous event. Size variations of Gephyrocapsa from approximately 1250 ka are discussed and fluctuations of small size Gephyrocapsa could represent cyclic changes of the Kuroshio and Oyashio currents.

Flowstones from the Račiška Pečina Cave (SW Slovenia) Record 3.2-Ma-Long History

Establishing a chronology of events is a critical step in reconstructing the palaeoclimate and it is important for all types of environmental records, including speleothems. Here, we analysed a unique series of flowstones deposited between 3.2 Ma (marine isotope stage (MIS) Km3) and 0.08 Ma (MIS 5). The studied flowstones are located in a classic karstic environment, the Račiška Pečina Cave in south-western Slovenia. Further, a detailed chronology of events was constructed based on oxygen isotope stratigraphy (OIS), combined with magnetostratigraphy and U-series dating. Two curves were selected as reference records where the LR04 record was used as the global curve and a Mediterranean record was used as the regional curve. The Račiška Pečina profile was divided into two segments separated by a principal disconformity. The lower segment correlated better with the regional Mediterranean curve, while the upper segment was with the global LR04 curve. These findings suggest that the main factors controlling environmental conditions in the cave area changed between 3.2 and 0.8 million years ago.

On the relationship between paleomagnetic secular variation and excursions – records from MIS 8 - ODP Leg 172

Summary Paleomagnetic secular variation (PSV) and excursion data obtained across MIS 8 (243–300 ka) from the western North Atlantic Ocean—ODP Sites 1060–1063 show composite high-resolution PSV records (both directions and relative paleointensity) developed for each site and inter-compared. Two methods of chronostratigraphy allow us to date these records. First, we used published results that compared the calcium carbonate records of ODP Leg 172 sediments and tuned them with Milankovich cyclicity. We also compared our paleointensity records with the PISO-1500 global paleointensity record that was dated with oxygen isotope stratigraphy. We prefer the PISO-1500 record to date our cores. Two excursions are preserved in our PSV records—Excursions 8α and 9α. Our revised age estimates for both excursions are 8α (236.7–239.8 ka) and 9α (283.7–286.9 ka). We have compared shipboard measurements of the two excursions with u-channel measurements of selected excursion intervals. Excursion 8α is interpreted as a ‘Class II’ excursion (local reversal) with in-phase inclination and declination changes; Excursion 9α is a ‘Class I’ excursion with 90° out-of-phase inclination and declination changes. Averaged directions (after removal of true excursional directions) and relative paleointensity in 3 ka and 9 ka overlapping intervals show significant PSV directional variability over 104 yr time scales that is regionally correlatable among the four sites. A notable pattern of angular dispersion variability involves most time spent with low (∼10°) dispersion, with three shorter intervals of high (∼25°) dispersion. The relative paleointensity variability also shows significant variability over 104 yr time scales with three notable intervals of low paleointensity in all four records and a direct correspondence between the three low paleointensity intervals and the three intervals of high angular dispersion. The two magnetic field excursions occur in two of the three low-paleointensity/high-dispersion intervals. This suggests that the geomagnetic field operates in two states between reversals, one with regular to high paleointensity and low directional variability and one with low paleointensity and significantly higher directional variability and excursions.

Terrestrial stratigraphical division in the Quaternary and its correlation

Initially, terrestrial evidence formed the foundation for the division of Quaternary time. However, since the 1970s there has been an abandonment of the terrestrial stage chronostratigraphy, which is based on locally dominated successions, in favour of the marine oxygen isotope stratigraphy which largely records global-scale changes in ice volume. However, it is now clear that glacial records around the world are asynchronous, even at the scale of the continental ice sheets which display marked contrasts in extent and timing in different glacial cycles. Consequently, the marine isotope record does not reflect global patterns of glaciation, or other terrestrial processes, on land. This has led to inappropriate correlation of terrestrial records with the marine isotopic record. The low resolution of the latter has led to a preferential shift towards high-resolution ice-core records for global correlation. However, even in the short term, most terrestrial records display spatial variation in response to global climate fluctuations, and changes recorded on land are often diachronous, asynchronous or both, leading to difficulties in global correlation. Thus, whilst the marine and the ice-core records are very useful in providing global frameworks through time, it must be recognized that there exist significant problems and challenges for terrestrial correlation.

The current state and 125 kyr history of permafrost on the Kara Sea shelf: modeling constraints

The evolution of permafrost on the Kara shelf is reconstructed for the past 125 kyr. The work includes zoning of the shelf according to geological history; compiling sea level and ground temperature scenarios within the distinguished zones; and modeling to evaluate the thickness of permafrost and the distribution of frozen, cooled and thawed deposits. Special attention is given to the scenarios of the evolution of ground temperature in key stages of history that determined the current state of the Kara shelf permafrost zone: characterization of the extensiveness and duration of the existence of the sea during stage 3 of the marine oxygen isotope stratigraphy (MIS-3), the spread of glaciation and dammed basins in MIS-2. The present shelf is divided into areas of continuous, discontinuous-to-sporadic and sporadic permafrost. Cooled deposits occur at the western and northwestern water zones and correspond to areas of MIS-2 glaciation. Permafrost occurs in the periglacial domain that is within a zone of modern sea depth from 0 to 100 m, adjacent to the continent. The distribution of permafrost is mostly sporadic in the southwest of this zone, while it is mostly continuous in the northeast. The thickness of permafrost does not exceed 100 m in the southeast and ranges from 100 to 300 m in the northeast. Thawed deposits are confined to the estuaries of large rivers and the deepwater part of the St. Anna trench. The modeling results are correlated to the available field data and are presented as a geocryological map. The formation of frozen, cooled and thawed deposits of the region is inferred to depend on the spread of ice sheets, sea level, and duration of shelf freezing and thawing periods.

The Permian–Triassic boundary Lung Cam expanded section, Vietnam, as a high-resolution proxy for the GSSP at Meishan, China

The Lung Cam expanded stratigraphic succession in Vietnam is correlated herein to the Meishan D section in China, the GSSP for the Permian–Triassic boundary. The first appearance datum of the conodont Hindeodus parvus at Meishan defines the Permian–Triassic boundary, and using published graphic correlation, the Permian–Triassic boundary level has been projected into the Lung Cam section. Using time-series analysis of magnetic susceptibility (χ) data, it is determined that H. parvus arrived at Lung Cam ∼18 kyr before the Permian–Triassic boundary. Data indicate that the Lung Cam section is expanded by ∼90 % relative to the GSSP section at Meishan. Given the expanded Lung Cam section, it is possible to resolve the timing of significant events during the Permian–Triassic transition with high precision. These events include major stepped extinctions, beginning at ∼135 kyr and ending at ∼110 kyr below the Permian–Triassic boundary, with a duration of ∼25 kyr, followed by deposition of Lung Cam ash Bed + 13, which is equivalent to Siberian Traps volcanism is graphically correlated to a precession Time-series model, placing onset of this major volcanic event at ~242 kyr before the PTB. The Meishan Beds 25 and 26, at ∼100 kyr before the Permian–Triassic boundary. In addition, the elemental geochemical, carbon and oxygen isotope stratigraphy, and magnetostratigraphy susceptibility datasets from Lung Cam allow good correlation to other Permian–Triassic boundary succession. These datasets are helpful when the conodont biostratigraphy is poorly known in sections with problems such as lithofacies variability, or is undefined, owing possibly to lithofacies exclusions, anoxia or for other reasons. The Lung Pu Permian–Triassic boundary section, ∼45 km from Lung Cam, is used to test these problems.

Low to middle Pleistocene paleoclimatic record from the Kraków-Częstochowa Upland (Poland) based on isotopic and calcite fabrics analyses

The quality of paleoenvironmental reconstruction based on speleothem records depends on the accuracy of the used proxies and the chronology of the studied record. As far as the dating method is concerned, in most cases, the best solution is the use of the U-series method to obtain a precise chronology. However, for older periods (i.e., over 0.5 Ma), dating has become a serious challenge. Theoretically, older materials could be dated with the U-Pb dating method. However, that method requires a relatively high uranium content (minimum of several ppm), whereas typical speleothems from Poland (and all of Central Europe) have uranium concentrations below 0.1 ppm. Because the materials in Polish caves are problematic, we applied oxygen isotope stratigraphy (OIS) as a tool for speleothem dating. By using OIS as an alternative tool to create a chronology of our flowstone, it was found that the studied flowstone crystallized from 975 to 470 ka with three major discontinuities, so obtained isotopic record can be correlated with oxygen isotopic stages from MIS 24 to MIS 12. The observed isotopic variability was also consistent and confirmed with the petrographic observations of the flowstone.