Tiny 'marine snow' particles in the oceans defy the laws of physics

Tiny clusters of drifting debris called “marine snow” shower through the ocean every day, ferrying food and carbon from the bright surface to the dark seafloor.

New experiments now show that the smallest flakes plunge more quickly than their larger cousins, overturning a century of textbook fluid dynamics and hinting at fresh twists in the planet’s carbon budget.

Understanding “marine snow” – the basics

This “snow” consists of dead and decaying animals, plankton, fecal matter, mucus, and other tiny detritus that clump together into slow-sinking flakes.

Despite the whimsical name, marine snow plays a serious role in the ocean’s ecosystem – it serves as a crucial source of food for many deep-sea organisms that live far below sunlight’s reach.

Without this steady rain of carbon-rich material, life in the ocean’s abyss would be severely limited.

Beyond its ecological function, marine snow also influences Earth’s climate system. As it sinks, it transports carbon from the surface to the ocean floor, effectively removing it from the atmosphere for centuries or even longer.

This process, known as the biological carbon pump, helps regulate global carbon dioxide levels and mitigate climate change.

Smallest marine snow sinks faster

“It basically means that smaller particles can sink faster than bigger ones,” said Robert Hunt and colleagues at Brown University and the University of North Carolina at Chapel Hill, who led the study.

That counterintuitive sprint happens only in stratified fluids such as the ocean, where density increases with depth.

When a porous flake soaks up salt from the layers it crosses, the added mass beats the drag that normally slows tiny objects.

Because surface‐to‐depth density changes are gentle yet persistent, salt diffusion keeps boosting the flake’s weight all the way down.

Why is this important?

Speed matters. The ocean’s carbon biological pump moves about 10.2  billion tons of carbon a year from sunlit waters toward long‑term storage below the thermocline.

Faster‑than‑expected settling of small fragments may revise estimates of how much carbon actually reaches the deep, especially during spring plankton blooms when the ocean teems with fine detritus.

The same physics could alter the journey of microplastics. A recent global survey found that smaller plastic bits, under 0.004 inch, linger longer at mid‑depths than bigger shards.

If porosity or biofilm growth changes their salt uptake, these particles might fall sooner than models predict and redistribute pollution hotspots on the seafloor.

How marine snow sinks through layers

Hunt’s team built a four‑foot‑long tank whose density changed smoothly from fresh water at the top to salty water below, mimicking the open ocean’s gradient. Carefully controlled pumps dripped the two end‑member waters to create a stable linear stratification.

The researchers then 3D‑printed molds and cast agar shapes (spheres, disks, and rods) with diameters from 0.04 to 0.4 inch.

Like gelatin desserts, the agar particles were riddled with pores that quickly exchanged water and salt with their surroundings.

High‑speed cameras tracked each particle’s plunge through the tank. “We ended up with a pretty simple formula where you can plug in estimates for different parameters and get reasonable estimates of the sinking speed,” said Daniel Harris, an associate professor of engineering at Brown who oversaw the work.

The observations matched the new equation: among spheres, a one‑millimeter bead beat a four‑millimeter bead to the bottom.

For flat or rod‑shaped bodies, the thinnest dimension set the pace, allowing needle‑like fragments to blitz past bulkier shapes of the same mass.

Salt makes porous marine snow fall faster

Classical drag calculations assume an object’s density stays fixed as it falls. Porous bodies break that rule because water flows through them, letting dissolved salt accumulate inside.

The model hinges on porosity, solute diffusivity, and the vertical density gradient. If salt absorption boosts density faster than drag slows descent, size becomes a liability and small wins the race.

This diffusion‑limited regime likely dominates for fluffier aggregates made of phytoplankton remains, fecal pellets, or dust‑laden snow. Less porous grains of sand or shell should still follow the familiar bigger‑falls‑faster script.

Fast-sinking marine snow changes estimates

Biogeochemical models track how quickly carbon sinks, gets eaten, or dissolves on the way down. Underestimating the speed of fine aggregates could inflate estimates of mid‑water remineralization and shortchange deep‑sea sequestration.

NOAA notes that the ocean absorbs about 30 percent of the carbon dioxide humans emit each year, buffering climate change. Tweaking particle speeds could shift where, and how long, that captured carbon stays locked away.

Some climate engineers are exploring methods that encourage fast‑sinking flakes, from adding minerals that ballast algae to growing kelp for deep burial.

Knowing exactly which sizes and shapes drop quickest will sharpen those interventions and gauge possible ecological side effects.

Lessons for plastic pollution

Plastic fragments often pick up biofilms, turning them into tiny sponges that absorb salts and organic goo, much like marine snow.

If that boosts density, millimeter‑scale fibers might leave surface waters sooner than remote sensing suggests.

A plastic snowfall could ferry toxins to bottom feeders and alter food webs once thought safe from floating litter. Policy aimed at surface cleanup alone may miss this hidden pathway.

Researchers also found that elongated pieces settle according to their thinnest axis. That means synthetic fibers shed from clothing, typically long and narrow, may descend even faster than spheres of equal volume.

Testing in real ocean and improving models

Hunt and Harris plan to partner with oceanographers to drop custom particles over deep moorings and compare lab predictions with real‑world drifts. They also aim to explore how temperature gradients, common near polar ice and hydrothermal vents, tweak the balance between drag and diffusion.

Back on land, modelers are already folding the new equation into regional carbon budgets and microplastic fate simulations. Small marine snow flakes, once dismissed as slow movers, are entering the spotlight, reminding scientists that size is only skin deep when salt can seep inside.

The study is published in Proceedings of the National Academy of Sciences.

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