by Ana Samperiz Vizcaino, PhD student at Cardiff University and ICY-LAB cruise scientist
The Earth is in constant change. Over millions of years it has gone through consecutive periods of cooling and warming, with successive changes in seawater temperature, sea-level and ice sheet’s extent. Oceanographic instruments only provide records (of temperature, salinity, pH, …) from the last few decades. So, how do scientists know what the ocean was like thousands of years ago?
Historians and archaeologist use archives to decipher certain aspects of ancient civilisations. So do oceanographers, but they use a different type of archives. Marine archives, such as ocean sediments, corals and shells have been widely used to reconstruct past ocean conditions. Geochemical analysis can shed light on several environmental and hydrographic parameters. For example, the composition of the oxygen atoms within the carbonate (CaCO3) skeleton of these animals can be used to estimate the seawater temperature in which they lived. However, the use of these marine archives is complicated and have some limitations. It is for this reasons that there is a constant search to expand the suite of archives that can be used to understand the history of the oceans.
In a new paper, published recently in Earth and Planetary Science Letters, we explore the geochemistry of stylasterid corals. This family of deep-sea corals, although known since the 19th century, has barely been studied previously. Stylasterids are fascinating creatures that build branching calcareous colonies of a few centimetres high, despite their polyps (the living tissue in charge of defence, feeding and reproduction) being only ~ 2-3mm in size. Furthermore, they are the only known coral group able to build their skeleton out or aragonite or calcite (or even both!) and whether they can serve as marine thermometers was unknown until now.
More than 90 stylasterid specimens were collected during 10 different oceanographic expeditions, from the North to South Atlantic and the Galapagos islands (including the ICY-LAB cruise in 2017). We measured and examined the oxygen isotopes within the stylasterid skeletons and created their first temperature calibration. Calibrations acts as a sort of “Rosetta Stone” for oceanographers. In this case, it informs us of how much the amount of oxygen isotopes within the coral skeleton will change as a result of temperature variations. They are the first step needed towards reconstructing historical environmental conditions.
Our results demonstrated that seawater temperature is recorded in the skeleton of stylasterids. They showed that oxygen isotopes in these organisms is a more precise temperature proxy than in other deep-sea corals. While questions remain open regarding the cause of these differences, and we do not fully understand the biochemical processes involved in coral skeletal growth, this is great news for oceanographers. This work raises the potential use of stylasterids to produce records of past seawater temperature. And although more work will be needed to combine other geochemical analysis and improve the quality of stylasterid coral records, this work lays the first stone.
Article citation: Samperiz, Ana, Laura F. Robinson, Joseph A. Stewart, Ivo Strawson, Melanie J. Leng, Brad E. Rosenheim, Emily R. Ciscato, Katharine R. Hendry, and Nadiezhda Santodomingo. “Stylasterid corals: a new paleotemperature archive.” Earth and Planetary Science Letters 545 (2020): 116407