Q&A with OASES Chief Scientist Chris German
Beginning January 6, 2012, an international, interdisciplinary team of scientists will be spending nearly a month at sea aboard the R/V Atlantis and using the deep-diving, remotely operated vehicle (ROV) Jason to explore and sample newly discovered hydrothermal fields on the Mid-Cayman Rise—Earth’s deepest and one of its most slowly-spreading mid-ocean ridges. Chris German, Chief Scientist on the OASES cruise, sat down for a short question-and-answer session about the geologic, geochemical, and biological questions he and others will be addressing through the three-week cruise
Q: Why study the Mid Cayman Rise?
In a nutshell, it displays perhaps the broadest range of mid-ocean ridge geologic processes all active in the same place.
The Mid-Cayman Rise is part of the nearly 60,000 kilometer long mid-ocean ridge that encircles the planet. Places like the Mid-Cayman Rise are an essential part of efforts to understand the way plate tectonics works and the way Earth’s surface evolves over time. This is where fresh ocean crust is created, just as deep ocean trench subduction zones are where cold, old ocean crust is swallowed back into Earth’s interior.
For this reason, mid-ocean ridges are among the most seismically and volcanically active parts of the planet. In the past decade, however, geologists have realized that volcanic eruptions are not the only mechanism by which tectonic plates spread apart at mid-ocean ridges. Along the slower-spreading sections of the global ridge system – including the entire length of the Mid-Atlantic Ridge that is actively driving Europe and Africa apart from North and South America—approximately half of all the spreading along this “seam” is now recognized to be taking place without fresh magma generation.
Instead, the two plates are simply spreading apart along shallow-dipping faults that permit one plate to slide out from under the other, much like the way a long row of books tip over if you take away the bookend that is holding them upright. There appears to be a lot of this non-magmatic spreading going on at the Mid-Cayman Rise, but, intriguingly, we have also found lava patterns there that look just like fresh lavas flowing in Volcanoes National Park on the Big Island of Hawaii. The implications for geology and plate tectonics have yet to be fully understood.
Q. How does this connect to hydrothermal activity on the Mid Cayman Rise?
Hydrothermal activity occurs when cold seawater percolates down into young, relatively hot seafloor at mid-ocean ridges. This water interacts with the rocks below the seafloor and undergoes a series of chemical changes before it rises back up as a hot, chemically altered fluid.
The diverse geology at the Mid Cayman Rise leads us to believe we might find a diverse range of vent-fluids there. We also expect there to be differences caused by the pressures at which venting might occur—for example we already know of sites at about 2300 and 5000 meters depth, much deeper than many other vent sites.
The rock types present at the Mid Cayman Rise are quite diverse and may also affect the chemical reactions at the site—we are confident that one site we will visit is hosted in basaltic lava but that there are other sites, including one that is already precisely located, that appear to be rooted in ultramafic rocks that have been drawn up to the seafloor from deep within the Earth.
The eruption temperature of the fluids also plays an important role in the makeup of hydrothermal vent fluids, and there is a very strong possibility that we will set a world record by finding vents hotter than 409°C; in fact, theory predicts we may find fluids hotter than 480°C.
Q. What about the things that really makes the general public interested in hydrothermal vents—the animals that live there?
For the large animals at vents, the Mid-Cayman Rise may act as a sort of deep-ocean crossroads. On the Mid-Atlantic Ridge, due East (but distant) from this site, hydrothermal vents are dominated by blind shrimp that survive thanks to an evolutionary dependence on microbes that exploit chemical energy from hydrothermal vents to convert carbon that is present as dissolved CO2 in seawater into organic carbon. This sustains both the bacteria and the large host animals that harbor them.
At cold seeps in the adjacent Gulf of Mexico that have been attracting so much attention in the past year or two (including my own), we have found tubeworms that do something very similar. They also host and apparently nourish microbes that derive energy from the hydrocarbons that seep out of the sediments, permitting both the microbes and the host tubeworms to thrive.
From what we know so far, at least one vent site at the Cayman Rise can play host to organisms of both types—shrimp that look similar (but not identical) to shrimp from Mid-Atlantic Ridge vent species and tubeworms that bear some resemblance to those from the Gulf of Mexico. The Gulf is only 3 to 4 days by ship, but organisms like this, that are highly adapted to a very specialized environment in the deep ocean would almost certainly take a much longer and perhaps a much more circuitous path. We don’t how these organisms got here—or got away from here. That’s the kind of thing we are going to be investigating by diving with the ROV and collecting samples of the animals.
There’s also something really weird that can theoretically happen if you find a vent site hosted in the kinds of ultramafic rocks that are lifted all the way up to the seafloor from deep within Earth’s mantle along slow and ultraslow-spreading centers like the Mid-Cayman Rise. If the fluids are hot enough, say around 150 to 200°C, then any seawater-like fluid passing through such a system is predicted to have all its dissolved CO2 converted into organic carbon molecules. These are the kinds of compounds that astrobiologists might refer to as “prebiotic chemistry.” The important issue here is that such reactions could potentially take place in the complete absence of life, driven by geologic processes alone.
That by itself is interesting, but what’s really intriguing is that this is the kind of hydrothermal circulation that may well have been common throughout the first half of Earth’s entire history, until about 2 to 2.5 billion years ago. Just like the Hubble telescope, pointed at the most distant points of the Universe, hydrothermal sites like these may become a sort of lens that allows us to view back into Earth’s distant past and explain how life originated on Earth. We do know, after all, that the most primitive life forms we know of thrive in high-temperature, low-oxygen environments, so submarine vents are certainly a good candidate for that. But our studies may also give us unique insights into how life might recur elsewhere in the solar system, for example on the moons of Jupiter and Saturn, or elsewhere in the galaxy, such as the small, Earth-like planets that recently been discovered by NASA’s Kepler mission.