たい積物コアの炭素同位体比，Ｃ／Ｎ比およびＦｅＳ２含有量からみた名古屋港の古気候，古海水準変動

Abstract

Three embayment-sediment cores were taken from the bottom of west Nagoya harbor situated in Ise Bay on the coast of the Pacific Ocean, Central Japan. Three cored columns (B2, B3 and B18) obtained in lengths of 43.4, 52.5 and 51.5m were dated by 14C-method to be 9, 000 and 9, 400 and 9, 500 years B. P. at the deepest horizon, respectively. To study the environmental changes such as the climatic and sea-level fluctuation in the past geologic time, cored sediment samples were sampled approximately at intervals of 1m along each column and analyzed for the stable carbon isotopic composition (13C/12C) and C/N ratio of sedimentary organic materials and pyrite (FeS2) contents.Total organic materials in cores have a δ 13C-range from -22.1 to -27.1‰ relative to PDB-standard and a C/N ratio-range from 10 to 40 in the weight ratio, and sediments have a pyrite contents-range from 0.0 to 9.0mg of S/g. Large ranges of δ 13C values and C/N ratios are due to the past depositional history affected primarily by the relative contribution of terrestrial- and marine-derived organic materials to the bottom sediments. Besides a difference in the source of organic materials, δ13C value is affected by the sedimentary environment, temperature conditions. The large range for pyrite contents of sediments is due to a change of the source SO=4 concentration because authigenic pyrite is produced mainly by a bacterial reduction of SO=4 to sulfide.From a difference in source organic materials due to the depositional environments and the temperature dependence of kinetic isotope effects in the production process of organic materials by the photosynthesis, one can expect the tendency towards larger δ13C values and smaller C/N ratios (large contribution of marine planktons) of organic materials deposited during warmer and higher sea-level periods. On the contrary, small δ13C values and larger C/N ratios (large contribution of non-marine planktons and terrestrial plants) can be expected in the sediments deposited during colder and lower sea-level periods, glacial ages. Pyrite contents of sediments must be higher for warmer and higher sea-level depositional conditions, because sea water contains 1, 000 times of SO=4 in concentration as high as that of fresh water.Based on the above-mentioned principles which have been verified by studying the present bottom surface sediments from a river and Ise Bay near Nagoya harbor, vertical profiles of δ13C values, C/N ratios and pyrite contents of sediment columns have been examined. Fluctuations of δ13C, C/N and FeS2 versus depth and the 14C-age showed the completely same climatic and sea-level change patterns for three cored columns, and as expected an inverse correlation can be found between δ13C and C/N.From the fluctuation pattern of δ13C, C/N and FeS2, we can find the following feature indicating the sea-level and the climatic (temperature) changes during the end of the last glacial age (Wurm or Winsconsin) and the Holocene. A low sea-level and a cold climate can apparently be seen at the deepest horizon of sediment columns in the 14C-age of 8, 800 to 9, 000 years B. P.. After this period, the sea-level gradually rose, and the highest sea-level and climatic optimum appeared in the 14C-age of 6, 000 to 6, 700 years B. P. corresponding to “the Marine Transgression of Jomon Age”. This warm and high sea-level period is followed by a low sea-level and a cold climate corresponding to a small scale“Marine Regression of Yayoi Age”, a little ice age. After the little ice age, 1, 000 to 1, 500 years B. P., the sea-level gradually rose again toward the present.