Deep Future

by Curt Stager

  The more such energetic hydrogen ions there are floating around in solution, the more acidic the solution is and the more corrosive it is to materials such as seashells and limestone. Keeping such carbonate-rich materials in a solid state rather than dissolving in acidified water is a bit like trying to stack hay in a hurricane.

  Nobody expects the oceans to become fully acidic in a vinegary sense; it would take an unrealistically large and rapid CO2 release to do that. Rather, they’re shuffling closer to the acid-base border but staying on the basic side of the line. Chemists mark that border with a neutral pH value of 7, thereby segregating higher pH values into the realm of basic conditions and lower values into the realm of true acids (pH, or potentia hydrogenii, is the formal measure of hydrogen-ion concentrations). Marine scientists have watched with increasing concern as mean ocean pH has dropped during the last century. It might not sound like much, but because the numbers are drawn on a logarithmic scale, the recent pH drop of a tenth of a unit means that average marine acidity has already increased by about one-quarter. If the pH eventually falls by 0.3 to 0.4 units, as most scientists predict it will by this century’s end, it will represent more than a doubling of acidity.

  The most immediate victims of ocean acidification are likely to be organisms that build shells or other body structures with acid-soluble carbonate minerals like calcite and aragonite. Aragonite is the more soluble of the two, and species that rely upon it are at greatest risk. According to one source, much of the Southern Ocean’s surface will be acidic enough to destroy aragonite by 2030 AD, and shallow waters in the Arctic Ocean could reach that point even earlier. If we follow an extreme-emissions path, the polar seas will be aragonite-corrosive from top to bottom by 2100 AD and will stay that way for thousands of years.

  What kinds of organisms are at risk? A quick rundown of widely known sea creatures produces an inventory that many of us would be sorry to lose. Alaska king crabs and California abalones. Caribbean conch snails and spiny lobsters. Chesapeake Bay oysters and blue crabs. Classic Maine steamer clams and scallops. Irish cockles and mussels. Countless forms of starfish, sand dollars, sea urchins, and barnacles, and even the beautiful limestone edifices of coral reefs.

  It can be difficult to imagine how a creature that lacks hands or even much of a brain could craft a delicately sculpted shell from much the same stuff that Michelangelo chipped and smoothed to produce his marble masterpieces. Some aspects of the process remain mysterious even to experts, but the fundamentals are fairly straightforward. Seawater carries many dissolved substances: salt, for instance. It also contains calcium atoms that have been eroded from rocks and soils on land, as well as carbonate and bicarbonate molecules that are likewise released into watery solutions by mineral weathering. Marine calcifiers gather calciums and carbonates from the surrounding liquid and stick them together to build crystal lattices of calcite and aragonite. Or, more properly, specialized cells in their body tissues do it, bit by molecular bit. Slowly, the composite molecules of calcium carbonate accumulate on the thin outer lip of a shell or the rim of a polyp’s home cup on a branch of coral, spreading outward in orderly rows like raindrop rings dilating on the surface of a puddle.

  Marine calcifiers dedicate much of their food energy to secreting and maintaining their shells. If the pH of the seawater around them falls closer to the acidic range, it becomes more and more difficult to coax carbonates out of solution. In addition, it requires more energy to repair the corrosion that results when hordes of hydrogen ions raid the carbonate storehouses of a shell or coral formation. At some point on the acidity spectrum, more calcium carbonate pairs break apart than are built, and as more and more hydrogen ions attack them, shells begin to crumble brick by molecular brick.

  Thais snails and their barnacle prey exposed at low tide on the coast of Maine. Curt Stager

  We don’t yet know exactly which organisms can adapt to acidification and which cannot. We don’t even know the full life histories of most marine animals yet, much less their potential responses to freakish chemical changes in seawater. But we do know that, at least in theory, any acidity increase could make it more difficult for a shell-producer to lure carbonate into solid form and keep it there. And in the Darwinian struggle for existence, the extra physiological investments required to overcome that chronic erosion might mean the difference between breeding and dormancy, growth and stasis, or even life and death. In the worst case, a hapless creature’s hard parts might simply melt away altogether.

  A seminal overview published in 2005 by Britain’s Royal Society has shown that it is already too late to avoid some of these anticipated changes. Because of delays in the transfer of dissolved gas into the deep ocean, it will take many centuries for marine chemistry to return to normal even if we could halt all of our emissions right now. David Archer has estimated that the recovery process could take as little as 2,000 years or as many as 10,000, depending on whether we follow a moderate-or an extreme-emissions scenario and on how rapidly the neutralizing effects of geological weathering can restore the oceans to their present chemical balance. If atmospheric CO2 concentrations eventually reach 500 to 550 ppm, aragonite will dissolve in the cold circumpolar seas. If CO2 concentrations triple, then calcite will dissolve there, too. And an all-out 5,000-Gton emission could destroy both aragonitic and calcitic shells even in tropical waters.

  Why are chilly places so much more vulnerable than the rest? It’s because cold water can hold more dissolved gas than warm water does, so frigid high-latitude seas retain more CO2 in solution than those at lower latitudes. Because of this temperature effect, aragonite corrosion will strike the polar regions first and hardest during winter, their coldest season.

  Though most media sources still ignore this issue, planktonic life-forms that are barely visible to the naked eye have already become the poster children of ocean acidification alerts among marine scientists. Considering the value of beauty in spreading a message effectively, it helps that some of these, most notably the micromollusks known as pteropods (TERR-o-pods), are so lovely that some biologists call them “sea butterflies.” Although they share a phylum with snails, slugs, and clams, they don’t creep or burrow as most of their relatives do. They swim or, more properly, fly under water. Much of their soft tissue consists of wispy, parachutelike sails or even paired wings, as the English translation of their scientific name attests; pteropod means “winged feet.”

  Under a microscope, many pteropods resemble translucent baby snails swaddled in billowing sheets of cellophane. In fact, real snail larvae do look a lot like pteropods, but they become clunky bottom-dwellers when they grow up. Pteropods, in contrast, seem to stay forever young. They drift about in the sunlit upper layers of the sea in huge numbers, feeding on even tinier prey and soaking up dissolved minerals with which to build fragile shells of soluble aragonite. Plankton sampling nets the world over are now hauling in pteropods whose shells show disturbing signs of carbonic acid etching and pitting, and lab studies seem to confirm the cause. To a trained eye, this is a warning flag.

  Pteropods are well worth caring about in their own right, but they are ecologically important, too. In the icy waters surrounding Antarctica, a single bucket-haul can contain hundreds of thousands of them, and they support food chains that link them to penguins, seals, and whales. In Antarctica’s Ross Sea, microscopic pteropods can outweigh the immense swarms of shrimplike krill. Fortunately, some two-winged pteropods—called “sea angels”—lack shells and may therefore be immune to acid corrosion. Although they have no shell, they aren’t necessarily defenseless. At least one Antarctic species produces a toxin so potent that other planktonic animals occasionally seize a guardian “angel” and carry it around like a shield to protect them from larger predators. Acidification might simply favor angel-type pteropods over the shelled butterfly types, but that’s not necessarily good news in terms of marine ecology. Replacing shelled species with toxic ones could be disastrous for other creatures that rely on pteropods
for food.

  Meanwhile, another tiny calcifier may be in trouble, as well. Satellite images occasionally show milky clouds staining hundreds of square miles of ocean off the coast of Alaska. These are single-celled algae that, at certain stages of their life cycles, look like microscopic bowling balls plastered with cream-colored hubcaps. Their long scientific name, coccolithophore, refers to the spherical form (coccus) of the cells and the mineralized (litho) calcite armor plates that they carry (phoros) around with them.

  The cumulative mass of calcite discs floating in a single Alaskan coastal bloom can weigh more than a million tons. Like tiny plants, coccolithophores are photosynthetic and they represent a vital base of marine food chains in regions such as these. But although their platelike armor is made of calcite rather than the more vulnerable aragonite favored by pteropods, some are already beginning to dissolve.

  The Royal Society calculates that most polar ocean waters will soon be undersaturated with respect to aragonite; that is, aragonite will dissolve there if not maintained, at high physiological costs, by the organisms that secrete it. Undersaturation is then expected to spread to warmer latitudes, slowly inching toward the tropics from both poles. But long before the sparkling waves of Jamaica and Tahiti turn deadly to many of their residents, other processes will work less visible mischief in the deepest, darkest recesses of the marine world.

  Water contracts as it cools, so cold water is denser and heavier than warm water. Because of this, the long winter nights and record-breaking cold of Antarctica help to produce some of the world’s densest seawater. The freezing of briny water at the surface also leaves residual salt in solution, which further increases density. Gravity takes over and pulls the heavy, near-frozen liquid down to the bottoms of the southern ocean basins, where it creeps like a sluggish submarine flood that can be hundreds of feet thick on the abyssal plains and even thicker in the deeper trenches and troughs. So much Antarctic bottom water forms each year that it blankets more than half the seafloor beneath the Atlantic and Pacific oceans, stopping its northward crawl only after Arctic bottom water overruns it; sometimes the contact zone lies as far north as the Grand Banks of Maine and eastern Canada.

  Normally, this is a good thing. It’s too dark down there for plants or algae to photosynthesize, so deep-living marine animals would suffocate without that flow. A flowerlike sea anemone need only sit still down there as slow-moving bottom currents deliver food particles to it. And as those cold, watery breezes waft gently through the animal’s delicate tentacles, they also represent its sole source of oxygen soaked from the churning polar winds topside.

  Unfortunately for marine life in the Age of Humans, the other gases that tumble about in those polar winds also join oxygen in the deep-sea voyage. And that includes carbon dioxide.

  For many of us who think of greenhouse gases only in terms of climatic change, the polar downwelling zones may seem to be helpful storm sewers through which our CO2 problems can drain down and out of the air. But in this case, out of sight and out of mind does not mean out of existence. Every CO2 molecule that is pulled under in this manner represents one more carbonic acid molecule added to the ocean. And if it enters near the poles, chances are good that its acidity will sooner or later contaminate the deep-sea buffet line, a cold, dark realm that is unlike anything most of us have ever seen. Though the foundations of the oceans cover most of this planet, they are still largely unexplored by marine biologists. With little or nothing to tether our imaginations to, we tend to act as if such places don’t even exist. But they do.

  Despite the chill, the intense pressures, and the darkness of the deep, it is home to many species. Most of its denizens live on or within soft organic oozes, feeding on a rain of dead plankton and the occasional whale carcass. And the deep-sea bestiary is a bit light in the lovably lovely department, favoring instead the utterly fascinating. Octopi with arm suckers that glow in the dark. Vampire squids whose very name speaks volumes. Toothy, widemouthed female angler fish whose puny mates fuse permanently to their posteriors, dangling there like vestigial appendages. And in frigid waters off the coast of Antarctica, pale yellow, spindly-legged “sea spiders” the size of dinner plates that step daintily among the hulks of enormous lumpy sponges.

  Population numbers below a few thousand feet depth tend to be low because there is generally not enough food down there to support much living biomass. Even so, the biodiversity can be staggering. One stretch of soft sediment two miles down and several miles out from New Jersey and Delaware yielded nearly 800 species captured in narrow vertical core tubes. And with the help of submersibles, biologists have recently found an amazing wealth of colorful sponges, crabs, and fish along the walls of mile-deep canyons in the Bering Sea. Nets and dredges have pulled up so many new species in recent years that some scientists postulate a global deep-sea tally of more than 10 million.

  Most relevant to this story are the many creatures that don’t swim at all and whose hard parts contain carbonate minerals. Cameras attached to remotely operated vehicles relay images of feathery red crinoids that resemble flowers but whose closest familiar relatives are starfish. Bristly sea cucumbers dot the bottom like humpbacked porcupines. Tusk-shaped scaphopod snails wriggle through the muck in search of detritus to swallow, and deepwater cuspidarid clams break ranks with their filter-feeding, chowder-stocking kin in order to hunt fleshy prey. Hermit crabs scuttle about, lugging their snail shell homes with them. And perhaps most surprising of all, there are corals, lots of corals.

  Common wisdom holds that coral reefs exist only in warm tropical shallows, but in fact, two-thirds of all known coral species live in deep, cold habitats, far outnumbering the ones found in all the famous near-surface reefs of the Indo-Pacific and Caribbean. Cold-loving corals can be surprisingly abundant in waters ranging in depth from several hundred feet to more than two miles (3 km). Among the species turning up in dredges and submersible photos are precious black corals, gorgonian sea fans, ivory tree and red “bubblegum” corals, stony scleractinian corals, and twiglike bamboo corals. For some reason yet to be determined, large deepwater reefs are more common in the North Atlantic than in the North Pacific where the corals tend to be more solitary, forming diverse but diffuse aggregations scattered across the seafloor. In contrast, many Atlantic species fuse into mounds hundreds of feet tall, piling high atop the dead rubble left by their predecessors, and some form reefs whose dimensions are measured in miles. As with shallow reefs, these corals provide shelter and sustenance for a riotous wealth of other animals. One recent survey found more than 1,300 species living in, on, and around deep-dwelling thickets of Lophelia pertusa, the dominant reef builder of the northern Atlantic. This hidden trove of animal species rivals that of Australia’s Great Barrier Reef.

  The diversity and abundance of deepwater corals reflect the amount of planktonic food particles raining down on them and the nature of the waters bathing them. Under the right conditions, cells in the thin outer skins of individual polyps gradually build cuplike supporting structures of acid-sensitive aragonite. But their stony communal formations grow very slowly when food is scarce and the water is cold; some that were recently found in the lightless depths of the mid-Atlantic took thousands of years to reach their present size. For a colony that barely manages to keep its carbonate balance in the black, so to speak, it’s easy to deduce what acidification could mean.

  The Royal Society suggests that a rapid CO2 rise to 550 ppm by 2100 ad might halve the annual rate of aragonite calcification in tropical near-surface reefs. A team headed by Joan Kleypas of the National Center for Atmospheric Research in Colorado, more conservatively estimates that tropical coral growth could decline by a third or more under those conditions. But however these numbers actually play out, carbonic acid buildups will be especially troublesome for deep-sea corals because cold down-welling currents shuttle huge loads of CO2 to the bottom. Climate scientists Ken Caldeira and Michael Wickett estimate that most of the world’s deepest waters will co
ntinue to dissolve aragonite and calcite even if atmospheric CO2 concentrations level off at a mere 450 ppm, well below the peak values expected in our moderate-emissions scenario.

  Truth be told, we don’t know what acidification will do to most marine organisms during the next millennia of the Anthropocene. We’ve barely begun to discover and describe them, much less subject many of them to laboratory testing. But we already know enough to worry. According to the Royal Society, “ocean acidification will threaten the existence of cold-water corals before we have even started to understand and appreciate their biological richness and importance for the marine ecosystem.” The nets of deep-sea trawlers now smash blindly through dense, brittle tangles of ancient coral in pursuit of equally threatened deepwater fish such as orange roughy, but even in more rugged and inaccessible submarine terrain and even in an enlightened future that might prohibit such destructive practices, there will be no such refuge from acidification.

  On the other hand, some investigations into this subject sound more hopeful notes. A team of British biologists headed by Helen Findlay of the Plymouth Marine Laboratory recently found that certain bottom-dwelling limpets, snails, mussels, and barnacles somehow manage to increase the thickness of their shells under acidified conditions. Lab studies have shown that some species of plankton-dwelling coccolithophore algae can do this, too, and one North Atlantic sediment core analysis has even revealed a surprising trend toward heavier coccolith shells at the study site during the last century of increasing acidity.