Surtsey volcano forms the southernmost expression of Vestmannaeyjar, an oceanic archipelago created by basaltic eruptions in the offshore extension of the eastern Icelandic rift zone (Jakobsson et al., 2009). Explosive eruptions, followed by effusive lava flows from 1963–1967, constructed the island from a seafloor depth of about 130 m (Thors and Jakobsson, 1982; Fig. 1a). A cored borehole achieved in 1979 traversed hydrothermally altered basaltic tephra, tuff and intrusions to 181 m below the surface (m b.s.) but did not reach the pre-eruption seafloor (Jakobsson and Moore, 1982, 1986). Borehole temperatures varied from 25 to 141 ∘C in 1980. Investigations of 16S rRNA gene amplicons from fluids sampled in 2009 from the 1979 borehole (SE-01) identified microbial communities dominated by Archaea, primarily thermotolerant Methanobacteria at 172 m.b.s. (54 ∘C) and by thermophilic Archaeoglobus-related sequences at 145 m.b.s. (80 ∘C; Marteinsson et al., 2015). In summer 2017, the International Continental Scientific Drilling Program (ICDP) 5059 expedition, the Surtsey Underwater volcanic System for Thermophiles, Alteration processes and INnovative Concretes (SUSTAIN), recovered nearly 700 m of drill core from the basaltic deposits (Jackson et al., 2015, 2019).

While microbial life has been detected in native subsurface oceanic basalts (e.g. Lysnes et al., 2004; Lever et al., 2013; Jørgensen and Zhao, 2016), little is known about the factors that control habitability and the actual extent of the deep crustal biosphere, in terms of cellular mass and spatial distribution (Baquiran et al., 2016). Even less is known about microbial functions and their potential significance in this environment. Energy sources thought to sustain this biosphere could be provided through both chemical and physical processes occurring when volcanic rocks, which make up the majority of the hydrologically active oceanic crust, interact with seawater (Bach, 2016). Most observations of the deep biosphere hosted in basalt stem from subsurface observatories deployed at submarine drill holes (e.g. Orcutt et al., 2011; Edwards et al., 2012; Baquiran et al., 2016). While these observations have provided fundamental new knowledge, they focus on oceanic basalts several million years in age, and their accessibility is limited by costly oceanic expeditions, in situ technologies and long travel times.

The Surtsey subsurface observatory, by contrast, provides relatively straightforward access to an exceptionally young and pristine environment that records the initiation of subsurface microbial life in basaltic tephra in a broad range of temperatures, 25–125 ∘C, measured in the parallel 1979 (SE-01) borehole in 2017 (Fig. 1b), and varying hydrothermal fluid compositions (Jackson et al., 2019). The island is a protected UNESCO World Heritage Site that contains a unique scientific record of colonization of new land by plants, animals and marine organisms through long-term studies of primary biological succession (Baldursson and Ingadóttir, 2007). The time-lapse drill cores obtained in 1979 and 2017 and their associated fluids provide a first glimpse of subsurface marine microbial life in oceanic basalt. The steep temperature gradients in the young hydrothermal system – including a section that in 1979 exceeded the known temperature limit of life, 121 ∘C (Fig. 1b; Takai et al., 2008) – offer a unique opportunity to assess temperature dependency of microbial activity in the deep biosphere hosted by oceanic basalt.

The 5059-1-C (SE-02b) borehole began with rotary drilling for a steel HWT conductor casing with an outer diameter (OD) of 11.43 cm (4 1/2 in.) that extends to a casing shoe depth of 9.54 m.b.s. (Fig. 2a). The core was then drilled with a HQ3 bit (63.5 mm core diameter) to 191.64 m vertical depth from 18 to 26 August 2017 using seawater as the circulating fluid. To reduce contamination from live microbes, the seawater was passed through a 30 µm cartridge filter to remove fine debris and a WEDECO AQUA 4ALT UV light sterilization system at a flow rate ≤1.58 L s-1 before storage in 1000 L reservoir tanks. During pumping to the drill head the treated seawater was passed through a second UV light sterilization system (Weisenberger et al., 2019). Although great efforts were made to avoid contamination during drilling, a few applications of Florigel attapulgite mud were employed to secure borehole stability. After retrieval of the original HQ casing (88.9 mm OD), downhole logging of the borehole was undertaken by the ICDP Operational Support Group (Jackson et al., 2019; Weisenberger et al., 2019). The T-6061 anodized extruded aluminium NQ (69 mm OD) tubes that form the casing of the observatory were then were lowered to a landing depth of 181.25 m and hung in the borehole (Fig. 3c). A custom shoe was designed with a conical shape to prevent infill of residual tephra and drilling debris during installation (Fig. 2d).

The 50 threaded NQ aluminium pipes that comprise the casing have an OD of 69 mm (2 3/4 in.), a 6.35 mm (0.25 in.) wall thickness and a 57 mm (2 1/4 in.) inner diameter. These occur in three lengths, 3.45, 3.60 and 3.75 m, due to sourcing constraints encountered by the provider (Arconic Energy Systems, 7211 Spring Cypress Road, Spring, Texas 77379, USA). Five of the tubes are perforated (Figs. 2e, 4a). They have a pattern of five 6.35 mm diameter (1/4 in.) holes, spaced equally in 72∘ angular increments, on 29 equally spaced cross sections that extend over 1.21 m (4 ft) centred in the mid-section of each pipe (Fig. 4a). The perforated sections are placed at 37.01–38.63, 62.74–63.96, 106.11–107.33, 135.03–136.25, and 160.48–161.70 m.b.s. at the sites of the incubation experiments (Fig. 1b; Table 1).

The casing is designed to hang freely within the open walls of the SE-02b borehole. It is assembled with a proprietary thread design developed by Arconic Energy Systems, tailored to meet the requirements for the casing size and application loads. The threaded flush joints were wrapped with Teflon^™ tape and wrenched together by hand. The Teflon^™ tape concentrates a small amount of organic polytetrafluoroethylene (PTFE) compound at the joint. Paste lubricants were not used, since they could have been smeared along casing surfaces, potentially contaminating the observatory.

At the top of the borehole, the aluminium casing hanger attaches to the uppermost section of aluminium pipe (Fig. 2b). The hanger has a shoulder that is larger in diameter than the internal landing shoulder on the steel wellhead. An elastomer O-ring centres the aluminium casing hanger in the bore of the steel wellhead. A Teflon^™ shim is installed between the aluminium casing hanger and the steel landing shoulder to prevent corrosion of the aluminium. Polyvinyl chloride (PVC) centralizer tubes are installed between the aluminium casing and the HWT conductor casing to several metres depth also to minimize corrosion (Fig. 2b, c). A custom wellhead flange and hinged cover holds the Vectran^™ rope with the five incubation experiments, which are suspended in the centre of the perforated aluminium casing segments (Fig. 2c). The flange cover is installed 0.21 m above the reference ground level. The hinged cover prevents ingress of environmental contaminants, such as rainwater, and is secured with hex cap screws (Fig. 2c). The wellhead can easily be opened to retrieve the incubators during subsequent observatory experiments or data acquisition retrieval operations.

The Vectran^™ rope is currently equipped with 40 perforated incubation polyether ether ketone chambers or PEEKins (Fig. 3) designed to investigate chemical, mineral and microbial alteration over an expected 2-year period, from September 2017 to summer 2019. Each PEEKin has an outer diameter of 18 mm and an inner diameter of 14 mm. The perforations encourage the flow of in situ fluids and microbes into the chambers. Each PEEKin is attached with PEEK cable ties to an 8 mm diameter Vectran^™ rope hung from the borehole wellhead. The PEEKins are deployed as packages of eight (4×2) to ensure replicates at each of the five perforated segment depths (38, 64, 108, 137 and 162 m.b.s.). A HOBO temperature logger monitors in situ temperature every 30 min at each depth. Artificial and sterile tephra, melted and quenched from Surtsey lava flows and separate packages of olivine crystals with compositions similar to those of Surtsey basalt (Fo 90, 90 % forsterite, Mg2SiO4; 10 % fayalite, Fe2SiO4; Jakobsson and Moore, 1986), are used as substrates in the current PEEKin chambers. The samples and data will be collected from the observatory in summer 2019.

Recharge of the hydrothermal system at Surtsey is thought to occur through the inflow of cool seawater in porous, poorly consolidated tuff within the deeper zones of the submarine deposits; heating occurs as the water percolates upwards through the higher temperature rocks of the geothermal reservoir (Jakobsson and Moore, 1982, 1986; Ólafsson and Jakobsson, 2009). Pronounced deviations in several geophysical logs at 143–150 m.b.s. in the SE-02b borehole confirm this inflow zone, where cooler water of higher salinity enters the borehole (Jackson et al., 2019). Vertical convection could initiate within the subsurface observatory but, based on the small casing diameter and temperature gradients, the flow rate would not strongly influence the alteration of the granules over the 2-year incubation periods.

Analytical investigations will evaluate rates of glass and mineral alteration, identify pioneering microorganisms, community dynamics and stability, and, potentially, provide pristine textural biosignatures of microbe–mineral interactions. The samples will be further analysed in terms of microbial genomics, including (1) community profiling by high throughput sequencing of prokaryotic 16S ribosomal gene amplicons libraries, (2) shotgun sequencing of genomic DNA, and (3) quantification of key functional genes, such as 16S rRNA, mcrA and dsrB. Atomic force microscopy combined with vertical scanning interferometry (AFM–VSI) as well as synchrotron X-ray microdiffraction studies will be performed on the glass and olivine samples retrieved from the observatory.

The Surtsey subsurface observatory and its current experiments provide an unprecedented opportunity to study very young submarine basalt without the need for expensive and risky deep ocean operations. The current iteration of experiments and instruments deployed in the observatory will be retrieved in summer 2019. The observatory will be open to a broad scientific community from summer 2019 onwards, with priority granted to the SUSTAIN team members during the 4 years following the current experiment. The long-term management of the Surtsey subsurface observatory occurs through the commitment of the Surtsey Research Society. The borehole observatory is planned to remain operational and open for research for many decades. Information on access to the observatory and application procedures for future research projects is listed on the webpage of the Surtsey Research Society (http://www.surtsey.is).