Deep-sea hydrothermal vents were unknown to science until 1977, when oceanographers discovered them on a research expedition along the Eastern Galapágos Spreading Center. These scientists were amazed to see hot fluids spewing from the Earth’s crust through chimney-like features on the seafloor. Perhaps even more astounding were the thriving and unique ecosystems which surrounded these vents despite the lack of sunlight on the seafloor. The scientists excitedly collected clams, mussels, and other specimens. However, not anticipating a need to preserve biological samples, they had to store their samples in vodka! 

Hydrothermal vents are essentially underwater hot springs. Seawater percolates through fissures in the ocean floor where it is heated by hot magma, reaching temperatures as high as 400°C (750°F). As this hot seawater interacts with the ocean’s crust, it strips metals and other elements from the rocks, forming hydrothermal fluid. This fluid is then ejected back into the cold ocean-bottom water where it is instantly cooled, and the minerals precipitate into chimney-like structures. The fluid also contains hydrogen sulfide, which is used by chemosynthetic bacteria instead of sunlight to grow. 

In certain locations, hydrothermal systems also result in the formation of massive sulfide deposits, which may contain critical marine minerals. 

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Dr. Amy Gartman, head of the USGS Global Marine Minerals project, is a principal investigator on the recent expedition, Ultra Fine-Scale Seafloor Mapping. Mendenhall postdocs Dr. Hope Ianiri and Dr. Maria Figueroa conducted shipboard fieldwork along with an international team of scientists including geologists, chemists, and biologists aboard the R/V Falkor (too), operated by the Schmidt Ocean Institute.  

The research objectives of this expedition were multifaceted, though one major goal was ultra high-resolution mapping of the seafloor. Only some of the areas the team visited have been mapped previously, and the resolution of their new maps are orders of magnitude greater, allowing unprecedented detail of hydrothermal features.

Sample collection with SuBastian 

Based on these seafloor maps, the team then selected specific areas for further sample collection using the remotely operated vehicle (ROV) SuBastian. They collected geological and biological samples from hydrothermal environments ranging from high-temperature black smoking chimneys to inactive sulfide spires. By sampling hydrothermal environments spanning such a wide range of activity, they hope to gain insight into how hydrothermal systems change over time. 

Drs. Ianiri and Figueroa specifically were collecting rocks and sediment cores from hydrothermal features and the surrounding areas. Many of the rocks are pieces of hydrothermal chimneys, formed from the precipitation of elements in the hydrothermal fluids, and thus provide insight into the formation of marine minerals at this system.  

In contrast, sediments near the vents can accumulate from the deposition of hydrothermal plumes, collapse of hydrothermal features, or sedimentation through the water column deriving from primary production at the ocean’s surface. These sediments accumulate slowly over time, meaning deeper sediments are older. The scientists collected sediment cores up to 50 centimeters long, allowing them to tease apart these different sedimentation processes over thousands of years.

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Analyzing samples back in the lab 

After collection shipboard, Drs. Ianiri and Figueroa will bring the rocks and sediments back to PCMSC, where they will be further analyzed in the lab for a range of geochemical analyses by themselves and the Marine Minerals team.  

For example, the Marine Minerals team will measure major, minor, and trace elements and characterize the mineral composition of both the rocks and sediments. By looking at the mineralogy down a sediment core, they can begin to understand how hydrothermal processes and mineralization changed over time.

In contrast, Dr. Ianiri will make measurements on the organic carbon stored in the sediments. She will look for biomarkers, or compounds that act as “fingerprints,” for hydrothermal versus open-ocean sources. They will also measure the age of rocks and sediments, so they can learn how far back into the past these different processes occurred. By combining these analyses with other geological, chemical, and biological analyses made by colleagues on the expedition, the researchers will gain a holistic understanding of this hydrothermal system, including how the marine minerals form and the environmental settings and ecosystems where they occur.