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Sources & Sinks: A Tale of Coastal Biogeochemistry - BONUS COCOA

Insights into the biogeochemistry work of the COCOA project: from the water column to the sediment, from the field to the lab. Secrets of the coastal filter and a life in aquatic sciences.
01.04.2017 17:56

Equipment explained: part II - the GEMAX / GEMINI corer

Dana Hellemann

Sampling the sea floor is not a trivial task. Frequently, I get the same question: “How do you get your sediment samples, do you dive down there?” The bottom of the sea puzzles people. It is somewhere there, but you cannot see it. Thus, it is not unreasonable to assume diving as solution to the puzzle, as it lends an eye to the sampling process. This is how my colleagues from the benthosphere , who work on shallow coastal ecosystems, get their samples (see Sampling benthos).

Somewhere there, below the sea surface, is the sea floor. Even in the coastal zone, this "somewhere" might however mean many meters of water column.

Yet, with increasing water depth and subsequently decreasing light, sampling via diving becomes complicated. On the dark side of the sea floor we therefore use sediment corers operated from ships. As corers have no eyes, we work virtually blindfolded which can often feel like gambling (and thus holds a certain kick). Sediment maps are helpful for giving directions, especially in early planning phases; however, often they are extrapolated or outdated and when looking at the first sample coming on deck, you might find that the sediment type has changed over the years, e.g. due to a change of current regime or deposition environment.

Surprises for free, excitement guaranteed- that´s sediment coring!

Sediment coring is an exciting task, including joy & disappointment, puzzlement & frustration, and a decent amount of waiting (picture: Franziska Thoms, IOW).

The successful retrieval of a sediment sample from the sea floor depends largely on the right corer. Sediment types vary in sturdiness, resp. softness, e.g. comparing sturdy sand sediment with soft mud sediment; consequently, you have special corers for each type. If you have followed this blog for some time, you will have heard about MUC, HAPS, GEMAX, GEMINI, BOX – over the last 3 years, we have had them all.

Today you will meet the GEMAX/GEMINI corer, which is according to an essay by Boris Winterhalter the “ultimate corer for soft sediment”. Do I need to say more?

A GEMINI corer, ready to be launched.

The GEMAX / GEMINI corers are twin gravity corers for soft sediments. They base on the original Niemistö corer (Niemistö, 1974), which was highly successful with the only offset of a too small sample size. By doubling the core barrels a twin corer was born, self-evidently baptized GEMINI. Sample quality was further improved by increasing the diameter of the core barrels, which created the GEMAX.

Today, both GEMINI and GEMAX are standard corers all along the Baltic Sea coast. They consist of a stainless steel housing which is fitted by acryl core liners with sharpened steel cutters at their ends; those, as the name says, are capable to cut through the sediment. In the end, the core liners will hold the sediment sample. Both corers might look alike, but do not underestimate their difference: even 1 cm variation in core diameter makes a huge difference when trying to fit in a wrong inner core liner. From experience I can say: no, they don´t fit and no, they also cannot be squeezed in. Acryl is a quite solid material.

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A full GEMAX corer back at the sea surface. Its muddy condition is a true sign that it really was in the sediment.

The corer penetrates soft sediments vertically based on its own weight and the lowering speed of the winch. Upon upwards pull a closing mechanism is triggered that locks the sediment securely in the core liners, creating a nice sediment core of ~ 30-40 cm.

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And action: Laetitia, Wytze and Mathias while retrieving the full core liners. Requirements here: be quick, put the plug tight and don´t be afraid of getting muddy (left). My house, my car, my....sediment core: happy mood while subsampling a fresh core (right). (pictures: Ines Bartl, IOW) 


Some care needs to be taken for the core liners, which somehow have a tendency to roll over board. Your only solution if you want to have them back: diving- and the circle closes.

A dropped core liner being successfully brought back to the surface.

Next time in equipment explained: why a toilet brush is an essential tool in sediment coring.

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