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Ocean Carbonates: Global Budgets and Models

Ocean Carbonates: Global Budgets and Models. Michael Schulz (Research Center Ocean Margins, Bremen). 9:15 - 10:45 The Role of marine calcium carbonate in the global carbon cycle - "Carbonate-compensation" mechanism - Response times of the carbonate system

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Ocean Carbonates: Global Budgets and Models

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  1. Ocean Carbonates: Global Budgets and Models Michael Schulz (Research Center Ocean Margins, Bremen)

  2. 9:15 - 10:45 • The Role of marine calcium carbonate in the global carbon cycle- "Carbonate-compensation" mechanism - Response times of the carbonate system - Carbonate chemistry, alkalinity and control of pH - Biological "carbonate pump" 2.The modern oceanic calcium carbonate budget- Quantifying carbonate sinks - Quantifying carbonate sources (flux-based vs. alkalinity-based estimates)- Dissolution in the water column - Dissolution in sediments 10:45 - 11:00 break

  3. 11:00 – 12:30 2. cont'd- Global budgets- Plankton group-specific budgets 3. Modeling the oceanic calcium carbonate budget- Glacial-interglacial cycles- Response to changes in ocean gateways

  4. Course Material (this presentation) www.geo.uni-bremen.de/geomod  English Pages  Teaching European Graduate College in Marine Sciences (at the bottom of the page) “Script” (Powerpoint File)

  5. Basic Literature Iglesias-Rodriguez et al., 2002: Progress made in study of ocean's calcium carbonate budget. EOS Transactions, American Geophysical Union, 83(34), 365-375.http://usjgofs.whoi.edu/mzweb/caco3_rpt.html Milliman, J. D. and A. W. Droxler, 1996: Neritic and pelagic carbonate sedimentation in the marine environment: ignorance is not bliss. Geologische Rundschau, 85, 496-504. Schneider, R. R. et al., 2000: Marine carbonates: their formation and destruction. Marine Geochemistry, H. D. Schulz and M. Zabel, Eds., Springer Verlag,283-307.

  6. 1. The Role of Marine Calcium Carbonate in the Global Carbon Cycle Weathering feedback probably stabilizes atmospheric pCO2 at timescales ≥ 106 years Ruddiman (2001)

  7. Today: P = 4 × R D = 3 × R  B = R River Input R (Ca2+, HCO3-) Production P Dissolution D Burial B CaCO3 Compensation The burial rate of CaCO3 in deep-sea sediments is ultimately controlled by the dissolution rate, which adjusts to maintain steady state between river input (weathering) and burial. Example: (P = const.) R↓ →B initially too high (imbalance) →D ↑ →B ↓ until B = R Broecker and Peng, 1987: The role of CaCO3 compensation in the glacial to interglacial atmospheric CO2 change. Global Biogeochemical Cycles, 1, 15-29.

  8. Carbon-Cycle – Characteristic Timescales Reservoir Sizes in [Gt C] Fluxes in [Gt C / yr] Sundquist (1993, Science)

  9. CaCO3 Solubility and Saturation State of Seawater • Saturation state W: ksp: solubility product = f(pressure, T , S) W > 1: supersaturated W < 1: undersaturated • Seawater: Changes in [Ca2+] are small  changes in W largely controlled by D[CO32-] Zeebe and Wolf-Gladrow, 2001: CO2 in Seawater: Equilibrium, kinetics, isotopes. Elsevier.

  10. CaCO3 Solubility and Saturation State of Seawater Zeebe and Wolf-Gladrow (2001)

  11. Oceanic Carbonate Buffering System Average surface- Water composition CO2 0.5 % HCO3- 89.0 % CO32- 10.5 % Open Univ. “Seawater”

  12. The Concept of Alkalinity • Chemical definition: Total Alkalinity (TA) measures the charges of the ions of weak acids: • Physical definition (based on principle of electroneutrality): Alkalinity = charge difference between conservative anions and cations: • TA is a conservative quantity  concentration unaffected by changes in temperature, pressure or pH Zeebe and Wolf-Gladrow, 2001: CO2 in Seawater: Equilibrium, kinetics, isotopes. Elsevier.

  13. Charge Imbalance of Major Ions in Seawater Zeebe and Wolf-Gladrow (2001)

  14. Alkalinity as a Master Variable • From Total Alkalinity (TA) and SCO2 together with T and S, all other quantities of the carbonate system can be quantified  From measurements of TA and SCO2 the CaCO3 saturation state can be inferred

  15. Biogeochemical Effects on Alkalinity • Precipitation of 1 mole CaCO3 alkalinity decreases by 2 moles • Dissolution of 1 mole CaCO3 alkalinity increases by 2 moles • Uptake of DIC by algae  no change in alkalinity (assuming electroneutrality of algae, parallel uptake of H+ or release of OH–) • Uptake of 1 mole NO3– alkalinity increases by 1 mole (assuming electroneutrality of algae) • Remineralization of algal material has the reverse effects on alkalinity Zeebe and Wolf-Gladrow (2001)

  16. Biogenic Calcium Carbonate Production Raises Dissolved CO2 Concentration pH Reaction: (1) Biogenic carbonate uptake (2) More bicarbonate dissociates (3) More CO2 is formed

  17. The Calcium Carbonate Pump Atmosphere CO 2 CO 2 Biogenic CaCO3 Formation 3 Lysocline Ocean CaCO3 Dissolution CO 2- 3 Fig. courtesy of A. Körtzinger

  18. Carbonate Concentration and CO2 • CaCO3dissolution [CO32-] ↑ reacts with CO2 to form HCO3-  [CO2] ↓ • CaCO3precipitation [CO32-] ↓ HCO3- dissociates  [CO2] ↑ • As [CO32-] rises [CO2] drops and vice versa

  19. 2. Calcium Carbonate Budget of the Modern Ocean • Budget = sources minus sinks • Sources: production rate • Sinks: • Burial in sediments • Dissolution in the water column • Steady-state Budget (sources = sinks)?

  20. Neritic vs. Oceanic Carbonate Budgets • Neritic Environments • Benthic production predominates • Mainly aragonite and magnesian calcite • Production rates 40-4000 g m-2 yr-1 • Oceanic Environments • Pelagic production predominates • Mainly calcite • Production several orders of magnitude lower than neritic production (compensated by larger area)

  21. Deep-Ocean CaCO3 Burial Rate • Catubig, N. R., D. E. Archer, R. Francois, P. deMenocal, W. Howard, and E. F. Yu, 1998: Global deep-sea burial rate of calcium carbonate during the last glacial maximum. Paleoceanography, 13, 298-310. • Approach: Estimate CaCO3 burial from sediment mass-accumulation rates (MAR)

  22. Estimating Net CaCO3 Burial • Calcite MAR are rare, but large number of calcite concentration measurements in sediments • Basic idea: Constant dilution assumption: • Non-calcite MAR required to calculated calcite MAR; usually not known for each record  use regional estimate instead

  23. Percent Calcite Data –Locations of Modern Core Tops • Note poor coverage in Indian and Southern Ocean • To obtain global coverage Extrapolation via regional %CaCO3-depth relationships Catubig et al. (1998)

  24. Mass-Accumulation Rate Data: Locations of Modern Core Tops • Note poor data coverage. • Only 191 out of 349 data are utilized. Criterion: non- CaCO3 MAR uncorrelated with %CaCO3 in specified regions (otherwise violation of constant-dilution assumption) Catubig et al. (1998)

  25. Regional Modern CaCO3Mass-Accumulations Rates Global Burial Rate: 8.6 ± 0.5 × 1012 mol CaCO3/yr Catubig et al. (1998)

  26. Oceanic Carbonate Production • From sediment-trap data: • Milliman, J. D., 1993: Production and accumulation of calcium carbonate in the ocean: budget of a nonsteady state. Global Biogeochemical Cycles, 7, 927-957. • From changes in alkalinity: • Lee, K., 2001: Global net community production estimated from the annual cycle of surface water total dissolved inorganic carbon. Limnology and Oceanography, 46, 1287-1297.

  27. CaCO3 Production from Sediment Traps • Sediment traps at > 500-1000 m depth monitor CaCO3 production in overlying mixed layer • Mooring well below mixed-layer to minimize effects of turbulent mixing, horizontal advection and “swimmers” • Key assumption: No dissolution in upper water column • Database: ~ 100 sediment traps with deployment time ≥ 1 year

  28. Modern CaCO3 Production from Sediment Traps (at 1000 m depth) Trap Position Global: 24 × 1012 mol CaCO3/yr • Note poor data coverage • Isolines based on primary production contours (Berger, 1989) Milliman (1993); Milliman & Droxler (1996)

  29. Net CaCO3 Production from Alkalinity Data • Basic idea: Biological CaCO3 precipitation reduces alkalinity in the surface water (Lee, 2001) • Data: Global monthly surface-water alkalinity • Derived from SST-alkalinity relationship (Millero et al., 1998; Mar. Chem.) [too few direct measurements] • Mixed-layer depth (Levitus climatology ) and surface area for integration • Corrections for: • Freshwater exchange at sea-surface ( salinity normalized alkalinity) • Mixing of water masses ( vertical diffusion) • Biological NO3- uptake ( Derived from SST-NO3- relation; Lee et al. 2000 GBC)

  30. Modern Alkalinity-Based CaCO3 Production Lee (2001)

  31. Modern Alkalinity-Based Oceanic CaCO3 Production Global: 92 ± 25× 1012 mol CaCO3/yr Lee (2001)

  32. CaCO3 Dissolution in the Water Column • Discrepancy between sediment-trap and alkalinity-based production rates • 24 vs. 92 × 1012 mol CaCO3 / year • Suggests 74 % dissolution in the upper 1000 m of the ocean, i.e., well above the lysocline!  Sediment trap based fluxes ≠ Production rates

  33. CaCO3 Dissolution in the Water Column – Possible Mechanisms • Milliman, J. D. et al., 1999: Biologically mediated dissolution of calcium carbonate above the chemical lysocline? Deep - Sea Research Part I - Oceanographic Research Papers, 46, 1653-1669. • Dissolution within • guts and feces of grazers • microenvironments with microbial oxidation of organic matter (e.g. in marine snow)

  34. Estimating Water-Column CaCO3 Dissolution from Alkalinity Data • Basic idea: CaCO3 dissolution increases alkalinity in the subsurface relative to the “preformed” values (i.e., the alkalinity when the water was last at the surface) • Data: • Global depth-profiles of alkalinity (WOCE/JGOFS…) • Preformed alkalinity is estimated from conservative tracers (salinity, …) using multiple regression • Corrections for: • NO3- release during remineralization of organic matter ( estimated via AOU = O2,sat – O2,meas) • Alkalinity input from CaCO3 dissolution in sediments

  35. Alkalinity Datain the Atlantic Ocean Chung et al. (2003)

  36. Dissolution-Driven Change in Alkalinity(Atlantic Ocean) DTACaCO3 in mmol/kg Chung et al. (2003)

  37. Water-Column Dissolution Rates of CaCO3 • Atlantic Ocean: 11.1 × 1012 mol CaCO3 / yr (31 % of net production) • Chung, S.-N. et al., 2003: Calcium carbonate budget in the Atlantic Ocean based on water column inorganic carbon chemistry. Global Biogeochemical Cycles, 17, 1093, doi:10.1029/2002GBC002001. • Pacific Ocean: 25.8 × 1012 mol CaCO3 / yr (74 % of net production) • Feely, R. A. et al., 2002: In situ calcium carbonate dissolution in the Pacific Ocean. Global Biogeochemical Cycles, 16, 1144, doi:10.129/2002GBC001866. • Indian Ocean: 8.3 × 1012 mol CaCO3 / yr (~100 % of net production) • Sabine, C. L. et al., 2002: Inorganic carbon in the Indian Ocean: Distribution and dissolution processes. Global Biogeochemical Cycles, 14, 1067, doi:10.129/2002GBC001869. • Total: 45.2× 1012 mol CaCO3 / yr (~ 50 % of net production)

  38. A Global Oceanic CaCO3 Budget 92 96 45 38 (92-45-9) 9 Modified after Milliman et al. (1999)

  39. CaCO3 Dissolution at the Seafloor • Basic idea: Oxidation of organic matter in sediments releases metabolic CO2 and promotes CaCO3 dissolution – even above the seawater lysocline (Emerson, S. and M. Bender, 1981: Carbon fluxes at the sediment-water interface of the deep-sea: calcium carbonate preservation. Journal of Marine Research, 39, 139-162.)

  40. CaCO3 Dissolution at the Seafloor Jahnke et al. (1997 GBC) OM = Organic Matter

  41. Quantifying CaCO3 in Sediments • Diagenetic model of calcium carbonate preservation (Archer, D., 1996: A data-driven model of the global calcite lysocline. Global Biogeochemical Cycles, 10, 511-526.) • Input: Global distributions of: • CaCO3 mass accumulation rates • Organic carbon accumulation rates (“rain ratio”) • [CO32-] and [O2] at sediment-water interface • Total dissolution flux: 24-40× 1012 mol CaCO3 / yr  Consistent with global budget (requires 38× 1012 mol CaCO3 / yr)

  42. Group-Specific Contributions to Oceanic CaCO3 Budget (Sediment-Trap Data; Schiebel, 2002 GBC) Independent Estimates 0.01- 0.03 (1-3 %) 0.34- 0.84 (31-76%) Paramount role of foraminifers depends critically on poorly quantified mass dumps

  43. Neritic Carbonates – Coral Reefs • CaCO3 production is estimated from Holocene reef growth data, i.e., age-depth profiles (Milliman, J. D., 1993: Production and accumulation of calcium carbonate in the ocean: budget of a nonsteady state. Global Biogeochemical Cycles, 7, 927-957.) • ProdCaCO3 = SR × porosity × densityCaCO3 • Total Production: 9× 1012 mol CaCO3 / yr • Loss due to erosion and dissolution (poorly quantified) Total accumulation: 7× 1012 mol CaCO3/yr

  44. Neritic Carbonate Budget Estimation of CaCO3 production similar to reefs (Milliman, 1993) Total Production: ~ 25 × 1012 mol CaCO3 / yr Total Accumulation: ~ 15 × 1012 mol CaCO3 / yr Milliman and Droxler (1996)

  45. Slope-Carbonate Budget • “In terms of carbonate production and accumulation, however, [the slope environment] is practically undocumented” (Milliman, 1993) • Estimates based on shallow sediment-trap data (Milliman and Droxler, 1996): • Total Production: 5× 1012 mol CaCO3 / yr • Import from shallower depths: 3.5× 1012 mol CaCO3 / yr • Total accumulation: 6× 1012 mol CaCO3 / yr (based on the assumption that 20 % of the slope and 40 % of the imported CaCO3 is dissolved)

  46. A Global Marine CaCO3 Budget Total Neritic Accumulation ≈ Total Oceanic (“Pelagic”) Accumulation (Higher neritic production compensates for smaller area) Iglesias-Rodriguez et al. (2002; EOS 83(34))

  47. 3. Modeling the Oceanic CaCO3 Budget Aims: • Consistent budget at a global scale • Quantifying the interaction of the oceanic carbonate budget with the remaining carbon cycle • Estimating past budget variations

  48. Structure of a Global Biogeochemical Model Ridgwell (2001, Thesis)

  49. Modeling Deep-Sea Sediments Ridgwell (2001, Thesis)

  50. A Modeled Sediment Stack in the North Atlantic Heinze, C. et al., 1999: A global oceanic sediment model for long-term climate studies. Global Biogeochemical Cycles, 13, 221-250.

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