Rivers carry the dissolved and solid products of silicate mineral weathering, a process that removes
from the atmosphere and provides a key negative climate feedback over geological timescales. Here we show that, in some river systems, a reactive exchange pool on river suspended particulate matter, bonded weakly to mineral surfaces, increases the mobile cation flux by 50%. The chemistry of both river waters and the exchange pool demonstrates exchange equilibrium, confirmed by Sr isotopes. Global silicate weathering fluxes are calculated based on riverine dissolved sodium (Na
+) from silicate minerals. The large exchange pool supplies Na
+ of nonsilicate origin to the dissolved load, especially in catchments with widespread marine sediments, or where rocks have equilibrated with saline basement fluids. We quantify this by comparing the riverine sediment exchange pool and river water chemistry. In some basins, cation exchange could account for the majority of sodium in the river water, significantly reducing estimates of silicate weathering. At a global scale, we demonstrate that silicate weathering fluxes are overestimated by 12 to 28%. This overestimation is greatest in regions of high erosion and high sediment loads where the negative climate feedback has a maximum sensitivity to chemical weathering reactions. In the context of other recent findings that reduce the net
consumption through chemical weathering, the magnitude of the continental silicate weathering fluxes and its implications for solid Earth
degassing fluxes need to be further investigated.For decades, silicate weathering has been postulated to provide the negative climate feedback on Earth that prevents a runaway greenhouse climate like on Venus (
1). Silicate mineral dissolution with carbonic acid converts atmospheric
into carbonate, and releases essential nutrients to the terrestrial and marine biosphere (
2). There have been many attempts to quantify the silicate weathering flux (
3), mostly assuming that riverine dissolved sodium (
) is derived only from silicate minerals and rock salt. Here we show that there is a major addition of nonsilicate
to the critical zone from ancient seawater, weakly bonded to sedimentary rocks and supplied to waters via the cation exchange process. The implication is not only that the silicate weathering flux is overestimated at a global scale, but that this nonsilicate
is most important in regions previously thought to have the highest silicate weathering fluxes (so called weathering-limited regions) and greatest climate sensitivity.Cation exchange is a rapid chemical reaction between cations in the dissolved phase and mineral surfaces, particularly clays (
4). Major and trace cations such as calcium (
), magnesium (
), sodium (
), potassium (
), and strontium (
) form the cation exchange pool, which balances negative charges on river-borne clay particle surfaces. This exchange takes place on interlayer sites, between the tetrahedral and octahedral layers, or on exposed surfaces (
4). The importance of the cation exchange pool is well recognized in soils and aquifers (
4,
5), has significant implications for enhanced weathering (
6), and has been proposed as an important mechanism for buffering the composition of river waters (
7–
9). However, data on the riverine exchange pool are only available for two large river systems [Amazon and Ganges-Brahmaputra (
10,
11)], despite its significance in providing a source of elements that are immediately bioavailable (
12), and their potential for biasing the quantification of silicate weathering (
9).It is increasingly recognized that rapidly reactive phases have a strong influence on the chemistry of river waters (
13,
14). Cation exchange is a rapid reaction occurring continuously in soils, as riverine freshwaters evolve downstream interacting with particulate matter, and when they mix with seawater (
15,
16). Important examples of cation exchange are the “swapping” of divalent cations
and
with
, in particular when there is a major change in water composition such as when fluvial clays reach the ocean,
[1]As a result, marine sediments have an exchange pool that is dominated by
(
17). Subsequently, these marine sediments are uplifted and emplaced on the continents where
in the exchange pool is released by cation exchange with Ca-rich fresh waters (
9). This has major implications for estimates of silicate weathering fluxes and associated
consumption, because they are calculated using the
content of rivers (
3). Cerling et al. (
9) proposed that the
-rich exchange pool exerts an important control on natural waters, based on charge balance arguments from river water chemistry, but this hypothesis has never been rigorously tested (
18) by determining the flux and composition of the exchange pool of rivers around the world.In this contribution, we present a large dataset of fluvial sediment cation exchange capacity (CEC) and composition in several of the world’s largest river basins. By comparing with the concomitant dissolved load chemistry, we demonstrate that 1) the exchange pool in river sediments is in equilibrium with the river water; 2) the fraction of mobile elements in the exchange pool relative to the dissolved pool can be significant, particularly in rapidly eroding, weathering-limited catchments; and 3) given reasonable inferences on the composition of old marine sedimentary rocks, modern-day silicate weathering has been overestimated and carbonate weathering has been underestimated. The results reduce the estimated magnitude of the silicate weathering flux, but increase the supply of base cations (e.g.,
, which can be a limiting nutrient) to the biosphere, suggesting a greater role of organic carbon burial compared with silicate weathering for the long-term atmospheric
sink.
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