Dissolution of calcite in sediments -- metabolic dissolution

1. Dissolution of calcite in sediments --metabolic dissolution

2. 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

3. 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?

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

5. Sediments vs water column In the water column, A very large reservoir of dissolved CO32-, B(OH)4- … result of respiration: lower pH and CO32- 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 CO32-, B(OH)4- limited by diffusion from BW Abundant CaCO3 (s) in sediments ==> Dissolution is favored in sediments if pore waters are driven to undersaturation

6. Theoretical consideration of metabolic dissolution

7. Predicted [CO32-] vs. depth General… A specific case, supersaturated bottom water…

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

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

10. Review: oxic metabolism and calcite dissolution Corg CaCO3 Net Alk flux out No net Alk flux equilibration dissolution Fluxes of solids: solid lines Fluxes of solutes: dashed lines

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

12. Evidence - Pore waterIn 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

13. Metabolic dissolution : models For each solute: Assume: acid-base equilibrium in solution ! For CaCO3: For mass: Where: Assume: no calcite precipitation !

15. Supersaturated bw Undersat.

16. In situ MEPArcher et al. (1989) GCA 53, 2831-2845 Data from: Station 12 -- above CSH Station 13 -- below CSH

17. pH, modeled to give Calcite dissolution rate O2, modeled to give rate of Corg ox vs depth 12 13 Curves = dif. Corg rain rates Dash: diff. calcite sat.

18. In situ MEPNorth AtlanticHales et al., 1994, DSR 41, 695-719 From 2 sites: 5 -- 2159m 7 -- 4236m

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

21. Evidence -- Pore waterin situ whole-core squeezingMartin and Sayles, 1996, GCA 60, 243-263 In situ measurement of pore water TCO2, Alk, Ca2+ … demonstration of dissolution without model Either shipboard or in situ measurement of NO3-, shipboard O2 … quantification of ox. vs depth Model of coupled Corg ox. and calcite dissolution to quantify dissolution rate

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

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

24. 2nd in situ wcs result --Cape Verde Plateau, E. tropical Atlanticwell above CSH Lines = fits of model to data to quantify dissolution rate

25. Pore water evidence -- carbon isotopesMcCorkle The 13C of pore water TCO2 Organic carbon ~ -20 ‰ CaCO3 ~ 0‰ dissolution respiration Pore water DIC its 13C reflects the proportions of the 2 processes

26. Note: compare shipboard and in situ sampling

28. SummaryMcCorkle isotopic data

29. SummaryPore water pH, Alk, TCO2 data

30. Counter evidence?In situ benthic flux chambersJahnke & Jahnke, 2004, GCA 68, 47-59 Above CSH Below CSH Alk Alk Ca Ca

31. SummaryBenthic 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

32. One more approach --230Th activity changes near swiMartin, 2004

33. Best place to observe this effect:sites with very high %CaCO3Ontong-Java Plateau

34. Bottom water saturation state%CaCO3

37. If metabolic dissolution occurs,how important is it? 2 competing processes: diffusion across sediment-water interface CaCO3 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

38. Depth distribution of Corg oxidation in sedimentsin the deep seaMartin and Sayles (2006) DSR II, 53, 771-792 Then “e-folding depth is 1/p1 If

39. How does metabolic dissolution vary with depth distribution of oxidation and saturation state? Model test: Keep Corg and CaCO3 rain rates constant, vary sat. state of bw and e-fold depth of oxidation

40. What is a “best guess” of metabolic dissolution-- rain rate ratio constant at ~ 0.8-- 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