Paleoenvironmental Change in a Brackish-water Lake during the past 3000 years as recorded in the Sediment of Lake Nakaumi, Southwest Japan.

Major environmental change in a brackish-water lake can occur through natural or human influenced changes in landforms as well as through eustatic sea level change. The hydro-environment of Lake Nakaumi, southwest Japan, has been greatly affected by both man-made and natural landscape evolution. In this study, environmental change during the past 3000 years is discussed based on high-resolution sedimentologic and geochemical analysis of 3 cores collected from Lake Nakaumi. Lake Nakaumi is located near the Sea of Japan coast in the southwest Japan, and is a brackish-water lake of about 86 square kilometers. Presently Lake Nakaumi experiences seawater inflow from Nakaura Channel via Sakai Channel and low saline water and fresh water inflow from the Oohashi and Iinashi Rivers. This leads to complex two-layer water-mass structures in the lake. Three sediment cores (X, N1 and N2) were collected in order to examine the effect of the inflow of low saline water from Oohashi River, seawater from Nakaura Channel, and fresh water from Iinashi River, respectively. The cores are 3.5 - 4m in length, and consist of mud with shell fragments in most horizons. Sample were taken at 1cm interval and grain size, C, N, S elemental content, and carbon stable isotope ratios were measured. Mean grain size ranges from 6.5 to 8.0 phi, and averages 7.5 phi in most samples. Mean grain size in the N2 core coarsens above 70cm. In N1 core, total organic carbon (TOC) contents show a peak (ca.3.5%) at 380cm, and decreases in steps toward the top of the core. TOC content shows the lowest value (ca.1.5%) at 30-40cm. Above this horizon, TOC contents increase upward. A similar TOC pattern is seen in the other cores. Total Sulfur (TS) content follows change in TOC content. The TOC and TS maxima at 380cm (ca. 2700 yr. B.P.) in N1 core indicates the most reducing environment in the interval covered by the cores. This suggests that Lake Nakaumi was isolated by the evolution of a sand bar (Yumigahama Beach). After that, TOC contents decrease in response to the re-establishment of a marine water influx, and shallowing due to a significant increase in the sedimentation rate. Marine faunas were re-established at 2000B.P. in Lake Nakaumi. A temporary change in the mean grain size in N2 core sediments can be attributed to a diversion of the Iinashi River in 1665. Another excursion in mean grain size above 40cm may be due to another modification of the channel of the Iinashi River in 1840. The rapid increase in TOC above 30 - 40cm resulted from human modification of the lake, such as dredging in the Sakai Channel and the closure of the Honjo Area (16 km2) by embankments in a land reclamation project.