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October 14, 2020
Japan Agency for Marine–Earth Science and Technology (JAMSTEC)
Kobe University
Chiba Institute of Technology
Tohoku University
Kyushu University

Contribution of microbial activity during the initial formation of seafloor hydrothermal deposits ―Possibilities for rewriting models of subseafloor mineralization―

1. Key points

Pyrite grains with extremely low sulfur isotopic compositions were found from seafloor hydrothermal deposits in the middle Okinawa Trough.
Sulfur isotope values indicate that the pyrite grains are comprised of sulfur produced by the microbial reduction of seawater sulfate.
The initial process by which the hydrothermal deposits formed was partly induced and promoted by microbial activity.

2. Overview

Tatsuo Nozaki, at the Japan Agency for Marine–Earth Science and Technology (JAMSTEC), in collaboration with researchers at Kobe University, Chiba Institute of Technology, Tohoku University, and Kyushu University, analyzed samples collected from seafloor hydrothermal deposits (*1) in the middle Okinawa Trough (Figs. 1 and 2). Through these analyses, pyrite (FeS2) grains derived from microbial activity, with extremely low sulfur isotopic compositions (δ34S) (*2), were identified. Although speculated, the involvement of microbial activities in the precipitation, oxidation, and dissolution of sulfide minerals (*3), which constitute seafloor hydrothermal deposits, has not been confirmed from deposit formation processes. Therefore, in their study, Nozaki and colleagues performed detailed microscopic observations, measured mineral compositions, and analyzed in situ sulfur isotopic compositions in drill core (*4) and chimney (*5) samples obtained from seafloor hydrothermal deposits during a research cruise of the D/V Chikyu.

Owing to the sulfide maturation process (i.e., growth of mineral deposits) driven by repeated hydrothermal activity, pyrite grains in drill core samples exhibited different textures and shapes, from framboidal (*6) to colloform (*7) to euhedral (*8) (Figs. 3 and 4). In situ sulfur isotopic analyses of these pyrite grains using secondary ion mass spectrometry (SIMS) revealed that framboidal pyrite had an extremely low sulfur isotopic composition, as low as -38.9‰, and gradually shifted to a higher value as its texture and shape changed to colloform and then to euhedral (Figs. 5 and 6). The only known mechanism that could cause such extreme fractionation is the reduction of seawater sulfate by sulfate-reducing bacteria.

As mineralization progresses in seafloor hydrothermal deposits, framboidal pyrites with low sulfur isotopic compositions are often replaced by other constituent sulfide minerals, such as chalcopyrite (CuFeS2), galena (PbS), and sphalerite (ZnS) (Fig. 4). Therefore, by functioning as the starting material and nucleus for sulfide minerals in seafloor hydrothermal deposits, framboidal pyrite is thought to provide iron and sulfur. Since this initial material includes sulfur that has undergone microbial sulfate reduction, subseafloor microbial activity is thought to play an important role in the initial formation of seafloor hydrothermal deposits by inducing and promoting their formation. In situ multi-sulfur isotopic analyses on drill cores, including 33S and 36S isotopes from other marine areas, are expected to reveal the mechanisms, characteristics, and contribution of microbial activity to the formation of seafloor hydrothermal deposits.

The study was supported by Japan Society for the Promotion of Science through KAKENHI grants JP17K18814 and JP20H01999 and by a grant-in-aid from the Japan Mining Promotional Foundation. This study was also supported by the Cross-Ministerial Strategic Innovation Project, Council for Science, Technology and Innovation of the Japanese government and by the Integrated Ocean Drilling Program (IODP), through which samples were acquired.

Microbial sulfate reduction plays an important role at the initial stage of subseafloor sulfide mineralization
Tatsuo Nozaki1,2,3,4, Toshiro Nagase5, Takayuki Ushikubo6, Kenji Shimizu6, Jun-ichiro Ishibashi7, and the D/V Chikyu Expedition 909 Scientists
1. Submarine Resources Research Center, Japan Agency for Marine–Earth Science and Technology (JAMSTEC)
2. Frontier Research Center for Energy and Resources, The University of Tokyo
3. Department of Planetology, Kobe University
4. Ocean Resources Research Center for Next Generation, Chiba Institute of Technology
5. The Tohoku University Museum, Tohoku University
6. Kochi Institute for Core Sample Research, JAMSTEC
7. Department of Earth and Planetary Science, Kyushu University


Seafloor hydrothermal deposit: Mineral deposit produced by hydrothermal fluids associated with submarine volcanic activity.
Sulfur isotopic composition (δ34S): Determined by the 34S/32S of the sample and 34S/32S of a standard material, divided by the 34S/32S of the standard and multiplied by 1000 (‰). Thus, δ34S is expressed by the following equation:
δ34S = ((34S/32S)sample-(34S/32S)standard))/((34S/32S)standard) x 1000
The Vienna–Canyon Diablo Troilite (VCDT) was used as the standard reference material in this study.
Sulfide minerals: A group of minerals that are bound to sulfur. Drill core and chimney samples used in this study are mainly composed of sulfide minerals, such as chalcopyrite (CuFeS2), sphalerite (ZnS), galena (PbS), and pyrite (FeS2).
Drill core: Columnar geological samples obtained by drilling. Unlike the drill bits used in normal drilling operations, a core bit has a hole in the center. A column of rock that enters the hole of a drill bit is collected as a drill core sample.
Chimney: Chimney-shaped mineral deposits formed around hydrothermal vents when metal elements provided by seafloor hydrothermal activities precipitate as sulfide, oxide, silicate, and sulfate minerals on the seafloor.
Framboidal texture: A microscopic texture of spherical minerals arranged in a manner that resembles a raspberry.
Colloform texture: A mineral texture in which fine grains are deposited in a concentric stripe. It is a precipitated texture from colloids and is also referred to as a “colloidal texture.” Crystallization can proceed directly from the solution to form a colloform texture in some cases. Depending on the aggregate form, colloform textures exhibit various shapes, such as grape-like, kidney, and more spherical shapes; this texture is abundantly represented in minerals that form at low temperatures.
Euhedral: A form in which the crystal planes of minerals are well formed. Its antonym is anhedral.

Figure 1. Locations and bathymetries of sample collection sites. Drill core samples from Izena Hole were collected at the Hakurei Site, while drill core and chimney samples from the Iheya–North Knoll were collected at the Original Site.


Figure 2. Cross-sectional photographs of drill core samples from the Hakurei Site, Izena Hole and the Original Site, Iheya–North Knoll.


Figure 3. Pyrite grains exhibiting various textures and shapes depending on their maturity and stage of mineralization . A–C: Drill core samples from Izena Hole; D: drill core sample from the Iheya–North Knoll; E, F: chimney samples from the Iheya–North Knoll. Abbreviations; a = acicular, c = colloform, Cp = chalcopyrite, e = euhedral, f = framboidal, Gn = galena, Mrc = marcasite, Po = pyrrhotite, pseudo = pseudomorph, Py = pyrite, s = spherical, Sp = sphalerite.


Figure 4. Backscattered electron image of the framboidal pyrite being replaced with chalcopyrite and galena. The image shows drill core sample from the Iheya–North Knoll.


Figure 5. Examples of the sulfur isotopic compositions of each pyrite grain measured by SIMS. A, B: Drill core samples from Izena Hole; C: drill core sample from the Iheya–North Knoll; D: chimney sample from the Iheya–North Knoll.


Figure 6. Histograms of sulfur isotopic compositions (δ34S) of pyrite grains, where n indicates the number of pyrite grains analyzed.


(For this study)
Tatsuo Nozaki, Researcher, Research Institute for Marine Resources Utilization, Submarine Resources Research Center, Geochemistry Research Group
(For press release)
Public Relations Section, Marine Science and Technology Strategy Department, JAMSTEC
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