Marine Snow and the Biological Carbon Pump
Fecal Pellet Sinking Rates, Salp Mucous Houses, and the Carbonate Compensation Depth
Peering through the quartz window of his steel Bathysphere off Bermuda in the 1930s, William Beebe watched a continuous blizzard of white flakes drifting downward through black water. Noboru Suzuki and Kenji Kato coined the term marine snow in 1953. This steady fallout of dead plankton and fecal pellets exports 10 gigatons of carbon from the surface annually.
By Stokes's law of viscous drag ($v \propto r^2 \Delta \rho / \eta$), a solitary two-micrometer phytoplankton cell sinks at just millimeters per day. Bacteria consume 100 percent of such cells in the top 100 meters. Carbon reaches the abyss only because zooplankton compact millions of cells into membrane-wrapped fecal pellets that plummet at 100 to 1,000 meters per day.
Pure organic matter ($1.03\text{ g/cm}^3$) is barely denser than seawater ($1.025\text{ g/cm}^3$) and stalls in midwater unless weighted by mineral shells. Analyzing global sediment traps in 2002, Christine Klaas and David Archer proved the mineral ballast effect. Over 80 percent of organic carbon reaching the abyssal floor is carried down by dense calcium carbonate coccoliths and opal diatom silica.
How scattered microscopic diatoms stick together in turbulent water to form centimeter-sized snowflakes was solved in 1993 by Alice Alldredge, who discovered Transparent Exopolymer Particles. When nutrient-starved phytoplankton sit in bright sunlight at the peak of a bloom, they exude acidic polysaccharide gels. Those gels cross-link with calcium ions into an invisible cobweb glue.
Floating in the mesopelagic zone are finger-sized chordate tunicates called giant larvaceans (Bathochordaeus), which secrete meter-wide mucus filtration balloons around themselves to strain food particles. Bruce Robison showed in 2005 that the animal abandons its house whenever the mesh clogs after twelve hours. Discarded mucus mansions sink to the 4,000-meter floor at 800 meters per day.
Barrel-shaped pelagic salps (Salpa thompsoni) link into 15-meter jet-propelled chains that strain sub-micrometer particles across an internal mucus net. During an algal bloom, salps clone themselves to double in days and excrete brick-dense fecal pellets sinking at 1,500 meters per day. A single swarm can dump 4,000 metric tons of carbon into the abyss per night.
Deploying free-drifting sediment traps across the Pacific in 1987, John Martin discovered that sinking marine snow obeys a universal power-law decay curve: $F(z) = F_{100}(z/100)^{-0.858}$. Midwater bacteria and animals intercept and respire 70 percent of falling organic flakes within the top 500 meters. Only 1 to 5 percent ever touches the abyssal mud.
Why fecal pellets do not all shoot straight to the bottom untouched is due to specialized midwater copepods such as Oncaea and Oithona. Detecting the hydro-chemical wake of a falling pellet from millimeters away, these crustaceans either eat it (coprophagy) or shred its wrapper into slow-sinking crumbs (coprorhexy). Their grazing keeps nutrients recycling upstairs.
Alongside passive fallout, vertically migrating lanternfish, krill, and shrimp operate an active carbon elevator across the pycnocline. Migrants graze in the sunlit top 100 meters at night and swim down to 500 meters at dawn before digesting their meal. By defecating and respiring below the upper microbial gauntlet, they drive 20 percent of carbon export.
First mapped by the HMS Challenger expedition in the 1870s, the Carbonate Compensation Depth sits around 4,500 meters like an inverted alpine snowline. Above 4,500 meters, white calcareous ooze blankets mid-ocean ridges like snow. Below 4,500 meters, 450 atmospheres of pressure and cold, $\text{CO}_2$-rich water dissolve every falling limestone shell before burial, leaving brown clay.
The Carbonate Compensation Depth sits at 5,000 meters in the young North Atlantic yet rises to 4,200 meters in the North Pacific. Because the deep North Pacific sits at the 1,000-year tail end of the global conveyor, its water has accumulated massive respiratory carbonic acid. That extra acidity dissolves falling shells nearly a kilometer higher.
The ocean holds 38,000 gigatons of dissolved inorganic carbon—fifty times the atmospheric pool. Jorge Sarmiento and Robbie Toggweiler proved in 1984 what would happen if marine phytoplankton died tomorrow. Without the biological carbon pump raining snow downward, upwelling would vent trapped deep carbon and raise atmospheric carbon dioxide by 150 to 200 parts per million.