Abstract
In order to reveal sedimentary processes in shallow lakes, we collected samples from 118 points in Lake Kitaura-a shallow coastal lagoon in the eastern margin of the Kanto region, central Japan, and analyzed grain size distribution, TOC, TN and C/N ratio. We propose a new depositional model for shallow lakes controlled by wave action owing to wind. The distribution of sandy sediments is restricted to areas near the perimeter of the lake, and correlates with areas of low contents of TOC and TN. Alternatively, clay-rich sediments are most widely distributed, and show high concentrations of TOC and TN. The TOC /TN ratio is low, about 7 to 8 throughout the basin, suggesting that major organic matter originated from planktons in the lake. High TOC/TN ratios (over 8) recognized in the northern end of the lake evidently suggests the influence of tributary rivers. The bottom sediments of Lake Kitaura are characterized by a predominance of clayey sediments which cover most of the sublacustrine plain and a narrow zone of sandy sediments along the southern coastal area shallower than 2.5 m. The former sediments are transported as suspended matter reworked from the shallow and shore areas by wave action. The latter are lag deposits sifted by surface drift generated by prevailing wind. Low topography around the lake and small drainage area of each river appears to be the cause of the very small amount or lack of coarse-grained sediments around the river mouths in the narrow inlets. The sandy sediments in the southern margin of the lake are interpreted as relict sediments deposited during the last high sea-level period.
