Coral Growth and Old Water
There are some interesting and smart people aboard, and I will try to tell one of the stories I have pieced together, some from what I already knew, but much from what I have learned on the boat. The Deepwater Canyon Expedition, using the ROV Kraken 2, has documented in Baltimore Canyon several types of corals and many other invertebrates as well as fish. There are several different types of anemones, including a pink one that looks superficially like the Condylactus sp. found in the Caribbean. There are sponges, quill worms, a red squid, red crabs, spider crabs, several types of squat lobsters, shrimp, lots of krill-like shrimps, amphipods, and recently deep sea mussels. There are also some colonies of soft coral.
We are mainly interested in three families of corals: primnoids, plexaurids, and paragorgiids. The main deepwater coral we are finding on this expedition is Paragorgia arborea. It has a worldwide distribution, in the North Atlantic, Gulf of Alaska, and off the coasts of New Zealand and Argentina. Like all Paragorgiids, it is an octocoral, which refers to the 8 tentacles found on the polyps. The polyps are the individual animals that together form the coral colony. Another large group of corals, the hexacorals, have 6 tentacles per polyp. There are hard and soft hexacorals. The hard, stony hexacorals, which includes Lophelia, are the reef building corals in the tropics with which most people are familiar. We have not seen any Lophelia in Baltimore Canyon, although it is the primary reef-building species in the deep sea.
Paragorgiids are unusual in that they lack a stony skeleton typical of most branching gorgonians. The large colonies are supported by calcitic sclerites, also called spicules. Because of the calcite making up their supporting skeleton, Paragorgia can live deeper than Lophelia, which needs aragonite instead for its skeleton. Aragonite and calcite are both crystalline forms of calcium carbonate.
Deep sea corals like Lophelia are limited to how deep they can grow by the aragonite saturation horizon. In order for stony corals to grow, the water they live in needs to be saturated with aragonite, and deep ocean water is not saturated with aragonite. Most stony corals need aragonite saturation levels of 3.5-4.0. In the deep sea, the levels are sometimes just 1.0. Decreasing aragonite at depth may be a negative consequence of climate change for deep sea corals and our team is collecting some data to examine the aragonite levels.
Aragonite levels in deep ocean water are lower because the water is older. When the ocean water was at the surface it absorbed carbon dioxide from the atmosphere. Once the currents go deep, no more atmospheric carbon dioxide is absorbed. The age of the water is determined by measuring the ratio of the radio isotope carbon-14 to carbon-12 in dissolved carbon dioxide, as carbon-14 decays at a steady rate. Basically, deep ocean water has less carbon-14, because it has not been at the surface for a long time. Due to the lack of photosynthesis and the respiration of deep sea animals, old water is higher in carbon dioxide, and therefore lower in aragonite. The carbon dioxide pulls aragonite from the water, lowering the aragonite level, and raising the aragonite saturation horizon.
As the oceans absorb more of the carbon dioxide produced by the burning of fossil fuels, the depth of the aragonite saturation horizon able to sustain stony coral growth is rising. When the saturation horizon moves higher than some Lophelia live, they have to work harder to grow their skeletons. Since Lophelia often lives atop undersea mounts, if the saturation horizon rises above the mount, the Lophelia may be doomed. The animals can control the environment under their tissue and compensate up to a point, but if the aragonite saturation horizons are too high, or are high for too long, then there is a tipping point and the coral can no longer survive.
There are differences between the Atlantic and Pacific Oceans. Deep Atlantic Ocean water is younger, has more oxygen, and more aragonite. North Atlantic surface waters are generally 100-200 years old. Deep Atlantic Ocean water averages 200-300 years old. From there, the currents move into the Antarctic, the Indian, and Pacific Oceans. Deep North Pacific Ocean water is 1400-1800 years old. It has more carbon dioxide, less aragonite, and less oxygen than deep Atlantic Ocean water. Ocean water gets recharged with oxygen and equilibrates with the atmosphere as it passes through the Arctic Ocean and reenters the Atlantic Ocean.
When deep ocean water upwells along the coasts, the water delivers nutrients like nitrate and phosphate that bacteria and phytoplankton can use, which feed copepods and fish populations in turn. But other animals that need to fix calcium have a harder time. When oyster larvae develop into spat, the younger Atlantic Ocean water has a high enough aragonite saturation level for them to grow shells. But the deep Pacific Ocean water currently does not. Pacific oysters on the West coast of the United States have not successfully reproduced in the wild since 2004, and hatcheries for oyster spat have closed down. The water has too much carbon dioxide, and too little aragonite, for them to survive.
The old deep Pacific Ocean water that is preventing the oyster spat from forming shells has not yet increased in carbon dioxide levels. But the deep currents are upwelling and coming onshore, as changes occur in how ocean currents flow. The extra carbon dioxide the oceans are absorbing today will possibly take hundreds if not thousands of years, based on the age of old Pacific Ocean water, to return to pre-industrial levels.
I would like to thank Dr. Brendan Roark, a paleoceanographer from Texas A&M University, for several interesting conversations about ocean water. Dr. Roark unfortunately had to leave the cruise early. Dr. Sandra Brooke, Marine Conservation Institute, also helped me understand this interesting and technical topic.