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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April 21, 2015

Hello everybody! We are in the middle of our last bit of operations, and so far so good. We have a really special treat today on this blog with Christine Peirce taking the helm to explain the science. Take it away Chris!

– Nick

 Imaging the lower crust and upper mantle – a seismologist’s perspective

Throughout the cruise we have been deploying ocean-bottom seismographs (OBSs) onto the seabed, each recording its own multi-component dataset from three geophone sensors used to measure ground motion in three dimensions, and a hydrophone which measures pressure waves in the water column. These OBSs come from the University of Texas in the USA, University of Durham in the UK and Geomar in Germany.

Why do we record airgun seismic signals using seabed instruments?

There are two approaches to seismically imaging beneath the Earth’s surface using man-made seismic signals. In the first approach – reflection seismics – a multi-sensor streamer is towed behind the vessel which measures the man-made signals that travel near-vertically down into the sub-seabed and reflect from the boundaries between individual rocks layers due to their difference in density. The resulting images are in the two-way (there and back) travel time of the recorded reflections, and give a cross-sectional-like view of the sub-surface as if it was cut vertically through by a knife. Unfortunately, these images contain no information that allows them to be converted into true depth, so we cannot answer the question “How deep is this layer beneath the seabed?” or “How thick are these sediments?

To answer these questions we need to know the speed, or velocity, at which each seismic signal travels through each layer, including the water layer. The water layer is a relatively easy velocity to measure using a sound velocity tool suspended from a wire lowered to near the seabed and back again. The velocities of rock layers are not so easy to measure. However, with these velocities we can convert the measured reflection times into distance much as you would use the speed limits on roads and the distances between two points to work out the time it would take to travel between A and B.

This is where an ocean-bottom seismograph (or 36, which is the maximum we have had deployed along any seismic line during this cruise at any one time) comes in useful and we use the seismic refraction approach. An example type of OBS from the UK national pool is shown below.

UK_OBS_web

This is the primary sub-seabed imaging tool we have been using during the cruise. By synchronizing the OBS’s internal clock to the same clock against which we time the airgun shots, we can measure the time it takes for signals to travel from the airgun array to each OBS on the seabed, and if we know their distance away from the shots, we can work out the speed the signals travel through each sub-surface layer. We use GPS for this purpose as it can equally well provide an accurate time source as it can tell you where you are at any point.

obs_schematic_orig

The figure above shows how the method works and how it can be used in conjunction with reflection surveying “killing two birds with one stone” and making cost-effective use of the expensive ship time that that has been awarded for this project. So, we have also been towing a short hydrophone streamer to record reflections from any sediment layers.

We have been using a G-airgun array to generate the several thousand seismic signals that we have generated so far during the cruise, all of which the OBSs have recorded. In our work area, the crust is somewhere from a few km thick beneath the ocean, to up to 20 km thick or more as we approach the Honduran continental margin. The sea water throughout the work area is also very deep at more than 5000m in many places and so the crust-mantle boundary – or Moho as it is named after the eminent seismologist Mohorovicic – could be just anywhere from a few km below the sea surface at the mid-Cayman Spreading Centre, up to 25 or more beneath the Honduran continental margin. So to image the deepest part of this boundary, we need to propagate seismic signals to more then 25 km below the surface and to at least 75 km laterally to see these signals returning from depth where they have travelled through the mantle, to our instruments located on the seabed.

The photograph below shows the G-airgun array behind the FS Meteor’s stern, with the air bubbles for the sources just breaking the surface by their towing floats.

airgun_array_web

An example G-airgun source refraction data plot is shown below with an outline interpretation added.

example_data_trim_annot_web

Analysis of the OBS data allows us to build a model of the sub-surface in terms of the speed at which the seismic sound signals we generated travel through the rocks to the OBSs on the seabed. An example from an old piece of oceanic crust in the Atlantic is shown below. In the Cayman Trough the oceanic crust is thought to be very different from this, with very little or no gabbro. We will find out if this is true when we analyze the data back in the lab.

crustal_model_example_annot_web

The OBSs also record large global earthquakes travelling through the work area as well as small local earthquakes originating along faults located at the Mid-Cayman Spreading Centre. We have recorded many of these small local events throughout the cruise with our OBSs, during both shooting and non-shooting periods as the examples show below with an earthquake recorded while airgun shooting shown at the top, and an earthquake that occurred during a non-shooting period shown at the bottom.

airgun_shots_equake_web

local_equake_web

– Christine Peirce (Durham, UK)

April 19, 2015

Greetings! It’s been a good few days during which we completed our fifth profile. At greater than 160 OBS deployments and recoveries over the entire cruise, we have set some personal records and provided plenty of fodder for poking fun of Ingo (“you haven’t worn us down…. yet”). Between operations we handle all the data management, and some analysis, but we alsoingochris take some moments off to lie on the “steel beach”, the open deck above the bridge where people read and listen to music. Yours truly did so yesterday and inadvisably fell asleep, so now I am the color of the “lobsters” (“the lobsters” are our term for the orange hydrophones). We also have access to the gym or various outdoor corners of the ship for exercise, and even have a sauna though it hasn’t had much use in this balmy Caribbean weather. Movie nights have become another tradition when we have downtime. So far we’ve done “North by Northwest,” “the Big Lebowski” and “The Matrix.” We also have some guest bloggers signed up and a T-shirt design in process, so come back for updates!

For the profile (number 5) to the east of the spreading center, we just shot a section of densely spaced (2 km as opposed to our usual 5 km) “lobsters” as well as OBS. Cord Papenberg from GEOMAR will use these to try out some inversion techniques that ought to tell us a lot about the lithospheric structure in the older crust that was spread off axis.

Remember how I described the suspenseful planning of our final deployments? As the end of the acquisition program comes closer, we have a pretty good read on how this will play out. You see, in mid-ocean ridge systems we are very concerned about how symmetric the spreading is. Does one side spread faster than another? Does one side have a thicker crust than the other? Some of these questions tie into a question surrounding the style of faulting on the ridge. Draw a line at an angle on a piece of paper. One side will taper outward, and one inward, no? Well, imagine this line is a fault cutting the crust. This can impose asymmetry if the ridge is extending by faulting. We were having this discussion and it became clear that both operationally, in terms of time and resources, and scientifically, we should conduct an OBS profile on the conjugate (the counterpart) to profile 5 found west of the spreading center. This current plan, about to begin, should allow us to test some really important hypotheses about symmetric vs. asymmetric spreading in ultraslow spreading centers.

I think this little story about our operations shows how people with different expertise work together at sea to respond to changing circumstances, thereby getting the best science done with the time and resources available. It’s why we can’t just mail these OBS to the ship and have the crew toss them over the side while we sit at home. Instead, marine science is an makingflagsactive, involved endeavor. What is really difficult about it is that every second counts – even now we are watching the clock and mentally recalculating our final deployment schedule – yet there are long stretches of waiting. It takes a special team to keep their cool in the face of that dilemma.

OK, I’ll be back at a future entry with more science explanations, and we’ll update the photos and images at some point as well. ‘Till then!

– Nick

April 13, 2015

Greetings readers. I have failed to recruit another blogger for this entry, but I can guarantee at least two additional voices will make an appearance here in the coming week or two. We also are in dire need of a cruise logo and T-shirt design: current idea involves the bathymetric profile and some anthropomorphized OBS at the bottom…

Given these important upcoming developments, I wanted to bring you a quick update. You see, research cruises invariably have a stressful moment at their halfway point or so when the amount of time relative to the science goals start to move toward one another, the novelty of the environment and work-flow starts to wane, and maybe even a personality conflict or two rises to the surface. Damon Teagle of Southampton referred to these times as “the hard yards” in reference to the months spent drilling the Pacific crust with IODP. That cruise was more than seven weeks long and one of three, so we hardly can complain a mere two weeks in or so. And in fact the largest disagreement I’ve had is with our engineer Anatoly Mironov about how best to use zip ties.

But here’s the suspenseful part. We will be pulling the air guns in from shooting our fourth profile by about 7 p.m. this evening. Then we need to recover the dozens of instruments, each one taking between one and two hours to recover given the rather long rise time of each (i.e. at roughly 55 meters per minute, 4 kilometers of rise time is 72 minutes!). As I said before, we are a slick operation that braves releasing our next OBS as soon as we set eyes on the previous one, and the Germans are even braver releasing the next OBS while the previous one is in the water column! Nonetheless, recovery will be a couple days…. Given that, it means we have a week to complete our final two profiles. However, if we make the next profile too ambitious, it could mean we do not get to do the final one. But if we don’t take some risk, we will not answer some key science questions. Ingo, Harm, Christine, and myself spend quite a bit of time scratching our heads about this dilemma, and discussing the relative science merits of each. I wouldn’t say we all completely agree, but we are a great team willing to listen to each other, offer opinions and advice, and ultimately trust Ingo as chief scientist to make the right call (no pressure Ingo).

Regardless, by tonight we’ll be recovering, which means we’ll be pretty busy for the next couple days. After that I’ll keep you posted on our operations, and explore some more science, and get a guest blogger or two on here.

– Nick

April 9, 2015

Thanks Florian! And hello again everybody.

We’re finishing shooting our second and third profiles, and it is probably a good time to tell you a bit more about the science we are doing here. Throughout this blog I’ve been trying to write for a general audience of scientists and non-scientists alike, but assuming the targeted reader: (a) does not need to be convinced of the intrinsic value of basic research, and (b) is willing to glide past technical terms to get the big picture. But that doesn’t mean that a bit of effort isn’t needed on my part to convey the richness of this story.

If one looks at an image of the Earth – using Google Earth for example – you will find that more than two-thirds of its surface is oceanic. The reason the oceans are deeper than the continents is that they are more magnesium and iron rich (“mafic”), and therefore denser as the lithosphere “sinks” into the more viscous asthenosphere. Then, as subduction zones (e.g. under Seattle) pull the oceanic plates, mid-ocean ridges make new crust where the asthenosphere rises up (“upwells”), melts with the dropping pressure, and forms the crust via intrusions and eruptions of that melt. Think about that episode of “I Love Lucy” where they have to keep putting chocolates onto the conveyer belt….

The fastest spreading ridge in the world is the East Pacific Rise, and it forms about a 6-kilometer thick crust of gabbro (the oceanic equivalent to granite) and basalt that spreads off the axis at well over 10 centimeters per year, about the rate that your fingernails grow. In contrast, the Mid-Atlantic Ridge is quite slow, spreading at less than 3-4 centimeters per year. The two systems are remarkably different, with the Atlantic hosting high massifs of exhumed (“dug up”) mantle and lower crustal rocks in areas where, for reasons we don’t really understand, the crust was either not completely formed, or is completely absent! Some of these are well over the size of Mt. Rainer, so they are not trivial objects, and in at least one place seafloor vents on top of them host a spectacular ecosystem.

In 2005, I was on cruise where we drilled into one of these “core complexes,” the Atlantis Massif, and found that most of its center was made of lower crust, rather than exhumed mantle. Though the second such result, it was a bit of a surprise, and we have learned a lot about how melts get introduced into the crust from the mantle from that project, among other things. But if one looks at the gravity anomalies and seismic images of the Atlantis Massif and structures like it, many of them appear to be made of intrusive crust with bits and pieces of mantle scattered within them and adjacent to them. This is certainly not true for all of them, however, and some parts of the Mid-Atlantic Ridge have exhumed mantle and other parts look like normal oceanic crust. A lot of research is going into understanding what determines the kind of oceanic lithosphere we observe, but it seems to have something to do with the duel between mantle-melting and breaking, or faulting of the crust.

A really peculiar aspect of all of this, however, is that when geophysicists sort of gloss over some of the complications I remarked on before, and look at the seismic velocities of different ocean basins, they find that the thickness of areas with crustal velocities stay more or less the same, overall. The big exception is where spreading rates dive below about two centimeters per year, at which point the crustal thickness drops precipitously. The easy explanation for this is that the mantle upwelling is less vigorous in these places, so it melts less easily, and less crust is created. Unfortunately, it is difficult to say much more about these areas because most of the places in the world where this happens are relatively remote, such as along the Arctic ridge system (the Gakkel), and the Southwest Indian Ocean. The Cayman spreading center is thus pretty important because it is relatively accessible, relatively short, yet has all of the features I described: core complexes, basalt fields, hydrothermal vents, etc…Our work here is thus to use the seismic velocities we get via the OBS-recorded refractions to determine the nature of the crust and mantle here, which should allow us to test some hypotheses for why core complexes form, why ultraslow spread crust is so thin (to absent), and how these ultraslow spreading centers evolve over time.

There’s more to it than all of that, and I can hopefully write an entry about how this all affects the price of milk, but I’ll leave it there for today. For an operations update, I can tell you that we are ahead of schedule! By preparing our deployments while recovering the previous profile, and efficiently getting the guns deployed, we shaved off half a day. Doesn’t sound like much, but at sea it can turn into two extra days by the end of it all (knock on wood), and every minute counts!

-Nick

April 8, 2015

For me as one the few geologists on board, this cruise is a great opportunity to learn more about how seismic data is gathered and processed – as I have to admit I did not really focus on geophysics during my studies. Now, having completed the first profile and on our way to the second and third, I think I already learned a couple of things so far:

At first I was really surprised to see how many different sizes and shapes the devices have, we – in need of a better word – just throw overboard to sit on the seafloor to collect our data. There are yellow and orange drum shaped floating bodies made of wood we call “lobsters” or simply just “drums”. Then, we have some really large devices looking like giant Easter eggs, some of them mounted on three legs, others have just one. There are glass spheres and block-shaped devices, but basically all devices share the same purpose to listen to and to record the sound waves we later produce to let us see beneath the seafloor.

Getting all the devices to their designated position though is not as trivial as it may sound. Weights have to be constructed and attached to the OBHs and OBSs. There is also a lot of screwing, heaving, maneuvering – not only the ship itself, but also the devices on board – and also programming involved during the preparation of what may appear a simple operation (shooting a profile). It also seems to prove useful to always have a considerable amount of duct tape and zip ties with you when going at sea.

Once all listening devices are deployed the second part of the operation is executed by first bringing out the airguns, a device which basically consists of a long string of tubes and cables and is towed behind the vessel. I was lucky to be part in both deploying and recovering the device so I can tell you that there are not only the actual “shooting” devices, that are connected by quite heavy tubes and cables and are later fed by them, but also the cable string as thick as my thigh that is not easy to carry or even to pull. Thus, this procedure always requires a lot of teamwork and good communication between all involved persons, crew as well as technicians and other helpers, such as me. Then, the “shooting” begins. The airgun canisters are filled with air at a maximum pressure around two hundred bars by a compressor located on the backdeck. Once every minute the air is released as a shot under water by the 12 airguns we use, simultaneously. This, on one hand generates a muffled sound like a canon heard from a long distance but on the other hand also generates a tremor that you can feel regardless where you are on the ship. These waves or more precisely the reflections of these waves from penetrating the different subsurface layers can later be used to do our geologic research. This however is done later during the processing and interpretation of the data.

After the profile is finished, the ship turns again and with the airguns back on deck, the recovery operation starts. Each device has its unique code that can be sent on a specific frequency and when the so-called “releaser” receives this code, it lets go of the weight that is holding it and begins its ascent. It seems pretty obvious when you think about it, but I was quite surprised at first when I heard that it can take up to one hour or even longer for one device to come back up again from sometimes more than over five or six thousand metres water depth here in the study area. There is of course always the risk that one of the devices does not come back – which can have a number of reasons – and when you look in the faces of everyone, there is always a smile when finally the red colour of the flag during daytime or the flashing light of the beacon during nighttime is spotted from the monkey island above the bridge. Sometimes, you can see an even wider smile when the device and the stored data with it are safely back on board again. And I have to say it did not take very long and I am now also thrilled every time we are waiting for our equipment to come back to us to the surface.

If you are still reading now I would like to thank you and hope I did not use too many technical terms describing our operations. As for life on board here on the R/V METEOR I can only say it is absolutely fantastic. I have not really said before but here on board, there are three different working groups (one American, one British and one German group), all contributing to this project concerning the seismic investigation of the Cayman trough and the slow-spreading ridge. The first few days were more characterized by intensive preparations and maybe to some point even nervousness, with everyone hoping that everything would go as planned. But now, after a couple of days and after the first successful operation I think everyone has more or less settled in living and working here. As I already said before I am not a geophysicist, but everyone here on board is very friendly and kind explaining the things to me that I yet do not know about and also apart from that I think we are all working together very good. That also includes the crew here on board, which are very experienced with these kind of operations and are always willing to assist in any way they can, so that at the end of the day everyone is happy and we can all sit in the lounge having a beer together and get to know each other a little better. I, for myself, enjoy this cruise M115 very much so far and I am looking forward to the coming three weeks to work and live here with this amazing team of people on this interesting and sometimes challenging project.

Cheers from the R/V METEOR,

Florian Gausepohl, geologist, GEOMAR, Kiel (Germany)

April 6, 2015

Just a quick update, and I am leaning on some of our wonderful international collaborators to chime in in the coming days. And Terry Britt has done a nice job setting this up from the beach! Keep coming back for updates, and I hope to also get a library of photos at some point either here or on a UTIG website.

We’re halfway through recovering our first profile, and we have a good rhythm going. When the UTIG team began our recovery last evening, it was just past dusk. Jenny and I with the help of our German colleagues Michaela and Rabea spotted the OBS, a dull spot on the horizon. “Isn’t there a light attached to it?” Rabea asked… turns out our OBS came up on their side obscuring the light! Remember how we had to do some hardware store improvisation after some things went missing from the shipment in KingP1030871_smallston? My bet is that our flags, fashioned from the True Value hardware store supplies, are the culprits for the rolling OBS, though this is a point of heated discussion.

Regardless, it was a lovely night standing on the upper deck, spotting dull specks in the rolling seas under the moonlight. In some cases I could observe the OBS come to the surface completely upright, and the data so far look a-ok, so no sweat. As dawn came we had completed our six recoveries, approaching about one per hour; the trick is to send the release command so they can start their hour-and-a-half ascent early enough so as not to waste time, but not too late or they could surface and be carried away by the current.

Time is of the essence at sea, and if we can keep this steady pace up we will complete the well over 100 OBS drops giving us our first glimpse into the deep Cayman Trough. Stay tuned!

– Nick