Abstract
Concentrations of arsenic (As) and its chemical forms were determined on deep-sea sediments drilled at three sites of Nankai Trough, off the Kii Peninsula, Japan. Those sediments were analyzed to document the behavior of As in relation to methane hydrate formation and the deep biosphere.The analytical results showed the total As concentration of interstitial water (IW) and squeezed cake (SC) ranged from 0.9 to 380 ppb and from 3 to 14 ppm (average, 6.4 ppm), respectively. The sediments from Site C0002, of which sediment column was the longest down to 2200 m below the seafloor (mbsf) among the studied three drilling sites, were analyzed for the host phase transformation of As. The total concentration of As of IW and SC from 200 to 500 mbsf, where methane hydrate zone was included, was higher than those from the uppermost 200 m. Concentration of As was ultimately controlled by pH. Also, organoarsenicals, such as methylarsonic acid (MMA) and arsenobetaine (AsB), were detected in the sediment column, implying that these organoarsenicals appeared in relation to the in situ microbial activities. These observations suggest that As becomes mobilized directly or indirectly as a result of microbial activity in deep-sea sediments.
Highlights
Distribution and behavior of arsenic (As), one of the most common toxic elements, in the hydrosphere and subareal and under-water shallow sediments have been well documented as a part of geologic cycle of arsenic in the Earth’s surface and crust (e.g., Masuda 2018, and references therein)
Bulk As concentration The bulk As concentrations in squeezed cake (SC) from Hole C0002 ranged from 3 to 14 ppm (Table 1, Fig. 2a), which is within the range of reported concentration in seafloor surface sediments surrounding the Japanese islands; for example, an average concentration of As of 5.5 ppm has been reported for 95 surface sediment samples from Kumano-nada (AIST 2018)
Only bulk As concentrations were obtained from the deepest sediments, we inferred that As behavior did not largely change below ~ 500 mbsf in Hole C0002
Summary
Distribution and behavior of arsenic (As), one of the most common toxic elements, in the hydrosphere and subareal and under-water shallow sediments have been well documented as a part of geologic cycle of arsenic in the Earth’s surface and crust (e.g., Masuda 2018, and references therein). Biogeochemical processes affecting As in ocean water have been well documented (e.g., Neff 2002, and references therein); in marine organisms, As is present as both inorganic species such as arsenate (AsV) and arsenite (AsIII) (e.g., Sanders 1979; Raab et al 2005; Duncan et al 2015) and organic species, In general, the dominant As phases in sediments are the inorganic species: that is, arsenite in reducing environments and arsenate in oxidizing environments (Neff 2002). Neff (2002) documented As phase changes in marine sediments due to microbial decomposition and synthesis of organoarsenicals and reported that sulfides such as realgar (As4S) and pyrite (FeS2), in which As was included as an impurity element, and oxides such as arsenolite (As2O3) are the main final inorganic products fixed in the sediments Unpolluted nearshore marine and estuarine sediments usually contain from 5 to 15 μg/g As, but deep-sea sediments occasionally contain much higher concentrations, probably owing to precipitation from submarine hydrothermal solutions. Neff (2002) documented As phase changes in marine sediments due to microbial decomposition and synthesis of organoarsenicals and reported that sulfides such as realgar (As4S) and pyrite (FeS2), in which As was included as an impurity element, and oxides such as arsenolite (As2O3) are the main final inorganic products fixed in the sediments
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