page_title: Reverse weathering
section_title: Deltas
hierarchical_section_title: Reverse weathering / Locations of Study / Deltas
caption_attribution_description: English: Lake Abijatta, Langano and Shala
context_page_description: 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 authigenic clay minerals from the reaction of 1 biogenic silica with aqueous cations or cation bearing oxides or 2 cation poor precursor clays with dissolved cations or cation bearing oxides.
  Formation of cation abundant authigenic silicate clays is thought to occur through the following simplified reaction:
  Biogenic opal + metal hydroxides + dissolved cations + bicarbonate → clay minerals + H₂O + CO₂
  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 Amazon delta, Mississippi delta, and in the Ethiopian Rift lakes, making a global understanding of the process difficult. Much of the driving force behind research into reverse weathering stems from constraining the chemical mass balance between rivers and oceans.
context_section_description: In the Amazon river delta, about 90% of buried SiO₂ is used up during reverse weathering, while the creation of potassium ions in this location is about 2.8 μmol g⁻¹ year⁻¹. Nearly 7-10% of the potassium input from the Amazon River is removed from the ocean by the formation of potassium-iron rich aluminosilicates. In the Mississippi river delta about 40% of SiO₂ that is buried in the sediment is converted to authigenic aluminosilicates. The major difference in the two deltas is due to the sediments in the amazon delta being subject to a number of erosional and depositional processes, which creates an abundant amount of Fe oxides. Sediment typically resides in the region for 30 years on average, but the upper layer undergoes major physical reworking 1-2 times per year. Pore water data suggests that the formation of authigenic clays in the Amazon delta occur on the order of a few months to a few years. The limiting reactant of clay formation in this region is the quantity of available SiO2, since the river water generally has a high concentration of other reactants, such as iron, potassium, magnesium, and aluminum. Whereas in the Mississippi delta, the limiting nutrient for these reactions is Fe.
caption: Ethiopian Rift Valley Lakes
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