April 26, 2015

Greetings! We’ve been transiting to Guadeloupe while we pack up all the equipment, back up all the data, and finalize the reports. Jenny and Mahshid (from the UK group) led the charge on the logo and T-shirt design with great input from the rest of the students and techs.

CaySEIS project logo.
CaySEIS project logo.
CaySEIS project T-shirt design.
CaySEIS project T-shirt design.

I was encouraged to write a few words about the importance of the Cayman research to the broader Earth system. One of the key ways that the solid earth interacts with our oceans and atmosphere is through the cycling of hydrothermal fluids through the crust and upper mantle. By the late 1970’s scientists had worked out that a great deal of the heat flow from the crust and chemical species in seawater were unaccounted for, and that the likely culprit were reactions between basalt and seawater at elevated temperature. Black Smoker hydrothermal vents were thus predicted in advance of their discovery in 1979!

Now, black smokers are just one part of the overall fluid-flow picture, but they are hot and vigorous systems, and thus eject a great deal of particulates (sulfides, oxides, etc.) into the water column, and concentrate other chemical species into hydrothermal minerals in the crust. An impressive example is that all of the magnesium that flows into oceans from the continents should supersaturate the oceans, yet we don’t see towers of magnesian minerals forming on the seafloor. Instead, the hydrothermal system sequesters magnesium into silicate minerals in the crust, and the reactions in turn cause the black smokers to be relatively acidic. Sulfide deposits mined for copper and gold form in a similar way, albeit in a slightly different environment. And if geochemical exchanges don’t excite you, remember that all of this activity provides an environment in which LIFE can thrive! Indeed, some argue that life may have originated in a setting not unlike the seafloor hydrothermal vents, and indeed many of the world’s most primitive microbes thrive there.

After hydrothermal vents were discovered on fast-spreading ridges, there was some speculation that they may not occur on slower ones such as the Mid-Atlantic ridge, but this was soon disproved as workers found very large vent fields with a wide range of behaviors. For example, the “TAG” field sits deep in the Mid-Atlantic Ridge rift axis in an area nearly 5 kilometers on a side with a sulfide mound towering 50 meters above the seafloor and emitting black-smoker fluids in excess of 360°C. In contrast, “the Lost City Field” sits high on top of the Atlantis Massif, as a complex network of towers, some more than 60 meters high, all producing clear vent fluids of temperatures under 100°C. What is amazing about the Lost City is that it shows all the signatures of seawater reacting with the mantle, a process known as “serpentinization.”

So, the larger point is that we now know that there is a diversity of vent types and that the type of spreading center plays a big role in determining what those are. And because portions of the mantle are involved in some vent systems, a wide variety of elemental cycling seems to occur, including potential exchanges of greenhouse gases such as methane and carbon dioxide. Nobody has made a strong case that mantle-alteration in nature has played a large role in the Earth’s climate over time, but I doubt we can rule out the possibility!

Cayman is fascinating because we have now discovered the deep black smoker field, and a vent high “Mt. Dent” that seems to reflect a combination of seawater-basalt and seawater-mantle reactions. And these are occurring on one of the slowest spreading centers in the world. And if the mantle is generally near, if not actually exposed at the surface, there may well be other Lost City Fields here, or at least the same kind of geochemical reactions.

It turns out we can test some of these predictions about different heat sources and mantle serpentinization because each of these processes changes the density and velocity of the crust and upper mantle!

– Nick

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