Shallow water carbonate sedimentation Including partial reviews of : Carbonate chemistry (solubility, saturation state) Metabolic dissolution (impact of OM decomposition) Stoichiometry and flux balance (predictions, checks) Non-diffusive transport(s) (often important) And introducing - Ocean Acidification

% CaCO 3 vs. water depth “lysocline”? “calcite compensation depth”?

Milliman, 1993 Reefs + banks + carbonate shelves + Halimeda > 1/3 total accumulation

Substantial carbonate dissolution in shallow water carbonate sediments. Bottom water supersaturated with likely carbonate phases (calcite, aragonite, high-Mg calcite); the dissolution must be “metabolic”.

Morse et al., 2006 Histogram of mol % MgCO 3 for cements (open) and carbonate bank sands (filled). ( contrast w. “normal” biogenic calcite, < 1 mol % MgCO 3 )

Walter and Morse, 1984 Solubility of biogenic high-Mg calcite not well known; early estimates (Plummer and Mackenzie, 1974) may be too high, but...

Solubility of synthetic (open) and biogenic (filled) high-Mg calcites vs. mol % MgCO 3. Offset; scatter in biogenic values. (Hard to measure biogenic – no pure phase.) Morse et al., 2006

Histogram of solubility, relative to aragonite; Bahamas (& Bermuda?) dominated by more-soluble phase(s). Morse et al., 1985 Bahamas Bermuda Florida Bay Everglades mangrove

Andersson et al., Model response to ocean acidification. Today, low-latitude surface seawater substantially supersaturated w.r.t. calcite, aragonite, and high-Mg calcite (but depends on high-Mg solubility estimate used).

Stoichiometry of OM oxidation by oxygen, nitrate, and sulfate Predicted aragonite saturation state during oxygen respiration? Small alk decrease; larger CO 2 increase. Expected change in [CO 3 = ] for O 2 respiration?

Boudreau and Canfield, Closed system and open system (w. transport) models Model-predicted oxidant profiles.

Boudreau and Canfield, 1993 Closed system oxygen respiration cases.

Morse and Mackenzie, 1990 Predicted aragonite saturation state during closed-system sulfate reduction.

Burdige, 2006 (Morse and Mackenzie, 1990)

Boudreau and Canfield, 1993 Stoichiometry of OM oxidation by oxygen, nitrate, and sulfate Predicted aragonite saturation state during closed-system sulfate reduction: Remember simplified versions of alkalinity (~ HCO x CO 3 = ) and DIC (~ HCO CO 3 = ). Expression for [CO 3 = ] ? What is delta [CO 3 = ] for SR? Why should saturation state drop?

Boudreau and Canfield, 1993 Species that yield protons upon dissociation H 2 S is produced during SR Some H 2 S dissociates into HS - and H + Some H + reacts with CO 3 = to yield HCO 3 - [CO 3 = ] drops; can drive dissolution With continued SR, alk production overshadows H 2 S dissociation.

Boudreau and Canfield, 1993; Ku et al., 1999 Re-oxidation of reduced metabolites by pore water oxygen releases acidity close to the sediment-water interface. This lowers pore water pH and drives dissolution (though some of this acidity can be neutralized by bottom water alkalinity). S burial as FeS lowers the H 2 S concentration, and thus reduces the proton release due to H 2 S dissociation; this minimizes initial dissolution due to SR

Boudreau and Canfield, Closed system and open system (w. transport) models Closed system sulfate reduction cases.

Boudreau and Canfield, 1993 Model oxygen profiles, with and without reoxidation of reduced metabolites.

Boudreau and Canfield, 1993 Model oxygen profiles, with and without reoxidation of reduced metabolites. Saturation state vs. depth profiles steeper with reoxidation (right, dashed curves)

Walter et al., 1993 Observe “excess” DIC and Ca ++ for given sulfate depletion in Florida Bay and Bahamas sediments – sulfide reoxidation to sulfate.

Ku et al., 1999 Shallow-water metabolic dissolution (+ S and O stable isotopes) in Florida Bay; Sulfate reduction / reoxidation cycle equivalent to enhanced oxic respiration in terms of saturation state. Expected changes due to reaction stoichiometry – DIC:Ca = 2:1 DIC:Alk = 1:1 No net change in sulfate

Ku et al., 1999 Again, “excess” DIC and Ca for given sulfate depletion – sulfide reoxidation

Burdige and Zimmerman, 2002 Impact of seagrass on metabolic dissolution at Lee Stocking Island Non-diffusive O 2 input via roots drives CO 2 production and CaCO 3 dissolution

Pore water profiles in sandy carbonate sediments: DIC and alkalinity increase, pH and CO 3 = decrease. Burdige and Zimmerman, 2002 What about just below SWI?

Pore water profiles in sandy carbonate sediments: Oxygen decreases; no net change in sulfate. Burdige and Zimmerman, 2002

Pore water evidence of carbonate dissolution in these shallow sediments. Expect Ca:Alk ~ 0.5; lower value due to MgCO 3 ? Burdige and Zimmerman, 2002

Alkalinity production in excess of that predicted from stoichiomentric oxic respiration + metabolic dissolution alone. B & Z: Probably not a sulfate reduction / reoxidation cycle (no O 2 = 0 zone for SR). B & Z: Additional O 2 input from roots / rhizomes of sea grass drives additional oxic respiration, and metabolic carbonate dissolution. Burdige and Zimmerman, 2002

Can dissolution of shallow carbonate-rich sediments (including high-solubility high-Mg carbonates) offset the ocean acidification impact of fossil fuel CO 2 release? Probably not (e.g., Andersson, 2005; 1 m reactive depth, 50% porosity, 95:5 at 15%:80% CaCO 3, observed mineralogy…) but let’s look.

Morse et al., 2006 Devil’s Hole, Bermuda. Seasonally elevated pCO 2 (at levels comparable to anticipated anthropogenic increase in atmosphere) drives dissolution today. Saturation state for calcite, aragonite, and high-Mg calcite.

Morse et al., 2006

Multi-component shallow water dissolution model. Sequential dissolution of most-soluble phase; does not include metabolic dissolution (Andersson does); dissolution is a function of water residence time on banks.

Morse et al., 2006 Extreme forcing: ~ 3x pCO2 by 2100, then continued linear increase to ~ 1800 ppm by pCO 2 and pH Saturation state Biogenic solubilityPlummer, 1974

Morse et al., 2006 Bottom line: Depends on solubility of high-Mg calcite (and on impact of metabolic dissolution) but at 25 C, 18 mol % MgCO 3 only becomes undersaturated at pCO 2 >1400 ppm. Impact on marine calcifiers will happen much sooner. pCO 2 and pH Saturation state Biogenic solubilityPlummer, 1974