Abstract
The Ammonia/ event was recognized in the Sada River that is an artificial canal linking the Sea of Japan and Lake Shinji, southwest Japan. This event has been originally recognized by Nomura and Endo (1998) in Lake Shinji as a distinct increase of foraminiferal species Ammonia “beccarii ” in the 1980s. The cause of this event has not yet been cleared, but the implication suggested the cause related to the artificial dyke constructed in Lake Nakaumi through the large-scale land reclamation project in the 1970s to 1980s. By using 210Pb radioactivity, we identified the age of this event occurred in the latest 1970s or earliest 1980s, which are very near to the original proposal. The age determination is very critical. If we applied the CRS model, the event age was 1977±1.4 and if we applied the constant flux and constant sedimentation rate model, it was 1982. The intercalation of fine-grained sand near the top of core sediments, however, suggests the age using the both model is valid. The Ammonia event is intimately related to the increased total organic nitrogen and also the decreased TOC/TN ratios. Not only A. “beccarii ” , but also other foraminiferal species such as Haplophragmoides canariensis and thecameobians were increased in accordance with the event. Taking the associated increase of both foraminifera and organic nitrogen into consideration, we suggest the change of water quality to eutrophic water with high primary productivity is the major cause of the Ammonia event.
https://ror.org/01jaaym28 
