Terrestrial carbonates, such as tufas and stalagmites, are unique archives in terms of providing paleo-environmental information of land area, especially in mid-latitudes where records of ice cores and corals are absent. The oxygen and carbon stable isotopic values (δ18O and δ13C) are the most fundamental proxy records, which reflect relative changes in hydrological condition and vegetation, respectively. The recent challenge focuses mainly on how we quantitatively estimate temperature, precipitation and geochemical circulation in the past. Here, I first review the traditional δ18O studies using tufas and stalagmites, and then introduce my challenge to evaluate multiple carbon sources in karst aquifer. Finally, I extended the mixing and partitioning model to the multi-tracers, such as Sr/Ca and REE/Ca ratios. Multi-tracer analysis is one of solutions to understand details of a unique karst system, which is involved by complex geochemical circulation and associated paleo-environmental changes in the catchment area.
Air bubbles trapped in the Antarctic ice revealed that atmospheric carbon dioxide concentration during glacial time was estimated to be 80–90 ppm lower than the interglacial level. Deep-ocean holds about 60 times as much carbon as the atmosphere and has played a major role in regulating the atmospheric carbon dioxide concentration on glacial-interglacial timescales. This review examines glacial-interglacial variations in ocean circulation. It is likely that cold and salty abyssal water formed in the Southern Ocean distributed in the glacial ocean. Glacial Atlantic meridional overturning circulation was suggested to be weakening but still operating. North Atlantic origin deep water was above ca. 2000 m, exhibiting pronounced hydrographic boundary with underlying southern source abyssal water. The cold and salty abyssal water is expected to be isolated from overlying water masses and to be a carbon reservoir during glacial period. In order to seek the isolated abyssal reservoir with depleted radiocarbon, glacial deep water ventilation has been reconstructed based on radiocarbon age offset between co-existing benthic and planktic foraminifera. However, the reconstructed ventilation records indicated little possibility of the presence of the radiocarbon depleted deep water mass in the glacial ocean. For further study to reconstruct how much old deep water was distributed in the glacial ocean, spatial and temporal change in marine reservoir effect must be constrained.
In the Shibukuro and Tama River system, Akita, where river water is acidified by inflow of acidic hot spring water, mixing process of confluent river water is explained by changes in the isotope ratios of Sr and S as well as the chemical composition of dissolved components. Based on the relationships between Sr and S isotopic ratios and their reciprocal concentrations, the mixing ratio of waste water from a neutralization plant of acidic hot spring water with river water of Shibukuro River was estimated as 1 to 1, and at the confluence of Shibukuro and Tama Rivers the mixing ratio of these two was estimated to be 3 to 7 or 4 to 6. These mixing ratios were mostly consistent with those obtained from the concentrations of dissolved chemical components. These consistent mixing ratios suggest that the chemical composition of river water was mainly controlled by the mixing of the waste water and the tributary river waters, and the influence of precipitation of insoluble salts was negligible. The contribution of the waste water was about 10% at Tose located downstream from the confluence of Shibukuro and Tama Rivers.