On this expedition, even though I am not taking seafloor samples for my own research, I very much enjoy discovering what other scientists study in the Arctic. In 2008, I was invited by my Ph.D. advisor to join an expedition on the western European margin of the Fram Strait extending into the fjords of Svalbard in the northern North Atlantic. It was a particularly nice gesture on his part because I did not participate to the writing of the original proposal. I have always appreciated the scientist for his intellect and ability to break down difficult concepts in bits and pieces easier to understand, but also the man himself for his sense of humor and genuine kindness. Thus, meeting with him again and one of his MS students only meant good times.
Our goal was to study the habitats selected by particular foraminifera (microscopic organisms that form a shell of calcium carbonate) species in the upper centimeters of the marine sediment. Benthic foraminifera shells have been extensively used in paleoceanography. This is the field of the Earth Sciences that studies past (or “paleo”) oceans and their physical, chemical and biological characteristics at a given time. For instance, has the temperature of the ocean changed through time? If so, what processes can explain such changes? Were they localized or global? Did the change happen simultaneously between the two hemispheres or not? The questions are endless and the scientific community very active in finding answers.
Several tools and applications used to decipher past oceanic climate rest on benthic foraminifera (i.e. those living on the seabed), and particularly the chemical composition of their shells. It is broadly accepted that the geochemistry of said shells is sensitive to the habitats selected by particular foraminiferal species. In other words, where the forams live in the sediment is likely to affect the chemical make-up of the shells. Understanding these habitats, then, is essential for the proper interpretation of results based on benthic forams.
A traditional view, albeit too simple, is that forams’ habitats are vertically layered in the sediments. Forams needing lots of oxygen to thrive will be found at the interface between the water and sediment, while species adapted to more oxygen-depleted environments will bury deeper in the sediment. The first few centimeters of a marine sediment core should return different living species stacked on top of each others, each situated at a particular sediment depth with less and less oxygen as you go down into the core. Species are then supposed to acquire the geochemical characteristics found at these depths.
We collected oxygen concentration in the sediment with micro-sensor slowly pushed into it, counted live forams and visually inspected the surface of the sediment for the presence of tubes and burrow structures. Where worms had foraged the sediment, leaving behind tubes and burrows, oxygen was available deeper, and in higher concentrations. From our study, it is clear that forams are not tied to particular sediment depths but instead follow, or concomitantly escape, the “oxygen trail” created throughout the sediment.
How this is exactly affecting the tools developed off the shell chemistry of forams is still unclear. A better understanding of what exactly controls foraminiferal shell chemistry awaits better techniques for sampling their environment along the tubes and worms. Finally, more investigation of how species select for their habitat in different oceans will certainly be needed as well.
- Mathieu Richaud