Rocks and water are not just scenery. They absorb carbon dioxide from the air, transport it through rivers, and set off reactions that later play out on the seafloor. They play a crucial role in the global carbon cycle.

A new study argues that these reactions form a single, connected system stretching from high ridges to deep marine basins.

The researchers concluded that we must treat land and ocean together if we want an accurate picture of how much carbon is stored or released.

Linked systems shape carbon

Jeremy Caves Rugenstein, an associate professor of geosciences at Colorado State University (CSU), studies long-term climate and weathering feedbacks. His team links what rivers carry off the land to how marine sediments process that cargo.

“Here we propose that the Earth’s silicate weathering occurs along a continuum linking mountains to the deepest sedimentary environments and forward to reverse weathering,” said Gerrit Trapp-Müller, a geoscientist at Utrecht University (UU).

The experts show why downstream chemistry depends on where particles come from, how fast they were eroded, and the conditions they meet along the way.

Weathering drives carbon chemistry

Chemical silicate weathering transforms reactive rock minerals into dissolved components that flow to the sea. These reactions consume atmospheric carbon dioxide as bicarbonate, which can later become carbonate minerals on the seafloor. This process has major implications for the global carbon cycle.

In marine muds, a different process – known as reverse weathering – forms new clays and releases carbon dioxide. These reactions alter the water’s alkalinity, which chemists use to track the acid–base balance that governs carbon chemistry.

The formation of marine clay minerals, also called reverse silicate weathering, plays a central role in controlling seawater pH.

The new study shows that these forward and reverse reactions are not independent. River particles inherit a history and continue reacting in floodplains, shelves, and deep basins. The balance can flip depending on the mineral mix and the local environment.

Small swings, big impacts

On land, a 2021 global analysis estimated atmospheric carbon dioxide consumption by continental silicate weathering at 0.133 to 0.169 gigatons per year. That is not insignificant, and it operates continuously over time.

However, other studies suggest these fluxes may be overestimated by roughly 12 to 28 percent at the global scale. This caution matters because even small percentage changes translate into long-term climate differences.

The continuum study also highlights a recurring control. When weathering rates on land are high relative to erosion, many river particles have already lost most of their reactive cations. This shift nudges marine sediments toward reverse reactions.

Observed modern ratios of weathering to erosion cluster between 0.03 and 0.09, and changes within that range can shift the balance on the ocean side.

The ocean’s role in weathering

“Authigenic clay formation can proceed even faster, within weeks to months,” wrote Sonja Geilert, GEOMAR researcher and lead author of a 2023 study. That was observed after extreme rainfall delivered fresh reactive minerals from land to Peru’s shelf.

Marine sediments occupy the intersection of biological activity, mineral inputs, and water chemistry. In shelf muds rich in iron and aluminum, reverse weathering can accelerate and reduce alkalinity. In areas supplied with fresh volcanic ash or mafic minerals, forward weathering can dominate and increase alkalinity.

Rates also depend on transport. When waves and currents churn the seabed, or when organisms pump water through burrows, reactions accelerate.

When sedimentation is steady and deep, the balance can flip with depth as conditions shift from oxygen-poor to sulfur-rich to methane-rich layers.

Land gains, ocean losses

Enhanced weathering on farm soils has attracted attention as a carbon removal strategy. With national-scale modeling, this approach could have sizable potential. The promise is real, and co-benefits for soils add to its appeal.

The new continuum view adds a necessary caveat: much of the material weathered on land may eventually feed reverse reactions at sea.

This means that some carbon-cycle gains on land could be reduced or delayed downstream. Verifying net removal requires tracking not only what dissolves in fields but also what happens to the products once rivers carry them to coasts.

The details depend on rock type, particle size, rainfall, and transport pathways. Basalt-rich amendments can drive land reactions toward greater alkalinity release.

However, coastal settings rich in aluminum and iron can still push marine balances toward reverse weathering. Careful monitoring must follow the chain from source to sink.

New directions for climate models

The continuum concept encourages climate models to couple land, shelf, and deep-ocean weathering more tightly. This includes representing the observed short timescales for clay formation in dynamic margins and capturing how biology and hydrodynamics alter surface area and transport.

Better constraints will come from isotope tracers, mineral fingerprints, and direct measurements of alkalinity changes in sediments. The goal is not only to tally global fluxes but also to identify when and where the balance flips – and why.

Weathering also shares the stage with other carbon-cycle players. Oxidation of ancient organic carbon on land can add carbon dioxide, while burial of reduced sulfur in anoxic muds can help preserve alkalinity. Untangling these threads across shared landscapes and seascapes remains a challenge.

Policymakers and engineers should anticipate regional variation. Strategies that succeed in one basin may require a different approach in another. Verification rules must reflect that spatial complexity without losing sight of measurable carbon outcomes.

The study is published in the journal Nature Geoscience.

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