Secrets of the seafloor - Blogpost 5 from RV SONNE, March 14, 2018

The multicorer for the upper 40 centimeters

With the multicorer, Matthias Zabels team can take twelve sediment cores at once. These cover the upper sediment layers: starting from the bottom water layer and the upper sediment layer, they reach a depth of 40 centimeters. This sounds like an easy task, but definitely it is not.
Matthias Zabel explains the problem: "With a winch we lower the multicorer on a steel cable to the seafloor. When almost there, it is important that the winch driver reduces the speed so that there will be a soft landing. Today we are working at a water depth of 8066 meters. It took more than two hours just to reach this depth. You have to keep in mind that at that depth, the device is slightly drifting off. Any ship movement affects the cable movements. Therefore, for the last few meters, you have to be very careful. As a rule, I have a close look at the cable tension displayed on a computer screen. When the multicorer hits the ground, this tension suddenly decreases: this is the landing. Now it is like hit and run, you have to start the winch immediately so that the device does not bounce up and down like a yo-yo. Upon my call via radio, the winch driver drives the winch up again immediately. This cooperation works wonderfully. I'm thrilled, I have never had so many good cores on an expedition. "

Iron, po­tassium, alu­minum and sil­icon are ter­ri­gen­ous, i.e. of land-based ori­gin. Mar­ine com­pon­ents are car­bon­ates and most of the or­ganic ma­ter­ial. The lat­ter ori­gin­ates from the food web, the basis of which is the bio­mass of pho­to­syn­thetic mi­croor­gan­isms (phyto­plank­ton) in the up­per light-flooded parts of the wa­ter column.

Microorganisms decompose almost everything

The northward-setting Humboldt Current along the coast of Chile causes nutrient-rich deep water to rise. This is important for the food web. Only if sufficient nutrients such as ammonium, nitrate, phosphate and metal ions such as iron are available, the photosynthetic algae can multiply and serve as a food source for others. When they die, they sink to the bottom of the sea, decomposing into flaky lumps as they descend. Below, these remnants form part of the sediment. But that's not the end of their story, because on and in the seabed other microorganisms draw energy from the remnants. Over the centuries the sediment layer grows steadily.
From above, new debris and particles continuously cover the older layers. In the uppermost layers, certain microorganisms use the abundant oxygen penetrating from the water column into the sediment. Oxygen is an effective oxidizer. Below, the microorganisms rely on alternatives such as nitrate, manganese, iron and sulfate.
By examining the depth profile of these substances, parameters can be calculated that provide information about the biological activities in the sediment. As a rule of thumb, the steeper the drop in the profile, the more active are the microorganisms. Finally, only about 0.1% of the biomass that rained down from above is stored in the sediments.

The Atacama Trench: From the west, the Nazca plate slides under the South American plate. Through tectonically triggered landslides sand from the mainland ends up in the trench (left). This can be seen in the depth profiles of the gravity cores (see insert). The eastern trench side benefits from the algae blooms. This is typical for upwelling areas. Here, nutrient-rich deep water rises and boosts biomass production. The diagram on the right shows the unusually steep sediment profiles of oxygen and other substances in the sediment. These testify to the high bioactivity of the trench.