The Mission and Instruments

The previously mentioned hydrothermally-derived metals are what the company funding this cruise, Nautilus Minerals, is after. We are mapping and sampling hydrothermal vents in the northeastern portion of the Lau basin primarily to determine good locations for future seafloor mining operations. There are three main techniques used to study these vents: AUV mapping, camera tows, and dredging.

AUV Mapping

AUV stands for autonomous underwater vehicle. Autonomous means that it is not manned and is not remotely controlled (which would be called a ROV, or remote operated vehicle). They basically program the sub with a preset area to survey, send it down, and it does all of the mapping itself. The AUV uses sonar to map the seafloor, and because it is so close to the seafloor, it can map at a very high resolution and sea feature on the scale of a few cm. Sonar basically just means bouncing sound waves off the seafloor and measuring the time it takes for the sound waves to return, which is converted to distances, and processed into a map of the topography of the seafloor. The AUV mapping is the primary operation that will be used to map the hydrothermal vents. The AUV is owned by a German company called GeoMar, who have a number of their employees on the ship to operate and maintain it.

The AUV on the dock, before being loaded onto the ship.

Camera Tows

Another method of studying these vents is using a camera that is mounted inside a metal frame and towed from a cable attached to the ship, cleverly named the TowCam. The camera is programmed to take photos every few seconds, giving us images of the seafloor which can be correlated with the mapping from the AUV to actually see what type of material the AUV is mapping.

The TowCam system onboard and ready to be deployed.

Dredging

This is actually a very old technique for collecting seafloor samples, which hasn’t really changed much over the last ~50 years or so. The dredge is basically a rectangular metal frame with teeth around the outside attached to a chain bag. It is attached to the ship with a long cable and basically just dragged along the seafloor to rip up and collect rock samples. It’s probably the most crude and primitive sampling technique, but it has the advantage of typically giving us lots of rocks, sometimes more than we really need. One problem with it is that you can’t know exactly where the sample came from, but it gives you a general idea of the composition of rocks in the dredging area. This is the part of the cruise that I am involved with. Regan and I are assisting Ken Rubin, one of the geochemists from UH, in collecting and analyzing the samples. We also help with deploying and retrieving the dredge from the back deck of the ship, so we get to do some work out on the deck as well.

The dredge with the contents of the fourth dredge haul

Since this is the part I’m involved in, I can give some more details on what we do with the samples after dredging. We first sort through and divide up the rocks into different types, so far we have only found two types: boninites (a gray to black lava rock that looks very similar to basalt), and rhyolite pumice (very light gray volcanic rock with an extremely large number of gas bubbles, making it very light weight so that it actually can float in water). Then, the geochemist will pick ~5 samples that seem to be the best for further analysis. We set those aside, give them numbers and do a brief description of them for the logs. Then, we try to chip the fresh glass off of the rocks, which is typically the outer part of the rock that cooled quickly and therefore has very few crystals in it. These glass chips are cleaned and saved for a microprobe analysis, which is not something that I know a whole lot about since I am not a geochemist. Then we cut the rock on a rock saw to see a fresh interior surface, since the outside of the rock is often covered in a manganese coating. Further cutting is done to basically produce a ~domino-sized piece that will be used to make a thin section, so that the rock can be viewed under a microscope and more precisely identified. Some chunks are saved to be crushed for a whole rock chemical composition analysis and so we can date the rocks once they are brought back to the lab at UH. I won’t be involved in any of the detailed chemical analyses, which I am perfectly happy with. I prefer to work at much larger scales, and geochemistry is probably my least favorite branch of geology. I’m glad there are people who enjoy doing it though, because it is extremely important for understanding things like the melting history of the rock, the composition of the mantle source that it came from, and the processes involved in creating the rock, among other things. Eventually, we will see samples from the hydrothermal vents that will get the Nautilus people excited, but for now we have only seen volcanic rocks.

By request, some more details on what geochemistry tells us and why it is important. (Sorry mom, I think your comment may have been deleted when I edited this post to add a picture and another section.) This is a very complex topic, but I'll try to give some examples of things that rocks can tell us. At a basic level, the chemistry tells us whether it's an igneous rock (solidified magma, either on the surface (volcanic) or within the crust (plutonic)), a sedimentary rock, or a metamorphic rock, but it gets MUCH more detailed than that. The rocks basically tell a story about the history of where they formed, what processes formed them, and what they have experienced since they formed. In igneous rocks, the composition and texture of the rock tell us whether the rock formed at a spreading center, an arc volcano, a stratovolcano like Mt. St. Helens, or in a huge frozen magma chamber like the granitic rocks of the Sierras. If there is a high water content, it usually means that the rock formed in a subduction zone, where the water from the subducting slab is added to the mantle. If there are lots of crystals, that means it sat around in a magma chamber within the crust for a long period of time before being erupted. If it is all crystals, it is likely a plutonic rock which cooled entirely within the crust and was never erupted onto the surface. If it has no crystals at all, that means it cooled very quickly after being erupted and did not spend much time in a magma chamber (the classic example being obsidian). If it has lots of vesicles (bubbles), that means that there was a lot of gas within the magma, and if there are very few bubbles it means either the gas escaped or there was not much gas in the magma to begin with. Certain trace elements can indicate whether a rock formed from melted sediment, continental crust, oceanic crust, or normal mantle, and whether there was some influence from a different process, like subduction or maybe a nearby hot spot. Ratios of radioactive isotopes typically don't change during the melting process and give you a more direct idea of the local composition of the mantle, which can help distinguish rocks that otherwise may be very similar. Isotopes are also used to date the rocks. In my thesis work, I used the ratio of two lead isotopes to determine how the influence of the subducting slab changed along the spreading centers that I was studying, which in turn gave insight into the structure of the mantle below and how the water-rich melt is distributed in the mantle. Metamorphic rocks were originally sedimentary or volcanic, but have been exposed to heat and/or pressure, causing them to change composition and texture, but not enough to melt them entirely. When you see a certain mineral in a metamorphic rock, it not only tells you about the composition of the original rock, but also how much pressure and heat that the rock was exposed to. Some minerals only form during weathering processes or when the rock is exposed to water, so they can tell you how long the rock has been exposed to the elements and what has happened to it since being exposed. In sedimentary rocks, which I probably know the least about, the chemistry tells you mostly about the composition of the rock that it was derived from and what the rock has been exposed to after being formed. That's a very basic introduction, let me know if there are more specific questions, although there is only so much I can answer in a blog.

Sonar Mapping

The Kilo Moana, similar to nearly every scientific research vessel, is equipped with a multibeam sonar system that pretty much constantly collects bathymetry and sidescan data. Multibeam just means that instead of a single vertical beam (which is what early vessels were equipped with), there are a few hundred beams that cover a wide swath of seafloor, typically ~5-10 km wide, increasing in width with seafloor depth. To collect bathymetry data, the device simply measures the return time of the sonar signal and converts this to distance, which gives a depth measurement for each beam and allows us to see the topography of the seafloor. Sidescan data is also collected with the same system, and instead of measuring the return time, it measures the intensity of the reflected sound beams, also called the backscatter. This type of data is a little more complex to interpret, but it is mainly affected by two things: the type of material, and the relief of the seafloor. Hard materials, such as a fresh lava flow, reflect most of the sound and produce a strong return, while less dense materials such as sediment or lava flows with a rough, broken up surface produce a weaker return. Also, because the beams are scanning toward the sides, the more perpendicular the seafloor is compared to the angle of the beam, the higher the return. If the seafloor is sloped away from the beam, you get almost no sound reflected back to the device. The resulting images basically look like a black and white image of the seafloor and can be used to identify areas of recent volcanic activity and features like fault scarps. Faults typically show up well in this data for two reasons: 1) most faults in the environment we are looking at are normal faults, where one block drops down relative to the other, exposing the hard rock under the sedimented seafloor, which produces a stronger return than the surrounding sediment, and 2) if the fault is oriented so that the scarp faces the instrument, you get a strong return off the face of the scarp.