Reconstruction of palaeo-environmental changes based on ecological information of diatoms in shallow marine sediments: Example of a holocene delta sequence on the nobi plain, central Japan

Ecological information about diatoms is useful for reconstruction of palaeo-environments influenced by marine transgression and regression. Each diatom has a different habitat, especially regarding salinity. In the coastal region, salinity changes occur controlled by marine transgression and regression. And, these salinity changes cause changes of diatom assemblages. If diatoms are in the sediments deposited under the influence of marine transgression and regression, reconstruction of palaeo-environmental changes based on ecological information of diatoms can be done. We analyzed diatom assemblages from sediment cores from the Nobi Plain, central Japan. Five diatom assemblage zones were identified: (1) at the beginning of the Holocene, freshwater species were dominant; (2) then, marine and brackish-marine species increased, indicating transgression; (3) in the middle Holocene, proportions of marine and brackish-marine species became almost constant, with marine species dominant; (4) marine species began to be replaced by freshwater species, indicating marine regression as a result of delta progradation; and (5) freshwater species again became dominant. These diatom assemblages correlate with previously defined lithological units: zone 1 and 2 with fluvial to coastal plain deposits, zone 3 with inner bay or prodelta deposits, zone 4 mainly with delta front slope deposits, and zone 5 with delta front platform deposits and delta plain and flood plain deposits. And, the percentage of marine diatom species is almost positively correlated with electrical conductivity (EC-which is considered to be a proxy of salinity, although there are the effects of compaction and grain size distribution), especially in inner bay deposits. In summary, if there are many diatoms in sediments with good storage condition, we can directly reconstruct salinity with high accuracy. Thus, the ecology of diatoms is useful for reconstruction of palaeo-environmental changes, especially those controlled by marine transgression and regression.