Andreia Plaza-Faverola
When I was a kid and still followed my parents in road trips to enjoy Venezuela’s nature, my mother would keep us quiet by playing with us a game that, when I think about it, was a very rational game. One of us would tell some facts. The rest of us would reconstruct the story behind the facts by making questions. I still recall 2 of those stories, which I recently taught to my kids in a road trip in Norway.
Just as in this game we use facts from data and observations to reconstruct a story behind. There is more than one story that can fit the facts but one will be the closest to the truth. The Earth is so bast and inaccessible that geosciences rely significantly in this way of reasoning. Often, we can just content with being sufficiently close to a truth.
The SEAMSTRESS project wants to understand the processes behind seafloor degassing in the Arctic Ocean. Why is there methane coming out to the oceans today? When did it start to happen? What forces it?
The sediments under the ocean are fill with water and gas. In some sub-seafloor regions, there is more gas either because there is gas leaking from deeper into the Earth or because there are large amounts of microbial communities consuming organic matter near the seafloor.
Illustration by Frances Cooke (see post earlier this year)
Understanding the processes that control the release of gas from the seafloor to the oceans is critical for better modeling submarine avalanches (towards the possibility of predicting tsunamis accurately), deciding whether geological settings are suitable for sequestration of carbons (carbon capture and storage as a way of mitigating human controlled climate change), to understand the evolution of large marine ecosystems in the deep sea, and for better simulating scenarios of future global climate changes.
Getting data exactly from where the water and the gas start to be generated and follow these movement of fluids through sediment all the way to the seafloor is hardly possible. We therefore rely on data from sediments near the seafloor. In a research cruise in 2021 with our colleagues from Ifremer we gathered pressure and temperature data from the upper 10 meters of sediments beneath the ocean floor. The instrument to collect this type of data is called a piezometer and it is a thin, long rod (ca. 7 cm in diameter and 7 m long) that has mechanical sensors every meter. These sensors can measure the temperature in the sediment fluids and also differences in the pressure that fluids in the sediment are experiencing sidewise compared to the pressure caused by the column of water above. In sediments that have compacted slowly without any major change in sedimentation speed, the pores are well interconnected and the fluid inside the pores is all interconnected with the ocean column. This creates a stable pressure field and no differences in the pressure field are measured. Similarly, the temperatures in the sediments would normally increase with depth and stay relatively constant in an unaltered setting. When differences in pore pressures and temperatures are measured the fun starts: we can search for a fitting story.
In our experiment we leaved the piezometers to record data over 3-4 days at 6 different locations along the continental margin off the west Svalbard coast. We noticed that the pressure that resisted penetration of the instrument into the sediment was less and less as we moved farther away from Svalbard (closer to the mid-ocean ridge). This trend doesn’t seem to be explained by changes in the type nor size of the sediment grains. Interestingly the amount of gas that is currently being released from the seafloor along the investigated transect is also decreasing in the same direction. We believe this differences in pressures within the upper sediments are reflecting changes in the way gas is transported towards the seafloor. We infer a damage of the sediment cohesion due to a less focused (more expansive) migration of the gas through the pores of the sediment. This changes in the mode of gas transport appear to be determined by the property of faults and cracks in the Earth. Whilst larger and more open cracks allow focused and fast gas transport out into the ocean, less open cracks favor slower transport of gas laterally through the sediment which result in more damaging of the sediment itself. How the gas moves through the sediments, whether it creates cracks or if it damages the internal structure of the pores, are questions we can just try to answer via lab experiments, the type of inferences we do by interpreting surficial data, and by simulating gas transport mathematically. Every new data set interpreted brings us just closer to a reality about the Earth interior…
The study is published in the Journal of Geophysical Research (JGR) Solid Earth https://doi.org/10.1029/2022JB025868