Takayoshi Kawai

Orbit-related long-term climate cycles revealed in a 12-Myr continental record from Lake Baikal

Quaternary records of climate change from terrestrial sources, such as lake sediments and aeolian sediments, in general agree well with marine records. But continuous records that cover more than the past one million years were essentially unavailable until recently, when the high-sedimentation-rate site of Lake Baikal was exploited. Because of its location in the middle latitudes, Lake Baikal is highly sensitive to insolation changes and the entire lake remained uncovered by ice sheets throughout the Pleistocene epoch, making it a valuable archive for past climate. Here we examine long sediment cores from Lake Baikal that cover the past 12 million years. Our record reveals a gradual cooling of the Asian continental interior, with some fluctuations. Spectral analyses reveal periods of about 400 kyr, 600 kyr and 1,000 kyr, which may correspond to Milankovitch periods (reflecting orbital cycles). Our results indicate that changes in insolation were closely related to long-term environmental variations in the deep continental interior, over the past 12 million years.

Simultaneous multielement determination of trace metals in lake waters by ICP emission spectrometry with preconcentration and their background levels in Japan

An analytical method combining a preconcentration followed by determination with inductively coupled plasma emission spectrometry (ICPES) was developed for monitoring background levels of trace metals in natural waters. Complexes of trace metals (Al, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Cd and Pb) and 8-hydroxyquinoline were absorbed to a C18 chemically bonded silica gel disposable plastic column. They were eluted with methanol and digested with nitric acid. With this preconcentration procedure, about 500-fold preconcentration is possible. Simultaneous multielement analysis with ICPES ensures the necessary sensitivity to measure the trace metals in Lake Mashu water at concentration ranges from μg l−1 (ppb) to ng l−1 (ppt). Lake Mashu is a deep oligotrophic lake in Hokkaido, Japan and famous for the highest transparency in the world. The trace metal concentrations in Lake Mashu were extremely low: Fe 2.6; Al 1.2; Mn 0.76; Zn 0.63; V 0.15; Cu 0.069; Ti 0.06; Pb 0.05; Ni 0.027; Cd < 0.006; and Co < 0.004 μg l−1. The suitability of Lake Mashu for a background level monitoring station for global environmental pollution was assessed by the comparison to reference data for other lakes in the world.

Photosynthetic pigments and perylene in the sediments of southern basin of Lake Baikal

Depth profiles of carotenoids, chlorophylls and related pigments in the sediments of the southern basin of Lake Baikal were analyzed. The photosynthetic pigments were found to be preserved through the sediment cores which spanned several hundreds years. Pigments originating from green algae, diatoms, Cryptophyceae and cyanobacteria were abundant in the sediments and a major part of chlorophylls a and b were preserved as pheophytin. Biogenic perylene was detected throughout the core layers and its presence indicates that the sediment had been anoxic.

Last-Glacial to Post-Glacial 10Be fluctuations in a sediment core from the Academician Ridge, Lake Baikal

10Be concentrations, sediment composition and sediment accumulation rates were determined in a sediment core from the Academician Ridge of Lake Baikal. The 10Be concentrations vary from 5.07 × 108 atoms/g in the last glacial section to 11.25 × 108 atoms/g in the section representing the post-glacial climate stage. 10Be fluxes, on the other hand, increase toward the last-glacial stage coinciding with an increase of the sediment accumulation rate. These facts imply that climate-related sedimentation is related to both flux and concentration of the nuclide in the sediment core. During the last-glacial stage, arid conditions in the lakes watershed caused a marked increase in flux of clay particles with low 10Be content to the lake, resulting in an increased flux but lowered concentration of 10Be in the sediments. It is suggested that the 10Be record of the bottom sediments of Lake Baikal can be used as a proxy indicator of climate change in central Asia.

Long‐term climato‐limnological oscillation during the past 2.5 million years printed in Lake Baikal sediments

The physical properties of long sediment cores (BDP96) obtained from the Academician Ridge of Lake Baikal, eastern Siberia, show that the changes in the climato-limnological environment of the continental interior during the past 2.5 m.y. clearly reflected global climatic change. Oscillation in water content and grain size, which are closely related to biogenic productivity, coincides with marine δ 18O variation. The values for water content and grain size are large during interglacial periods and small during glacial periods. Milankovitch parameters were also imprinted in the sediments over the past 2.5 m.y.: a 400-ky period as well as a 100-ky period due to eccentricitical parameters is found in addition to other orbital parameter (a 40-ky period of obliquitical and a 20-ky period of precessional parameter). This indicates that change in solar insolation was closely related to long term variations in climato-limnological regime in this area.

Magnetic record of Lake Baikal sediments: chronological and paleoclimatic implication for the last 6.7 Myr

Magnetic remanence vectors for 1472 samples taken from a 601 m core through Lake Baikal sediments are reported along with a complete magnetic susceptibility profile obtained from a pass-through system. Matching the stable remanence directions to the standard geomagnetic polarity time scale (GPTS) provides a robust chronology from the present back to V6.7 Ma and yields a remarkably constant sediment accumulation rate of 3.9 cm/kyr. For earlier times ^ represented by depths s 270 m ^ correlation to the GPTS is more problematic. Susceptibility fluctuations reflect climatic changes that can be matched to the marine oxygen isotope pattern for the last 6.7 Myr. Spectral analysis of the resulting susceptibility time series then indicates that, for the most part, the Milankovitch obliquity signal dominates. However, when the temporal evolution of the frequency content is investigated by analyzing sequences of time windows, a complex picture emerges in which eccentricity and precession power appear during some intervals. Furthermore, there is persistent evidence for significant power in a ‘non-Milankovitch’ band between 28 and 35 kyr. A 2003 Elsevier Science B.V. All rights reserved.

Mantle helium flux from the bottom of Lake Mashu, Japan

Abstract Most of the 52 water samples from Lake Mashu, Hokkaido, Japan show large anomalies in their3He/4He ratios (up to 4.6 times the atmospheric ratio) and4He/20Ne ratios, indicating continuous injection of mantle He due to hot spring activity at the lake bottom. The3He/4He ratio of the injected mantle He is estimated at3He/4He= 9.43 ± 0.17 × 10−6 , which is close to the highest value reported for volcanic gases from the Japanese Islands. Accumulation of mantle He in the near-bottom water can be reasonably explained by a diffusion process with a constant He flux from the lake bottom off4He = 9.2 × 107 atoms/cm2/s andf3He = 8.7 × 102 atoms/cm2/s. The heat/3He ratio ( 1.7 × 10−7J/atom ) falls in the range of values reported for submarine hydrothermal systems (0.4–26 × 10−7 J/atom), whereas the C3He/ ratio ( 1.8 × 1011 ) is about two orders of magnitude higher than the MORB average ( 2 × 109 ). The mantle He injected from the lake bottom has a fairly short mean residence time in the lake water, and is estimated at about 2–4 years. Overturn of the lake water, which occurs twice a year, probably plays an important role in the rapid transportation of the mantle He from the bottom to the surface.

Onset of current Milankovitch-type climatic oscillations in Lake Baikal sediments at around 4 Ma☆

Abstract Low summer insolation would increase Northern Hemisphere ice sheets only if winter snow persisted all year. Lake Baikal records reveal that such a climatic threshold may have been reached at around 4.0 Ma. Insolation minima at about 3.9, 3.8 and 3.7 Ma, which were obliquity-related, may have triggered Milankovitch-type climatic oscillations characterized by orbital cycles (with periodic major Northern Hemisphere ice sheets). Intensification of these oscillations at about 2.8 Ma was probably connected to another decrease in insolation (i.e. another obliquity-related insolation minima), and seemingly triggered larger oscillations in the prevailing climatic regime of the late Pliocene and Pleistocene.

Overexpression of DnaK from a halotolerant cyanobacterium Aphanothece halophytica enhances growth rate as well as abiotic stress tolerance of poplar plants

The DnaK/Hsp70 family is a molecular chaperone that binds non-native states of other proteins, and concerns to various physiological processes in the bacterial, plant and animal cells. Previously, we showed that overexpression of DnaK from a halotolerant cyanobacterium Aphanothece halophytica (ApDnaK) enhances tolerance to abiotic stresses such as high salinity and high temperature in tobacco plants. Here, we tested the transformation of poplar (Populus alba) with ApDnaK for enhancing the growth of transformed poplar plants. Under control growth conditions, transgenic poplar plants exhibited similar growth rates with the wild-type plants during young seedlings under low light intensity, whereas they showed faster growth, larger plant size, and higher cellulose contents when poplar plants were grown under high light intensity. Transgenic young poplar plants exhibited more rapid recovery from the stresses of high salinity, drought, and low temperature compared with those of the wild type plants when poplar plants were grown under low light intensity. These results suggest that ApDnaK could be useful to enhance the growth rate as well as to increase the stress tolerance.

Climato-limnological signals during the past 260 000 years in physical properties of bottom sediments from Lake Baikal

The St.16 core obtained from the Academician Ridge of Lake Baikal in eastern Siberia may span about 260 000 years, and some physical properties of the core samples are closely related to aquatic paleoproductivity and climatic change. The median of grain size, grain density, and water content fluctuate synchronously. They also are connected with change in the abundance of biogenic silica (diatoms). The physical parameters indicate that there were high aquatic productivity periods around ‘interglacial’ periods (MIS 5 and 7; 70 000-125 000 yr B.P. and 180 000-250 000 yr B.P.). Comparatively large clastics were transported from outside of the lake through various routes (ice rafting, etc.) in addition to fluvial routes during the ‘glacials‘ or stadials’. There are ca. 20 000 yr, 40 000 yr and 100 000 yr periods in the variations of physical properties. These are related to the three Milankovitch parameters of solar insolation.