Carbonate Solubility Solubility Dissolution mechanisms Dissolution rate expressions   Saturation state in the ocean T, P, and CO2 release In situ [CO3–2] and the saturation horizon Sedimentary evidence of carbonate dissolution Kinetic and/or thermodynamic controls

Carbonate Solubility   CaCO3 <=> Ca+2 + CO3–2 Define satauration state as: Ώ = IAP / Ksp With IAP (ion activity product) = [Ca+2][CO3–2] for the solution, and with the apparent solubility product: for a solution in equilibrium with solid CaCO3 (T,P,S)

  K’sp depends on mineralogy (calcite < aragonite < Mg-calcite,…) as well as T and P At equilibrium: Ώ = IAP / Ksp = [Ca+2][CO3–2] = 1 When Ώ > 1, solution is supersaturated When Ώ < 1, solution is undersaturated

Often, we take [Ca+2] as a constant function of salinity, and express Ώ in terms of the saturation carbonate ion concentration   so, if [Ca+2] constant, Define DCO3–2 as ([CO3–2] in situ - [CO3–2]sat)

Controls on saturation state: Pressure and temperature effects on carbonate solubility Solubility higher in deeper and colder water   Pressure, temperature, and respiration effects on [CO3–2] in situ [CO3–2] lower in deeper, colder, and “older” water

in situ saturometer pH sensor downstream of a cartridge filled with crushed carbonate; cycle pump, observe pH increase as carbonate dissolves. Ben-Yakov et al., 74

Profile of saturation carbonate ion concentration, based on in situ saturometer (crosses) and calcite lysocline (circles); strong increase in solubility with increasing depth. Broecker and Peng

Calcite solubility as fn. of T and P Ingle, 1975 Broecker and Peng

Use observed K’sp in 0-15 cm interval of in situ pore water profiles (Alk + pH => [CO3–2], Ca+2) to estimate solubility. Sayles, 1985

Solubility vs depth for Sayles’ pore water data consistent with lab solubility studies

Solubility vs depth comparison. Broecker and Peng

What is the relationship between degree of undersaturation and dissolution rate?   CaCO3 <=> Ca+2 + CO3–2 Define the mass-normalized dissolution rate: “k*” is the reaction rate (%/day) ((A/V)*k) “n” is the reaction order “*” to account for % CaCO3 in bulk sediments, and for surface area Can express dissolution rate in terms of carbonate ion concentration R = k* ([CO3–2]sat - [CO3–2] in situ )n Where [CO3–2]sat is the saturation carbonate ion concentration (T,P)

  Morse and Arvidson - High order kinetics consistent with control of dissolution by one of the surface processes (adsorption, migration, reaction, migration, desorption) Hales and Emerson, Sayles and Martin Shallow depth at which porewaters reach saturation implies low-order kinetics; otherwise final approach to saturation would take much longer (deeper).

An unresolved puzzle:   Laboratory dissolution studies (Keir, 1980) and theoretical arguments (Morse) suggest that k* ~ 1000 / day and n = 4.5 strongly non-linear fast dissolution really fast at low Ώ But ~ all porewater dissolution studies give k* ~ 1 - 10 / day (102 to 103 slower) n = 3, or even 1 (Hales and Emerson, 1997) when n=4.5 is assumed

Recalculate with new constants: n = 3.2 Reassess solubiliy too: n = 1.3 Hales and Emerson

Why do we care? Influences of dissolution rate law include: vertical profile of dissolution in sediments impact of CO2 release by benthic decomposition total dissolution rate shape of lysocline

Saturation state in the ocean Influence of T, P, and CO2 release In situ [CO3–2] and the saturation horizon

Pressure, temperature, and respiration effects on [CO3–2] in situ . [CO3–2] lower in deeper, colder, and “older” water. Aging dominates.

How does [CO3–2] respond to changes in Alk or DIC?   CT = [H2CO3*] + [ HCO3–] + [CO3–2] ~ [ HCO3–] + [CO3–2] (an approximation) Alk = [OH–] + [HCO3–] + 2[CO3–2] + [B(OH)4-] – [H+] ~ [HCO3–] + 2[CO3–2] (a.k.a. “carbonate alkalinity”) So (roughly): [CO3–2] ~ Alk – CT CT ↑ , [CO3–2] ↓ Alk ↑ , [CO3–2] ↑

Photosynthesis CO2 + H2O => “CH2O” + O2   D SCO2 = -1 D Alk = 0 So D [CO3–2] = D Alk - D SCO2 = 0 – (-1) = +1  [CO3–2] increases, Ώ increases. Respiration  “CH2O” + O2 => CO2 + H2O D SCO2 = +1 So D [CO3–2] = D Alk - D SCO2 = 0 – 1 = -1  [CO3–2] decreases, Ώ decreases.

Zeebe and Wolf-Gladrow

Calcification  Ca+2 + CO3–2 => CaCO3   D SCO2 = -1 D Alk = -2 So D [CO3–2] = -2 – (-1) = -1  [CO3–2] decreases, Ώ decreases. Dissolution:  CaCO3 => Ca+2 + CO3–2 D SCO2 = +1 D Alk = +2 So D [CO3–2] = 2 – 1 = +1  [CO3–2] increases, Ώ increases.

% CaCO3 vs. water depth “lysocline” – onset of dissolution “calcite compensation depth” – dissolution rate = rain rate

Takahashi and Broecker ’80; GEOSECS data

in situ Aragonite Calcite saturation

High aragonite/calcite ratio in sinking flux: Another puzzle: NPac sediment traps – loss of carbonate above the aragonite saturation horizon. High aragonite/calcite ratio in sinking flux: 0.2 to 20, most > 1 Betzer et al., 1984

High-solubility phase(s)? Magnesian-calcite Carbonate flux in NAtl sediment traps – loss of carbonate above the aragonite saturation horizon. High-solubility phase(s)? Magnesian-calcite Milliman et al., 1999

Sedimentary evidence of carbonate dissolution Kinetic and/or thermodynamic controls

Sediment evidence for dissolution?

Here, core-top aragonite distribution seems to match the depth of the aragonite saturation horizon.

Farrell and Prell

Adelseck 1977 – lab study of selective dissolution of planktonic foraminifera Assemblage change only after substantial carbonate dissolution; not a sensitive indicator

Berger et al., 1982 – Selective dissolution in EqPac Differential dissolution: a range of susceptibility Thermodynamics? (different solubilities?) Kinetics? (different area/g, crystallinity?)

% CaCO3 % Coarse % whole plankt. Peterson and Prell (1985) – “composite dissolution index” in core-top sediments of the eastern equatorial Indian Ocean % benthic % radiolaria % whole G menardii

Peterson and Prell – a “composite dissolution index” compared with water column saturation state. What assumption(s) underlies downcore (paleo) application of CDI?

Lohmann – size-normalized shell weight. As individuals grow, the mass:size relationship is consistent. Within a size class, negative deviations from expected relationship reflect dissolution. (Shells thin (and lose mass), without a significant change in size.)

Size-normalized shell weight suggests dissolution starting well above the calcite saturation horizon. Downcore (paleo) application thwarted by…?