Shoji Horie

Organic geochemistry of a lacustrine sediment (Lake Haruna, Japan)

Abstract Total organic matter, lipids, humic compounds, pheopigments, aliphatic and aromatic hydrocarbons, aliphatic mono- and dicarboxylic acids and sterols were analyzed for a 133-cm core sediment sample taken from a mesotrophic freshwater lake, Lake Haruna, Japan. Vertical distributions of major organic compounds can be interpreted in terms of the variation of their possible precursor materials (autochthonous or allochthonous) rather than their post-depositional chemical changes. Variations in the net production of the lake in the past 2500 years was estimated, indicating that a drastic change of the eutrophic level of the lake from oligotrophic to mesotrophic occurred approximately 1000 years ago.

Regulation of the monsoon climate by two different orbital rhythms and forcing mechanisms

The East Asian monsoon is responsible for transferring huge amounts of heat and moisture between the land and the adjacent ocean. Significant changes in its capacity to do this will have direct impacts on regional climatic gradients and global atmospheric circulation. Determining the mechanisms that force long-term variation in monsoon behavior is therefore important for understanding global climate change. Competing theories vary in the degree of importance attached to glacial forcing, other orbital rhythms, and internal feedback mechanisms as primary drivers of change. There is, however, no convincing explanation as to why different proxy records from closely neighboring regions are tuned to different orbital rhythms. Here we present quantitative climatic reconstructions for the past 450 k.y. based on a long pollen record from Lake Biwa in Japan. The data suggest that continental and oceanic air mass temperatures respond predominantly to the 100 k.y. orbital rhythm, whereas the land-ocean temperature gradient and monsoon vigor oscillate mainly at the 23 k.y. insolation cycle. We suggest that the mechanisms for this behavior lie in the differential response of land and ocean to solar forcing, and conclude that the 100 k.y. signal dominates monsoon intensity only when the amplitude of solar forcing falls below a threshold level.

An organic carbon isotopic record of glacial-postglacial change in atmospheric pCO2 in the sediments of Lake Biwa, Japan

A 12 m piston core of sediment from Lake Biwa, Japan, provides a high-resolution record of sedimentary organic matter accumulation over the Pleistocene-Holocene transition. Organic carbon concentrations are generally 1–2% in sediments deposited since 15 kyr ago. C/N ratios indicate that the organic matter is predominantly from aquatic production. A shift in organic carbon isotopic composition from −21‰ to −25‰ occurs over the glacial-postglacial boundary. Changes in lake productivity, organic matter preservation, sources of organic matter, and water temperature can be eliminated as probable causes of the observed organic carbon isotopic shift. Annual overturn of lake waters and rapid dissolution of biogenic carbonates maintain equilibrium between atmospheric CO2 and dissolved CO2 in this lake. The isotopic composition of organic matter buried in the sediments of Lake Biwa consequently appears to record the shift in atmospheric pCO2 from 200 ppm to 280 ppm which occurred at the Pleistocene-Holocene transition.

Preliminary results of the first scientific drilling on Lake Baikal, Buguldeika Site, Southeastern Siberia

The Baikal Drilling Project (BDP) is a multinational effort to investigate the paleoclimatic history and tectonic evolution of the Baikal sedimentary basin during the Late Neogene. In March 1993 the Baikal drilling system was successfuly deployed from a barge frozen into position over a topographic high, termed the Buguldeika saddle, in the southern basin of Lake Baikal. The BDP-93 scientific team, made up of Russian, American and Japanese scientists, successfully recovered the first long (>100 m) hydraulic piston cores from two holes in 354 m of water. High quality cores of 98 m (Hole 1) and 102 m (Hole 2), representing sedimentation over the last 500,000 years, were collected in 78 mm diameter plastic liners with an average recovery of 72% and 90%, respectively. Magnetic susceptibility logging reveals an excellent hole-to-hole correlation. In this report the scientific team describes the preliminary analytical results from BDP-93 hole 1 cores. Radiocarbon dating by accelerator mass spectrometry provides an accurate chronology for the upper portion of Hole 1. Detailed lithologic characteristics, rock magnetic properties and inorganic element distributions show a significant change to the depositional environment occuring at 50 m subbottom depth, approximately 250,000 BP. This change may be due to uplift and rotation of the horst block in the Buguldeika saddle. The sedimentary section above 50 m is pelitic with varve-like laminae, whereas the section below 50 m contains a high proportion of sand and gravel horizons often organized into turbidite sequences. Accordingly, high resolution seismic records reveal a change in sonic velocity at this depth. It is inferred that sedimentation prior to 250 ka BP was from the west via the Buguldeika river system. After 250 ka BP the Bugnldeika saddle reflects an increase in hemipelagic sediments admixed with free-grained material from the Selenga River drainage basin, east of Lake Baikal. Variations in the spore-pollen assemblage, diatoms, biogenic silica content, rock magnetic properties, clay mineralogy and organic carbon in the upper 50 m of BDP-93-1 reveal a detailed record of climate change over approximately the last 250,000 years. These variables alternate in a pattern characteristic of glacial/interglacial climatic fluctuations. The present age model suggests that the climate signal recorded in Lake Baikal sediments is similar to Late Quaternary signals recorded in Chinese loess sections and in marine sediments. Copyright

Topography and morphometry of Lake Bo-ga-ike in Niigata Prefecture, Central Japan

Bo-ga-ike is a small lake that lies at the northern foot of the Sekida mountain range, Niigata Prefecture. This lake is conspicuous that its maximum depth of 28 m. is so great for 0.15 sq. km. in its superficies. The bathymetric map of this lake basin together with its morphometric data are shown in Fig. 2 and Table 1. The assumption that this lake is volcanic in origin is difficult to accept immediately, because it is surrounded by Tertiary tuffaceous sand- and mud-stones. However, the results recently obtained, that there were found the volcanic ejects consisting of pyroxene andesite in the area around the lake and that were shown by the topographic observations, have made the writer to suggest that this basin is a maar in its origin.