Oceans, often perceived as vast blue expanses, are far from uniform. Even on clear days, a constant rain of organic matter and mineral particles – often described as “marine snow” – drifts downwards, playing a critical role in the ocean’s ecosystem and, increasingly, in understanding the planet’s climate future. Scientists are now focusing intensely on the composition of this marine snow, particularly the trace elements it carries, to unlock insights into ocean health and the global carbon cycle.
This seemingly delicate snowfall isn’t just aesthetic; it’s a complex biogeochemical process. The majority of iron in the ocean, a crucial micronutrient for phytoplankton growth, is actually particulate, not dissolved, according to research from the GEOTRACES program. Understanding the characteristics of these particulate trace elements – how they form, where they accumulate, and how they interact with marine life – is becoming increasingly vital as climate change alters ocean conditions. The study of marine snow and its components is a key area of focus for oceanographers seeking to predict how the ocean will respond to increasing carbon dioxide levels and warming temperatures.
The GEOTRACES Initiative and Particulate Matter
The GEOTRACES program, an international effort to map the distribution of trace elements in the ocean, has been instrumental in shifting scientific understanding towards the importance of particulate matter. Launched in 2010, GEOTRACES has expanded measurements of trace elements, recognizing their roles as key nutrients, potential contaminants, and indicators of ocean processes and past conditions. As Benjamin S. Twining notes in a 2024 Oceanography article, the program has been “particularly valuable for growing our understanding of this fraction” of ocean material. Read more about the GEOTRACES program here.
Traditionally, oceanographic research focused heavily on dissolved elements. However, GEOTRACES data demonstrates that chemically labile particulate iron – the form most readily available for biological use – is often more abundant than dissolved forms, especially near continents and in the deep sea. This finding underscores the need to incorporate the particulate fraction into both conceptual and numerical ocean models. The program’s work highlights that simply measuring dissolved nutrients provides an incomplete picture of ocean fertility.
Microplastics Join the ‘Snow’
Whereas trace elements are a natural component of marine snow, human-generated pollutants are increasingly becoming entangled within it. Microplastics, defined as plastic particles less than 5 millimeters in size, are now ubiquitous throughout the world’s oceans. A 2021 study estimated up to 24.4 trillion pieces of microplastics are floating in the upper layer of the oceans alone. These particles originate from sources ranging from cosmetic microbeads to the breakdown of larger plastic debris, including fishing nets, and tires.
Divers are playing a crucial role in monitoring this pervasive problem, leveraging their unique access to underwater environments. Microplastics pose significant health risks to marine creatures, often being mistaken for food and causing physical harm. They similarly enter the food chain, potentially impacting human health. The presence of microplastics in marine snow adds another layer of complexity to the already intricate dynamics of the ocean’s particle cycle.
Global Distribution and Accumulation
Recent research, published in Nature in April 2025, reveals that microplastics are now present throughout the entire water column, from the surface to the deepest trenches. The study, compiling data from over 1,200 sampling stations, confirms massive microplastic accumulation in subtropical gyres – large rotating ocean currents like the North Pacific and South Atlantic Gyres. While these accumulations don’t form “plastic islands” as previously hypothesized, concentrations can exceed 10,000 particles per cubic meter in some locations.
Researchers have categorized microplastics into size groups, finding that smaller particles (1–100 μm) tend to distribute more evenly throughout the water column due to their slow sinking rate. In the Atlantic Ocean, microplastics in the 32-651-μm size category in the top 200 meters average 2,200 particles per cubic meter, representing an estimated mass of 11.6-21.1 million metric tons. More details on the distribution of subsurface microplastics can be found here.
Connecting the Surface and the Deep
Understanding how particles, both natural and anthropogenic, sink and collect in the deep ocean is crucial. Research published in 2024 highlights that the physical motions of seawater influence particle collection in the deep ocean, classifying particles into nine groups based on their trajectories and associated flow structures. This study demonstrates the complex interplay between ocean currents and the fate of sinking particles.
As scientists continue to investigate the composition and dynamics of marine snow, the implications for climate modeling and ocean health become increasingly clear. The ongoing research promises to refine our understanding of the ocean’s role in the global carbon cycle and the long-term consequences of plastic pollution. Future studies will likely focus on the interactions between trace elements, microplastics, and marine organisms within the marine snow, providing a more holistic view of this critical ocean process.
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