Dissolution of calcite in sediments -- metabolic dissolution

CaCO3 accumulation in marine sediments CaCO3 is the predominant biogenic sediment constituent over much of the ocean Changes in the marine alkalinity budget can drive changes in atmospheric PCO2 involve changes in the accumulation rate of CaCO3 in marine sediment ==> The study of climate change is linked to the study of CaCO3 in marine sediments

CaCO3 dissolution ~ 50% of calcite dissolution in the ocean occurs at the sea floor Up to 50% of seafloor calcite dissolution may occur in sediments that lie above the calcite saturation horizon … what drives this dissolution?

“Metabolic” calcite dissolution Oxic respiration results in the release of acids to solution : Acids are neutralized by (and similar reactions for neutralizing H + )

Sediments vs water column In the water column, A very large reservoir of dissolved CO 3 2-, B(OH) 4 - … result of respiration: lower pH and CO 3 2- in deep water over its ~ 1000 yr trip from NAtl to NEPac How are sediments different? spatial scale… 8 cm mixed layer, w~1cm/ky… res time ~ 8000 y \ supply of dissolved CO 3 2-, B(OH) 4 - limited by diffusion from BW Abundant CaCO 3 (s) in sediments ==> Dissolution is favored in sediments if pore waters are driven to undersaturation

Theoretical consideration of metabolic dissolution

Predicted [CO 3 2- ] vs. depth General… A specific case, supersaturated bottom water…

Evidence -- Solid phase Measurements Of %CaCO3 in NA sediments Calcite sat. horizon

Evidence -- Pore water Organic matter oxidation by O2: With no dissolution: With dissolution, add to this reaction: Now:

Review: oxic metabolism and calcite dissolution C org No net Alk flux CaCO 3 Net Alk flux out equilibration dissolution Fluxes of solids: solid lines Fluxes of solutes: dashed lines

Pore waters from Atlantic Both above and below CSH Lines: predicted, with dissolution

Evidence - Pore water In situ microelectrode profiling ** measure pore water pH and O2 at high resolution ** interpret profiles using a model of coupled oxic metabolism / calcite dissolution ** the combination allows (1) qualitative demonstration of metabolic dissolution (2) quantification of dissolution rate

Metabolic dissolution : models For each solute: For CaCO 3 : For mass: Where: Assume: acid-base equilibrium in solution ! Assume: no calcite precipitation !

Supersaturated bw Undersat.

In situ MEP Archer et al. (1989) GCA 53, Data from: Station above CSH Station below CSH

O 2, modeled to give rate of Corg ox vs depth pH, modeled to give Calcite dissolution rate Curves = dif. Corg rain rates Dash: diff. calcite sat.

In situ MEP North Atlantic Hales et al., 1994, DSR 41, From 2 sites: m m

Clear demonstration that dissolution must occur … if model is correct! Quantification of dissolution by fits of model output to pH data With NO dissolution

Evidence -- Pore water in situ whole-core squeezing Martin and Sayles, 1996, GCA 60, In situ measurement of pore water TCO 2, Alk, Ca 2+ … demonstration of dissolution without model Either shipboard or in situ measurement of NO 3 -, shipboard O 2 … quantification of ox. vs depth Model of coupled C org ox. and calcite dissolution to quantify dissolution rate

Ceara Rise, w. tropical Atlantic Site A: just above CSH

In all: lines are fits of model to data to constrain fluxes and parameter values Ceara Rise, Site A

2nd in situ wcs result -- Cape Verde Plateau, E. tropical Atlantic well above CSH Lines = fits of model to data to quantify dissolution rate

Pore water evidence -- carbon isotopes McCorkle The  13 C of pore water TCO 2 Organic carbon ~ -20 ‰ CaCO 3 ~ 0‰ Pore water DIC its  13 C reflects the proportions of the 2 processes respiration dissolution

Note: compare shipboard and in situ sampling

Summary McCorkle isotopic data

Summary Pore water pH, Alk, TCO 2 data

Counter evidence? In situ benthic flux chambers Jahnke & Jahnke, 2004, GCA 68, Above CSH Below CSH Alk Ca Alk Ca

Summary Benthic flux chamber data ? No dissolution in sediments with %CaCO3 > 30%? -- ? precipitation at interface? -- ? Incorrect speciation model -- calcite surface chemistry -- ? Or are chambers just not sensitive enough

One more approach Th activity changes near swi Martin, 2004

Best place to observe this effect: sites with very high %CaCO 3 Ontong-Java Plateau

Bottom water saturation state %CaCO 3

If metabolic dissolution occurs, how important is it? 2 competing processes: diffusion across sediment-water interface CaCO 3 dissolution Balance between them depends on: Ratio, Corg rain : CaCO3 rain -- today, constant at ~ 0.8 depth distribution of Corg oxidation in sediments saturation state of bottom water

Depth distribution of Corg oxidation in sediments in the deep sea Martin and Sayles (2006) DSR II, 53, If Then “e-folding depth is 1/p 1

How does metabolic dissolution vary with depth distribution of oxidation and saturation state? Model test: Keep C org and CaCO 3 rain rates constant, vary sat. state of bw and e- fold depth of oxidation

What is a “best guess” of metabolic dissolution -- rain rate ratio constant at ~ observed Corg ox. Distribution in sediments Dissolution rate increases with Increasing rain at given saturation CaCO3 burial efficiency is ~ constant At given sat., but decreases with Decreasing bw. saturation “mde” ~ constant at given sat., But increases as sat. decreases