By now, if you have been following the other Deep Links blogs, you will hopefully have become familiar with one of the main purposes of the expedition: collecting deep-sea organisms to examine genetic connectivity between different deep-sea populations. But there is another way in which we can use the corals we have collected, to help us learn how the oceans and climates of the past were different to those of today. I am a palaeoceanographer (that is, I study past oceans), so far specialising in cold-water coral research, and since joining the ship I have been on the lookout for samples of stony (properly called ‘scleractinian’) corals that could help me.
Scleractinian corals will be familiar to anyone who has watched a coral documentary or been snorkelling or diving on tropical reefs. They all create a hard skeleton of aragonite (calcium carbonate) and in tropical waters they form the frameworks of these coral reefs, home to a huge diversity of life. These corals also do well in the deep sea, relying on food floating in the water to sustain them, not usually forming full reefs but existing as single coral colonies or individuals. The image below shows a group of deep-sea corals nestling on a rock six hundred metres beneath the surface. You can see quite large individual corals called Desmophyllum dianthus (which each have their own skeleton) in between clumps of the branching colonial coral Lophelia pertusa (where lots of single animals live within the same skeleton).
It is the hard skeletons that are of increasing interest to palaeoceanographers like me. As the corals grow, the chemical composition of their skeletons is – at least in some part – dependent on the conditions of the seawater around them. For example, in living corals, the lithium to magnesium ratio (i.e. the concentration of lithium divided by the concentration of magnesium: written Li/Mg) in the skeleton is related to water temperature. As temperature goes up, the Li/Mg ratio goes down.
We can use this relationship by employing the skeletons of long-dead corals as a kind of thermometer of the oceans, going back beyond when we had actual thermometers to take measurements with. Dead coral skeletons can be collected and dated and their Li/Mg ratio measured, allowing reconstructions of ocean temperature stretching back many thousands of years. We call the Li/Mg ratio a ‘proxy’ for temperature, because it is not a direct measurement but an estimate based on chemistry.
However, a problem with such methods is that seawater temperature is unlikely to be the only control on the Li/Mg ratio in coral skeletons. For example, corals living at different locations can have very different Li/Mg ratios, even if they were found growing at the same temperature. This means that something else must be going on. These effects also vary between different coral species. The skeletal Li/Mg ratio may be partially controlled by the speed at which the corals grow their skeletons, or the volume of organic matter trapped within the skeleton. Such variability in coral chemistry can make reconstructing past ocean temperatures very difficult, because you cannot be sure that what you are recording is actually temperature, rather than some other change in conditions or coral growth.
The more species and sample locations we can use to test the relationship between the Li/Mg ratio and temperature, the easier the interpretation of our results will become. This means that the live corals we collect on this expedition can serve multiple purposes, being not only useful for genetics but also for enhancing our knowledge of coral chemistry and thereby informing our investigations of past oceanic and climatic change.
Text by Peter Spooner. Peter has just completed his PhD in deep-sea coral geochemistry at Bristol University.