In situ Raman observations reveal that the gas fluxes of diffuse flow in hydrothermal systems are greatly underestimated

Much progress has been achieved in understanding the seafloor hydrogeology, fluid and mineral geochemistry, and microbial biogeochemistry of hydrothermal systems through decades of marine scientific investigations (Von Damm, 1990; Fisher et al., 2007). However, most of this understanding is based on studies conducted at high-temperature focused flow hydrothermal vents (Koschinsky et al., 2008). The more extreme fluid temperatures and eruption conditions of focused flow vents are more likely to draw the attention of researchers. However, compared with focused fluids, diffuse fluids have larger discharge areas, longer subsurface residence times, and more suitable ambient temperatures and flow velocities for microbially mediated biogeochemical transformations (Wankel et al., 2011; Luther et al., 2012). Moreover, heat flux studies indicate that the heat output from diffuse flow is an order of magnitude greater than that from focused flow (Schultz et al., 1992; Bemis et al., 2012). However, observations of volatile component concentrations and their fluxes in hydrothermal diffuse flow are scarce, which greatly limits our understanding of their role in hydrothermal material release fluxes, hydrothermal ecosystem sustenance, and global ocean chemical cycling.

In recent decades, gas-tight fluid sampling has been an effective method of measuring chemical components of hydrothermal fluids and has played a significant role in research on geochemical processes of hydrothermal systems (Seewald et al., 2002). However, gas escape and sulfide precipitation resulting from changes in temperature and pressure during sampling measurement challenge accurate assessment of gas fluxes of hydrothermal systems. New deep-sea in situ observation technology is likely to be the key to advancing hydrothermal system research. High-temperature-resistant electrochemical sensors have obtained dissolved concentrations of H₂S and H₂ in high-temperature hydrothermal fluids successfully (Ding et al., 2001). The applications of in situ mass spectrometry to hydrothermal volatile gas measurement have revealed that hydrogen is an important energy source for hydrothermal vent symbiosis and that subsurface microbial communities have influence on geochemical fluxes from diffuse hydrothermal fluids (Wankel et al., 2011; Petersen et al., 2011).

In situ Raman spectroscopy is an ideal approach for measuring volatile gases in hydrothermal fluids due to its advantages of being nondestructive and noninvasive and not requiring sample pretreatment (Brewer et al., 2004). We report results from submersible-based deployments of an in situ Raman insertion probe (Brewer et al., 2004; Zhang et al., 2017) to quantify geochemical fluxes of low-temperature diffuse flow sites and high-temperature focused flow sites in the Iheya North hydrothermal field (mid–Okinawa Trough, East China Sea) and to evaluate their contributions to ecosystem sustainability (Fig. 1).

The Iheya North hydrothermal field is situated at a depth of ~1000 m within the mid–Okinawa Trough, a typical arc–back arc hydrothermal system (Fig. 2). The North Big Chimney is the most active hydrothermal site in the Iheya North hydrothermal field and has the highest temperatures and highest flow rates (Kawagucci et al., 2011). Interstitial water collected from sediment drilled in the Iheya North hydrothermal field during Integrated Ocean Drilling Program (IODP) Expedition 331 suggested that the subseafloor hydrothermal fluid regime was chemically stratified with respect to chloride concentration (Takai et al., 2012).

We used a K-type Omega thermocouple with an upper range limit of 650 °C and a precision of 0.75% to measure the temperature of hydrothermal fluids, and used the Raman insertion probe system that was developed on the basis of the Deep Ocean Raman In Situ Spectrometer to determine the composition of hydrothermal fluids in major vents in the North Big Chimney mound, Iheya North hydrothermal field (Fig. 1). Flow rates of focused flow were determined by placing a customized ruler marked with 10 cm intervals vertically on the orifice. For low-velocity diffuse areas, the customized ruler and turbine flow meter were used to determine their flow rates (Ramondenc et al., 2006). The diffuse and focused flow areas were estimated according to the boundary of diffuse and focused flow sites determined through video and the ultrashort baseline positioning system of the remotely operated vehicle (ROV). These in situ measurement processes were carried out mainly in dives 192, 193, and 194 by ROV Faxian during the 2018 cruises of R/V Kexue (Figs. 1A–1C). Fourteen (14) hydrothermal vents were selected for in situ measurements; their distribution covers the top, middle, and bottom of the North Big Chimney mound (Fig. 2).

The highest temperature of the hydrothermal fluids in the Iheya North hydrothermal field measured in this study is 302 °C (Fig. S1 in the Supplemental Material¹). According to the measured fluid temperatures of 14 survey zones, the temperature of the North Big Chimney mound displays spatial variations over a wide range: the temperature increases from 30 °C at the bottom to a maximum of 302 °C at the top. The low-temperature diffuse flow area covers the majority of the North Big Chimney mound and is found mainly at the mound bottom. The middle-temperature diffuse flow area is located primarily at the middle of the mound and encircles the high-temperature diffuse flow area. The focused flow region is located in a small area at the top of the mound, and the largest focused eruption vent is situated at the highest position of the North Big Chimney mound, with fluid as hot as 302 °C (Figs. 2B and 2C).

The end-member concentrations of CO₂, CH₄, and H₂S at the 14 vents are in the ranges of 12.2–767.1, 3.0–50.9, and 3.6–22.7 mmol/kg, respectively (Fig. S6), indicating that the chemical characteristics of hydrothermal fluids in the North Big Chimney mound vary spatially (Fig. 3; Tables S1 and S2). The in situ observation data reveal that the gas concentrations of low-temperature and middle-temperature diffuse flows are much higher than those of hightemperature diffuse flow and focused flows in the North Big Chimney (Fig. 3). The gas content variation in hydrothermal fluids emitted from vents in the North Big Chimney mound can be explained by subseafloor phase separation. The end-member Cl concentrations for the site 1, 6, and 14 vents are calculated to be 355.0, 427.6, and 661.8 mmol/kg, respectively (Fig. S7). The Cl values of the vent fluid at the site 1 and 6 vents are lower than those of ambient seawater and are accompanied by a higher content of volatiles, suggesting that the fluid was derived from a low-salinity vapor phase produced by phase separation (Foustoukos and Seyfried, 2007). In contrast, the vent fluid at site 14 has a higher Cl value and lower volatile content than ambient seawater, suggesting that the fluid came from the high-salinity brine phase.

Based on the analysis of the video images, we estimate that the diffuse venting and focused venting occur over 5%–20% and 1%–5% of the entire area, respectively (Table S3). It is important to clarify that it is difficult to accurately determine the size of the diffuse and focused venting zones, given that these estimates are largely visual and therefore highly subjective. The emission fluxes of CO₂, CH₄, and H₂S are shown in Figure S8, where the fluxes of CO₂, CH₄, and H₂S in the focused flow area are 1.2–5.8 × 10⁹, 7.0–35.0 × 10⁷, and 7.0–35.1 × 10⁷ mol/yr, respectively, while their fluxes in the diffuse flow area are 3.1–12.6 × 10¹⁰, 1.4–5.8 × 10⁹, and 9.3–37.0 × 10⁸ mol/yr, respectively. The fluxes of CO₂, CH₄, and H₂S from the diffuse flow are one to two orders of magnitude higher than those from the focused flow.

Recent electrical resistivity tomography research of seafloor massive sulfide deposits in the Iheya North hydrothermal field clarified a semilayered resistivity structure, interpreted as that in addition to the sulfide deposits exposed on the seafloor, there is another deep-seated sulfide layer at ~40 m depth below the seafloor (Fig. 2C). A less permeable cap rock layer exists between these two sulfide layers (Ishizu et al., 2019). We speculate that the upwelling CO₂-rich hydrothermal fluid in the North Big Chimney mound is trapped under a less permeable cap rock layer, which results in the precipitation and accumulation of massive sulfide deposits below the cap rock (Fig. 2C). When these CO₂-rich hydrothermal fluids are ponded and cooled, exsolution and segregation of CO₂-rich fluids occurs. For the CO₂-rich phase of the H₂O-CO₂ system, there is a negative correlation between the phase equilibrium temperature and CO₂ fraction concentration under the same pressure conditions (Fig. S9) (Dubacq et al., 2013), which explains the negative correlation between the CO₂ content and hydrothermal fluid temperatures in the North Big Chimney mound. H₂S and CH₄ are stripped from the CO₂-poor liquid and hence are also enriched in the CO₂-rich phase.

Our observations show that the proportion of volatiles carried by diffuse flow ranges from ~90% to 95% of the total volatile flux in the North Big Chimney mound, which is attributed to the extensive diffuse flow distribution in the Iheya North hydrothermal field and the high gas volatile content in diffuse flow. The gas contents of arc–back arc hydrothermal fluid are much higher than those of a mid-ocean ridge (MOR) due to the subduction of hydrated oceanic crust and the involvement of felsic magmas (Nakamura and Takai, 2014). In addition, the water depth of arc–back arc hydrothermal vents is usually shallower than that of MOR hydrothermal systems, which makes it easier for phase separation to occur. When CO₂-rich hydrothermal fluids undergo CO₂ phase exsolution and segregation, the negative correlation between the volatile fraction and equilibrium temperature in the H₂O-CO₂ equilibrium phase makes the relatively low-temperature diffuse flow more advantageous in terms of volatile component distribution (Dubacq et al., 2013) (Fig. S9).

Sediment-associated hydrothermal fluids generally contain high gas contents due to the thermal decomposition of organic matter. Moreover, microbial methanogenesis occurring in low-temperature sedimentary recharge and discharge zones is also an important source of volatile CH₄ in hydrothermal fluids (Proskurowski et al., 2008). Thus, the low-temperature diffuse area of the sediment-associated hydrothermal system has an advantage in the release of volatile gases. Combining these factors, we speculate that the distributions of gas volatile fluxes of the sediment-associated hydrothermal system and the arc–back arc hydrothermal system probably have similar patterns to those in the Iheya North hydrothermal field.

The observation data from the Mid-Atlantic Ridge, East Pacific Rise, and Juan de Fuca Ridge suggest that the fraction of heat output attributed to diffuse flow may range from 70% to 95% of the total output (Schultz et al., 1992; Rona and Trivett, 1992; Ramondenc et al., 2006; Shank et al., 1998; Baker et al., 1993; Goto et al., 2007; Mittelstaedt et al., 2012) (Fig. S10). Even in the Lilliput field at 9°33¢S, almost 100% of the heat flux is carried by diffuse flow (Haase et al., 2009). Does the proportion of volatile flux for diffuse and focused flows follow the same rule? Considering that there are common parameters in the calculation of heat flux and volatile flux, we tried to estimate the proportion of volatile flux released by diffuse flow and focused flow using the reported heat flux, fluid temperature, and gas concentration data. Here, we define R as the ratio of heat flux (H) to volatile flux (V); then, we can estimate the ratio of diffuse flow and focused flow in terms of volatile fluxes (see methods in the Supplemental Material). From a large number of previous studies, we searched for data containing the temperature and volatile concentration of both diffuse and focused fluids in the same hydrothermal field. Finally, nine eligible hydrothermal fields (regions) were chosen, and their volatile flux ratios of diffuse flow to focused flow are displayed in Figure 4 (Butterfield and Massoth, 1994; Koschinsky et al., 2002; Lupton et al., 2006; Proskurowski et al., 2008, Craddock, 2009; Foustoukos et al., 2009; Bourbonnais et al., 2012; Reveillaud et al., 2016). As we can see from the statistics of hydrothermal volatiles, nearly half of the volatile flux ratio (V_d/V_f, where d refers to diffuse flow and f refers to focused flow) lies in the 1–10 interval, and the other half of the volatile flux ratios are >10. This indicates that in most hydrothermal regions, the hydrothermal volatile flux of the diffuse flow area is higher than that of the focused flow area, even by one to two orders of magnitude. Although our proposed model for calculating volatile fraction flux ratios is unlikely to be applicable in all hydrothermal regions, available statistics indicate that volatile gases released from low-temperature diffuse flow are probably much higher than those released from high-temperature focused flow on a global scale.

The in situ Raman observations of the Iheya North hydrothermal field in the Okinawa Trough back-arc basin indicate that the volatile flux of the diffuse flow is one to two orders of magnitude higher than that of the focused flow, and the phase separation process plays an important role in the distributions of gas volatile fluxes in diffuse flow and focused flow. Considering the fluid characteristics and volatile flux together, we believe that the contribution of diffuse flow in supporting hydrothermal ecosystems is much greater than that of focused flow and that the role played by diffuse flow should be reassessed.

We thank the science editor Kathleen Benison and three anonymous reviewers for their prompt work and valuable suggestions. This research was supported by the following grants: the National Natural Science Foundation of China (grants 92058206, 42106184, and 41822604); the Strategic Priority Research Program, Chinese Academy of Sciences (CAS) (grants XDA22050102 and XDA19060402); the Open Fund of the Key Laboratory of Marine Geology and Environment, CAS (grant MGE2021KG06); the Young Taishan Scholars Program (grant tsqn201909158); the China Postdoctoral Science Foundation (grant 2020M682245); the Natural Science Foundation of Shandong Province (grant ZR2021QD046); and the Special Research Assistant Project of CAS.