Diffusion of Chloride in Sediments of a Brackish Lake : Distributions of Chlorinity in Pore Waters of Drillcores from Lake Hamana

Abstract

Three drill sediment cores, which penetrated throughout the Holocene, were collected in 1985 from the bottom of a coastal brackish lake, Lake Hamana, Japan. Chlorinity in pore waters extracted form the sediment samples was determined. In 2 cores by the ocean side, chlorinity is low in the upper and lower silt layers and high in the middle fine sand layers. However, the core in the most-inland site is composed of silty clay at the whole depth, and chlorinity gradually decreases from 16.67 g kg^&lt-1&gt in the surface layer, closely to that in the lake bottom water, to 1.35 g kg^&lt-1&gt at the bottom depth of 5.32 m. Using non-steady state diffusion-advection models, the distributions of chlorinity in cores from the ocean and most-inland sides were analyzed. The comparisons between measured and predicted distributions suggest as follows; (1) from the ocean side core the salt water is laterally transported from the sea to the lake through the sandy sediment layer, and (2) from the inland side core the elapsed time while chloride in the lake water diffuses into the sediment is estimated to be 200-300 years at least, and therefore the recent increase in salt concentration of the lake water can be attributed to the change of the mixing rate of oceanic water and fresh lake water by mainly giant tsunami disasters since the early 18th century.