The activities, fluxes and inventories of ²¹⁰Pb and ¹³⁷Cs in sediments collected from freshwater lakes, brackish-water lakes and seabeds in the East Asia are studied to elucidate their geochemical behaviors and characteristics concerning sedimentation environments. The surface concentrations varied widely by location. The average flux of excess ²¹⁰Pb increased from freshwater lakes to brackish-water lakes to seawater, and the average flux of ¹³⁷Cs showed an inverse trend. The inventories of excess ²¹⁰Pb and ¹³⁷Cs were in good correlation in samples from the same lake or adjacent areas. The inventory ratio (¹³⁷Cs/excess ²¹⁰Pb) was in the order of samples from seawater, increasing in brackish-water, further increasing in freshwater, and samples from Chinese lakes showed higher values. It is inferred that the inventory ratio may become a kind of index of sedimentation environment, geological, and/or radiochemical events.

View full abstract

The Plio-Pleistocene Gotsu Group is distributed in the western part of Shimane Prefecture, southwest Japan. The sedimentary environment of the Shimanohoshi Formation in the middle of the Gotsu Group has been elucidated as downriver sediment. However, detailed sedimentary facies of the Shimanohoshi Formation have not yet been understood because of the difficulty involved in the lateral correlation of lithofacies of this formation. In this study, I clarified the sedimentary environment of the Shimanohoshi Formation in the type locality around Mt. Murogami-yama, which is located in the east of the Gonokawa River, on the basis of the sedimentary facies, distributions, and paleocurrent directions of the formation.
Sedimentary facies of the Shimanohoshi Formation are classified as follows: (i) facies Gmm (debris-flow deposits), composed of unsorted massive matrix-supported or clast-supported conglomerates; (ii) facies Gb (mass-movement deposits), composed of unsorted clast-supported angular conglomerates directly covering basement rocks; (iii) facies St (braided-river deposits), composed of very coarse to coarse-grained sandstone with trough cross-bedding, showing paleocurrents in the west-northwest direction; (iv) facies Sm (subaqueous gravity-flow deposits), composed of massive or cross-bedded, very coarse to fine-grained pebbly sandstone with a muddy matrix; and (v) facies Fsm (lake deposits), composed of mudstone with fine laminae.
The succession around Mt. Murogami-yama comprises alternating debris-flow deposits (Gmm) and braided-river deposits (St) along with some lake deposits (Fsm). On the basis of the distribution of the dominant debris-flow deposits (Gmm) and braided-river deposits (St), the sedimentary environment of the Shimanohoshi Formation is inferred to be a debris-flow-dominated alluvial fan with a west-northwestward braided river.

View full abstract

This study was carried out on sediment samples collected by piston-coring in two areas of the eastern margin of the Japan Sea. One area is located at open sea conditions in the Oki Trough, offshore Kanazawa city, and the other is located in the enclosed bay conditions of the Joetsu Basin, offshore Joetsu city. Using these samples it was possible to differentiate the source of the organic matter in the sediments of Holocene and late Pleistocene time on the basis of δ¹³C_org and TOC/TN ratios coupled with palynofacies analysis. The Holocene sediments are characterized by high TOC and TN contents, low TOC/TN ratio, and heavier δ¹³C_org values, which indicate a predominant marine organic matter production, probably due to warming and inflow of warm ocean currents and coastal currents along the East China Sea. These currents carried abundant phytoplankton from the Pacific Ocean as a result of the sea level rise. Occurrence of particulate organic matter shows abundant primary productivity during the Holocene under marine conditions. On the other hand, the LGM sediments are characterized by low TOC and TN contents, high TOC/TN ratio, and lighter δ¹³C_org signatures, which are characteristic of terrestrial organic matter, probably due to seaward migration of shorelines and strong input of freshwater with terrestrial materials. This terrestrial influence decreased gradually from the LGM to the Holocene because of the sea level rise and consequent increase in the marine organic matter.

View full abstract