The depositional facies and radiocarbon ages of a long Holocene sediment core (KS) from Cambodia are reported here to clarify the sedimentary environments of the Mekong River delta system at the time of Holocene maximum flooding by the sea. The 30.7-m-long KS core, from the upper Mekong River lowland about 20 km southeast of Phnom Penh, penetrated five depositional facies, A to E in ascending order. Facies A is cross-laminated fluvial sand. Facies B is laminated, very fine sand with mud drapes. Facies C to E are salt marsh, flood-plain and natural-levee deposits, characterized by a succession of peat, organic clay, and reddish brown silt. Facies B dates to 9.0 to 7.5 ka and is interpreted as aggradational tidal deposits, 16.5 m thick, deposited during a rapid rise of sea level; these deposits became overlain by an accumulation of salt marsh and flood-plain deposits during the subsequent period of slowly rising and then falling sea level. The maximum flooding surface lies in the Facies B interval. The presence of tidal deposits implies that the core site, about 230 km inland from the present river mouth, was near the shoreline and experienced strong tidal influences during the early Holocene.
Micropaleontological analysis of nearshore to offshore sediments recovered from the southwestern coast of Thailand was performed to clarify the submarine processes of sediment transport and deposition during the 2004 Indian Ocean tsunami. The distribution pattern of benthic foraminifers showed seaward migration after the tsunami event. Agglutinated foraminifers, which are characteristic of an intertidal brackish environment, were identified in the post-tsunami samples from foreshore to offshore zones. These suggest that sediments originally distributed in foreshore to nearshore zones were transported offshore due to the tsunami backwash. On the other hand, the distribution pattern of planktonic and benthic species living in offshore zones showed slight evidence of landward migration by the tsunami. This suggests that landward redistribution of sediments by the tsunami run-up did not occur in the offshore seafloor of the study area. Our results and a review of previous studies provide an interpretation of submarine sedimentation by tsunamis. It is possible that tsunami backwashes induce sediment flows that transport a large amount of coastal materials seaward. Thus, traces of paleotsunami backwashes can be identified in offshore sedimentary environments as the accumulation of allochthonous materials. This can be recognized as changes in benthic foraminiferal assemblages.
The Holocene history of the Japan Sea was reconstructed using foraminiferal assemblages and stable isotope records from two piston cores recovered from west of the Tsugaru Strait, with the overall goal to resolve a controversy regarding flow in and out of the Japan Sea during the Holocene. In the northern core the planktonic foraminifer N. pachyderma changed from left‐coiling to right‐coiling at 8.3 ka. However, in the southern core the left‐coiling form remained dominant until 4.8 ka. Some indices of the stable isotopic records also changed at 8.3 and 4.8 ka. These results suggest that the Tsushima Current started to inflow into the Japan Sea at 8.3 ka, but a branch of the Oyashio Current continued to inflow until 4.8 ka. We attribute the coexistence of the two currents between 8.3 and 4.8 ka to the opposite direction flows between the shallow and the deep portions of the Tsugaru Strait driven by baroclinic components.
To clarify spatial and seasonal differences in net plankton and zoobenthos in Lake Tonle Sap, Cambodia, quantitative surveys were carried out at 14 stations in the north and south basins in high-and low-water seasons during 2003-2005. In the phytoplankton communities, a diatom Aulacoseira granulata dominated throughout the lake in the high-water seasons, while blue-green algae, mostly composed of Microcystis, surpassed other algae in the low-water season when the lake water was very turbid and the Secchi disk readings were only a few centimeters. In the low-water seasons, a bloom of floating blue-green algae occurred everywhere, especially prominent in the coastal areas. Protozoans and rotifers dominated the zooplankton communities. In the open-water stations, diversity was higher in high-water seasons in phytoplankton, while it was not significantly different between seasons in zooplankton. Composition of plankton communities in Lake Tonle Sap appears to have changed little since the 1950s, at least in phytoplankton, while the phytoplankton density appears to be higher in the present study. Among the macrozoobenthos, mollusks, oligochaetes and chironomids dominated in density, and mollusks exceeded others in biomass in both basins and seasons. The total densities of macrozobenthos were not high, being fewer than 1,300 m -2 throughout the stations and seasons. Possible reasons for the low zoobenthos abundance in the lake may include high predation pressures by benthivorous fish or unfavorable unstable and flocculant substrates.
Faunal composition of aquatic invertebrate communities associated with submerged parts of several species of macrophytes were studied in different areas in littoral Lake Tonle Sap in Cambodia, with special reference to those in root systems (interrhizon) of a free-floating water hyacinth (Eichhornia crassipes). Nine phyla of invertebrates were collected, of which oligochaetes, shrimps and Limnoperna mussels were abundant along with meiobenthic crustaceans. The macrophyte-associated invertebrates in Lake Tonle Sap might be unique in having abundant sessile animals, such as sponges, bryozoans and Limnoperna mussels. The Limnoperna mussels attached to macrophytes were more abundant in offshore and inundated forest than in secluded vegetational stands toward the shoreline. It suggests that water movement can be an important factor determining the distribution and abundance of the sessile animals by controlling larval dispersions and might be associated with the hydrological characteristic of the lake, i.e., the lake opens to the large Mekong River with drastic seasonal changes in water level.
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