waves at seas — Bad weather approaching, North Sea. Photo: Celine Golda (NGU)

Hide and seek in the North Sea: where Biology meets Geology

Cruise diary: During the November cruise with «G.O. Sars» we have seen a lot of sandy mud with some occasional patches of fine gravel on the seafloor. Although soft bottom sediments are not always the most exciting for geologists; they may provide important clues about biological traces and activity in the sediments.

Published: 02.12.2024 Author: Celine Golda, Daniel Hesjedal Wiberg, Henning Jensen and Nicole Baeten (NGU). Camille Saint-André, Ragni Olssøn and Hilde Elise Heldal (IMR)

The soft sediments provide the perfect habitat for benthic organisms - animals living on top or just below the seabed - within which they crawl, hunt, hide, defecate and create homes in the form of tubes and burrows.

While they are in the process of creating their homes, they mix the overlying sediments with deeper layers during a process called bioturbation (see figure below). This movement of sediments and overlying waters by benthic organisms. like bristle worms, bivalves and crustaceans etc., play an important role in the cycling of nutrients, trace metals, carbon and oxygenation of the sediments. Species with high bioturbation activity thus make the sediments habitable for other organisms which would not be able to live there otherwise.

Understanding the interplay between the organisms, the sediments and the chemical components is crucial in our monitoring of marine environments, as bioturbation may lead to the transportation of contaminants and trace metals deeper into the sediments.

Figures — Processes (A-C) and some of the traces also known as Lebensspuren (1-4) from sea organisms on the seabed A.) Resuspension of sediments B.) Bioturbation by organisms that move and feed throughout the sediment profile, mixing the sediments C.) Bio-irrigation through suspension feeders (e.g. bivalves) exchanging water from porewaters with the overlying water column 1.) Tracks from sea urchin 2.) Burrow mounds from bristle worms 3.) Tracks from Norwegian lobster Nephrops norvegicus 4.) Burrow entrance, waste bank and shaft formed by Nephrops norvegicus Sketch by: Celine Golda (NGU) (modified after Burdige, 2006 and Baden & Neil, 2003)

Tracks — Living sea urchin Gracilechinus acutus forming tracks and resuspending overlying sediments with its sharp spines. Scale: inside tube diameter is 105 mm. Photo: Celine Golda (NGU)

Burrows in the North Sea. The squat lobster Munida sarsi on the right, use the burrows for shelter. Photo: MAREANO/IMR

To identify biological structures from bioturbation in the sediments, sediment cores obtained from the multicorer on MAREANO-cruises may provide useful insights. Firstly, the sediment cores are visually inspected for changes in grain size, colour, sedimentary and biological structures (see figure below).

Then the cores are sliced into 1cm intervals and analysed for both inorganic and organic contaminants at NGU and IMR respectively.

Sediment layers on the seafloor build up gradually, as particles sink through the water column and accumulate, taking along contaminants and other anthropogenic inputs. Changes in the patterns of chemicals and contaminants at different depths in the sediments may be linked to the activities of organisms that mix the sediment. This mixing could make it harder to accurately determine the current levels of contaminants and the age of the sediment, as recent, newly deposited sediments are mixed with older, deeper sediments.

To account for this, upon our return home to NGU, the sediment cores will be scanned with X-rays (XRI), allowing us to observe until which depth the sedimentary structures and burrows occur (see figure). This inspection allows us to piece together the true history of the sediments.

References:

Baden, S. P., & Neil, D. M. (2003). Manganese accumulation by the antennule of the Norway lobster Nephrops norvegicus (L.) as a biomarker of hypoxic events. Marine environmental research, 55(1), 59–71. https://doi.org/10.1016/s0141-1136(02)00216-7

Burdige, D. (2020). Geochemistry of Marine Sediments. Princeton University Press. Retrieved from https://www.perlego.com/book/1563906/geochemistry-of-marine-sediments-pdf (Original work published 2020)