When mantle rocks come into contact with seawater, the resulting reaction, called serpentinization, transforms the rock and generates methane, hydrogen, and small organic molecules. These organic compounds should be a buffet for microbes, but serpentinization fluids also are highly alkaline and lack oxygen, metals, and other molecules that microbes use to “breath.” The details of how microbial communities colonize these challenging environments are still unknown.
To better understand the geological processes leading to serpentinization and how microbes survive in these systems, DCO members Gretchen Früh-Green (ETH-Zurich, Switzerland) and Beth Orcutt (Bigelow Laboratory for Ocean Sciences, USA) led International Ocean Discovery Program (IODP) Expedition 357 in late 2015. An international team of researchers drilled holes across an actively serpentinizing site in the North Atlantic Ocean called the Atlantis Massif. This dome of mantle material ascended from about four kilometers deep, through a fault zone in the seafloor. The researchers describe the expedition in a new paper in Lithos .
“We were trying to see if it were possible for microbes to live in a subsurface environment using the components generated by serpentinization,” said Früh-Green. “The process might take place on other planets, like Mars, so one of our goals is trying to understand how to create life with rock and water.”
During the expedition, researchers employed several new drilling and sampling technologies. Instead of IODP’s standard shipboard drilling apparatus, the researchers alternated between two seabed drills that sit on the seafloor and are operated remotely. They drilled cores from 17 holes at nine sites, reaching a maximum depth of about 16 meters beneath the seafloor. The seabed drills also enabled them to plug some of the drilling holes. The recent Return to Lost City expedition revisited some of these holes to collect fluids from deep within the boreholes, without contamination from seawater.
A sensor package and water sampling system was attached to the drills that continuously measured several environmental conditions such as temperature and pH, as well as dissolved methane and hydrogen. They detected these gases within many of the boreholes as they drilled. “The fact that we can see methane and hydrogen signals almost everywhere indicates that serpentinization is a major process that is still going on in the Atlantis Massif and it’s contributing to a flux of methane and hydrogen into the water column,” said Früh-Green.
The researchers also used Niskin bottles attached to the drills to collect drilling fluids, which were primarily seawater, but also had traces of fluids from deep in the massif. These fluids can give hints to the geochemical processes occurring within each drill site.
The cores collected from the expedition show that the area is surprisingly heterogeneous. It contains a mix of rocks, including serpentinized mantle rocks that have been intruded by magma during seafloor spreading, and brought up to the surface through faults. By analyzing the minerals in the cores, the researchers could piece together the magmatic and structural processes that formed this mix of rock types.
The researchers also are interested in how microbial communities adapt to local variations in rock type, and collected samples across the massif. Through the use of added synthetic tracers, they could check that the samples were not contaminated by seawater. With DCO funding, they brought the samples to Yuki Morono and Fumio Inagaki (both at Japan Agency for Marine-Earth Science and Technology, Japan), who have developed methods that count cells in low-biomass materials in an ultra-clean lab.
“The cell counts are really low,” said Orcutt. Cells occurred in the range of 10 to 100 or sometimes even 1000 cells in a volume of sample about the size of a sugar cube. For comparison, other expeditions that drilled into subseafloor basalt yielded cell densities of about 1000 to 10000 cells in the same volume. “What is limiting life in these subsurface serpentinizing rocks? We don’t know the answer to that yet. That’s what everyone is still working on,” said Orcutt.
The science team has already begun analyzing the small amounts of biomass they collected using sequencing techniques to investigate which microbes live in these systems and how they’re surviving. By linking the microbiological and geochemical processes occurring in the massif, the team can come closer to understanding the role of serpentinization in the global carbon cycle. “We hope to really bring everything together now,” said Früh-Green.
Additional DCO researchers on the project include Marvin Lilley (University of Washington, USA), Javier Escartin, Mathilde Cannat, Bénédicte Ménez (all at Institut de Physique du Globe de Paris, France), Esther Schwarzenbach (Freie Universität Berlin, Germany), Morgan Williams (CSIRO Mineral Resources, Australia), Sally Morgan (University of Leicester, UK), Susan Lang (University of South Carolina, USA), Matthew Schrenk (Michigan State University, USA), Marianne Quéméneur (Aix-Marseille University, France), and William Brazelton and Katrina Twing (both at University of Utah, USA).
Main image: Researchers drilled cores from nine sites across the Atlantis Massif, a dome of mantle material dragged onto the seafloor near the Mid-Atlantic Ridge. Credit: Image courtesy of ECORD/IODP Expedition 357