The anthropogenic change of sedimentary system recored to short core in brackish Lake Ogawara, North Japan

The sedimentary environment of brackish lake is fluctuated by the effect of sediment flux from rivers, tidal, climate change, and human activities. Each brackish lake has a specific character. The Lake Ogawara is located in east part of Aomori Prefecture, is connected to the Pacific Ocean through Takase River. In this study, we performed the investigation in the Lake Ogawara at 2011, to clarify a characteristic of recent sedimentary environments. In addition, the short coring at 2012 is performed to show a changes of sedimentary environments. The investigation is made up of 110 detailed sampling localities in grid and 100 water quality measurement sites in a traverse line through the lake system. In addition, the short cores were sampled at sites of Og20, 33, 64, 84, 95 and 97. The water column of Lake Ogawara divided into 3 water masses, as an epilimnion (0-10m), a metalimnion (10-18m), and a hypolimnion (deeper than 18m). The environments in metalimnion and hypolimnion show the anoxic to euxinic condition. The upper part of the metalimnion shows pycnocline dependent on water temperature and salinity, and the lower part shows pycnocline dependent on mainly salinity. Surface sediments are observed well-sorted sand shallower than 10m, and black organic mud with lamination deeper than it depth. As a result of CNS element anaysis of surface sediments, the total organic carbon (TOC) contens increase toward deep, and show very high value (around 8%) in metalimnion and hypolimnion. Total sulfur (TS) content shows 1-2% of values deeper than metalimnion. In spite of euxinic condition, TOC/TS ratio is high in comparison with the normal marine. This suggests the exhaustion of metal ions such as iron or undersupply of sulfate ion. In Og84, 97 cores, the upper part shows the black (L*=6). But lower part shows relative high lightness (L*=15 to 20). The mean grain size of the upper part (black layer) is 6.5φ, the lower part shows the 8.0φ. TOC contents shows the around 7% in the upper part of black, however, tends to decrease in horizon deeper than it. TS contents of 2.5% in the upper part of black, showed the 0.4% deeper than it. As the results of analysis, the lower part of Og84 and 97 cores are the oxidative environment. The upper part of black, it is changed to reduced environment. In addition, the grain size distribution is changed to polymodal of the upper part from monomodal of the lower part. The sediment of coarse grain size is supplied from Pacific Ocean side by density current flow due to inflow of sea-water. The change of grain size distribution is suggested the development of the density current flow, and the metalimnion is formed.