The paleoenvironmental and climatic recorded in core sediments of Lake Abashiri in the east part of Hokkaido, Japan

In the coastal area of the Sea of Okhotsk in the east part of Hokkaido located to for subarctic zone, many brackish-water lakes are distributed. Especially, the Okhotsk brackish-water lake group around Abashiri City is constituted by major lake in Japan such as Lake Abashiri, Lake Mokoto, Lake Tofutsu, and Lake Notoro. The each lake shows a different present environment and history. Therefore, the change that is common in those lakes seems to be the change concerning the climate. In this study, paleoenvironmental and climatic changes after the Little Ice Age in Abashiri region is discussed by sedimentologic and geochemical high-resolution analysis of the 4m-sediment cores collected from the Lake Abashiri. The water column of Lake Abashiri has a distinct halocline around 5m depths, and is divided into oligohaline epilimnion and polyhaline hypolimnion by its pycnocline. The hypolimnion in Lake Abashiri shows the euxinic conditions throughout the annual. Therefore, surface sediment of below water depth 5m shows the black organic mud with the lamination. The 10AB-5C core collected from Lake Abashiri shows the length of 332cm. This core is composed of muddy sediment with a distinct lamination through all horizons. The Ta-a tephra (AD 1739) and Ko-c2 tephra (AD 1694) are found at the horizon of 250 cm, and 291 cm, respectively. Sedimentation rate based on these ages was 0.92cm/yr between Ko-c2 tephra and Ta-a tephra, and was 0.91cm/yr between surface and Ta-a tephra. The result of observation used by Soft X-ray photograph, the lamina-set of high, low, intermediate density layers was observed. The lamina set of 44 was recognized between Ko-c2 tephra and Ta-a tephra. This is suggested that this set is annual lamina (varve). If its so, it is considered that high density layer indicate the high precipitation term of summer season judging from precipitation pattern in Abashiri Area. Particularly, the year of flood events tend to show a thick and distinguished layer in lamina-set. The high-density lamina-set group is repeated with 20 to 30 year cycle. The horizon of high-density lamina-set group is correlated with the term showing several years continuously high precipitation. This lamina-set group is observed from 400 years ago, the climate system would have been continued. In order to clarify the formation process of the lamina set in Lake Abashiri, we are observed by a sediment trap. Sediment traps are set up above 2m from lake bottom in water mass under euxinic environment. Result of this observation, sediment flux is increased in early spring (ice melting) and late summer (flood) seasons. Sediment flux in early spring season is higher than late summer season. Total organic carbon (TOC) contents of trapped sediment are higher than surface sediments except for trapped sediment in early spring season. These are suggested that low-density layer indicate in early spring (ice melting) season. In winter (ice) and early summer seasons, the sediment flux is low, and TOC contents show the high value. This is suggested that the limited supply of inorganic sediment. In this time, the diatom lamina may been formed in lamina set.