17:15 〜 19:15
[AOS16-P05] The sectional distributions of Te(IV) and Te(VI) in the eastern Indian Ocean
キーワード：東部インド洋、微量金属、海洋地球化学的サイクル、テルル
Tellurium (Te) is assigned as one of the technology-critical elements (TCEs) used for industrial products such as DVD. Marine ferromanganese crusts on the seafloor sediments contain high concentration of Te and the enrichment mechanisms from seawater are subjects of importance and interest from both geochemical and economic perspectives [1]. In seawater, Te exists as two oxidation states of Te(IV) and Te(VI). Clarifying its distribution and redox transformation in the marine environment is crucial for better understanding of biogeochemical cycles of Te in the ocean. While a previous study has clarified vertical profiles of Te(IV) and Te(VI) in the subarctic western North Pacific [2], knowledge of Te in open ocean waters remains limited. The basin-scale distribution of Te(IV) and Te(VI) is required to be clarified in the various marine environments including oxygen-depleted regions.
In the eastern Indian Ocean, unique hydrographic and geochemical properties, such as large river runoff to the Bay of Bengal, strong stratification, oxygen-depleted waters, seasonal monsoon currents and anthropogenic aerosol inputs to surface waters have been observed. Thus, we have clarified the sectional distributions of dissolved Te(IV) and Te(VI) in the eastern Indian Ocean for the first time. Seawater samples were collected during KH-18-6 cruise of R/V Hakuho-Maru. Dissolved Te(IV) and Te(VI) were analysed by a method based on Mg(OH)2 coprecipitation, anion exchange resin column separation and an isotope dilution ICPSFMS [2].
The distributions of Te(IV+VI) in the surface water varied across different surface currents. The highest Te(IV+VI) of 2.1 pmol kg−1 was observed at St.8 within the South equatorial current (SEC), while the lowest Te(IV+VI) of 1.5 pmol kg−1 was observed at St.5 in Wyrtki Jet region. A correlation between Te(IV+VI) and dissolved Pb [3] was observed only at St.1–3 in the Bay of Bengal [3]. These results suggest that Te in surface waters has different sources and sink from dissolved Pb which is mainly derived by anthropogenic aerosol inputs. Instead, Te(IV+VI) appears to be affected by surface currents transported from various regions. In subsurface, the distributions of Te(IV)/Te(VI) ratios corresponded to horizontal water mass transport. Our data shows that Te(IV)/Te(VI) ratios in subsurface waters can be used as a tracer for water mass transport and mixing processes occurring shorter timescale than the residence time of Te. In intermediate and deep waters, both Te(IV) and Te(VI) were scavenged by particulate matter, showing a distinct north-south concentration contrast. In the Northern Hemisphere, relatively low Te(IV+VI) were observed with particularly low Te(IV) in the oxygen-depleted waters. The distributions of Te(IV+VI) may be influenced by scavenging enhanced by high particulate loads into the Bay of Bengal. Although a correlation between Te(IV)/Te(VI) ratios and dissolved oxygen were not observed, the lowest ratios were found in the oxygen-depleted waters. The distribution of Te(IV) in the oxygen-depleted waters may be affected by possible transformation of Te(IV) to organic Te in surface waters and Te(0) on the surface of particulate matter in the oxygen-depleted waters [4], followed by slow oxidation of both species to Te(IV). In the Southern Hemisphere, relatively high Te(IV+VI) was observed with particularly high Te(VI) at St.9 and 10. The distributions of Te(IV+VI) may be influenced by low scavenging rates arising from the low productivity [5], and by potential Te inputs from Australian aerosols to the subtropical gyre, followed by surface water subduction.
References
[1] Hein et al. (2003) Geochim. Cosmochim. Acta. 67, 1117–1127. [2] Fukazawa et al. (2024) Anal. Chim. Acta. 1300, 342430. [3] Ikhsani et al. (2003) Mar. Chem. 248, 104208. [4] Harada and Takahashi (2008) Geochim. Cosmochim. Acta. 72, 1281–1294. [5] Grand et al. (2015) Glob. Biogeochem. Cycles. 29, 375–396.
In the eastern Indian Ocean, unique hydrographic and geochemical properties, such as large river runoff to the Bay of Bengal, strong stratification, oxygen-depleted waters, seasonal monsoon currents and anthropogenic aerosol inputs to surface waters have been observed. Thus, we have clarified the sectional distributions of dissolved Te(IV) and Te(VI) in the eastern Indian Ocean for the first time. Seawater samples were collected during KH-18-6 cruise of R/V Hakuho-Maru. Dissolved Te(IV) and Te(VI) were analysed by a method based on Mg(OH)2 coprecipitation, anion exchange resin column separation and an isotope dilution ICPSFMS [2].
The distributions of Te(IV+VI) in the surface water varied across different surface currents. The highest Te(IV+VI) of 2.1 pmol kg−1 was observed at St.8 within the South equatorial current (SEC), while the lowest Te(IV+VI) of 1.5 pmol kg−1 was observed at St.5 in Wyrtki Jet region. A correlation between Te(IV+VI) and dissolved Pb [3] was observed only at St.1–3 in the Bay of Bengal [3]. These results suggest that Te in surface waters has different sources and sink from dissolved Pb which is mainly derived by anthropogenic aerosol inputs. Instead, Te(IV+VI) appears to be affected by surface currents transported from various regions. In subsurface, the distributions of Te(IV)/Te(VI) ratios corresponded to horizontal water mass transport. Our data shows that Te(IV)/Te(VI) ratios in subsurface waters can be used as a tracer for water mass transport and mixing processes occurring shorter timescale than the residence time of Te. In intermediate and deep waters, both Te(IV) and Te(VI) were scavenged by particulate matter, showing a distinct north-south concentration contrast. In the Northern Hemisphere, relatively low Te(IV+VI) were observed with particularly low Te(IV) in the oxygen-depleted waters. The distributions of Te(IV+VI) may be influenced by scavenging enhanced by high particulate loads into the Bay of Bengal. Although a correlation between Te(IV)/Te(VI) ratios and dissolved oxygen were not observed, the lowest ratios were found in the oxygen-depleted waters. The distribution of Te(IV) in the oxygen-depleted waters may be affected by possible transformation of Te(IV) to organic Te in surface waters and Te(0) on the surface of particulate matter in the oxygen-depleted waters [4], followed by slow oxidation of both species to Te(IV). In the Southern Hemisphere, relatively high Te(IV+VI) was observed with particularly high Te(VI) at St.9 and 10. The distributions of Te(IV+VI) may be influenced by low scavenging rates arising from the low productivity [5], and by potential Te inputs from Australian aerosols to the subtropical gyre, followed by surface water subduction.
References
[1] Hein et al. (2003) Geochim. Cosmochim. Acta. 67, 1117–1127. [2] Fukazawa et al. (2024) Anal. Chim. Acta. 1300, 342430. [3] Ikhsani et al. (2003) Mar. Chem. 248, 104208. [4] Harada and Takahashi (2008) Geochim. Cosmochim. Acta. 72, 1281–1294. [5] Grand et al. (2015) Glob. Biogeochem. Cycles. 29, 375–396.