Coral reefs are increasingly threatened by human activities (e.g., sewage discharge and industrial effluents). Composition of coral skeleton is expected to quantitatively record the change in marine environments in which skeletal calcium carbonate is precipitated. Recently, trace elements, particularly heavy metals, in the skeleton of massive and annually banded corals have been successfully used as a proxy of marine pollution at regional and local scale. Critical problems, however, exist due to lack of a well-established method for analyzing trace elements in coral skeleton. Large range in reported values for heavy metal concentrations in coral skeleton could be attributed to differences in pretreatment procedures of skeletal materials rather than the real extent of marine pollution. In this review, we summarize recent progress of studies on the topics and key factors for the further application of coral proxies in order to reconstruct the history of marine pollution.

View full abstract

In the Tamagawa hot spring area, three different types of geothermal activity are found, which are hot water of acid Cl-SO₄ type at Obuki, fumarole steam containing CO₂ and H₂S but no SO₄ and HC1 and hot water of acid SO₄ type. The chemical composition and hydrogen and oxygen isotopic ratio of these hot water and steam have been observed since 1978. The hydrogen and oxygen isotopic ratios are clearly different among these three types, and it is explained that the Obuki hot water is mixture of volcanic steam and local meteoric water, the fumarole steam boils away from the Obuki hot water at about 150℃ underground, and the hot water of SO₄ type is surface water heated by steam similar as the fumarole steam. Significant variations of chemical and isotopic compositions have been observed on the Obuki hot water during the present observation as well as the observation by others since 1951. The relationship between the Cl content and the isotopic ratios reveal that the variations are caused by change in the mixing ratio of volcanic steam and meteoric water. Soon after the explosion of Mt. Yake from 1948 to 1951, the proportion of volcanic steam increased from 14 to 25%, and then had kept rather constant. In 1977, the proportion started decreasing sharply, and then, in 1992, returned to the state before the explosion, though slight increasing happened again since 1997. As regards the SO₄ content, a remarkable change, which is not due to the change in the mixing ratio of volcanic steam and meteoric water, happened from 1972 to 1991. In practical geochemical monitoring on geothermal water of volcanic gas origin, it is necessary to note that variation in SO₄^2- content is not always due to the change in the contribution of volcanic gas.

View full abstract

Elemental and isotopic compositions of noble gases trapped in ultramafic rocks (xenoliths and orogenic peridotites) provide valuable information regarding the processes that affected the terrestrial mantle. Here I present some of the examples we found in fresh suites of xenoliths from southeastern Australia and in orogenic peridotites from Horoman complex in northern Japan. The great majority of SE Australian xenoliths yielded MORB-like ³He/⁴He ratios by crushing gas extraction, indicating that fluids derived from the convecting upper mantle (= MORB source mantle) clearly dominate the continental lithospheric mantle. In contrast, noble gas extraction by stepheating revealed the presence of significantly radiogenic ²¹Ne (and ⁴He to a lesser extend) in crystal structures of U-Th-bearing minerals such as apatite, amphibole and clinopyroxene. The presence of radiogenic noble gas component is clearly a consequence of mantle metasomatism by fluid/melt enriched in incompatible elements. Separation of CO₂ + noble gas-rich fluid from the incompatible-element-rich melt during the ascent would be a plausible explanation for the preservation of mantle source signature in fluid inclusions. This in turn suggests that noble gases in fluid inclusions of ultramafic rocks can be used to infer the source of metasomatic agent which sometimes is difficult to speculate upon. This notion can further be confirmed by the occurrence of plume-like neon in metasomatic apatite in the xenoliths in which the coexisting minerals showed MORB-like noble gases signatures. It was also found in the orogenic peridotites from Horoman complex that they were affected by metasomatic fluids from the dehydrating slab which introduced recycled-atmospheric argon into the mantle wedge. We note that this recycled component could have been stored in mantle wedge for a significant period of time to become part of continental lithospheric mantle. Somewhat lower ⁴⁰Ar/³⁶Ar ratios in xenoliths from the subcontinental mantle might be due to an involvement of a recycled component during the formation of the metasomatizing melt. These observations suggest that the continental lithospheric mantle have multiple noble gas components of distinct origins; a mantle component (both from degassed- and less-degassed-sources), an in situ radiogenic component and a recycled atmospheric component. It was also noted that Kola carbonatites of supposed lower mantle origin also showed some indication of a recycled volatile component. This issue certainly has important implications for bettering our understandings on the origin of noble gas heterogeneity observed in samples from the deeper mantle origin.

View full abstract