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[MIS34-15] The change of sedimentary environment for past of 350 years in the Lake Mokoto, Hokkaido, Japan
Keywords:Coastal Lagoon, Lake Mokoto, lamina, Ta-a tephra, Ko-2c tephra, anoxic
Mokoto has two-layer structure of intermediate salinity surface waters and high salinity bottom water. The bottom water in Lake Mokoto shows the anoxic conditions in summer season.
The 09Mk-2C cores collected from Lake Mokoto shows the length of 3.87m. In Lake Mokoto, there were the Ta-a tephra (AD 1739) at the 3.5m depths and Ko-2c (AD 1964) at the 3.87m depths.
The cores collected from Lake Mokoto consist of organic mud with the lamination in all cores. In 09Mk-2C core, the core top 185 cm shows the black (L value: ca 5), and it seems to indicate the seasonal anoxic environment as present. The organic mud below 100cm depth shows black (L values: ca 13-15). In the observation by the soft X-ray photograph, the cyclic lamina set is observed in the core from Lake Mokoto. It is considered that this cyclic lamina set is the verve.
Phosphorus contents in 09Mk-2C core show the relatively high values (ca 0.1wt%) above192cm depth. However, Phosphorus is not detected below this horizon up to 290cm depths. Same Trend was observed to 09Mk-1C core. According to result of this core, the increasing of phosphorus contents may be caused by drainage of pollution from stock farm. The horizon of phosphorus increasing is estimated to AD1957 from the number of cyclic lamina set. In report, domestic animal increased at AD1955 in the drainage basin. This timing is consistent with phosphorus increasing. The thickness of lamina set is about 7mm before late 1960s for the age of cyclic lamina set. After that, the thickness of lamina set is about 25mm.It is suggest that Lake Mokoto is aggrading by sediment from drainage basin under high sedimentation rate.