Cool, alkaline serpentinite formation fluid regime with scarce microbial habitability and possible abiotic synthesis beneath the South Chamorro Seamount

Shinsuke Kawagucci,Jeff S Seewald,Junichi Miyazaki,C Geoff Wheat,Yuki Morono,Ken Takai

doi:10.1186/s40645-018-0232-3

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.

Highlights

Serpentinization of ultramafic rocks that are the major components of the oceanic lithosphere produces highly reductive fluids, in which transformation of inorganic carbon species to organic matter occur (e.g., McCollom 2013)
The fluid chlorinity (503–510 mmol/kg) of Circulation Obviation Retrofit Kit (CORK) fluids was distinguishable from seawater level (540 mmol/kg) and identical to those of the Deep porewater (DPW) (510 ± 5 mmol/kg) (Fig. 3a)
CORK fluids ranged from 12.1 to 12.3 in pH values, which were slightly lower than the highest pH of 12.5 ± 0.1 in the DPW (Fig. 3b)

Summary

Introduction

Serpentinization of ultramafic rocks that are the major components of the oceanic lithosphere produces highly reductive fluids, in which transformation of inorganic carbon species to organic matter occur (e.g., McCollom 2013). Forearc serpentinite mud volcanoes are formed by extensive serpentinization of the mantle, triggered by the injection of slab-derived fluids into the overlying mantle wedge in a subduction setting. These serpentinization products ascend to form the largest mud volcanoes on Earth (Fryer, 2011). Deep-sea serpentinite mud volcanoes are outstanding settings, compared to other serpentinization-associated geofluid systems (Table 1), for studying abiotic organic syntheses (i.e., present-days’ chemical evolution) and for finding the boundary between microbiologically habitable and uninhabitable zones (i.e., limit of life and the biosphere) in natural environments of the Earth.

Results

Discussion

Conclusion