Yugo Ono

Asian summer monsoon instability during the past 60,000 years : magnetic susceptibility and pedogenic evidence from the western Chinese Loess Plateau

Abstract The 28 m high-resolution Shajinping loess section in Lanzhou on the western Chinese Loess Plateau records a 60 ka, millennial summer monsoon variation. The record shows that Asian summer monsoons have rapid episodic pulse enhancements spanning only ca. 1–2 ka in high-frequency domain and having sub-Milankovitch cycles of progressive weakening in low frequency domain in the last glaciation. Soil formation seems to occur with a surprisingly fast response to these summer monsoon enhancements, resulting in weakly or moderately developed paleosol sequences. Both the pattern and timing of the summer monsoon enhancements show that they can be correlated to most major warm (Dansgaard–Oeschger) episodes and long-term cooling (Bond) cycles of the North Atlantic climatic records, indicating a possible teleconnection between tropic oceanic air masses and the North Atlantic climatic system. But differences exist for the transition of MIS 2/3 and the Holocene, where extraordinarily heavy dust-input events and fairly variable climatic fluctuations occur for the former and latter, respectively. A westerlies-swing model is proposed to interpret this link.

Snowline elevation and eolian dust flux in the Japanese islands during isotope stages 2 and 4.

Abstract Changes in snowline elevation and dust flux clearly contrast the paleoenvironments of the Japanese Islands during Isotope Stages 2 and 4. A lower snowline elevation and a smaller dust flux during Isotope Stage 4 suggests a climate with a cold and dry summer dominated by winter monsoonal precipitation between 70 and 55 ka. A higher snowline elevation and a larger dust flux during Isotope Stage 2 indicates a climate with a cold and very dry summer with a minimized winter monsoon precipitation especially around 18 ka. A drier climate during Stage 2 can be explained by (1) a lowering of sea surface temperature in the Japan Sea by blocking the warm Tsushima Current during a sea-level lowering; (2) the extension of sea ice over the northern part of the Japan Sea and the Sea of Okhotsk; and (3) weakening of the summer monsoon and the southern migration of the polar front. The increase in dust flux in both Stages 2 and 4 suggests an intensification of westerlies and winter monsoon during these stages. A bigger dust flux in Stage 2 than Stage 4 reflects (1) stronger westerlies and winter monsoon, and (2) the emergence of continental shelf which became an important dust source.

Thermal regime of sporadic permafrost in a block slope on Mt. Nishi-Nupukaushinupuri, Hokkaido Island, Northern Japan

In this study, we discuss the predominant factors that determine the ground temperature regime of an active layer in a block slope. The distribution of the bottom temperature of snow cover (BTS) measurements, warm funnels at the top of the slope, and ground temperature changes on the block slope indicate continuous air circulation during the winter. In the spring, snowmelt water flows to the valley bottom and refreezes, adding superimposed ice onto the perennial ice that fills the voids between coarse blocks. At the study site, the ground temperatures showed a simultaneous, abrupt increase at all depths in the active layer. These results strongly suggest that air circulation in winter, as well as the ice formation processes in the spring, control the thermal regime of the active layer of the block slope with mean annual air temperature (MAAT) above 0 °C.

Origin and derived courses of eolian dust quartz deposited during marine isotope stage 2 in East Asia, suggested by ESR signal intensity

The ESR signal intensity of fine quartz (finer than 20 μm) during marine isotope stage (MIS) 2 in the Japanese Islands is spatially. Under the assumption that the values of ESR signal intensity of eolian fine quartz reflect those of the quartz at the in-source areas, the source area and trajectory of eolian dust are tentatively reconstructed. In northern Japan, where the ESR signal intensity is high (10–12), the eolian dust seems to have originated from Siberia and Mongolia where the Precambrian rocks with a high ESR signal intensity are widely exposed. In central and southern Japan, where the ESR signal intensity is medium (5.8–8.7), the eolian dust seems to be supplied from central Asia where Paleozoic–Mesozoic rocks with a medium ESR signal intensity are widely exposed. For the southernmost islands of Japan, where the ESR intensity signal increases again (9.7–13.4), seem to have an eolian dust supply from areas south of Himalayan Range where the Precambrian rocks are widely exposed. On the basis of ESR intensity signals, three major courses of eolian dust transport in MIS 2 are proposed: the winter monsoon in eastern and northern Japan, the summer subtropical jet in western and southern Japan, and the winter subtropical jet in the southernmost islands of Japan.

Timings and causes of glacial advances across the PEP-II transect (East-Asia to Antarctica) during the last glaciation cycle

A comparison of glacial advances through the last glacial cycle between Northern and Southern Hemispheres in the PEP 2 transect (East Asia and Australasia) revealed (1) a relatively good synchrony of glacial advances during the LGM across the whole transect except Antarctica, (2) a rough synchrony in glacial advances in the Karakorums, Himalayas, Japan, and New Zealand during Marine Isotope Stages (MIS) 3 and 4, although the age control is still poor, and (1) a greater glacial extent in MIS 3 and 4 than in MIS 2. The LGM advances are driven by Northern Hemisphere temperature forcing, but the MIS 3 and/or MIS 4 advances appear to be dominantly controlled by the effects of moisture availability or lack of thereof. In mainland Australia and Antarctica glaciers advanced only in the most humid phases. In Himalaya and Karakorum, where the moisture is supplied both by a Summer monsoon and the westerlies, timings of glacier advance coincide with both Summer monsoon enhancement and westerly intensification. In Japan the glacier extent is controlled by monsoon changes, sea-surface temperatures (SSTs) in the Japan Sea (moisture source) and by zonal shifts in westerly circulation. In New Zealand, either insolalion changes or changes in the intensity of westerly circulation are invoked, Lis the main track of the westerlies did not change greatly during glacial times

Timing of late Quaternary vegetation response in the 30–50° latitude bands in southeastern Australia and northeastern Asia

Abstract Quaternary climatic change in part results from Milankovitch forcing, and is accordingly a boundary condition commonly applied in global climate modelling. Orbital parameters result in different receipts of insolation across a year, and result in hemispheric differences in climate, for example, in the seasonal expression of climate. This effect may be dampened by the effects of the ocean, topographic and other forcing factors. In this study a comparison is made of the late Quaternary records from mid-latitude southeastern Australia and northeastern Asia along the Pole-Equator-Pole (PEP-II) transect of the Australasian hemisphere. A review of the major climate features and vegetation of the regions is given. The study areas have a good geographic coverage of palaeoecological sites and from these it is possible to estimate the timing of the major vegetation shifts over the last 25,000 years; including through the period of Last Glacial Maximum, recovery from this to events around the Pleistocene-Holocene transition, and the timing of peak warming in the Holocene. There appears to be no difference in the timing of vegetation response in the two regions at Last Glacial Maximum or around the Pleistocene-Holocene boundary. However, the peak of Holocene warming appears to significantly precede its expression in southeastern Australia. Data from all periods show significantly different variations in responding to climate change along the PEP-II transect. These findings have to be considered in relation to the range and quality of the existing records, and in the great differences in taxonomy and community structure between the two regions.

Southern migration of westerlies in the Northern Hemisphere PEP II transect during the Last Glacial Maximum

Abstract A southward shift of continuous and discontinuous permafrost zones and vegetation belts of Taiga, broad-leaf deciduous and evergreen forests on the islands forming Japan suggests a southern migration of the Northern Hemisphere (NH) summer westerly subtropical jet by around 3–5° of latitude at 135–140°E during the global Last Glacial Maximum (LGM). Since this jet induces the Asian (NH) summer monsoon, the southern shift caused a cooler and drier summer, and the spread of deciduous forest in Southern Japan. The expansion of mountain glaciers on the islands of Japan also supports the southern migration of the subtropical westerly jet. At present, the ablation of snow occurs by heat and precipitation carried by the NH summer monsoon. By comparison, the position of the NH winter westerly subtropical jet remained unchanged, presumably because its course was constrained by the Himalayan–Tibetan Plateau. The northern limit of the Kuroshio Current, corresponding to the boundary of the subtropical and subarctic water in the northwestern Pacific Ocean, also migrated by around 3° during the global LGM. The southern migration of the westerly subtropical jet suggests that the current situation where its location shifts during the NH summer to north of the Tibetan Plateau did not occur during the coldest phases of the global LGM, although it did occur intermittently during Marine Isotope Stage (MIS) 3.

Climate, human, and natural systems of the PEP II transect

Abstract The purpose of this introduction is to provide a background to the natural and human environmental systems on the Austral-Asian (or PEP II) transect. Aspects of these systems are discussed in greater detail in the papers that follow in this volume. Focusing on the distinctive characteristics of the PEP II transect, this paper begins with an overview of the main physical features, reviews the nature of the present climate systems and concludes with an outline of the patterns of human settlement across the region and the diversity of environmental interactions between these and human societies.

The Late Quaternary History of Atmospheric Trace Gases and Aerosols: Interactions Between Climate and Biogeochemical Cycles

Human activities perturb the atmosphere and thereby influence the global climate. Prominent examples are greenhouse trace gases and sulfate aerosols which both affect the radiative balance at the surface of the Earth. This is also true for black carbon and organic carbon aerosols emitted by burning of biomass and fossil fuel, as well as eolian mineral dust originating from changes in land use and land cover.

A 60 000-year loess-paleosol record of millennial-scale summer monsoon instability from Lanzhou , China

The 28-m high-resolution Shajinping loess section in Lanzhou on the Chinese western Loess Plateau records a detailed history of millennial Asian summer monsoon change since the earlier last glaciation. Summer monsoon proxies of soil magnetic susceptibility, carbonate content and soil color show that Asian summer monsoon experienced a series of rapid episodic pulse enhancements spanning only ca. 1–2 ka and have sub-Milankovitch cycles of progressive weakening in low frequency domain since ca. 60 kaBP. Soil formation responds to these by deepening colors, accumulating organic matter, increasing biological channels and leaching certain carbonates. The pedogenic degree generally increases with the degree of enhancement of summer monsoon. These may reflect the impact of the last glacial fast climatic change in the North Atlantic region on Asian summer monsoon.