Rain looks deceptively simple. Water falls from the sky. It lands. It runs away.
But every raindrop has passed through an atmosphere containing an astonishing mixture of material — dust, sea salt, soot, mineral particles, biological fragments, pollutants and countless microscopic aerosols.
Some of that material is incorporated into clouds. Some is captured by falling raindrops.
And eventually, much of it reaches the Earth's surface.
Scientists call this process wet deposition.
It is happening every time it rains. And it raises a fascinating question: Could rainfall be quietly contributing to the sediment layers that record Earth's history?
The atmosphere isn't empty
The air around us may look perfectly clear, but it contains enormous numbers of microscopic particles.
These atmospheric aerosols can originate from deserts, soil, oceans, volcanoes, vegetation, fires, industry, vehicles and biological activity.
Some particles are so small that they can remain suspended in the atmosphere for days or even weeks.
Others are considerably larger and fall more quickly.
Rain provides one of the most effective mechanisms for removing these particles from the atmosphere.
A major scientific review of wet deposition describes rainfall and snowfall as important mechanisms for scavenging airborne particulate elements and transporting them to the surface.
The atmosphere is therefore not simply transporting gases.
It is continually transporting solid material.
Where does a raindrop get its particles?
There are several different processes involved.
Inside clouds, microscopic aerosol particles can act as cloud condensation nuclei. Water vapour condenses around suitable particles, helping cloud droplets form. As droplets grow, they can collide with other droplets.
Ice particles can also become involved, particularly in colder clouds.
Eventually precipitation develops.
But the process doesn't necessarily stop once the raindrop leaves the cloud. As it falls through the atmosphere, a raindrop can collide with and capture additional airborne particles.
Scientists refer to this process as below-cloud scavenging.
In other words, a falling raindrop can effectively sweep material out of the air.
What is being deposited?
The list is surprisingly diverse. Atmospheric wet deposition can contain:
- mineral dust
- soil particles
- sea salt
- iron
- aluminium
- calcium
- magnesium
- potassium
- phosphorus
- biological particles
- soot and other carbonaceous material
- industrial pollutants
- trace metals
Some of these substances are dissolved in the rainwater. Others remain as tiny solid particles suspended within it.
The distinction matters because a raindrop isn't simply carrying water. It can carry a tiny sample of the atmosphere through which it has travelled.
Dust from another continent can fall with your rain
One of the most remarkable aspects of atmospheric transport is distance. A dust particle doesn't necessarily fall close to where it originated.
Large quantities of mineral dust are lifted into the atmosphere in arid regions and transported by winds.
Some eventually fall back to Earth thousands of kilometres away.
When precipitation encounters that airborne dust, some of it can be incorporated into the rainfall.
This means that a layer of material accumulating in one location can contain particles originating somewhere else.
The atmosphere therefore acts as a gigantic planetary conveyor belt.
A particle can leave a desert, travel across an ocean and eventually become part of material deposited on another continent.
But does every raindrop contain sediment?
No. This is an important qualification. Not every raindrop contains a significant amount of solid material.
Particle concentrations vary enormously depending on location, weather, air pollution, wind conditions and the history of the air mass. Rain falling after a prolonged dry period can also behave differently from rain falling in an already clean atmosphere.
The size of the particles matters too.
Research shows that larger atmospheric particles are generally scavenged more efficiently by precipitation than many very fine particles, although the behaviour varies by chemical composition and particle type.
So the idea isn't that every raindrop is carrying a visible grain of dust.
It is that precipitation provides a continuous pathway by which atmospheric material reaches the ground.
Where does the material go?
Once deposited, the story becomes more complicated.
A particle might simply remain on the ground. It could become incorporated into soil. It might dissolve. Plants could absorb some of its chemical constituents. Wind could lift it back into the atmosphere.
Or the next rainfall could wash it into a stream.
Eventually it might reach a lake, estuary or ocean. And there it may become incorporated into sediment.
This is where wet deposition becomes particularly interesting from a geological perspective.
Atmospheric deposition is not automatically sediment. But it can become part of the enormous chain of processes that eventually produces sediment.
Lakes are natural collectors
Consider a lake. Rain falls directly onto its surface. Dust falls onto the surrounding landscape and is washed into the lake. Streams bring material from the surrounding catchment.
Wind deposits airborne particles. Organic material accumulates. Year after year, these materials settle towards the bottom.
Under the right conditions, the lake can preserve layers recording changes in climate and environment.
Atmospheric particles therefore have an opportunity to become part of the sedimentary archive.
Researchers have measured atmospheric particulate deposition at lakes specifically to understand its contribution to sediment budgets. At Lake Tahoe, for example, researchers found substantial particle deposition through both dry and wet processes, with wet processes dominating seasonal particle deposition during periods of high precipitation.
A raindrop can therefore become part of geological history
Imagine a microscopic mineral particle. It begins in a desert thousands of kilometres away.
Wind lifts it into the atmosphere. It travels for days. Eventually it encounters a cloud. The particle is incorporated into a droplet or captured by falling precipitation. The raindrop reaches the ground. The particle enters a stream. The stream carries it into a lake. It settles onto the lake floor.
More material accumulates above it.
Eventually the lake sediments are buried.
Over geological timescales, those sediments can become rock. The particle has gone from dust to sediment to geological record.
It sounds almost insignificant.
But multiply that one particle by billions upon billions.
The cumulative effect becomes much harder to ignore.
How much material are we talking about?
This is where the answer becomes complicated. There isn't one universal number because atmospheric deposition varies dramatically from one environment to another.
Deserts and dusty regions can receive enormous quantities of mineral material. Remote locations can receive far less. Urban and industrial areas can receive additional anthropogenic particles.
The 2021 synthesis of global measurements found large geographical differences in wet deposition of elements such as aluminium and iron. It also found that industrial, urban, agricultural and dusty environments can receive substantially greater particulate fluxes than remote locations.
This means that the contribution of atmospheric material to a sediment deposit can depend heavily on where that sediment formed.
A remote lake surrounded by little eroding terrain may receive a surprisingly important proportion of its mineral input from the atmosphere. A river-fed lake surrounded by eroding mountains may receive comparatively little.
Rainfall isn't the only mechanism
There is another important distinction. Atmospheric material reaches the ground through both wet deposition and dry deposition.
Wet deposition occurs through rain, snow and related precipitation processes. Dry deposition occurs when particles settle or are transported directly onto surfaces without precipitation.
In some environments, dry deposition can actually dominate.
The relative importance depends on particle size, climate, surface conditions and precipitation patterns.
The global review of particulate-element deposition found that dry deposition often dominates for crustal elements, particularly mineral material, although the balance varies substantially between elements and locations.
So if we want to understand atmospheric contributions to sediment, we can't look at rainfall alone.
We have to consider the entire atmospheric deposition system.
Snow can be an even more effective collector
Rain isn't the only precipitation capable of scavenging atmospheric particles.
Snow can be extremely efficient at collecting airborne material.
The same synthesis found that elemental scavenging ratios for snow were approximately three times those measured for rain, although the precise behaviour varies with particle and environmental conditions.
This has potentially important implications for places such as polar regions and mountain environments.
Snow can accumulate atmospheric material over an entire season. When it melts, that material is released onto the landscape.
In some environments, snow therefore becomes a temporary storage system for atmospheric particles.
Rain can also carry a chemical history
The material arriving in precipitation isn't necessarily inert dust.
Atmospheric particles can contain chemically important elements. Some are nutrients. Some are pollutants.
Some can become bioavailable after being deposited.
Iron is a particularly interesting example because atmospheric dust can transport iron over enormous distances, including across oceans.
The amount that actually becomes biologically available depends on chemical form, atmospheric processing and solubility.
Researchers have found considerable differences in the solubility of elements deposited through precipitation. In the 2021 review, reported solubilities ranged from roughly 8% for iron to 94% for calcium under the studied conditions.
So rainfall doesn't merely move particles.
It can redistribute chemically active elements around the planet.
Could rainfall influence what we see in ancient sediment?
Absolutely.
But scientists have to be careful.
When researchers find a particular mineral or element in an ancient sediment layer, they need to determine where it came from.
Was it carried by a river? Blown in by wind? Produced biologically? Deposited by volcanic activity? Transported through groundwater?
Or delivered through atmospheric precipitation?
Often the answer is a combination of several sources.
Geochemical signatures can sometimes help researchers distinguish between them. The ratios of particular elements and isotopes can provide clues about the material's origin.
This is one reason sediment cores are so valuable.
They don't simply tell us that material accumulated. They can sometimes tell us where that material came from.
The atmosphere is part of the sediment cycle
We tend to imagine sediment as something produced by erosion. Mountains weather. Rivers carry fragments. Coastlines erode. The material settles.
That's certainly true.
But the sedimentary system is more complicated.
The atmosphere is part of it.
Particles can move from land to atmosphere, from atmosphere to ocean, from ocean back into the atmosphere and eventually back onto land.
Rain and snow provide important pathways connecting the atmosphere to Earth's surface.
In that sense, the planet is constantly recycling material between its major environments.
The intriguing question
This brings us back to the original thought: How much of the material eventually preserved in Earth's sediments arrived through the atmosphere?
There isn't a single global percentage that answers the question.
And it would be misleading to suggest that rainfall is secretly responsible for most sediment.
In many environments, erosion and river transport are vastly more important. But in others, atmospheric deposition can be significant — sometimes surprisingly so.
And because atmospheric particles can travel enormous distances, they can introduce material into environments that otherwise receive very little mineral input.
A geological record written in raindrops?
Imagine a sediment layer at the bottom of an ancient lake. Researchers might examine it and discover minerals that originated hundreds or thousands of kilometres away.
Perhaps some arrived through rivers. Perhaps some were blown directly into the lake. And perhaps some arrived suspended within rainfall.
The final layer doesn't necessarily preserve the journey of each individual particle.
But it preserves the accumulated result.
Year after year. Storm after storm. Rainfall after rainfall. Tiny particles descend from the atmosphere and become part of the Earth's surface.
Most disappear into the background of everyday life. Some are recycled. Some are washed away. Some are consumed by living organisms.
And some eventually become locked into sediment.
Given enough time, even an apparently insignificant process can leave a geological signature.
So, the next time it rains, consider what is actually falling from the sky.
It isn't just water.
It is also a tiny sample of the atmosphere — carrying material from the landscape, the oceans and sometimes from places thousands of kilometres away.
And some of those microscopic particles may eventually become part of the geological record of our planet.
Not in one dramatic event. But drop by drop. Storm by storm. Layer by layer.
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