Homeward bound

We are now sailing back over a friendly North Sea, as smooth as glass under a warm summer sun. Anna had her early morning coffee under a pink sunrise, with dolphins hopping around the ship.

The cruise work is mostly done. Deckhands are cleaning the ship from the last traces of North Sea mud. The scientists are now busy storing last samples, cleaning equipment, transferring data to hard-discs, packing and inventing cruise reports.

A beautiful creature from the bottom of the North Sea, found in a box-core

We aimed to determine the natural activity of the sea bottom. The seafloor here is flat and compacted mud with lots of pockmarks. These are 3-4 m deep valleys of about 100 m wide, formed by explosive methane release. It is possible that these pockmarks are more active ecosystems as they collect detritus and maybe are fuelled by ongoing methane release. We are curious if these pockmarks have a different faunal community, if they take up more oxygen and release more CO2, and if they expel compounds to the water column. Fauna is studied from box cores. Oxygen uptake rates are measured in situ by the so-called eddy covariance method. Sediment activity is measured by oxygen uptake using microsensors and sulphate reduction rates by radiotracer incubations. Possible effects on the water column chemistry are detected by CTD, a device for sampling and analysing the water column. First analyses indicate that the pockmarks are equally as active in oxygen uptake as the normal flat sediments. But sulfidic sediments and an area where drilling has occurred have a twice as active aerobic microbiology. Measurements on sulphate reduction will start next week in the home laboratory.

It was a good cruise. Yesterday the last box-core was taken (number 76!) and the last of the 8 gravity cores. Dirk Koopmans deployed the eddy covariance system 5 times. Satisfyingly, the instrument collected many hours of unique data on the biological activity of the seafloor, and how this depends on current and position. The data will keep him busy in the coming weeks.

Last recovery of the eddy co-variance lander

The lab-on-chips sensors from Allison and Martin worked like a charm, highly reliably. A battery of 4 is still working, continuously analysing seawater on transect from Goldeneye to Kiel. They will be the last ones to be packed, in the harbour.

Lab on chip sensors from Allison Schaap are measuring continuously the seawater composition

So, a fond farewell from the Poseidon as we make our way back to Kiel.

Dirk de Beer

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