The possibility of a paleotemperature proxy, skeletal magunesium/strontium (Mg/Sr) ratio analyzed using ICP-AES technique, was examined on a modern Porites choral skeleton from Sesoko Island, the Ryukyus. The Mg/Sr ratio showed clear cyclic variations along a growth axis in the skeleton, fluctuating between 0.40 ∼ 0.59 (atomic ratio). The annual nature of the variations was confirmed by comparison with annual banding observed in X-radiographs, together with density measurements. Variations in Mg/Sr ratio were in phase with those in δ¹⁸O of skeletal aragonite. Given a calendar in the skeleton based on temperature dependency using annual δ¹⁸O variations, Mg/Sr ratio is strongly correlated with sea surface temperature, suggesting that the ratio have a potential as a paleo-thermometer. Mg/Sr ratio is an good proxy of temperature, compared to conventional Mg/Ca and Sr/Ca ratios due to the convination effect of directly opposed parameters. Mg/Sr ratio also has an advantage in measurements, because there is no necessity of calcium measurements, which might cause some trouble due to small amplitude of the variation when Ca is dominant. The Mg/Sr ratio using a convenient ICP-AES technique, yielding a precision of ±1.02% (2σ) for Mg/Sr ratio, enables to reconstruct paleo-temperature within the precision of ±0.36°C.

抄録全体を表示

Change of M₂ tide and tidal current due to the reclamation in Tokyo Bay is investigated using a three-dimensional numerical model. The sea surface area and volume of Tokyo Bay decreased by 25% and 8%, respectively, from 1923 to 1983 due to the reclamation. As a result, M₂ tidal amplitude in the bay and its water exchange volume across the bay mouth decreased by 8% and 35%, respectively. The surface sediment around the mouth of Tokyo Bay changed from sand to silt due to the decrease of tidal current amplitude there.

抄録全体を表示

Use of a stable isotope ¹⁵N to study microbial processes associated with nitrogen dynamics in marine environments has several advantages including estimates of in situ rates of nitrogen metabolisms and of transfer of nitrogen within ecosystem. First part of this review summarizes application of ¹⁵N methodology to evaluate microbial processes of ammonification, ammonium assimilation, nitrification, and denitrification in marine sediment as well as in water column. ¹⁵N nitrate isotope dilution was successfully applied to show the coupling between sedimentary nitrification and denitrification, which occurs efficiently in the aerobic-anaerobic micro-sites in the vicinity of organic detritus deposition. In global scale, the amount of denitrified nitrogen in coastal sediments estimated by ¹⁵N method is comparable to that in the whole water column, indicating its global importance as the oceanic sink of available nitrogen. Microbial populations in the upper ocean also play a significant role on nitrogen dynanics including ammonium regeneration and its assimilation. In Antarctic coastal waters of spring bloom condition, bacterial net production of ammonium was observed, although they also assimilated ammonium simultaneously. High concentration of amino acids possibly supplied from spring bloom would support this bacterial metabolism. At the later part, I describe our recent finding of non-living organic particles of sub-micron size range (0.38∼1.0 μm) in the ocean, abundance of which is in the order of 10 million per ml at the surface layer. Since the abundance of those particles is positively correlated with chl a and other biological parameters, we assume its biological origin. Laboratory experiments and field observations show several biological processes of their production including grazing of micro flagellates on bacteria, lysis of bacteria by viral infection and exudates of bloomphytoplankton. Also, flexible and fragile natures of those particles suggest that they are marine snow type organic aggregates of submicron seze. This finding fulfills the previous gap between dissolbed and particulate organic materials in the ocean, and suggests an importance of "detritus ladder" in marine ecosystem.

抄録全体を表示