Reverse weathering
Reverse weathering generally refers to the formation of a clay neoformation that utilizes cations and alkalinity in a process unrelated to the weathering of silicates. More specifically reverse weathering refers to the formation of
Formation of cation abundant authigenic silicate clays is thought to occur through the following simplified reaction:
cations (K+, Mg2+, Li+, etc.) + bicarbonate (HCO3) → clay minerals + H2O + CO2[2]
The formation of authigenic clay minerals by reverse weathering is not fully understood. Much of the research done has been conducted in localized areas, such as the
Methods of analysis
The process and extent of reverse weathering has been inferred by several methods and proxies.
In-situ measurements of biogenic silica and silicic acid (a product of weathering) have been used to analyze the rate and extent that reverse weathering occurs within an aquatic system.[6][7] Uptake of biogenic silica as a result of reverse weathering would be observed as a relative low concentration of dissolved SiO2 compared to the overlying water.
Laboratory observations of reverse weathering have been conducted using incubations and flow through reactors to measure opal dissolution rates[2][3] The clay was studied using scanning electron microscopes, x-ray, and transmission electron microscopes.[1] It was observed that the clay formed quickly, and using this amount of time and the known content of the sediment, concentration of potassium ions consumed by this process in rivers around the globe was estimated.[1]
Laboratory experiments can also include incubation experiments, in which sediment samples obtained from natural environments are enclosed in sealable containers with varied concentrations reverse weathering reactants (biogenic silica in the form of diatoms, cations, metals, etc.).[2]
Using an
Lithium isotope concentration within planktonic foraminifera has been used to infer past changes in silicate and reverse weathering rates over the last 68 million years.[8] Removal of lithium from seawater is mainly dependent on its assimilation within marine sediments and variations are believed to be indicative of the relative rates of silicate weathering and reverse weathering, in addition to other factors. Foraminifera with low lithium content suggest that reverse weathering may have been more prominent during that time period.[8]
Controls
Thermodynamics
Formation of authigenic silicate clays through reverse weathering was shown to be thermodynamically favorable during studies of Amazon delta sediments.[3] Primary controls on the formation of authigenic silicate clays are on the supply of reactants in solution. Areas of limited biogenic opal, metal hydroxides (e.g. aluminate (Al(OH)4−)), or dissolved cations limit production of authigenic silicate clays.[2] Metals, cations, and silica are largely supplied by the weathering of terrigenous materials, which influences the thermodynamic favorability of reverse weathering.[9]
Kinetics
Kinetically, formation of clay minerals by reverse weathering can be relatively rapid (<1 year).[3] Due to the short formation timescale, reverse weathering is seen as a reasonable contributor to various ocean biogeochemical cycles.[3]
Influence on global cycles
The carbon cycle
The process of creating authigenic clay minerals through reverse weathering releases carbon dioxide (CO2).[9] However, release of bicarbonate by silicate weathering exceeds the quantities of CO2 produced by reverse weathering. Therefore, while reverse weathering does increase CO2 during production of authigenic clay minerals, it is overwhelmed by the concentration of HCO3− in the system, and will not have a significant effect on local pH.[9]
The silica cycle
In recent years, the effect of reverse weathering on biogenic silica has been of great interest in quantifying the silica cycle. During weathering, dissolved silica is delivered to oceans through glacial runoff and riverine inputs.[2] This dissolved silica is taken up by a multitude of marine organisms, such as diatoms, and is used to create protective shells.[2] When these organisms die, they sink through the water column.[2] Without active production of biogenic SiO2, the mineral begins diagenesis.[2] Conversion of this dissolved silica into authigenic silicate clays through the process of reverse weathering constitutes a removal of 20-25% of silicon input.[11]
Reverse weathering is often found to occur in river deltas as these systems have high sediment accumulation rates and are observed to undergo rapid diagenesis.[12] The formation of silicate clays removes reactive silica from the pore waters of sediment, increasing the concentration of silica found in the rocks that form in these locations.[12]
Silicate weathering also appears to be a dominant process in deeper methanogenic sediments, whereas reverse weathering is more common in surface sediments, but still occurs at a lower rate.[3]
Study locations
Deltas
In the
The effect of reverse weathering has also been observed in paleo-delta systems. In the Ainsa basin, a palaeo-deltaic system was formed during the Eocene and uplifted through the orogeny of the Pyrenees. Isotopic geochemical differences were observed between palaeo sediments deposited in the marine conditions and those from alluvial environments.[14] The lithium isotope signature (δ7Li) and the silicon isotope signature (δ30Si) are systematically lighter in marine sediments than that in alluvial sediments,[14] implying authigenic clay formation in the marine sediments. Additionally, in the marine sediments the δ7Li signature is correlated to iron contents, suggesting the coupling of iron diagenesis and reverse weathering processes in the marine environments. This coupling can be achieved in reduced environments through the following reactions:[14]
H4SiO4 + Fe2+ + 2HCO3- → FeO-SiO2 (Fe-rich clays) + 3H2O + 2CO2
Ethiopian rift lakes
Reverse weathering in the
Hydrothermal vents
Some hypothesize that
Some researchers hypothesize that reverse weathering could play a role in the silica cycle at hydrothermal vents.[5] Low temperature hydrothermal vents release silicic acid from the Earth's crust, and before it is able to exit the seabed, it cools and precipitates out as clay, such as a smectite.[11] The extent to which reverse weathering at hydrothermal vents adds to the overall silica cycle is a hot topic.[16][17][11]
History
In 1933,
Today, there is much debate over the significance of reverse weathering. The global extent of the process has not yet been measured, but inferences can be made by using specific local examples.[22]
References
- ^ S2CID 128993379.
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- ^ Morifuji, Naoto; Nakashima, Satoru (2016). "Hydrothermal transformation of biogenic silica as studied by in situ infrared spectroscopy" (PDF). Goldscmidt Conference Abstracts.
- ^ .
- ^ S2CID 42591236.
- ^ .
- S2CID 5672525.
- ^ PMID 22809182.
- ^ ISBN 9780080983004.
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- ^ S2CID 245366429.
- ^ ISBN 9781118663998– via American Geophysical Union.
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- ^ a b Ristvet, Byron (1978). "Reverse Weathering Reactions Within Recent Nearshore Sediments, Kaneohe Bay, Oahu". Defense Nuclear Agency.
- S2CID 220099254.
- ISBN 978-0-521-83313-4.
- ^ Holland, H.D.; Turekian, K.K., eds. (2014). "Sedimentary Diagenesis, Depositional Environments, and Benthic Fluxes". Treatise on Geochemistry. Vol. 8 (2 ed.). Oxford: Elsevier. pp. 293–334.