Researchers analyzed marine sediments to determine the weathering patterns over the last two billion years.
Introduction
Have you ever watched a heavy rainstorm carry dirt down the street? This is an example of physical weathering. Rocks are broken down into smaller pieces in a process called erosion. Water, wind, and changes in temperature can contribute to erosion. And rocks that are at higher elevations or in the mountains can erode more quickly. Particles eroded from rocks move from place to place in a process called particle transport. Eroded particles can even make their way through rivers to the ocean. There, they become part of marine sediments.
Rocks can also change through chemical reactions. Chemical weathering happens when rain and soil water interact with the minerals in rocks. Some of these minerals dissolve, releasing molecules and elements that plants and animals need to grow.
The shapes of mountains and rivers on the continents are controlled by weathering. Weathering is also responsible for the movement of elements – like carbon – around the Earth. But researchers don’t all agree on how weathering has changed over Earth’s history. This is because it’s hard to find and analyze reliable samples of sediment from billions of years ago.
We wanted to better understand the long-term history of weathering. So, we designed a study to analyze the particles produced by weathering over geologic time. These particles are found as ancient marine sediments within sedimentary rocks.
Methods
We used the Sedimentary Geochemistry and Paleoenvironments Project database to find information about the chemistry of ancient marine sediments. We also looked at 2,000 of our own samples. The samples contained sediments that were up to 2,000 million years old. We looked at the amounts of zirconium (Zr), rubidium (Rb), sodium (Na), and aluminum (Al) in the samples. These elements are related to weathering.
We calculated the ratio of Zr to Al (Zr/Al) to look at physical weathering. Zr is a metal that is fairly stable. Zr/Al in marine sediments is low when chemical weathering is high or when Zr gets trapped on land instead of being transported to the ocean. Zr/Al in marine sediments is high when a large amount of this trapped Zr is transported from the continents to oceans. This can happen because of intense physical weathering, erosion, and particle transport.
We also calculated the ratios of Rb to Al (Rb/Al) and Na to Al (Na/Al) to look at chemical weathering. Rb and Na are parts of minerals that are removed from rocks easily, although Na is more easily removed than Rb. The Na/Al and Rb/Al ratios in particles transported to the oceans should be small when chemical weathering is intense. This is because more of the Na and Rb have been removed from the rock during chemical weathering.
We then looked at Zr/Al and Rb/Al over time. We wanted to see if the values changed at regular intervals – and thus, if their timing matched any known geological processes.
Results
Physical Weathering
We saw that before 650 million years ago, Zr/Al was close to the average in the continental bedrock. This means that physical weathering, erosion, and particle transport were efficient at moving Zr into the oceans. Chemical weathering was stable but not intense. But around 650 million years ago, Zr/Al became more variable. Values were both higher and lower than in the continental bedrock (Fig. 1).
Chemical Weathering
We found that Rb/Al decreased around 300 to 400 million years ago. Also, Na/Al was low for all of our samples.
Timing
Our elemental ratios did change at regular time intervals. Zr/Al and Rb/Al both peaked about every 35 million years, 50 million years, and 200 million years.
Discussion
We saw a shift in weathering around 650 million years ago. This timing matches the origin of modern plate tectonics and deep subduction (Fig. 2). Subduction happens when one plate slides under another into the mantle. This can cause high mountains to form, which mainly tend to be eroded by physical weathering. So, when subduction is intense, erosion and particle transport of Zr is high. When subduction is less intense, chemical weathering is more important.
We know that the intensity of subduction and plate movement changes at regular intervals. For example, it takes the mantle about 200 million years to circulate. And this matches one of the time intervals at which Zr/Al and Rb/Al regularly peaked! Our data shows good evidence that weathering might be related to patterns of subduction.
We think increased chemical weathering 400 million years ago corresponded to the evolution of plants with roots. Roots stabilize the soil and produce acidic chemicals, which can increase chemical weathering.
Conclusion
Our research shows that geological processes can have huge impacts on weathering. And you can see the impacts no matter where you live! Take a walk to a nearby park or natural space. Can you see any places that have eroded? Is there a creek or river? Is the water clear, or can you see sediment? Over a few years you might see the bank of the river start to erode and wash away. Just imagine these processes happening at a continental scale for millions and millions of years. What you see now could change a lot in that much time!
