The depth where coral skeletons start dissolving has risen more than 100 metres off California since 1994

In June we published a short piece here saying that the aragonite saturation horizon is rising and that deep-water corals are dissolving because of it. The direction was right and we gave no figures, no region and no source, which meant a reader had no way to check the one thing that matters: how fast, and where.

What the horizon is

Corals build skeletons out of aragonite, a form of calcium carbonate. Whether aragonite dissolves or holds depends on how much carbonate ion is available in the water around it, and that falls with depth: colder, higher-pressure, more carbon-rich water holds less. Somewhere down the water column there is a level below which aragonite starts to dissolve rather than form. That level is the aragonite saturation horizon.

It is not a fixed depth. Carbon dioxide the ocean absorbs from the atmosphere makes surface water less saturated, and the horizon moves upward through the water column as it does. An animal that cannot move is either above it or below it.

How fast it is moving, and where

Feely and colleagues measured decadal change in aragonite and calcite saturation across the Pacific and published it in Global Biogeochemical Cycles in 2012. Across the basin the horizons are migrating upward at about 1 to 2 metres a year.

The number that should interest anyone on this coast is regional rather than basin-wide. In the California Current, between 135° and 120° west, the saturation horizon west of the continental shelf rose by more than 100 metres between the 1994 WOCE survey and the repeat occupation. That is not a gentle basin-scale drift; it is a large local displacement over about fifteen years, and the paper is explicit that in this region decadal changes in circulation can dominate over the direct chemical signal.

Which matters for how the story is told. Upwelling brings old, carbon-rich water onto the shelf here. The California Current is one of the places where ocean acidification stopped being a projection first, and part of the reason is physical delivery, not only chemistry.

What it means for the animals

Cold-water corals are not the reefs in the postcards. They grow in the dark, without symbiotic algae, feeding on organic matter that drifts down, and they grow slowly — a colony can be older than any building in California. They also build the physical structure that other deep species live in, so losing them is losing habitat, not just an organism.

The commonly cited projection is that around 70 percent of known cold-water coral locations will sit in undersaturated water at some point this century, with some crossing much earlier. Treat that as an estimate over a poorly mapped population: the deep sea is surveyed thinly, and a projection about places nobody has visited carries the uncertainty of the map underneath it.

Why a Sacramento paper covers this

Because the same water arrives here. The California Current feeds the upwelling that supports the salmon runs the Delta depends on, and the shellfish industry on this coast was the first in the world to be measurably damaged by acidified water, in Oregon and Washington hatcheries. The deep corals are the part of the system with the least ability to move and the longest memory. What happens to them is a reading on the water that arrives at the Golden Gate.

What we do not know

Whether a given colony dissolves depends on local chemistry, on how much organic matter is raining down, and on whether the animal can maintain saturation at the site of calcification — several deep species appear to hold on below the horizon for a while. 'Below the horizon' is a chemical threshold, not an obituary, and the distance between the two is where the actual research is.