International Ocean Discovery Program
Expedition 399 Preliminary Report
Building Blocks of Life, Atlantis Massif1
Andrew McCaig, Susan Q. Lang, Peter Blum, and the Expedition 399 Scientists
1 McCaig, A., Lang, S.Q., Blum, P., and the Expedition 399 Scientists, 2024. Expedition 399 Preliminary Report: Building Blocks of Life, Atlantis Massif. International Ocean Discovery Program. https://doi.org/10.14379/iodp.pr.399.2024
Abstract
International Ocean Discovery Program (IODP) Expedition 399 collected new cores from the Atlantis Massif (30°N; Mid-Atlantic Ridge), an oceanic core complex that hosts the Lost City hydrothermal field (LCHF). Studies of the Atlantis Massif and the LCHF have transformed our understanding of tectonic, magmatic, hydrothermal, and microbial processes at slow-spreading ridges. 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 reached 1415 meters below seafloor (mbsf) through a series of primitive gabbroic rocks. A series of 17 shallow (<16.4 mbsf) holes were also drilled at 9 sites across the south wall of the massif during Expedition 357, recovering heterogeneous rock types including hydrothermally altered peridotites, gabbroic, and basaltic rocks. The hydrologic regime differs between the two locations, with a low permeability conductive regime in Hole U1309D and a high (and possibly deep-reaching) permeability regime along the southern wall.
Expedition 399 targeted Hole U1309D and the southern wall area to collect new data on ancient processes during deformation and alteration of detachment fault rocks. The recovered rocks and fluids are providing new insights into past and ongoing water-rock interactions, processes of mantle partial melting and gabbro emplacement, deformation over a range of temperatures, abiotic organic synthesis reactions, and the extent and diversity of life in the subseafloor in an actively serpentinizing system. We sampled fluids and measured temperature in Hole U1309D before deepening it to 1498 mbsf. The thermal structure was very similar to that measured during Expedition 340T, and lithologies were comparable to those found previously in Hole U1309D. A significant zone of cataclasis and alteration was found at 1451–1474 mbsf. A new Hole U1601C (proposed Site AMDH-02A) was drilled on the southern ridge close to Expedition 357 Hole M0069A, where both deformed and undeformed serpentinites had previously been recovered. Rapid drilling rates achieved a total depth of 1267.8 mbsf through predominantly ultramafic (68%) and gabbroic (32%) rocks, far surpassing the previous drilling record in a peridotite-dominated system of 201 m. Recovery was excellent overall (71%) but particularly high in peridotite-dominated sections where recovery regularly exceeded 90%. The recovery of sizable sections of largely intact material will provide robust constraints on the architecture and composition of the oceanic mantle lithosphere. The deepest portions of the newly drilled borehole may be beyond the known limits of life, providing the means to assess the role of biological activity across the transition from a biotic to an abiotic regime.
Borehole fluids from both holes were collected using both the Kuster Flow-Through Sampler and the new Multi-Temperature Fluid Sampler. Wireline logging in Hole U1601C provided information on downhole density and resistivity, imaged structural features, and documented fracture orientations. A reentry system was installed in Hole U1601C, and both it and Hole U1309D were left open for future deep drilling, fluid sampling, and potential borehole observatories.
Plain language summary
The Earth’s mantle is a thick (1802 miles; 2900 km) layer of dense rock that makes up most of the planet’s mass. It has long been a goal to drill through the Earth’s crust and into the upper mantle, but so far this has not been achieved, even in the oceans where the crust is relatively thin (6 km). In some places, mantle and lower crustal rocks have been brought to the seafloor by faulting associated with plate tectonics. One such place is the Atlantis Massif, an underwater mountain in the middle of the Atlantic Ocean. Mantle rocks are rich in the mineral olivine, and when they are altered by seawater they produce the mineral serpentine. Hydrogen is produced as a by-product of the reaction and can further fuel the generation of compounds such as methane, short-chain hydrocarbons, and organic acids in the absence of life. Hence, we can call these compounds “the building blocks of life” because when life began on Earth in the distant past, they may have been the precursors to more complex compounds, such as DNA, necessary for life to exist. At the present day, this suite of compounds may be feeding ancient forms of microbial life far below the seafloor.
The goals of Expedition 399 were to recover rocks and fluids to provide new insights into how underwater mountains such as the Atlantis Massif form, to document the nonbiological reactions between water and rocks that may represent ancient systems that preceded life on Earth, and to assess the extent of life in the subseafloor. We deepened preexisting Integrated Ocean Drilling Program Hole U1309D by 83 m to reach a new depth of 1498 meters below seafloor and sampled fluids to understand the geochemical regime. The recovered rocks were gabbroic, typical of the lower crust, and temperatures near the bottom of the borehole were ~140°C, similar to previous measurements. Drilling revealed a 23 m zone of fracturing and alteration.
A new Hole U1601C was also drilled on the southern wall of the massif and achieved a total depth of 1267.8 meters below seafloor with excellent overall recovery of 71%. It primarily consists of a long section of mantle rocks with intrusions of gabbro that cooled from injections of magma from depth. Previous drilling into this type of rock has not been nearly as successful and has only reached a maximum depth of 201 m. The recovery of sizable sections of largely intact mantle rocks will provide robust constraints on the structure and composition of the oceanic mantle lithosphere and the reactions that produce hydrogen. Downhole logging measurements were carried out to provide a continuous record of temperature, density, porosity, seismic velocity, and distributions of fractures. Water samples were collected from several depths within the borehole to determine the chemical and biological characteristics of fluids. Both Holes U1601C and U1309D were left open for future fluid sampling and potential borehole observatories.