抄録
Diatom, elemental (organic carbon and nitrogen) and stable carbon isotope analyses were carried out on a sediment core from Meike-Pond in the Oki Islands, western Japan, to reconstruct past environments. Five radiocarbon dating results show that the 148 cm long sediments of homogeneous organic mud were deposited in the last millennium. The main diatom floras were Fragilarioid taxa (e.g. Staurosira, Pseudostaurosira) and the genus Aulacoseira, with their species and variety showing different trends in relative abundance. The limnetic environment in the lower half of the core (980— 1450 CE) was characterized by lower trophic level probably due to growths of aquatic plants and weaker water mixing. A warmer climate with high aquatic productivity was also inferred for the period 980— 1140 CE. Inflow of mineral particles that would reflect temporal increases in rainfall were observed at 1110 CE and 1450—1550 CE. Basin sedimentation and the subsequent lowering of water level promoted water mixing and eutrophication especially from 1550 CE. During periods presumably from the mid- 20th century, the diatom flora changed markedly indicating water acidification and oligotrophication. Compared with previous research, we inferred that the deposition of artificial airborne ash particles and acid rain caused this. This water acidification has continued to the near present, as acidic indicators have increased towards the top of the core.
https://ror.org/01jaaym28 