Stories from the depths – A Brief Guide to reading sediment archives

Humans have affected their environment for a long time. The ancient people respected the Earth.  However, they already had an impact on the environment and its state. Over the past decades (and even longer) increased anthropogenic activities have altered both marine and terrestrial environments worldwide. Nowadays it is difficult to find a place on Earth where human influence would not be visible. We have even influenced our planet’s climate.

How the changes we make in the marine environment, for example, can be studied?

Topography picture shows deeper areas.
Fig. 1. Seabed topography after marine sand extraction at an area around 1,5 x 1,5 km2. Geologian tutkimuskeskus (GTK).

Seabed sediments are one of the archives which can be studied (and read) to obtain information from the past environmental changes. Where and how these past environmental archives, “books” would then be found? And how to read them?

Deep basins in the open sea, sheltered depressions in the coastal areas, as well as the river estuaries, are often suitable places for hunting these treasure chests of environmental change research. These geological records that have accumulated nearly continuously on the seafloor, provide unique information on environmental changes over the past centuries and even millennia.

The exact sites, where the best and most representative sediment deposits could be found, need to be surveyed and located by using acoustic-seismic sounding methods, like sediment echo sounder. The sediment echo sounder provides information e.g. on water depth, and on the thickness and internal structures of soft (mud and clay) sediment units.

Echo sounder profile shows that sediments are in 30-55 meters deep.
Fig. 2. Sediment echo sounder profile showing seabed sediment deposits. The length of the profile is around 1 km. The depth scale (in meters) is shown in the left side of the profile. Picture: GTK.  

When the exact site is chosen, we need to take a sample from the seabed. Over the years, various methods for seabed sampling have been invented, like grab samplers, box corers, piston corers, vibrohammer corers and gravity corers. In the SEAmBOTH project we have used GEMAX gravity corer, because it can recover undisturbed samples even from the fluffy, soft mud.  

The coring itself is a relatively quick operation, especially in shallow waters. The corer is lowered to the seabed using the crane and wire, and then lifted up.

A person lifts a sediment sample from the sea on the deck of the boat.
Fig. 3. Seabed sediment sampling with the GEMAX corer onboard r/v Geomari. Photo: GTK.

On the deck, the acrylic sample tubes are removed from the metal corer body. One sample tube immediately leaves to the gentle handling of geologist. Because the acrylic sample tube is transparent, it is possible to make the first observations and descriptions straight through the tube liner. Through the liner it is easy to see the soft surface sediments and their structures.

Sediment sample on the deck in see-through container.
Fig. 4. Surface sediment core from the Bothnian Bay, close to Tornio, summer 2019. Photo: Aarno Kotilainen, GTK.

The next step is to split the description sediment core in half lengthwise, and then to set the splitted core at the operation table, like a patient, unknown patient (suom. tuntematon potilas).

At its best, the splitted sediment core can reveal the detailed sedimentary structures, like sediment layers deposited in different seasons. The geologist describes all the details and structures seen in the sediment core, and photograph the core. In the past, you had to wait the photograph, until the film was developed. Sometimes it took weeks until you knew if the picture was successful. Everything is different nowadays, in the digital age. Thus, today, we do not run out of the film.

Sample on the deck, showing different layers.
Fig. 5. Splitted sediment core. Photo: GTK.

Once the sediment core has been described, the subsampling from the parallel sample tube begins. The sediment core can be sliced e.g. into 1 cm thick subsamples using a slicing device, and packed in plastic bags and boxes. The subsample volumes depend on what kind of analyses will be done. And in case more samples are needed from the same site, then new sediment cores will be taken. The packed subsamples will be stored in the vessel’s cold store, waiting for landing and laboratory analyses.

Subsamples can be studied in laboratories for virtually anything, like grain size, chemical composition, mineralogy, microfossils, or, for example, microplastics. Analysing the age of the sediment core is essential for studying the changes in the marine environment.

One person holds a plastic bag for the sample, while other one slices it.
Fig. 6. Slicing the sediment core. Photo: GTK.

Seabed sediment cores from the Baltic Sea, also from the SEAmBOTH project area, the Bothnian Bay, have revealed the human influence, mentioned already in the beginning of this article. Unfortunately, our emissions (e.g. nutrient and heavy metal loading) have also been archived into the seabed. Thus, it is essential to map where the concentrations of harmful substances are most prominent in the bottom sediments. The detailed knowledge can be used to direct seabed construction (e.g. dredging) to the most suitable location and to choose the right method for its safe implementation, while minimizing the environmental risks.

In the SEAmBOTH project we study and provide information on the biology and geology of the seabed as well as changes in the marine environment, including anthropogenic loading.

Aarno Kotilainen, GTK

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