Microbial sulfate reduction plays an important role at the initial stage of subseafloor sulfide mineralization

Abstract

Seafloor hydrothermal deposits form when hydrothermal fluid mixes with ambient seawater, and constituent sulfide minerals are usually interpreted to precipitate abiogenically. Recent research drilling at Izena Hole and Iheya North Knoll in the middle Okinawa Trough (East China Sea), combined with secondary ion mass spectrometry determinations of δ34S in pyrite grains, provides compelling evidence that the initial stage of subseafloor sulfide mineralization is closely associated with microbial sulfate reduction. During the sulfide maturation process, pyrite textures progress from framboidal to colloform to euhedral. Pyrite δ34S has highly negative values (as low as –38.9‰) in framboidal pyrite, which systematically increase toward positive values in colloform and euhedral pyrite. Sulfur isotope fractionation between seawater sulfate (+21.2‰) and framboidal pyrite (–38.9‰) is as great as –60‰, which can be attained only by microbial sulfate reduction in an open system. Because framboidal pyrite is commonly replaced by chalcopyrite, galena, and sphalerite, framboidal pyrite appears to function as the starting material (nucleus) of other sulfide minerals. We conclude that framboidal pyrite, containing microbially reduced sulfur, plays an important role at the initial stage of subseafloor sulfide mineralization.

Mineral-scale variation in the trace metal and sulfur isotope composition of pyrite: implications for metal and sulfur sources in mafic VMS deposits

“…Based on the expected fractionation factors between SO 2 and pyrite (after Sakai 1968), SO 2 disproportion would have had to occur at a temperature of ~ 120 °C to account for the measured depletion in 34 S to − 17.1‰, assuming a starting SO 2 composition of 0‰. Alternatively, at low temperatures (< 120 °C) BSR of seawater sulfate can generate large depletions in 34 S up to − 38.9‰ in pyrite (Habicht and Canfield 1997;Wortmann et al 2001;Farquhar et al 2003;Rouxel et al 2008;Alt and Shanks 2011;Nozaki et al 2021).…”

Tidally Modulated Temperature Observed Atop a Drillsite at the Noho Hydrothermal Site, Mid‐Okinawa Trough

“…Various active hydrothermal sites have been discovered and explored in the Okinawa Trough (e.g., Glasby & Notsu, 2003; Halbach et al., 1989; Ishibashi & Urabe, 1995; Ishibashi et al., 2015; Kawagucci et al., 2010; Kimura et al., 1988; Kinoshita & Yamano, 1997; Kinoshita et al., 1990; Nozaki et al., 2021a, 2021c; Sakai et al., 1990; Takai et al., 2011, 2012; Yamano et al., 1989). According to Masaki et al.…”

“…Various active hydrothermal sites have been discovered and explored in the Okinawa Trough (e.g., Glasby & Notsu, 2003;Halbach et al, 1989;Ishibashi & Urabe, 1995;Ishibashi et al, 2015;Kawagucci et al, 2010;Kimura et al, 1988;Kinoshita & Yamano, 1997;Kinoshita et al, 1990;Nozaki et al, 2021aNozaki et al, , 2021cSakai et al, 1990;Takai et al, 2011Takai et al, , 2012Yamano et al, 1989). According to Masaki et al (2011), who analyzed heat flow data from 78 sites in the Iheya-North hydrothermal field in the mid-Okinawa Trough obtained by remotely operated vehicle (ROV) dive surveys, heat flow within the active hydrothermal area varies greatly from 0.01 to 10 W/m 2 whereas heat flow on the eastern sedimented slope is 0.1-1 W/m 2 .…”

Pyrite Dissolution in the Cretaceous Yogou Formation of the Niger (Chad) Basin: Implications for Basin Evolution under a Rift Tectonic Setting

“…Syngenetic Py is formed in the near-bottom water column and sediment–water interface that is saturated with H 2 S , and preserved in the underlying sediment, while diagenetic Py is usually formed from OM decomposition in an anaerobic sedimentary environment that has sufficient H 2 S and water. Py in the marine environment is formed largely by microbial sulfate reduction in the subsea environment. , In a petroliferous basin, Py has an intimate relationship with OM, as both Py and OM degradation in sediments is through microbial reduction of seawater sulfate, , and the sulfide product is preserved either as Py or inorganic sulfur (S) compounds …”

“…Py in the marine environment is formed largely by microbial sulfate reduction in the subsea environment. 16,17 In a petroliferous basin, Py has an intimate relationship with OM, as both Py and OM degradation in sediments is through microbial reduction of seawater sulfate, 17,18 and the sulfide product is preserved either as Py or inorganic sulfur (S) compounds. 18 Py is a hard-to-dissolve mineral stable in an anoxic environment across a range of pH and Eh values.…”