Kawagucci et al. Progress in Earth and Planetary Science (2018) 5:74 Progress in Earth and https://doi.org/10.1186/s40645-018-0232-3 Planetary Science RESEARCH ARTICLE Open Access Cool, alkaline serpentinite formation fluid regime with scarce microbial habitability and possible abiotic synthesis beneath the South Chamorro Seamount Shinsuke Kawagucci1,5* , Junichi Miyazaki1, Yuki Morono2, Jeff S. Seewald3, C. Geoff Wheat4 and Ken Takai1 Abstract South Chamorro Seamount (SCS) is a blueschist-bearing serpentinite mud volcano in the Mariana forearc. Previous scientific drilling conducted at SCS revealed highly alkaline, sulfate-rich formation fluids resulting from slab-derived fluid upwelling combined with serpentinization both beneath and within the seamount. In the present study, a time-series of ROV dives spanning 1000 days was conducted to collect discharging alkaline fluids from the cased Ocean Drilling Program (ODP) Hole 1200C (hereafter the CORK fluid). The CORK fluids were analyzed for chemical compositions (including dissolved gas) and microbial community composition/function. Compared to the ODP porewater, the CORK fluids were generally identical in concentration of major ions, with the exception of significant sulfate depletion and enrichment in sulfide, alkalinity, and methane. Microbiological analyses of the CORK fluids revealed little biomass and functional activity, despite habitable temperature conditions. The post-drilling sulfate depletion is likely attributable to sulfate reduction coupled with oxidation of methane (and hydrogen), probably triggered by the drilling and casing operations. Multiple lines of evidence suggest that abiotic organic synthesis associated with serpentinization is the most plausible source of the abundant methane in the CORK fluid. The SCS formation fluid regime presented here may represent the first example on Earth where abiotic syntheses are conspicuous with little biotic processes, despite a condition with sufficient bioavailable energy potentials and temperatures within the habitable range. Keywords: Forearc serpentinite mud volcano, South Chamorro Seamount, Limit of biosphere, Present-days’ chemical evolution, Radio-isotope-tracer carbon assimilation estimation Introduction et al. 2016). Serpentinization-associated geofluid systems, Serpentinization of ultramafic rocks that are the major typically characterized by an abundance of H2 and/or highly components of the oceanic lithosphere produces highly re- alkaline fluids, have been discovered at a range of tectonic/ ductive fluids, in which transformation of inorganic carbon geological settings on the modern Earth where ultramafic species to organic matter occur (e.g., McCollom 2013). rocks encounter water circulation. The most notable exam- Serpentinization-associated geofluid systems have been ples include slow-spreading mid-oceanic ridges (Charlou et recognized as modern analogs of plausible settings for the al. 2002;Konnetal.2015), ocean core complexs (Kelley et origin of primordial life (e.g., Russell et al. 2014)aswellas al. 2001;Früh-Greenetal.2003), ophiolites (Barnes et al. ancient microbial communities (e.g., Takai et al. 2006;Ueda 1978; Schrenk et al. 2013), coastal springs (Barnes et al. 1967), onshore volcanic hot springs (Homma and Tsukahara, 2008;Sudaetal.2014), inner trench slope along * Correspondence: [email protected] 1Department of Subsurface Geobiological Analysis and Research (D-SUGAR), the southern Mariana forearc (Ohara et al. 2012;Okumura Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 et al. 2016a;Onishietal.2018), and forearc serpentinite Natsushima-cho, Yokosuka 237-0061, Japan seamounts (Mottl et al. 2004; Fryer, 2011). 5Institute of Geochemistry and Petrology, ETH Zürich, Zürich, Switzerland Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Kawagucci et al. Progress in Earth and Planetary Science (2018) 5:74 Page 2 of 20 Forearc serpentinite mud volcanoes are formed by exten- geological and biogeochemical processes associated with sive serpentinization of the mantle, triggered by the injection subduction (Shipboard Sci. Party 2002). The serpentinite of slab-derived fluids into the overlying mantle wedge in a mud porewater, with the exception of taken near the sea- subduction setting. These serpentinization products ascend floor, exhibited a highly basic pH of 12.5 and had a com- to form the largest mud volcanoes on Earth (Fryer, 2011). position rich in dissolved sulfate and carbonate. The Deep-sea serpentinite mud volcanoes are outstanding set- alkaline porewater chemistry is indicative of slab-derived tings, compared to other serpentinization-associated geofluid fluids that had reacted with mantle rocks when ascending systems (Table 1), for studying abiotic organic syntheses (i.e., through the overlying plate (Fig. 1b), but few studies have present-days’ chemical evolution) and for finding the bound- investigated the origin of the abundant sulfate in the deep ary between microbiologically habitable and uninhabitable (Mottl et al. 2003, 2004; Hulme et al. 2010). In addition, zones (i.e., limit of life and the biosphere) in natural environ- porewater samples from shallow zones ~ 20 mbsf exhibited ments of the Earth. For example, the impermeable mud vol- sulfate depletion and high alkalinity. The chemistry of these cano allows for decreased entrainment of photosynthesis- shallow fluids has been interpreted to reflect the oxidation derived organic matter and seawater-derived microbial of CH4 (and H2) derived from the deep by a shallow, sub- populations into the internal part of the volcano where seafloor microbial community including sulfate-reducers pristine mud and fluid ascend from the deep. As such, if we (Mottl et al. 2003; Aoyama et al. 2018). However, the gas can obtain these pristine fluids contributing to the formation composition of the porewater could not be accurately de- of serpentinite mud volcanoes, then they can be used to ad- termined, since the standard ODP sampling retrieval, hand- dress the unique geochemical and microbiological processes ling, and on-board procedures resulted in a significant loss associated with these volcanoes. of dissolved gases. Microbiological analyses including The South Chamorro Seamount (SCS) is located at the phospholipid analysis (Mottl et al. 2003), cell count, and southern end of a serpentinite mud volcano chain in the cultivation attempts (Takai et al. 2005) were carried out, Mariana forearc, where the Pacific plate subducts beneath but the community structure and potential microbial func- the Philippine Sea plate (Fig. 1a). To date, the SCS is the tions could not be determined. only known location of active blueschist-bearing serpentin- A cased borehole observatory, the Circulation Obviation ite mud volcanism that hosts lush deep-sea chemosynthetic Retrofit Kit (CORK), was deployed during the ODP Leg 195 communities sustained by the serpentinite fluid seepage at Hole 1200C on the SCS (Shipboard Sci. Party 2002; (Fryer and Mottl, 1997). The Ocean Drilling Program Wheat et al. 2008). When the observatory seal was opened (ODP) Leg 195 drilled the summit of the SCS (Site 1200) in on 20 March 2003, 2 years after its installation, 2001 and obtained core samples in order to examine over-pressurized fluids discharged from the CORK outlet Table 1 Deep-sea serpentinization-associated geofluid fields Name of field South Chamorro Seamount (SCS) Shinkai Seep Field Lost City Rainbow Location 13°47 ′N–146°00′E 11°40′N–140°03′E 30°07′N–42°07′W 36°14′N–33°54′W Tectonics Forearc seamount chain Non-accretionary trench slope Oceanic core complex, MAR Non-transform offset, MAR Driving force Serpentine mud volcanism (Hydrothermalism?) Hydrothermalism Hydrothermalism Fluid source Slab-derived fluid Seawater penetration Seawater penetration Seawater penetration Approach ODP, CORK, Dive Dive Dive, IODP Dive Depth (m) 2960 5800 700–800 2300 T max 2 – > 91 365 pH 12.5 (Brucite chimney) 10.7 2.8 H2 (mM) 0.01 (possibly < 40) – 15 16 CH4 (mM) 37 (possibly < 65) – 2 2.5 Note The strongest alkaline Active alkaline fluid seeping Hot alkaline fluid discharging Black smoker acidic fluid deep-sea geofluid from brucite chimney from tall carbonate chimney, discharging from sulfide system so far identified. surrounded by macrofauna only identified on the modern chimney with abundant See text for details. colony. seafloor at this moment. H2. Similar vent sites have already been found on the modrern seafloor. Ref See text Ohara et al. 2012 Kelley et al. 2001 Charlou et al. 2002 Stern et al. 2014 Früh-Green et al. 2003 Okumura et al., 2016a Proskurowski et al. 2008 Onishi et al. 2018 Kawagucci et al. Progress in Earth and Planetary Science (2018) 5:74 Page 3 of 20 b a Fig. 1 Topography and schematic illustration of the Mariana forearc and the SCS. a Seafloor topography of the Mariana forearc region. b Schematic illustration of the ODP Hole 1200C focused in the present study pipe (this fluid is referred to as the ‘CORK fluid’ hereafter) at The present study presents results from four subse- a flow rate of ~ 0.2 L/s. The endmember