Tectonic plate movements wrinkle and stretch Earth’s crust, pushing up mountain ranges in some regions and spreading the crust in other regions until it splits. Along mid-ocean ridges, slow spreading under the right conditions can expose rocks from the lower crust and upper mantle. Seawater percolating through cracks and fractures in exposed mantle rocks oxidizes the minerals in these rocks, releasing ions and chemical compounds into the nearby seawater.
These rock reactions excite scientists because they may produce nutrients for subsurface microbial life in the depths of the ocean where sunlight doesn’t penetrate. The conditions these reactions create may be analogous to conditions early in Earth’s history or those that exist on other planets.
In late 2015, Expedition 357 of the International Ocean Discovery Program (IODP) set out to collect cores from across the Atlantis Massif, a prominent, nearly 4,000-meter-high underwater mountain on the Mid-Atlantic Ridge (Figure 1). The Atlantis Massif is one of the best-studied oceanic core complexes. Its exposed mantle-derived rocks are reacting with seawater and producing methane- and hydrogen-rich, alkaline waters [Karson et al., 2006; Proskurowski et al., 2006]. Along the massif, where these warm alkaline waters exit through fissures in the seafloor, impressive white towers of carbonate rise up tens of meters, creating a distinctive environment that researchers have dubbed the “Lost City” [Kelley et al., 2005].
IODP Expedition 357 aimed to gain a deeper understanding of the interlinked magmatic, tectonic, and mineral-forming processes that led to the exposure and alteration of mantle rocks. The expedition also investigated the types of subsurface life that make their homes in this extreme, alkaline, carbon dioxide–poor environment.
Pivotally, our team of researchers found organisms within mantle rocks in core samples we collected from the seafloor under the North Atlantic Ocean, as recently reported by Fruh-Green et al. . Identifying the organisms in this ecosystem is a work in progress, but this biosphere may be larger and more diverse than we had previously understood.
Rock Reactions Create an Extreme Habitat for Life
The formation of slow-spreading mid-ocean ridges is influenced by irregularly occurring magmatic activity and asymmetric crustal stretching along major faults. In some areas with this type of spreading, the upper crustal layers can split apart, and lower crustal and upper mantle rocks can be pulled up to the surface in the gap, producing a domed structure referred to as a core complex.
Seawater can percolate through cracks and fractures in mantle rocks exposed in the core complex, causing iron- and magnesium-bearing minerals to be oxidized and hydrated in a process known as serpentinization, named after the serpentine minerals produced in the process (Figure 2). Serpentinization is a fundamental process that drastically changes the geophysical properties of the oceanic lithosphere and can produce alkaline fluids (pH 9–12) with high concentrations of hydrogen, methane, and formate ions (HCO2–), which could support microbial life.
Many Firsts for Ocean Drilling
Using seabed drills for the first time in the history of the international ocean drilling program, we cored 17 holes across nine sites. This was also the first time that seabed drills, usually a soft-sediment tool, were used to penetrate hard rocks on the seafloor. We recovered as much as 75% (in some cores) of shallow mantle sequences from the top 17 meters of a core complex detachment fault zone—an unprecedented level of recovery for hard-rock drilling [Früh-Green et al., 2017, 2018]. Also new to the drilling program was a suite of sensors mounted on the seabed drills that measured dissolved methane and hydrogen, temperature, pH, and oxidation reduction potential during coring operations, providing real-time information about fluids in the basement.
We collected bottom water samples before and after drilling and analyzed them for dissolved hydrogen and methane levels to evaluate the influence of serpentinization in the region. Finally, a high-resolution multibeam bathymetric survey provided a picture of the surface of the detachment fault at the top of the Atlantis Massif, the Mid-Atlantic Ridge axis to the east, and the Atlantis Fracture Zone to the south. The survey revealed small-scale patterns, such as corrugations and striations, late-stage faults, and mass wasting (Figure 1).
Documenting a Heterogeneous Lithosphere
The cores we recovered preserved a variety of contacts between various types of rock, deformation features, and alteration characteristics (Figure 3). The cores highlight a highly variable distribution of magma-derived rocks with a range of alteration styles and extents of deformation. The cores also show that the rocks in this region have a high degree of serpentinization and alteration due to interaction with seawater or other fluids, which points to silica mobility and channeled fluid flow within the detachment fault zone.
We compared the coarse-grained, iron- and magnesium-rich gabbroic rocks recovered during this expedition with similar rocks recovered from the central dome of the Atlantis Massif during IODP Expeditions 304/305 [Ildefonse et al., 2007]. The distribution of rock types between the southern ridge and the central dome of the massif reflects variations in the distribution of magmatic melt within the oceanic core complex. Overall, comparison of the recovered rock types, crosscutting alteration and deformation relationships, and interpretation of the new bathymetric map provide a record of the history of this region: early gabbroic intrusion in the shallow mantle, progressive seawater infiltration with multiphase serpentinization and silica mobilization along the detachment fault zone [Rouméjon et al., 2018], injection of dolerite dikes, and recent basalt volcanism.
In Situ Records of Active Processes
The complementary water sampling program and data from the custom sensor package on the seabed drills confirmed that seawater is still circulating through the mantle-derived rocks at the Atlantis Massif and these rocks are still being altered to form serpentinite. The drill-mounted sensors registered increases in releases of methane and hydrogen that correlated with decreases in oxidation reduction potential at most sites [Früh-Green et al., 2017].
This suggests that the drills penetrated horizons that released serpentinization-derived fluids and volatiles into the drilling fluid. We found elevated hydrogen and methane concentrations in most water samples, particularly at sites near Lost City (Figure 4). The elevated concentrations of volatile species reflect active serpentinization processes occurring within the subseafloor and the release of hydrogen and methane into bottom seawater.
We also designed and installed novel borehole plug systems to enable future sampling of the subsurface fluids and provide long-term information about fluid compositions and microbial processes. A U.S.-led research expedition visited these plugged holes in September 2018 with the remotely operated vehicle Jason (funded by the National Science Foundation) to further investigate the serpentinization and microbiological processes operating in this system.
Rock Reactions Fueling a Deep Biosphere
The gases that we found could support a subseafloor biosphere fueled by chemolithotrophic reactions; that is, the chemical alteration of the rocks could provide nutrients for living organisms. We sampled roughly 15% of the recovered core for microbiological investigation, and we were able to confirm the presence of life in this subseafloor environment through cell counts, which revealed cell densities on the order of 10–1,000 cells per cubic centimeter of rock [Früh-Green et al., 2018].
We’re currently evaluating the diversity of life and testing the range of metabolisms supported by reactions between fluid and rock in this subseafloor biosphere using a variety of laboratory incubations and analyses. Considering the extent of ultramafic environments on the seafloor and the longevity of seawater circulation during serpentinization, the ultramafic-hosted biosphere may be greater than previously appreciated, and these samples provide a first glimpse of the magnitude and diversity of this ecosystem.
Overall, IODP Expedition 357 proved a successful trial of using seabed drills for ocean crust drilling and provided unprecedented samples from an oceanic core complex to study crustal accretion, uplift processes, serpentinization, and the deep biosphere.
These results were made possible by the IODP Expedition 357 project managers (Carol Cotterill and Sophie Green), the MeBo and British Geological Survey seabed drill teams, and the entire science party (N. Akizawa, G. Bayrakci, J.-H. Behrmann, E. Herrero-Bervera, C. Boschi, W. Brazelton, M. Cannat, K. G. Dunkel, J. Escartin, M. Harris, K. Hesse, B. E. John, S. Q. Lang, M. D. Lilley, H.-Q. Liu, L. Mayhew, A. McCaig, B. Ménez, S. Morgan, Y. Morono, M. Quéméneur, A. S. Ratnayake, S. Rouméjon, M. O. Schrenk, E. M. Schwarzenbach, K. Twing, D. Weis, S. A. Whattam, M. Williams, and R. Zhao). Figure 1 was prepared postcruise by Javier Escartin, Marvin Lilley evaluated the hydrogen and methane data, and cell count data were provided by Yuki Morono.