Reverse weathering describes authigenic clay‑forming reactions in aquatic sediments and pore waters whereby new clay minerals precipitate from dissolved or particulate precursors, effectively consuming dissolved cations and alkalinity by pathways distinct from continental silicate breakdown. Two principal mechanistic routes have been identified: neoformation via interaction of biogenic silica (SiO2) with aqueous cations or cation‑bearing oxides (for example Al(OH)4−), and transformation of cation‑poor precursor clays through uptake of dissolved cations (e.g., K+, Mg2+, Li+) or metal hydroxides to yield authigenic clay phases. The net chemistry can be summarized stoichiometrically as SiO2 + Al(OH)4− + dissolved cations + HCO3− → clay minerals + H2O + CO2, illustrating the coupled removal of cations and bicarbonate and the concomitant release of CO2 to solution. Key reactant species invoked in models and experiments therefore include silica, aluminous hydroxides, a suite of alkali and alkaline earth cations, and bicarbonate, with clay, water and carbon dioxide as primary products. Empirical evidence for reverse weathering is robust at a few well‑studied depositional settings—notably the Amazon and Mississippi deltas, a palaeo‑delta in Aínsa‑Sobrarbe (Pyrenees), and Ethiopian Rift lakes—but such geographically limited observations constrain the ability to scale processes globally. Interest in reverse weathering arose from attempts to close the river–ocean chemical mass balance: early models that omitted authigenic clay formation predicted oceanic concentrations of alkali metals and bicarbonate higher than observed, implying an additional sink. That interpretation has been modified following recognition of significant hydrothermal fluxes that also remove cations and alkalinity, so partitioning between vent losses and authigenic sequestration remains unresolved. Consequently, the global significance of reverse weathering is still uncertain because of the chemical complexity of potential reactants, strong dependence on local depositional and diagenetic conditions, and the need to apportion sinks among competing processes when constructing oceanic chemical budgets.
Methods for quantifying reverse weathering combine field chemistry, controlled laboratory experiments, mineralogical imaging and high‑precision elemental/isotopic analyses to resolve rates, mechanisms and broader significance. In situ approaches exploit concentration gradients of biogenic silica and dissolved SiO2 between pore waters and the overlying water column: depressed dissolved Si relative to overlying waters is interpreted as net uptake of silica into authigenic clay phases. These field measurements are complemented by laboratory incubations and flow‑through reactor experiments that isolate mechanistic steps—opal (biogenic silica) dissolution and subsequent precipitation of authigenic clays—and yield empirical dissolution and formation rates under controlled conditions.
Incubation experiments using natural sediments in sealed vessels allow systematic variation of key reactants (biogenic silica, dissolved cations such as K+, and trace metals) to determine reaction velocities and stoichiometries of clay formation; measured changes in sediment composition and pore waters provide direct estimates of ion consumption (notably K+). Mineralogical and textural characterization of experimentally formed clays using scanning electron microscopy, X‑ray methods and transmission electron microscopy documents mineral identity, grain morphology and apparent formation timescales. Rapid clay textures observed by these methods, when scaled by sediment inventories, have been used to infer the magnitude of potassium removal by reverse weathering in fluvial and marine settings.
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Elemental concentrations and bulk isotopic compositions of pore waters and sediments are typically measured by ICP‑OES, while MC‑ICP‑MS provides high‑precision isotopic ratios for metals and dissolved silica. A complementary long‑term proxy is the lithium isotope composition preserved in planktonic foraminifera: because Li removal from seawater is dominated by sedimentary uptake, variations in foraminiferal Li isotopes over multimillion‑year records have been interpreted as changes in the balance between silicate weathering and reverse weathering.
Integrating these lines of evidence—SiO2 gradients, reactor‑derived opal dissolution rates, rapid authigenic textures from microscopy and X‑ray analyses, experimentally constrained K+ stoichiometries, and Li isotope trajectories—permits quantitative constraints on the spatial distribution, temporal variability and global importance of reverse weathering across aquatic environments, marine sediments and river systems.
Targeted investigations of Amazon delta sediments indicate that the formation of authigenic silicate clays via reverse weathering is thermodynamically favored under the in situ sedimentary conditions documented in that system. However, the realization of these favorable reaction pathways is tightly governed by the chemical composition of porewaters: the rates and extent of clay authigenesis track the local concentrations of dissolved reactants rather than thermodynamic potential alone.
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Critical solution-phase reactants include dissolved silica (principally derived from the dissolution of biogenic opal), metal hydroxide complexes such as aluminate (Al(OH)4−), and a range of dissolved cations; deficiency in any one constituent constrains clay production. These solutes are supplied largely through weathering and mechanical breakdown of terrigenous material delivered to the delta, so the intensity and character of continental weathering directly set the chemical inventory available for reverse-weathering reactions. Accordingly, authigenic clay formation is spatially heterogeneous within the delta: zones with reduced inputs of biogenic silica, lower metal-hydroxide concentrations, or depleted cation pools correspond to diminished authigenesis, underscoring a direct geochemical linkage between terrestrial weathering processes and in‑sediment mineral formation in this coastal–estuarine environment.
Kinetics
Reverse weathering in marine environments—the authigenic formation of clay minerals via kinetically driven reactions—occurs on subannual timescales, with observed formation often completing in less than a year. This rapidity distinguishes reverse weathering from slower, multi‑year or purely geological diagenetic processes and indicates that clay authigenesis can proceed on seasonal to interannual timesteps.
Because clay formation can unfold within months, reverse weathering is dynamically coupled to short‑term oceanic variability and can actively alter the chemistry of both seawater and freshly deposited sediments on ecologically and climatically relevant timescales. Consequently, it supplies a measurable flux of elements and compounds between the water column and the sedimentary reservoir and therefore must be incorporated into assessments of oceanic budgets and short‑term biogeochemical dynamics.
The formation of authigenic clay minerals during marine reverse weathering liberates CO2 locally as a by‑product of mineralization. However, this CO2 input is counterbalanced by prior silicate weathering, which supplies dissolved bicarbonate (HCO3−) to the marine system; the molar production of HCO3− from silicate weathering generally exceeds the CO2 released during authigenic clay formation. As a result, the additional CO2 generated by reverse weathering is effectively masked by the larger bicarbonate pool and does not drive a substantial local pH decline.
This relationship is integral to the coupled marine silica–carbon system, in which silica transformations, authigenic clay precipitation, and exchanges among dissolved inorganic carbon species (CO2 and HCO3−) are interdependent (see schematic adapted from Treguer et al., 1995). Geographically, the balance between terrestrial silicate weathering—providing HCO3− to coastal and open‑ocean waters—and in situ reverse weathering—producing CO2 during clay authigenesis—sets local alkalinity and buffering capacity; in most settings the bicarbonate input dominates the immediate chemical equilibrium, stabilizing pH despite CO2 release from clay formation.
Dissolved silica generated on land—delivered to the ocean chiefly by glacial meltwater and river discharge—constitutes the reactive silicon pool that sustains marine siliceous biota, especially diatoms. These organisms sequester dissolved Si into opaline biogenic tests that, upon death, sink and contribute to the sedimentary Si inventory. Where biogenic silica production ceases or burial outstrips supply, diagenetic alteration progressively converts unstable opaline silica into more stable mineral phases. A key diagenetic route is reverse weathering, in which dissolved and biogenic silica is incorporated into authigenic silicate clays within pore waters and sediments; this pathway effectively withdraws silicon from the oceanic reactive pool and from direct burial as biogenic Si. Reverse weathering constitutes a major sink in the global silicon cycle, removing on the order of 20–25% of silicon inputs to marine systems. River deltas are particularly important loci for this process because rapid sediment accumulation and intense early diagenesis favor clay authigenesis and consequent depletion of pore-water reactive silica. The relative importance of reverse weathering versus silicate weathering is strongly modulated by sedimentary redox and depth: surface, more oxidized layers tend to host reverse-weathering reactions, while deeper, methanogenic (anoxic) horizons favor silicate-weathering processes, with differing reaction rates and net effects on silicon fluxes.
Deltaic environments are hotspots for reverse weathering, where dissolved silica and other solutes are removed from pore waters through formation of authigenic clays and aluminosilicates. The Amazon delta exemplifies an extreme case: roughly nine out of ten grams of buried SiO2 are reincorporated into authigenic minerals, with authigenesis producing potassium at rates near 2.8 μmol g−1 yr−1 and accounting for an estimated 7–10% of the river’s dissolved K flux being sequestered as K–Fe–rich aluminosilicates. By contrast, the Mississippi delta converts a substantially smaller fraction of buried silica—about 40%—into authigenic aluminosilicates. These inter‑delta differences trace primarily to sediment composition and transport dynamics: the Amazon’s intense erosional–depositional reworking generates abundant reactive iron oxides and other labile phases that favor Fe‑rich clay and K–Fe‑aluminosilicate formation, whereas iron supply is more limiting in the Mississippi system. Rapid sediment turnover in the Amazon—mean residence times near 30 years and surface reworking on the order of one to two times per year—maintains reactive surfaces and supports authigenesis on short diagenetic timescales (months to a few years), consistent with pore‑water observations. The identity of the rate‑limiting reagent therefore differs by system: silica availability constrains clay formation in the Amazon (where Fe, K, Mg and Al are abundant), whereas iron availability is the primary constraint in the Mississippi. Paleo‑delta records corroborate these process links: Eocene marine deposits of the Ainsa palaeo‑delta preserve systematically lighter δ7Li and δ30Si relative to contemporary alluvial sediments, a signature attributable to substantial authigenic clay formation; the correlation of δ7Li with iron content in those marine strata further implicates Fe‑driven diagenesis in promoting simultaneous Si and Fe incorporation into authigenic phases. Mechanistically, reduced pore waters can couple ferrous iron and dissolved silica into Fe‑silicate solids, a reaction that consumes bicarbonate and releases CO2 (e.g., H4SiO4 + Fe2+ + 2 HCO3– → Fe‑silicate + 3 H2O + 2 CO2). Analogous Fe‑coupled reverse‑weathering pathways have been observed beyond modern deltas, including lacustrine and rift settings such as Ethiopian Rift Valley lakes, indicating the broader environmental significance of these diagenetic processes for local sediment geochemistry and global element cycles.
In the Ethiopian Rift lakes reverse weathering is unusually evident because low evaporite precipitation permits unambiguous, in situ development of authigenic aluminosilicate (clay) minerals. Geochemical surveys there consistently record an alkalinity deficit; combined mass‑balance and rate calculations attribute slightly more than half of that deficit to the formation of aluminosilicate phases via reverse weathering reactions, making these lakes a valuable natural laboratory for quantifying the process under lacustrine conditions.
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Measured clay‑formation rates from the Rift have been used as empirical benchmarks to infer potential global and marine reverse weathering fluxes. However, simple extrapolation of lacustrine rates to the ocean yields an implausibly large oceanic clay production, indicating that lacustrine kinetics and boundary conditions cannot be directly transferred to marine settings without modification.
In the deep ocean reverse weathering follows a different trajectory: low bottom‑water pH and attendant chemical constraints often prevent reaction sequences from running to completion and converting dissolved constituents fully into stable aluminosilicate phases. To account for observed deep‑sea clay abundances and inferred formation rates, researchers therefore invoke additional sources—notably hydrothermal alteration and other diagenetic processes on the seafloor—so that marine clay production appears to be a composite outcome of incomplete deep‑sea reverse weathering plus hydrothermal and diagenetic inputs rather than a simple analogue of lacustrine reverse weathering.
Hydrothermal vents such as those on the East Scotia Ridge are deep-sea loci where seawater altered by fluid–rock interaction issues as mineral-rich discharge and has been implicated as a site of reverse weathering. In this view, vent-derived solutes are not simply added to the ocean but may be converted back into solid, authigenic phases within or immediately beneath the seafloor. Water–rock reactions release ions and silicic acid that can react with the surrounding substrate; these in situ reactions remove species from solution and produce clays and other mineral precipitates, so that the ionic composition of ambient seawater in the vicinity of a vent differs from that of the fresh hydrothermal fluid.
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Low-temperature hydrothermal systems are particularly important for the silica budget: they liberate dissolved silica from the crust, and before this dissolved silica escapes the seabed it can cool and be immobilized as clay minerals (notably smectite), transferring dissolved silica into a solid sink. Recognition of substantial hydrothermal solute fluxes represents a historical shift from the earlier paradigm that treated continental rivers as the sole significant source of ocean salts; hydrothermal circulation is now understood to supply large quantities of dissolved minerals and to exert a measurable control on ocean salinity and ion inventories.
Despite clear mechanistic pathways for hydrothermal-driven reverse weathering and authigenic clay formation, the quantitative importance of these processes for the global silica cycle and long‑term ocean chemistry remains contested. Whether vent-associated reverse weathering constitutes a major sink for dissolved constituents or a more locally confined modifier of seawater chemistry is an active area of research with implications for regional salinity, biogeochemical cycling (especially of silica), and the composition of the seabed.
History
Early conceptual groundwork for reverse weathering dates to Victor M. Goldschmidt (1933), who proposed that interactions between igneous lithology, associated volatiles, and the ocean could produce marine sediments and alter seawater composition, thereby linking lithospheric inputs to sedimentary and chemical evolution of the oceans. Building on this lineage, Lars G. Sillén (1959) argued that formation of silicate phases in the marine environment could exert a primary control on seawater chemistry and pH by removing or transforming dissolved constituents that regulate ocean composition. At the time of Sillén’s proposal, however, the thermodynamic parameters for clay‑mineral formation were largely unknown, preventing robust quantitative evaluation of such reactions.
Frederick Mackenzie and Robert Garrels (1966) integrated these ideas into a formal hypothesis of “reconstitution reactions,” coining the reverse weathering concept to describe authigenic mineral formation in marine settings as a process that can partially counteract continental weathering. Since its articulation, reverse weathering has been repeatedly invoked to explain diverse observations—shifts in seawater composition, changes in sediment mineralogy, and mechanisms for ocean pH buffering. Despite extensive application in case studies, the global magnitude and geographic extent of reverse weathering remain unresolved; direct, worldwide quantification is lacking, so current assessments depend largely on inferred signals from localized and regional investigations.