Abstract The early to mid-Aptian was punctuated by episodic phases of organic-carbon burial in various oceanographic settings, which are possibly related to massive volcanism associated with the emplacement of the Ontong Java, Manihiki, and Hikurangi oceanic plateaus in the southwestern Pacific Ocean, inferred to have formed a single plateau called Ontong Java Nui. Sedimentary osmium (Os) isotopic compositions are one of the best proxies for determining the timing of voluminous submarine volcanic episodes. However, available Os isotopic records during the age are limited to a narrow interval in the earliest Aptian, which is insufficient for the reconstruction of long-term hydrothermal activity. We document the early to mid-Aptian Os isotopic record using pelagic Tethyan sediments deposited in the Poggio le Guaine (Umbria-Marche Basin, Italy) to precisely constrain the timing of massive volcanic episodes and to assess their impact on the marine environment. Our new Os isotopic data reveal three shifts to unradiogenic values, two of which correspond to black shale horizons in the lower to mid-Aptian, namely the Wezel (herein named) and Fallot Levels. These Os isotopic excursions are ascribed to massive inputs of unradiogenic Os to the ocean through hydrothermal activity. Combining the new Os isotopic record with published data from the lowermost Aptian organic-rich interval in the Gorgo a Cerbara section of the Umbria-Marche Basin, it can be inferred that Ontong Java Nui volcanic eruptions persisted for ∼5 m.y. during the early to mid-Aptian.
The concentration of total metal sulfide throughout a water column over a submarine hydrothermal vent in Kikai Caldera south of Kyushu Island, Japan, at ~350 pmol/kg, was higher than that reported in previous studies, at <50 pmol/kg below the halocline. Seawater filtered at 0.2 μm pore size and unfiltered seawater exhibited almost identical metal sulfide concentrations throughout the water column, indicating that most metal sulfide existed in dissolved and particulate forms with diameters <0.2 μm. By using a mass balance calculation with the observed sulfide species of free and metal sulfides and carbonyl sulfide, we showed that ~70% of the metal sulfide supplied from hydrothermal vents were contained in the water column beyond the halocline without undergoing oxidative loss even after mixing into overlying oxic seawater. Our findings clearly indicate that sulfide and trace metals emitted from hydrothermal vents form a stable metal-sulfide complex with diameters <0.2 μm. These results also strongly support the recently proposed theory such that metal-sulfide complexation/nanoparticles play an important role in the long-distance transportation of trace metals in the ocean.
Coastal areas are socioeconomically important but are susceptible to disturbances by both natural and man-made events. On March 11, 2011, the eastern coast of Japan was seriously inundated by a massive tsunami following the 2011 Tohoku earthquake. The tsunami caused a major disturbance around the coastal area. However, an understanding of the consequences of such an event is often hampered by a lack of knowledge of prior conditions. Furthermore, field observations during and immediately after the event are often particularly difficult. The present study demonstrates that environmental reconstruction by geochemical and growth pattern analyses of mussel shells successfully revealed transitional (daily) environmental changes caused by the Tohoku tsunami. A pronounced surge in shell Mn/Ca ratios observed immediately after the tsunami implies a drastic emission of pore water following sediment disturbance as well as a large input of terrestrial material through backwash. The subsequent decrease of the high Mn/Ca peak indicates a prolonged tsunami disturbance effect over ca. 40 days, with the stabilized shell Mn/Ca ratios observed thereafter (being higher than that prior to the tsunami) suggesting that the latter had altered the coastal environment, allowing for greater susceptibility to terrestrial input following ground subsidence and loss of coastal levees. Shell Mn/Ca patterns provide evidence for the tsunami-generated release of materials stored in sediments, such as organic, nutrient, and pollutant materials, which are then suspended in the water column for sufficient periods to allow for incorporation into geochemical cycles. Although the greatest environmental disturbance occurred immediately after the tsunami, the effects lasted for longer than several months thereafter.
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