Seafloor Fluid Flow

By Chris German

In addition to our studies of the geology and geophysics of the seafloor at Pito Deep, we will also be keeping a keen eye (and nose) out for evidence of fluid flow, particularly around the deepest parts of the section.  Wherever tectonic plates are pulled apart (as at mid-Ocean Ridges) or are pushed together (at Subduction Zones) we know that active fluid flow arises at the seafloor and, further, that the chemicals released in those fluids can provide the energy to fuel primitive ecosystems, independent of sunlight.  The most famous of these systems, hydrothermal vents, were first discovered nearly 40 years ago but with so little of the Earth’s seafloor yet explored there remains much to discover.  For example, on Mid-Ocean Ridges we have found an increasing diversity of temperatures, flow-rates and geochemical compositions of expelled fluids along ridge axes as we have continued to explore from the world’s fastest spreading ridges (where the largest time-integrated heat-flux occurs) to the very slowest ridges on Earth such as the Mid Cayman Rise.  There, the wide diversity of rock types exposed at the seafloor, because of the complex tectonic regime, gives rise to a broad spectrum of vent-types.  Further, the resultant geodiversity of the vent-types present gives rise to an enhanced microbial biodiversity: different fluid compositions beget different organisms using different metabolisms.  

For this expedition, therefore, we are interested to investigate what different kinds of fluid flow might arise from the novel geologic setting of the Pito Deep.  We already know that active black smoker venting exists on nearby Pito Seamount and it will be exciting to revisit that site for the first time in more than 20 years, document its temperature and observe how it might have changed.  But before that, we also want to find out whether there are other kinds of fluid flow associated with the Pito Deep itself.  These might also manifest themselves as high temperature vent systems (our preliminary maps have already revealed fresh looking seamounts in the very deepest parts of our study area) or they may occur in the form of lower temperature and more diffuse flow.  Somewhat amazingly, it seems that there have been no systematic investigations for such fluid flow along any fracture zone/transform fault since the first discovery of venting in the late 1970s – even though these represent one of the three key forms of tectonic plate boundary.  Thus, anything we discover (or, indeed, fail to discover) on this cruise will represent an important scientific step forward.  

So, how will we conduct our work?

Put simply, we will put in situ sensors that can sniff for chemical evidence of fluid flow on everything that we put into the ocean.  Whenever Sentry is deployed to map or photograph the seafloor it will not only be equipped with its sonar, camera and the magnetometers brought by Co-PI Jeff Gee.  It will also be measuring the composition of the ocean, close above the seabed - continuously monitoring the ocean composition for changes in temperature, salinity, optical clarity and the redox equilibrium/disequilibrum of the water that the vehicle is passing through.  Why these for parameters in particular?  Hydrothermal vents, by definition, emit water (hydro-) that is anomalously hot (-thermal).  So measuring for temperature anomalies can be an excellent way to detect whether you are close to a vent or not.  But while the hottest vents can exceed 400°C, the fluids they emit are immediately diluted by a factor of approximately 1000:1 within a matter of seconds and ultimately to a factor closer to 10,000:1 within the next hour as their turbulent plumes billow upward 100m or more bove the seabed.  Consequently, by the time the plume stops rising and begins to disperse, an initially very hot fluid is mixed down into something that is made up of 99.99% local seawater at 2°C and any thermal anomaly becomes hard to detect.  For salinity, the problem is even worse.  The total dissolved salts in a vent-fluid erupted from the seafloor rarely vary by more than a factor of 2 from the starting seawater composition so it becomes very difficult to use salinity to distinguish vent-influences from ordinary seawater.  However, there are important chemicals including iron (Fe), manganese (Mn) and methane (CH4) that are one million fold higher in vent fluids than in deep ocean waters.  Dilute those concentrations by 10,000:1 and you still end up with plumes of water that are enriched to about 100 times higher than the surrounding ocean = easy to spot from more than a mile away if you choose the right measurements!  It is these properties that we will seek to exploit.  

First, the Fe emitted from vents precipitates very quickly to form mineral particulates (the black smoke that give black smokers their common name).  In the case of high temperature vents, the deep ocean far from land is so optically clear that one can often detect these “smoky” plumes, using in situ optical sensors, as they disperse for up to 10km or so away from their source as lenses of water, 50-100m thick, that are perched 100-300m above the seafloor – a depth which all of our sampling and survey equipment will pass through daily on the way to/from the seabed.  The only other part of the ocean that has similar concentrations of particles is the sunlit upper 200m where photosynthesis occurs.

As one gets closer and closer to a vent source, not only does the concentration of particles get stronger but, once one gets to within ≤1km of the source, a second “sensor” kicks in – our Oxidation-Reduction Probe.  Many chemical reactions in the deep sea, particularly those associated with seafloor fluid flow, revolve around the exchange of electrons to restore chemical imbalances generated when seawater full of oxygen percolates down into, and reacts with, the rocks that have recently been delivered from the mantle (recent in a geological time-frame), that form young ocean crust.  In the fresh fluids emitted by such systems, chemical (hence electrical) disequlibrium often exists which can then be discharged through the flow of electrons.  Our ORP sensor, therefore, acts as a “one size fits all electrode” that can detect the presence of chemical species that are present and out of equilibrium.  While it cannot tell us exactly which chemicals are present, the size and shape of the electrical response recorded by the sensor is an excellent way to telling when we are getting close to a seafloor fluid flow source.

For this expedition, we will use these sensors routinely on Sentry.  The data will be logged continuously while the dives are underway and right after each dive, a copy of the sensor data will be sent ashore for analysis by non-sailing collaborator Chris German who will analyse the data for any evidence of when and where Sentry passed close to any sites of fluid flow.  In parallel, we will also collect the same kinds of data in vertical profiles from self-logging instruments called MAPRs which will be clamped to the wire for our dredge stations (used to collect rocks from the seafloor between Sentry and Jason dives).  Again, data sets will be piped ashore after each operation so that Chris can analyse those data too for all of temperature, ORP and optical back-scatter anomalies indicative of seafloor fluid flow.  As Chris feeds his interpretations back to the ship each day, that information will be available to be merged with the detailed maps and images of the seafloor coming from the Sentry surveys and may, ultimately, help us to select future Jason dive tracks that not only conduct transects to map, image and sample the geology of the seafloor but potentially also to locate new sites of seafloor fluid flow.