The sudden eruption of submerged calderas can potentially pose imminent danger to nearby communities. Hence, it is crucial to closely monitor the changes in volatile effluents in these environments. Unlike subaerial volcanoes, underwater volcanic activity is difficult to observe. Wakamiko, a submerged caldera in southern Japan shares a magma source with the active subaerial volcano, Mt. Sakurajima. Following years of inactivity, the volcanic eruptions in Mt. Sakurajima became more frequent in 2009; however, it is undetermined how the neighboring Wakamiko caldera has behaved in relation to this upsurge in activity. In this study, we assess the state of hydrothermal venting inside Wakamiko following recent disturbances in Mt. Sakurajima. Based on gas bubbles, seawater, and sediment pore water measurements of samples obtained in 2015, it appears that acidic fluids enriched in He continue to be discharged to the seafloor. Excess He in the water column shows no apparent change since 2010; however, the helium isotopic ratio of the magmatic source continues to be elevated at ∼7.2 Ra. This ratio for the magmatic component is higher than previously observed in 1986 (∼6 Ra). The difference could be related to changes in the degassing activity as well as the mantle composition of the magmatic source. Using the new data from sediment pore water, we estimate the diffusive fluxes of He and He through the seafloor at 16.05 atoms cm s and 1.52 × 10 atoms cm s, respectively. We also calculated the vent He flux at 3.38 × 10 atoms cm s. With the use of the He flux, we calculated the diffusive CO flux from Wakamiko at 1.41 × 10 mol yr and the vent flux at 3.72 × 10 mol yr. The available data that we have are still limited and a more frequent and longer observation is necessary to deduce the temporal changes in Wakamiko and its relation to the volcanic activity of Mt. Sakurajima.

中文翻译：

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监测日本南部活跃排气的海底破火山口的岩浆活动和挥发通量

水下火山口的突然喷发可能会对附近社区造成迫在眉睫的危险。因此，密切监测这些环境中挥发性废水的变化至关重要。与地面火山不同，水下火山活动很难观察到。若美湖是日本南部的一个水下破火山口，与地下活火山樱岛山共享一个岩浆源。经过多年的不活动，樱岛山的火山喷发在 2009 年变得更加频繁；然而，尚不清楚邻近的若美子火山口对这种活动激增的反应如何。在这项研究中，我们评估了最近樱岛山扰动后若见湖内部热液喷发的状态。根据 2015 年获得的样品的气泡、海水和沉积物孔隙水测量结果，富含 He 的酸性流体似乎继续排放到海底。水柱中He过量自2010年以来没有明显变化；然而，岩浆源的氦同位素比继续升高至~7.2 Ra。岩浆成分的这一比例高于 1986 年观测到的值 (∼6 Ra)。这种差异可能与脱气活动以及岩浆源地幔成分的变化有关。利用来自沉积物孔隙水的新数据，我们估计了 He 和 He 通过海底的扩散通量分别为 16.05 原子 cm s 和 1.52 × 10 原子 cm s 。我们还计算了 3.38 × 10 个原子 cm s 处的排气 He 通量。通过使用 He 通量，我们计算出 Wakamiko 的扩散 CO 通量为 1.41 × 10 摩尔年，排放通量为 3.72 × 10 摩尔年。我们现有的数据仍然有限，需要更频繁、更长时间的观测来推断若美子的时间变化及其与樱岛火山活动的关系。