How to find a hydrothermal vent
Hi everyone, we are now at the end of Week 5 of our MOOC: How do we explore the oceans? The most exciting part of my PhD was the opportunity to take part in two research cruises to explore and study previously unknown hydrothermal vent sites in the Southern Ocean. But, how did we known where to start looking for these deep-sea phenomena?
Hydrothermal vents are known to occur along tectonic boundaries in the Earth’s crust, so the >56,000 km-long mid-ocean ridge system is a great place to begin our search.
As you can see from the map above, very few hydrothermal sites have been discovered along the Circum-Antarctic Ridge (which surrounds the Antarctic continent). In fact, the ChEsSO expeditions to the East Scotia Sea aboard the RRS James Cook during 2009-2010 were the first to confirm the presence of active hydrothermal activity along the East Scotia Ridge and the Bransfield Strait (indicated by the red squares and star on the map). Before these expeditions, even fewer hydrothermal sites were known from high southern latitudes.
Once we are at sea in a region of potential hydrothermalism, there are three steps involved in the detection of a hydrothermal vent field.
Step 1: locate the hydrothermal plume using a CTD profiler.
In my previous post, I explained that the black ‘smoke’ which can be seen billowing from the top of a hydrothermal vent is actually hot fluid that is full of metals. This metal-rich fluid rises into the water column as it cools to form a hydrothermal plume; so if we can detect the hydrothermal plume, we known that there must be a vent nearby.
Those of you that have been following the MOOC will know that a CTD profiler is a common oceanographic tool used to survey the temperature and salinity structure of the ocean. The water in a hydrothermal plume is warmer than the surrounding deep-sea water, so positive temperature anomalies are one clue that helps us to track a plume. A CTD is often fitted with other equipment, such as a Light Scattering Sensor (LSS) and Niskin Bottles. The LSS allows us to measure the amount of light that is being transmitted through the water column: more light will be scattered in a hydrothermal plume that contains lots of particulate material, so the LSS is another useful tool for finding a plume. Hydrothermal activity releases lots of metals (e.g. iron, zinc, copper, and lead), gases (e.g. methane), and other compounds (e.g. hydrogen sulfide) into the water column that are normally present in seawater at very low concentrations. Using the Niskin Bottles, we can take water samples from different depths and analyse these back onboard the ship to see if the concentration of things like methane and hydrogen sulfide are present at concentrations higher than those of the ‘background’ seawater. If the CTD reveals the presence of temperature, plume particle, and chemical anomalies in the water column, we can be pretty sure that we are in the right area to start our search for a hydrothermal vent at the seabed.
Step 2: map the seafloor.
Many different techniques and technologies are used to map the seabed. During the ChEsSO cruise to the East Scotia Ridge, we used a multi-beam echosounder (mounted to the bottom of our remotely operated vehicle (ROV) ‘ISIS’) to produce detailed bathymetric maps of the ocean floor, from which we could identify potential vent fields.
Step 3: photographic reconnaissance and ROV sampling.
When the previous two stages confirm the presence of a hydrothermal vent, we can begin homing in on a smaller area of the seabed to video with the ROV. This is always a very exciting time to be on watch! Once the ROV has been deployed, you can spend many hours watching the dark, deep-ocean and endless seabed before the ROV-mounted camera is suddenly engulfed by a dense plume of black ‘smoke’ and you have finally found the hydrothermal vent.
Although finding the vent is a successful mission in itself, the real work is yet to begin! From here we must observe and record as much as we can about the vent environment. We use a temperature probe to measure the maximum temperature of the emitted vent fluid (which can be as high as 400°C), a titanium water sampler to extract some of the vent fluid, coring equipment to take sediment samples, suction hoses to hoover animals from the vent and surrounding seabed, and sometimes we can use the manipulator arms of the ROV to break small sections off of the vent itself to investigate its mineralogical composition.
