Abstract
Foraminiferal events induced by man-made impacts are modern analogues that are significant in interpreting pre-historic environmental changes. The Honjo area of the brackish Lake Nakaumi of Japan is an ideal place to examine the process of foraminiferal disappearance. The Honjo area is mostly enclosed and has a maximum depth of 5.5m. Its environment has been changed artificially by a land reclamation project, with the construction of dikes, a barrier gate, and sediment dredging. Three steps of assemblage change are recognized by the time of foraminiferal disappearance based on the thanatocoenoses. In the first step, the original marine assemblage was destroyed abruptly, over a time interval of less than 3 years. A typical brackish assemblage appeared in the second step, and continued to develop over the next 7-10 years. Foraminiferal assemblages disappeared in the third step. These three steps correspond to salinity changes of the Honjo waters. The first step was caused by the cessation of marine water upstream, and the second reflects more restricted circulation of brackish water, with higher salinity bottom water and less saline surface water. The third and final step was caused by collapse of the halocline, which induced thorough mixing of the bottom and surface water, and wind-stressed winnowing of bottom sediments. This resulted in the oxidation of deposited organic and sulfide materials. The final step is more significant for the survival of brackish foraminifera. Foraminiferal disappearance in the Honjo area was completed in maximum 10 years.
