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Lacustrine sediment gravity flow deposits (L-SGFDs) originate from flood inflows and slope failures triggered by earthquakes. Previous studies have mainly focused on sandy L-SGFDs and have described these deposits as hyperpycnites. However, very few studies exist concerning fine-grained L-SGFD, therefore the detailed sedimentary processes associated with these deposits remain unclear. In addition, homopycnal flows and hypopycnal flows are also expected to occur but clear descriptions of these deposits are not available in the literature. In this study, microscopic internal structures in flood-induced L-SGFDs within several diatomite formations in Japan were examined and a model of the sedimentary processes associated with these deposits is established.The middle Pleistocene Hiruzenbara Formation is a diatomite deposit from a paleo-dammed lake and includes a wide variety of intercalated L-SGFDs. The L-SGFDs in this formation may be classified as deposits induced by floods and slope failure. For comparison, we also examined L-SGFDs within the middle Pleistocene Miyajima Formation, a caldera fill deposit in northeast Japan, and the middle Miocene Choujabaru Formation on Iki Island in the Japan Sea. Flood-induced L-SGFDs in this study may be further sub-classified as a thicker erosional type (>1 mm) and a thinner non-erosional type (=<1 mm). Most L-SGFDs classified as the thicker erosional type show graded bedding, occasionally including an internal erosion surface. L-SGFDs that show an internal erosion surface consist of a lower inversely graded unit and an upper graded unit, separated by the internal erosion surface. This type are suggested to be hyperpycnites deposited from the increasing and decreasing discharge stages of a hyperpycnal flow. As the lower unit in some L-SGFDs of this type is observed to be structureless with many rip-up clasts, there is a possibility that the lower unit was deposited from a debris flow eroding the underlying layer. In that scenario, a dilute turbulent flow may evolve into a concentrated laminar flow by entraining material from the underlying layer, which is then deposited as a massive structureless unit from a freezing debris flow during the increasing discharge stage. The upper graded unit may then have formed from a more dilute flow over the lower unit. Some L-SGFDs interpreted as hyperpycnites were draped by very thin clay-rich layers. The origin of these capping clay-rich layers is suggested to be related to deposition from a dilute homopycnal or hypopycnal flow separated from a main flow, and which spread on the lake water surface or along the thermohaline boundary. The thinner non-erosional type of L-SGFD deposit may include a graded unit and constitute very thin deposits with no clear grain size trend. These types of deposits show good lateral continuity and may be correlated over 1-2 km, although the thickness of these layers is typically thinner than 1 mm. L-SGFDs of this type are suggested to be homopycnites or hypopycnites. These deposits are thought to originate from homopycnal and hypopycnal flows spreading on the surface or the thermocline of a lake, with sediments subsequently settling on the lake floor. [ABSTRACT FROM AUTHOR]

The Ishigaki Island, with its uplifted coral reefs in the south, is located 270 km east of Taiwan Island and has a subtropical marine climate. The Shirahosaonetabaru Cave Site, one of the largest Palaeolithic human sites in Southeast Asia is located near the southeastern coast of the island. We analysed the annual layers and geochemical data of a stalagmite obtained from the site and correlated the palaeoenvironmental proxies to the meteorological record of the island. The results obtained were as follows: (1) About 600-year annual layers with an average thickness of about 0.4 mm were consecutively counted in the stalagmite. (2) Six growth direction changes were observed along the section of stalagmite. However, no clear correlation could be found between the historical earthquakes encountered by the island and the stalagmite growth. (3) Time-series of relative fluorescent intensities of the annual layers have several minimal intervals corresponding to those of the solar radiation intensities. Biological activities producing fluorescent fulvic acids may be affected by solar radiation. (4) The oxygen isotope ratios suggest relatively lower temperatures between AD 1400 to 1700 followed by increasing temperatures after AD 1830. The variation corresponds to the Little Ice Age and the following warming. (5) The carbon isotope ratios suggest frequent changes in C3 and C4 plants vegetation. After AD 1900, the contribution of C4 plants increased probably due to the change in land use for pasture. (6) Based on a comparison between meteorological observation data and the annual layer characteristics, it was found that annual layer thickness and relative fluorescent intensity have larger values with their larger variances when the variation in yearly outside cave temperature was larger. These facts suggest that the active cave air ventilation during the intervals with highly-fluctuating outside cave temperature may have enhanced the growth rates of stalagmite. (7) The fluorescent intensity in the annual layer before AD 1840 was dominated by the pattern gradually increasing upwards in a layer. The palaeoclimate during the Little Ice Age may have caused such kind of pattern. For the last 100 years, this type has formed when the stalagmite growth rate was high with their wide variances and the outside cave temperature and precipitation largely fluctuated over a year, or when the precipitation was large but its coefficient of variation was low. During these intervals, it is suggested that the growth rate of the stalagmite and the co-precipitated amount of fulvic acids increased after the summer because of high precipitation in the late summer and autumn. Thus, it can be concluded that the precipitation in the late summer and autumn was higher in the Northwest Pacific region during the Little Ice Age. [ABSTRACT FROM AUTHOR]