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Earth's Systems / Biological processes / Terrestrial ecosystem functioning / Living soil system / Mineral weathering and parent material / Pedogenic carbonate accumulation and calcrete formation

Pedogenic carbonate accumulation and calcrete formation

In dry climates Ca from dust and rain re-precipitates as calcite when soil water is lost by evapotranspiration, forming nodules and then cemented calcic horizons at the mean depth of wetting. The carbonate buffers pH near 8 and stores inorganic carbon.

Boundary: where this process starts

Starts with: Dust and rain deliver Ca2+ and carbonate to the surface of arid and semi-arid soils

Boundary: where it ends

Ends after: Calcite equilibrium buffers pH at 7.5 to 8.5; phosphate precipitates as Ca phosphates and Fe, Zn and Mn availability falls; Soil inorganic carbon stores on the order of 700 to 950 Pg C in the top metre globally

PARTICIPANTSRESOURCESCalciteDustSoil waterPlant rootsRainCalciteWaterCalciumCarbon dioxideCarbonic acidImpact +3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.water moves down then evaporImpact +4 of 5. Click for details and evidence.Impact +4 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.↻ reinforces · moderate · 0.3 Plugged horizoImpact 03 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Impact 02 of 5. Click for details and evidence.Impact +3 of 5. Click for details and evidence.Dust and rain deliver Ca2+ and carbonate to the surface of arid and semi-arid soils: makes CalcitemakesDust and rain deliver Ca2+ and carbonate to the surface of arid and semi-arid soils: uses WaterDust and rain deliver Ca2+ and carbonate to the surface of arid and semi-arid soils: makes CalciummakesInfiltrating water dissolves carbonate in the wetter upper profile as soil CO2 raises carbonic acid: uses up Calciteuses upInfiltrating water dissolves carbonate in the wetter upper profile as soil CO2 raises carbonic acid: uses WaterInfiltrating water dissolves carbonate in the wetter upper profile as soil CO2 raises carbonic acid: uses up Carbon dioxideuses upInfiltrating water dissolves carbonate in the wetter upper profile as soil CO2 raises carbonic acid: uses up Carbonic aciduses upEvapotranspiration removes water and CO2 degasses, so Ca2+ + 2HCO3- precipitates as CaCO3: makes CalcitemakesEvapotranspiration removes water and CO2 degasses, so Ca2+ + 2HCO3- precipitates as CaCO3: uses up Wateruses upEvapotranspiration removes water and CO2 degasses, so Ca2+ + 2HCO3- precipitates as CaCO3: uses up Calciumuses upEvapotranspiration removes water and CO2 degasses, so Ca2+ + 2HCO3- precipitates as CaCO3: makes Carbon dioxidemakesNodules and coatings concentrate at the mean wetting depth, which rises with mean annual precipitation: makes CalcitemakesNodules and coatings concentrate at the mean wetting depth, which rises with mean annual precipitation: uses WaterPores fill and cement into a plugged horizon (Gile stage III to IV) that limits infiltration and root penetration: makes CalcitemakesPores fill and cement into a plugged horizon (Gile stage III to IV) that limits infiltration and root penetration: uses WaterCalcite equilibrium buffers pH at 7.5 to 8.5; phosphate precipitates as Ca phosphates and Fe, Zn and Mn availability falls: uses CalciteSoil inorganic carbon stores on the order of 700 to 950 Pg C in the top metre globally: uses Calciteaccumulates at soilsurfaceCalcite: accumulates at soil surfacedelivers calcium andcarbonateWater: delivers calcium and carbonatedelivered to soilsurfaceCalcium: delivered to soil surfacedissolved by carbonicacidCalcite: dissolved by carbonic acidcarries dissolvedcarbonate downwardWater: carries dissolved carbonate downwardforms carbonic acid withwaterCarbon dioxide: forms carbonic acid with waterdissolves carbonate inupper profileCarbonic acid: dissolves carbonate in upper profileprecipitates fromsolutionCalcite: precipitates from solutionremoved byevapotranspirationWater: removed by evapotranspirationprecipitated withbicarbonateCalcium: precipitated with bicarbonatedegasses from solutionCarbon dioxide: degasses from solutionconcentrates intonodules at wetting depthCalcite: concentrates into nodules at wetting depthsets depth of wettingWater: sets depth of wettingcements pores intoplugged horizonCalcite: cements pores into plugged horizoninfiltration limited byplugWater: infiltration limited by plugbuffers soil pHCalcite: buffers soil pHstores inorganic carbonCalcite: stores inorganic carbonStart: Pedogeniccarbonate accumulationand calcrete formationSTARTStart: Pedogenic carbonate accumulation and calcrete formationEnd: Pedogeniccarbonate accumulationand calcrete formationENDEnd: Pedogenic carbonate accumulation and calcrete formationDust and rain deliverCa2+ and carbonate tothe surface of arid a…📄1Dust and rain deliver Ca2+ and carbonate to the surface of arid and semi-arid soilsInfiltrating waterdissolves carbonate inthe wetter upp…📄1Infiltrating water dissolves carbonate in the wetter upper profile as soil CO2 raises carbonic acidEvapotranspirationremoves water and CO2degasses, so Ca2+…📄1Evapotranspiration removes water and CO2 degasses, so Ca2+ + 2HCO3- precipitates as CaCO3Nodules and coatingsconcentrate at the meanwetting depth, whi…📄1Nodules and coatings concentrate at the mean wetting depth, which rises with mean annual precipitationPores fill and cementinto a plugged horizon(Gile stage III to I…📄1Pores fill and cement into a plugged horizon (Gile stage III to IV) that limits infiltration and root penetrationCalcite equilibriumbuffers pH at 7.5 to8.5; phospha…📄1Calcite equilibrium buffers pH at 7.5 to 8.5; phosphate precipitates as Ca phosphates and Fe, Zn and Mn availability fallsSoil inorganic carbonstores on the order of700 to 950 Pg C in t…📄1Soil inorganic carbon stores on the order of 700 to 950 Pg C in the top metre globally
Line thickness = size of impact (1 to 5)% on a line = chance a fork takes that branch📄 = pieces of evidence on a stepNegative effectLoops backHypothesisResource used up (arrow into the step)Resource used, not used upResource made or released (arrow out of the step)
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Every step, with its evidence

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Dust and rain deliver Ca2+ and carbonate to the surface of arid and semi-arid soils
DustRainCalciumCalciteWater
Evidence (1) · added by Fern
  • Machette (1985), Calcic soils of the southwestern United States, GSA Special Paper 203:1-21: Eolian dust and rain are the main Ca sources for calcic horizons where parent material has little carbonate. (confidence 0.7)
Infiltrating water dissolves carbonate in the wetter upper profile as soil CO2 raises carbonic acid
Soil waterPlant rootsCalciteCarbonic acidCarbon dioxideWater
Evidence (1) · added by Fern
  • Breecker, Sharp & McFadden (2009), Seasonal bias in the formation and stable isotopic composition of pedogenic carbonate in modern soils, GSA Bulletin 121:630-640: Soil CO2 from root and microbial respiration drives dissolution and sets carbonate precipitation conditions. (confidence 0.7)
Evapotranspiration removes water and CO2 degasses, so Ca2+ + 2HCO3- precipitates as CaCO3
Plant rootsSunCalciteCalciumBicarbonateCalciteCarbon dioxideWater
Evidence (1) · added by Fern
  • Breecker, Sharp & McFadden (2009), GSA Bulletin 121:630-640: Pedogenic carbonate forms mainly in dry seasons when the soil dries and pCO2 is low. (confidence 0.75)
Nodules and coatings concentrate at the mean wetting depth, which rises with mean annual precipitation
CalciteCalciteWater
Evidence (1) · added by Fern
  • Retallack (2005), Pedogenic carbonate proxies for amount and seasonality of precipitation in paleosols, Geology 33:333-336: Depth to the Bk carbonate horizon correlates with mean annual precipitation in modern soils. (confidence 0.75)
Pores fill and cement into a plugged horizon (Gile stage III to IV) that limits infiltration and root penetration
CalcitePlant rootsCalcitePore spaceWater
Evidence (1) · added by Fern
  • Gile, Peterson & Grossman (1966), Soil Science 101:347-360: Carbonate plugs all pores in stage III and forms laminar caps in stage IV, restricting water and roots. (confidence 0.8)
Calcite equilibrium buffers pH at 7.5 to 8.5; phosphate precipitates as Ca phosphates and Fe, Zn and Mn availability falls
CalciteSoil solutionCalcitePhosphateHydrogen ionsIron
Evidence (1) · added by Fern
  • Lindsay (1979), Chemical Equilibria in Soils, Wiley: Calcareous soils sit at pH 7.5 to 8.4, where micronutrient cation solubility is very low and phosphate precipitates with Ca. (confidence 0.8)
Soil inorganic carbon stores on the order of 700 to 950 Pg C in the top metre globally
CalciteSoilCarbonCalcite
Evidence (1) · added by Fern
  • Lal (2004), Soil carbon sequestration impacts on global climate change and food security, Science 304:1623-1627: Soil inorganic carbon pool estimated at roughly 695-940 Pg C in the top metre. (confidence 0.65)