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

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Presentation on theme: "Ocean Carbonates: Global Budgets and Models Michael Schulz (Research Center Ocean Margins, Bremen)"— Presentation transcript:

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

2 9: :45 1.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: :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)  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), Milliman, J. D. and A. W. Droxler, 1996: Neritic and pelagic carbonate sedimentation in the marine environment: ignorance is not bliss. Geologische Rundschau, 85, Schneider, R. R. et al., 2000: Marine carbonates: their formation and destruction. Marine Geochemistry, H. D. Schulz and M. Zabel, Eds., Springer Verlag,

6 1. The Role of Marine Calcium Carbonate in the Global Carbon Cycle Ruddiman (2001) Weathering feedback probably stabilizes atmospheric pCO 2 at timescales ≥ 10 6 years

7 CaCO 3 Compensation River Input R (Ca 2+, HCO 3 - ) Production P Dissolution D Burial B Today: P = 4 × R D = 3 × R  B = R The burial rate of CaCO 3 in deep-sea sediments is ultimately controlled by the dissolution rate, which adjusts to maintain steady state between river input (weathering) and burial. Broecker and Peng, 1987: The role of CaCO 3 compensation in the glacial to interglacial atmospheric CO 2 change. Global Biogeochemical Cycles, 1, Example: (P = const.) R ↓ → B initially too high (imbalance) → D ↑ → B ↓ until B = R

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

9 CaCO 3 Solubility and Saturation State of Seawater Saturation state  ksp: solubility product = f(pressure, T, S)  > 1: supersaturated  < 1: undersaturated Seawater: Changes in [Ca 2+ ] are small  changes in  largely controlled by  [CO 3 2- ] Zeebe and Wolf-Gladrow, 2001: CO2 in Seawater: Equilibrium, kinetics, isotopes. Elsevier.

10 CaCO 3 Solubility and Saturation State of Seawater Zeebe and Wolf-Gladrow (2001)

11 Oceanic Carbonate Buffering System Average surface- Water composition CO % HCO % CO % 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  CO 2 together with T and S, all other quantities of the carbonate system can be quantified  From measurements of TA and  CO 2 the CaCO 3 saturation state can be inferred

15 Biogeochemical Effects on Alkalinity Precipitation of 1 mole CaCO 3  alkalinity decreases by 2 moles Dissolution of 1 mole CaCO 3  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 NO 3 –  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 CO 2 Concentration pH Reaction: (1) Biogenic carbonate uptake (2) More bicarbonate dissociates (3) More CO 2 is formed

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

18 Carbonate Concentration and CO 2 CaCO 3 dissolution  [CO 3 2- ] ↑  reacts with CO 2 to form HCO 3 -  [CO 2 ] ↓ CaCO 3 precipitation  [CO 3 2- ] ↓  HCO 3 - dissociates  [CO 2 ] ↑ As [CO 3 2- ] rises [CO 2 ] 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 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 CaCO 3 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, Approach: Estimate CaCO 3 burial from sediment mass-accumulation rates (MAR)

22 Estimating Net CaCO 3 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- CaCO 3 MAR uncorrelated with %CaCO 3 in specified regions (otherwise violation of constant-dilution assumption) Catubig et al. (1998)

25 Regional Modern CaCO 3 Mass-Accumulations Rates Catubig et al. (1998) Global Burial Rate: 8.6 ± 0.5 × mol CaCO 3 /yr

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, 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,

27 CaCO 3 Production from Sediment Traps Sediment traps at > m depth monitor CaCO 3 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 CaCO 3 Production from Sediment Traps (at 1000 m depth) Milliman (1993); Milliman & Droxler (1996) Note poor data coverage Isolines based on primary production contours (Berger, 1989) Trap Position Global: 24 × mol CaCO 3 /yr

29 Net CaCO 3 Production from Alkalinity Data Basic idea: Biological CaCO 3 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 NO 3 - uptake (  Derived from SST-NO 3 - relation; Lee et al GBC)

30 Modern Alkalinity-Based CaCO 3 Production Lee (2001)

31 Modern Alkalinity-Based Oceanic CaCO 3 Production Lee (2001) Global: 92 ± 25 × mol CaCO 3 /yr

32 CaCO 3 Dissolution in the Water Column Discrepancy between sediment-trap and alkalinity- based production rates  24 vs. 92 × mol CaCO 3 / 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 CaCO 3 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, Dissolution within –guts and feces of grazers –microenvironments with microbial oxidation of organic matter (e.g. in marine snow)

34 Estimating Water-Column CaCO 3 Dissolution from Alkalinity Data Basic idea: CaCO 3 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: –NO 3 - release during remineralization of organic matter (  estimated via AOU = O 2,sat – O 2,meas ) –Alkalinity input from CaCO 3 dissolution in sediments

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

36 Dissolution-Driven Change in Alkalinity (Atlantic Ocean) Chung et al. (2003)  TA CaCO3 in  mol/kg

37 Water-Column Dissolution Rates of CaCO 3 Atlantic Ocean: 11.1 × mol CaCO 3 / 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: /2002GBC Pacific Ocean: 25.8 × mol CaCO 3 / 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:10.129/2002GBC Indian Ocean: 8.3 × mol CaCO 3 / 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:10.129/2002GBC Total: 45.2 × mol CaCO 3 / yr (~ 50 % of net production)

38 A Global Oceanic CaCO 3 Budget Modified after Milliman et al. (1999) ( )

39 CaCO 3 Dissolution at the Seafloor Basic idea: Oxidation of organic matter in sediments releases metabolic CO 2 and promotes CaCO 3 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, )

40 CaCO 3 Dissolution at the Seafloor Jahnke et al. (1997 GBC) OM = Organic Matter

41 Quantifying CaCO 3 in Sediments Diagenetic model of calcium carbonate preservation (Archer, D., 1996: A data-driven model of the global calcite lysocline. Global Biogeochemical Cycles, 10, ) Input: Global distributions of: –CaCO 3 mass accumulation rates –Organic carbon accumulation rates (“rain ratio”) –[CO 3 2- ] and [O 2 ] at sediment-water interface Total dissolution flux: × mol CaCO 3 / yr  Consistent with global budget (requires 38 × mol CaCO 3 / yr)

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

43 Neritic Carbonates – Coral Reefs CaCO 3 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, ) Prod CaCO3 = SR × porosity × density CaCO3 Total Production: 9 × mol CaCO 3 / yr Loss due to erosion and dissolution (poorly quantified)  Total accumulation: 7 × mol CaCO 3 /yr

44 Neritic Carbonate Budget Estimation of CaCO 3 production similar to reefs (Milliman, 1993) Milliman and Droxler (1996) Total Production: ~ 25 × mol CaCO 3 / yr Total Accumulation: ~ 15 × mol CaCO 3 / yr

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 × mol CaCO 3 / yr –Import from shallower depths: 3.5 × mol CaCO 3 / yr –Total accumulation: 6 × mol CaCO 3 / yr (based on the assumption that 20 % of the slope and 40 % of the imported CaCO 3 is dissolved)

46 Iglesias-Rodriguez et al. (2002; EOS 83(34)) A Global Marine CaCO 3 Budget Total Neritic Accumulation ≈ Total Oceanic (“Pelagic”) Accumulation (Higher neritic production compensates for smaller area)

47 3. Modeling the Oceanic CaCO 3 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

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,

51 Modeled and Observed Modern CaCO 3 Content of Deep-Sea Sediments Heinze et al. (1999)  Even the most sophisticated biogeochemical models allow only for a crude approximation of the real world. Discrepancies are largely due to an inadequate resolution (e.g. MOR) and a lack of knowledge of the processes being involved. ModelObservations

52 Case Study I: Glacial-Interglacial Variations in Pacific Lysocline Depth Farrell and Prell (1989, Paleoc.))

53 Modeled vs. Reconstructed Glacial- Interglacial Lysocline Variations South Atlantic South Pacific 60 % (Farrell & Prell, 1989) Model-Forcing: Prescribed changes in NADW formation, terrestrial carbon storage, neritic CaCO 3 storage, among others (Ridgwell, 2001) Glac. Deposition Reduced Glac. Deposition Enhanced  

54 Evolution of Ocean Gateways Since the Eocene Fig. courtesy of W. W. Hay (GEOMAR, Kiel)

55 Heinze, C. and T. J. Crowley, 1997: Sedimentary response to ocean gateway circulation changes. Paleoceanography, 12, × Change in Lysocline Depth [m] Shallowing Deepening Modelled Lysocline Response to Closing of the Panama Gateway Reduced CaCO 3 Preservation

56 Farrell, J. W. and W. L. Prell, 1991: Pacific CaCO 3 preservation and  18 O since 4 Ma: Paleoceanic and paleoclimatic implications. Paleoceanography, 6, Reconstructed Lysocline Response in the Easter Equatorial Pacific Enhanced (!) CaCO 3 Preservation During Closure of Panama Gateway

57 Outlook Convergence of independent oceanic budget estimates seems achievable. Neritic budget still not better known than during the late 70’s. Within the uncertainties of the estimates, the modern budget is consistent with a steady state. The relative contributions of the various oceanic CaCO 3 producers to the oceanic budget remains elusive. Initial model studies provide interesting results. However, discrepancies with reconstructions clearly warrant further investigations and model improvements.


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