Last Friday I added another article to my publication list. The publication was in the PLOS ONE journal and caries the title: Linking microbiology and Geology: inactive pockmarks affect sediment microbial communities structure. I am very happy with this publication, since it is the result of my curiosity and it taught me a lot about marine sediments. If you are not working in either microbiology or geology, which is likely, then the title is not very understandable. Assuming you have no background in these fields , I imagine a flow of questions coming up inside your head:
– What is a pockmark?
– What is an (in-) active pockmark?
– What has this to do with microbes in sediments?
In this blog post and the next, I try to answer those questions and explain a bit more about the reasons behind our experiment.
What is a pockmark?
The rather short description in Wikipedia states:
“Pockmarks are craters in the seabed caused by fluids (gas and liquids) erupting and streaming through the sediments.” The one thing that is missing here, is when that happens. The geological literature is currently suggesting that a pockmark appears after the fluid /gas stream slows down or stops . Fluid and gas streams coming from the subsurface push the sediment upward and export fine-grained material elsewhere. When the fluid and gas stream slows down for some reason, the seabed sediment sinks because of the reduced pressure.
Pockmarks can be found on the seabed of all oceans, seas and even in Norwegian Fjords. Interestingly, they are often associated with subsurface oil / gas fields. This makes pockmarks very interesting for the oil industry.
The diameter of pockmarks ranges between ≈ 1 m to over 1500 m. Large pockmarks can be more than 150 m deep, but most pockmarks are not so big. For instance, the few of the 500 Oslofjord pockmarks we sampled in our study had a diameter of 50 meters and were around 5 m deep.
What is an (in-) active pockmark?
To answer this question we look at the pockmark definition. Pockmarks are active when they emit fluids or gas. These emissions are often composed of methane, other hydrocarbons (e.g petroleum), brine solutions, freshwater or a combination of these. As I mentioned earlier, pockmarks appear when fluid/gas streams slow down or stop. Most pockmarks are early in their live active and with time change to inactive pockmarks that do not emit any gas or fluid. This means that most pockmarks discovered are inactive.
For biological research the active pockmarks are of interest, since they harbor unique microbial communities which use methane as an energy source and provide food for unique macro-organisms. Nonetheless, inactive pockmarks are interesting as well. Many macro-organisms use inactive pockmarks in different ways.For instance, in regions with heavy seabed trawling, such as the North Sea, inactive pockmarks can provide shelter to corals and bottom dwelling fishes and crustaceans. In addition, the abundances of various animal species moving through the top sediment can be different inside and outside pockmarks. These mixing or bioturbating animals effect sediments by redistributing or ventilating the sediments.
Furthermore, some fishes tend to accumulate on the downstream edges of a pockmark. Fishes group there because of upward water streams (See figure on the right). That pockmarks effect the water stream has consequences for sedimentation and re-suspension rates of sediment material inside these structures. It was recently shown, that sedimentation rates within a Oslofjord pockmark was higher than above the surrounding sediments. In addition, re-suspension of the top fluffy layer of the sediment inside the pockmark was also higher, causing fine-grained material to be transported out of the pockmark . These processes could have effects on the microbial communities present in the seabed sediments.
What has this to do with microbes in sediments?
Microbes in seabed sediments are mostly involved in the breakdown of organic matter derived from the water column above it. This organic matter can be composed of sinking phytoplankton, marine snow, zooplankton fecal pellets or material from terrestrial sources. When sedimentation and /or re-suspension rates are different inside a pockmark compared to its surroundings, than it might be that the amount of organic matter available for sediment microbes bacteria is different as well. We therefore suspected that there would be a difference in the microbial community in pockmark sediments compared to non-pockmark sediments. With this in mind we set up and experiment to test if we could detect such a difference. In my next post I will discuss the main results of our experiment.
1] The physics of gas chimney and pockmark formation, with implications for assessment of seafloor hazards and gas sequestration. Cathles et al., 2010.
2] Numerical simulation of upwelling currents in pockmarks, and data from the Inner Oslofjord, Norway. Hammer et al., 2009.3] Sediment mapping and long-term monitoring of currents and sediment fluxes in pockmarks in the Oslofjord, Norway. Pau and Hammer, 2013.