Gas hydrate image

Frozen vestiges from a chilly past respond to climate change

Hannah Stewart

As climate change continues to warm the planet, it is also impacting some parts of the Black Sea that have not changed significantly since the last ice age, more than 20,000 years ago.

A multi-institutional, multiphase study found evidence that far below the Black Sea the gas hydrates could stir again.

Gas hydrates are ice-like minerals that form when certain gases combine with water and freeze into solids under moderate pressure. A recently published study by scientists from the University of Bremen’s MARUM Center for Marine Environmental Sciences and the GEOMAR Helmholtz Centre for Ocean Research Kiel in Kiel, Germany, found that gas hydrates in the Danube River’s deep-sea fan in the Black Sea are responding to climate change, a situation that offers both opportunities and risks.

Gas hydrate crystals
A scanning electron microscope image of gas hydrate crystals in a sediment sample. The scale is 50 micrometers (µm) or approximately 0.002 inches. Pic courtesy Woods Hole Coastal and Marine Science Center (public domain)

What is a deep-sea fan?

The Danube River is the second-longest river in Europe, stretches 1,770 miles, and passing through 10 countries. On its way, it picks up and carries sediment — tiny pieces of rock, sand, or silt. When the Danube finally empties into the Black Sea, the current slows down and loses the energy needed to carry sediments along. They drop to the sea floor, forming an underwater fan shape. The narrow end is at the mouth of the river and spreads outwards into the Black Sea.

According to Michael Riedel, a marine geodynamics research scientist at GEOMAR and the lead author of the study, “Within the European marginal seas, the Black Sea is considered the most [prominent] region, with large occurrence of gas hydrates.”

To study gas hydrates in the Danube deep-sea fan, 16 businesses, industries, and research institutions teamed up to launch the SUGAR initiative. Riedel’s work was the third and final stage of the project.

The importance of gas hydrates

The Danube deep sea fan overview
The Danube deep sea fan overview. Image courtesy the SUGAR project

In a recent email interview with Truly Curious, Riedel explained that, “SUGAR was aiming at conducting basic research on gas hydrates for three main objectives.”

The first is to determine their use as a fuel source. Gas hydrates are mainly formed from frozen methane, which has the potential to serve as an important source of energy.

The second is to determine the climate change threat.

“Methane hydrates represent a huge natural carbon pool locked in the marine realm and in permafrost settings,” says Riedel. Since methane is a powerful greenhouse gas, releasing all the carbon stored in the hydrate could potentially accelerate climate change and impact oceanic and atmospheric biochemistry.

The third goal of SUGAR is to determine the risk of hydrate destabilization – the risk of letting them loose. Methane hydrates occur mostly at a depth range of 600-2,000 meters (1,900-6,500 feet) within the sediment on the slopes or edges of continents. Environmental changes, such as rising sea levels or warming water at the bottom of the lake, could thaw and loosen the hydrates, allowing them to escape the sediment. The sudden release of pressurized gas could cause underwater earthquakes that could also trigger tsunamis.

The study

Typical kinds of gas hydrates in different geological environments
Thin (A) and thickly veined (B) sediment-displacing gas hydrates (white) in fine-grained sediment (grey); (C) pore-filling gas hydrates in sand; (D) gas hydrate mounds on the sea floor (hydrate has an orange coating from oil and is draped with grey sediment); (E) disseminated gas hydrates (white specks) in fine-grained sediment (grey); (F) gas hydrates (white) in coarse sands (grey). Image courtesy GRID-Arendal

For six weeks during the fall of 2017, Riedel and his team lived and worked on a German research ship, the METEOR, in the Black Sea. The team used the MARUM MeBo2000, a seafloor rig weighing about 90 tons to extract samples from below the seabed.

Tim Freudenthal, the operating head of MeBo, said, “When we started the development of the MeBo technology in 2003, we were quite naive… If we had foreseen all the pitfalls and challenges, we wouldn’t have tried it all… When operating the MeBo, I am always a bit astonished that this machine, 1,000 meters below us, in a very hostile environment (at least for organisms and technology not adapted to the conditions of the deep sea), really does what we want her to do!”

MeBo recovered sediments from the sea floor to define the composition, age, physical properties, and chemistry of the Danube’s deep-sea fan’s sediments.

Harbor test with the drilling rig MARUM-MeBo200. Photo courtesy
Harbor test with the drilling rig MARUM-MeBo200. Photo courtesy Torsten Klein / geomar.de

The rule of assumptions

Previous research on the gas hydrates in this region had used a technique called bottom-simulating reflector, or BSR. This helps estimate the bottom edge of the zone of stable gas hydrates. This is based on creating and watching the return of vibrations to judge the depth of the lower limit of the hydrates. The bottom of the stable hydrate zone is usually determined by pressure, temperature, and salinity.

“The system was not what we expected,” says Riedel. The team found that the BSR estimate did not accurately reflect the reality represented by the MeBo samples. In some parts of the Black Sea, the stability zone was deeper than the BSR, but in other places, it was much more shallow.

Riedel’s team hypothesized that this inconsistency could be an effect of Earth’s last ice age. During and before that time, the Black Sea was a freshwater lake. As the ice age ended and planetary temperatures rose, rising sea levels connected the Black Sea to the Mediterranean, making it much saltier. This change in temperature and salinity allowed some gas hydrates to travel and find a new equilibrium below the Black Sea.

However, methane is a “sluggish” gas that can only travel when sediment permeability is sufficiently high. In areas where permeability was low, the methane gas was trapped at the old depth established during the ice age. The variation in sediment permeability may be why methane hydrates appeared at different depths, regardless of BSR readings.

Researchers study samples in the RV METEOR lab
Researchers study drill cores in the RV METEOR lab. Pic courtesy Christian Rohleder / geomar.de

As Riedel put it, “We learned that the BSR is not an ideal indicator of the depth of the hydrate stability field within the Black Sea region.”

What next?

Speaking to Truly Curious, Riedel, Freudenthal, and Jörg Bialas, the chief scientist of a similar expedition in 2014, said that future research teams should test other sites along the channel-levee systems and identify high gas hydrate levels in different rocks. More data could show how the gas hydrate system evolved in response to past climate change. Understanding the history could help us predict how modern climate change could affect the hydrate stability zone, which, in turn, could pose risks to the surrounding environment.

Hannah Stewart

Hannah Stewart is an editorial assistant for a cancer research publishing house by day and a freelance science writer by night. She specializes in the environmental sciences with a focus on climate change.

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