International Ocean Discovery Program
Expedition 399 Scientific Prospectus

Building Blocks of Life, Atlantis Massif¹

¹McCaig, A., Lang, S.Q., and Blum, P., 2022. Expedition 399 Scientific Prospectus: Building Blocks of Life, Atlantis Massif. International Ocean Discovery Program. https://doi.org/10.14379/iodp.sp.399.2022

Abstract

International Ocean Discovery Program (IODP) Expedition 399 will collect new cores from the Atlantis Massif (30°N; Mid-Atlantic Ridge), an oceanic core complex that has transformed our understanding of tectonic and magmatic processes at slow- and ultraslow-spreading ridges. The exposure of deep mantle rocks leads to serpentinization, with major consequences for the properties of the oceanic lithosphere, heat exchange between the ocean and crust, geochemical cycles, and microbial activity. The Lost City hydrothermal field (LCHF) is situated on its southern wall and vents warm (40°–95°C) alkaline fluids rich in hydrogen, methane, and abiotic organic molecules. The Atlantis Massif was the site of four previous expeditions (Integrated Ocean Drilling Program Expeditions 304, 305, and 340T and IODP Expedition 357) and numerous dredging and submersible expeditions. The deepest IODP hole in young (<2 My) oceanic lithosphere, Hole U1309D, was drilled 5 km north of the LCHF and reaches 1415 meters below seafloor (mbsf) through a primitive series of gabbroic rock. In contrast, during Expedition 357 a series of shallow (<16.4 mbsf) holes were drilled along the south wall of the massif, one within 0.4 km of the LCHF, and serpentinized peridotites were recovered. The hydrologic regime differs between the two locations, with a low permeability conductive regime in Hole U1309D and a high likelihood of deep permeability along the southern wall.

Expedition 399 targets both locations to collect new data on ancient processes during deformation and alteration of detachment fault rocks. Recovered rocks and fluids will provide new insights into ongoing water-rock interactions, abiotic organic synthesis reactions, and the extent and diversity of life in the subseafloor in an actively serpentinizing system. We will deepen Hole U1309D to 2060 mbsf, where temperatures are expected to be ~220°C. The lithology is predicted to transition with depth from primarily gabbroic to more ultramafic material. Predicted temperatures are well above the known limits of life, so detectable hydrogen, methane, and organic molecules can be readily attributed to abiotic processes. A new ~200 m hole will be drilled on the southern ridge close to Expedition 357 Site M0069, where both deformed and undeformed serpentinites were recovered. We aim to recover a complete section through the detachment fault zone and to sample material that reflects the subseafloor biological, geochemical, and alteration processes that occur along the LCHF circulation pathway. Borehole fluids from both holes will be collected using both the Kuster Flow Through Sampler tool and the new Multi-Temperature Fluid Sampler tool. Wireline logging will provide information on downhole density and resistivity, image structural features, and document fracture orientations. A reentry system will be installed at proposed Site AMDH-02A, and Hole U1309D will be left open for future deep drilling, fluid sampling, and potentially borehole observatories.

Plain language summary

Where most of us live, on the continents, the Earth’s crust is normally 30–40 kilometers thick, and the underlying mantle cannot be reached by drilling. The crust is thinner underneath the oceans, typically 6–7 kilometers thick, but still no hole has yet been drilled through normal thickness crust and into the mantle. Seafloor spreading at mid-ocean ridges is normally a magmatic process: the crust is continually formed by eruption and intrusion, leaving a well-layered structure with volcanics underlain by sheeted dikes and then gabbros. However, in some places magmatism cannot keep up with the spreading rate, and large convex-up extensional faults expose mantle rocks and lower crustal gabbros on the seafloor. An underwater dome-shaped mountain formed by this process is called an oceanic core complex, and these locations allow us to directly sample mantle rocks and gabbros by drilling.

The Atlantis Massif is an oceanic core complex at 30°N on the Mid-Atlantic Ridge on the north side of the Atlantis I transform fault, which offsets the ridge by ~60 kilometers. The massif is capped by a corrugated fault zone, and is composed of a large gabbro intrusion into serpentinized mantle rocks to the south. These altered mantle rocks host the Lost City hydrothermal field, which is famous for carbonate chimneys the height of a house, venting alkaline fluids rich in hydrogen and methane. The hydrogen is formed by the reaction between seawater and mantle mineral olivine, and is a powerful source of energy that may have fueled the formation of the first building blocks of life on Earth. Before life could begin, small organic molecules must have formed abiotically. Scientists have suggested that vent fields such as Lost City may be an analog of systems where these prebiotic reactions occurred, leading to the early development of life. Similar systems may be present on “icy worlds” such as Enceladus, which is one of the moons of Saturn, and capable of supporting life.

One of the main aims of International Ocean Discovery Program Expedition 399 is to study the reactions between olivine and seawater that are believed to be actively occurring at depth in the massif today. We will deepen an existing hole (U1309D) that reaches 1415 meters below the seafloor and has a bottom temperature of 140°C. This temperature is above the currently known limit for microbial life, so we can be sure that any hydrogen, methane, or organic molecules produced at greater depths are not due to microbial processes. We hope to reach 2060 meters below the seafloor, where temperatures of ~220°C are predicted. We will sample fluids in the borehole to look at active exchange of chemical components between fluid and rock and production of hydrogen and methane. We will also drill a shallow hole closer to the hydrothermal field that we predict will access the subseafloor environments that reflect the processes leading to this remarkable system.

Other aims of the expedition are to study the processes of formation of the Atlantis Massif, including magmatism, deformation, and high temperature seawater-rock interaction, as well as the microbes living within the rocks and in the borehole waters. These are “extremophile” microbes that can live in highly alkaline environments at temperatures up to 100°C and high pressures. If environments similar to Lost City were the cradle of life, this is the type of life that would have formed.