Corals
Past environments
Paleontology is the study of the history of life, looking at how organisms evolved and what their lives were like in the past.
We can use various techniques to understand how the cold-water corals that we see today came to live where they are, and what the oceans surrounding them were like in the past. However, the only way we have of learning about the history of corals is by looking at their old preserved skeletons or fossils.
The element of luck is very important in the study of the history of corals, as finding well-preserved fossil locations is very hard. Plus, even when a key fossil is found, it is not that easy to identify the animal itself from only a fossil of its skeleton. So, paleontology is a challenging area of research for cold-water corals.
Thanks to Melanie Douarin at the University of Nantes for the information for this section.
Fossil corals
Although corals first appeared in the Cambrian period, 542 million years ago, the stony corals (Scleractinia), appeared approximately 237 million years ago. Scleractinian corals include the corals which make the enormous Great Barrier Reef, but also the cold-water corals which we are interested in. The difference is, cold-water corals do not have algae living in their cells (they are ‘azooxanthellate’), while most tropical corals obtain the majority of their energy from these algae.
The first azooxanthellate coral structures (corals lacking photosynthetic algae) are at least 145 million years old. So far, we know from geological records that Lophelia sp. and some other cold-water coral species appeared at least 23 million years ago. As we keep finding more and more of these fossilized corals, we think that cold-water corals started to become much more widespread from that point onwards.
Coral mound formation
Cold-water corals are important in the formation of cold-water coral reefs or mounds, which are impressive features on the sea floor. These reefs and mounds are commonly found along the European continental margin and occur from 50 m depth down to more than 2000 m. Some of these mounds are only a few metres high, such as the “shallow” reefs of Mingulay. But, some are more than 350 m high and up to a few kilometres in diameter, such as the giant mounds at Rockall bank in the Atlantic Ocean.
These mounds are formed by a complicated set of processes. First, a coral larva will settle on a hard surface, such as bedrock or a dead coral skeleton. Then, when environmental conditions are favourable (i.e. its warm enough, there’s enough food, the currents are the right speed), the corals start to grow and form complex reef structures. These reefs then attract more organisms, which use the reefs as a home and place to find food. The 3D coral skeleton will also attract sediment, which will accumulate around the framework and therefore the mounds will grow. Then, when conditions aren’t favourable for coral growth, the tops of the mounds may be eroded, as the corals can’t grow fast enough to keep up with the natural erosion of the currents and other organisms.
The periods of favourable and unfavourable environmental conditions for mound and coral growth are linked to geological time periods known as glacial and interglacial periods, which occur within an ice age. These periods last thousands of years; interglacial periods have warmer global average temperatures while glacial periods have cooler than average temperatures.
Interglacial periods, such as the last 11,000 years, have been favorable for coral growth and thus coral-mound development. However, to date, we haven’t found any evidence of the cold-water coral Lophelia pertusa from glacial periods in the Northeast continental margin. We therefore assume that cold-water corals could not grow in these environmental conditions, and so mound structures were not built like they were in interglacial periods.
Corals as archives
The global effects of climate change are now a major concern for us and future generations. To predict the future climate and the associated environmental consequences, it is important for scientists to have a good record and understanding of past climate variations. Much of this information is in our oceans, and paleo-oceanographers need robust records of water mass history to truly understand past climate variations. In the last few decades, oceanographic research has focused on looking at the relationship between deep and shallow water masses, and many scientists are now using cold-water corals to help them understand the past.
Cold-water corals can be used as archives because their skeletons contain information on the chemistry of the oceans at the time the skeleton was made. We can work out the age of the coral, how fast they grew, and what their environment was like, all by examining their aragonite skeleton. The coral Lophelia pertusa is particularly useful as an archive as is found at many depths, and grows relatively fast. In fact, it grows from 4 to 26 mm per year, which may not seem like much, but does mean that we can get a lot of detailed information about the past climate.
Dating corals
To work out how old coral skeletons are, we can examine their isotopes, which are atoms of the same element that differ in atomic mass. Isotopes can either be stable or unstable; stable isotopes have the same number of protons and neutrons and these remain the same over time, whereas unstable isotopes have too few or too many neutrons compared to protons.
A coral’s aragonitic skeleton is mainly composed of calcium, oxygen and carbon but it also contains other elements you may not even think of, such as uranium or magnesium. These elements have several isotopes, some stable and some unstable. Unstable isotopes are commonly referred to as radioactive isotopes, and are used to date geological features. As all the elements and their isotopes from the coral’ skeleton are, in theory, the same as the seawater in which the skeleton was formed, there is the same proportion of each element and their isotopes in the coral’s skeleton as in the seawater.
Unfortunately, things are a little bit more complicated than that. The growth of coral skeleton is also affected by the animal itself, as the animal can change the seawater properties during the process of skeleton construction, which is called biomineralisation. The biomineralisation process is still poorly understood and so we have to be careful when using cold-water corals as paleo-markers of past climate.
In the last few decades, most of the geochemical studies on cold-water corals have focused on two research areas.
The first area focuses on dating the absolute age of the coral skeletons using 230Thorium/Uranium (Th/U) and/or 14C, and using coupled 230Th/U-14C dating to reconstruct past ocean ventilation.
To do this, we can use sediment cores from coral mounds/reefs, which are cylindrical sections of sediment extracted using special underwater drills. These sediment cores allows us to see favourable periods for coral growth and mounds/reefs expansion in that one location. In other words, where there is a lot of coral growth, we know environmental conditions were favourable, and where there is no coral growth then conditions weren’t favourable (assuming that in the past, corals ‘favourable’ conditions were the same as today).
The second group of studies aims to gain a better understanding of the processes affecting the chemistry of cold-water coral skeletons and to establish new calibrations for using cold-water corals as palaeo-proxies, such as looking at the role of temperature in the biomineralisation processes.
Dating methods
Lophelia pertusa has a skeleton made of aragonite (carbonate), which allows the use of radiocarbon dating and 230Th/U dating.
Radiocarbon dating is a technique based on the decay of the radioactive isotope of carbon, 14C. The radioactive isotope of carbon, 14C, is produced by cosmic ray bombardment of 14N in the upper atmosphere. After their formation, 14C isotopes are oxidized to CO2 and exchange between the different carbon reservoirs. In the North Atlantic, where North Atlantic Deep Water (NADW) is formed, the 14C isotopes from the surface water are carried into the deep ocean, closing off carbon exchange with the atmosphere and allowing the radiocarbon decay clock to start ticking. The radiocarbon age on a marine (bio)carbonate includes both the age of the mineral and the age of the water mass from which the mineral formed.
As a dating method, once the coral skeleton is formed, the 14C isotopes present are in equilibrium with the ocean and start decaying to become stable. By measuring the remaining quantity of 14C compared to the carbon total, the period of carbonate formation can be estimated.
The other method, U/Th dating is based on the radioactive decay series of the isotopes of Uranium (238U), which is also present in coral’s skeleton. Eventually these isotopes will become stable as 230Th isotopes. The age of the aragonite mineralization can be calculated by measuring the present-day activity ratios of the isotopes from the radioactive decay series.
The difference between the ages obtained by these two methods (radiocarbon and 230Th/U) allows the estimation of the water mass age (reservoir age), which is a function of how quickly the ocean is overturning and mixing with atmospheric CO2. Since the aragonite of cold-water corals is ideally suited for analysis by both U-series and radiocarbon methods, several studies have used this technique to reconstruct past ventilation.
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