Ocean Carbonates: Global Budgets and Models Michael Schulz


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Ocean Carbonates: Global Budgets and Models

  • Michael Schulz

  • (Research Center Ocean Margins, Bremen)


9:15 - 10:45

  • 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



11:00 – 12:30

  • 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



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)



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.



1. The Role of Marine Calcium Carbonate in the Global Carbon Cycle



CaCO3 Compensation



Carbon-Cycle – Characteristic Timescales



CaCO3 Solubility and Saturation State of Seawater

  • Saturation state 

  • ksp: solubility product = f(pressure, T , S)

    •  > 1: supersaturated
    •  < 1: undersaturated
  • Seawater: Changes in [Ca2+] are small  changes in largely controlled by [CO32-]

  • Zeebe and Wolf-Gladrow, 2001: CO2 in Seawater: Equilibrium, kinetics, isotopes. Elsevier.



CaCO3 Solubility and Saturation State of Seawater



Oceanic Carbonate Buffering System



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.



Charge Imbalance of Major Ions in Seawater



Alkalinity as a Master Variable

  • From Total Alkalinity (TA) and CO2 together with T and S, all other quantities of the carbonate system can be quantified

  •  From measurements of TA and CO2 the CaCO3 saturation state can be inferred



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



Biogenic Calcium Carbonate Production Raises Dissolved CO2 Concentration





Carbonate Concentration and CO2

  • CaCO3 dissolution  [CO32-] ↑  reacts with CO2 to form HCO3-  [CO2] ↓

  • CaCO3 precipitation  [CO32-] ↓  HCO3- dissociates  [CO2] ↑

  • As [CO32-] rises [CO2] drops and vice versa



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)?



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)


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)



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



Percent Calcite Data – Locations of Modern Core Tops



Mass-Accumulation Rate Data: Locations of Modern Core Tops



Regional Modern CaCO3 Mass-Accumulations Rates



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.


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



Modern CaCO3 Production from Sediment Traps (at 1000 m depth)



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)


Modern Alkalinity-Based CaCO3 Production



Modern Alkalinity-Based Oceanic CaCO3 Production



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



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)


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


Alkalinity Data in the Atlantic Ocean



Dissolution-Driven Change in Alkalinity (Atlantic Ocean)



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)



A Global Oceanic CaCO3 Budget



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.)



CaCO3 Dissolution at the Seafloor



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)


Group-Specific Contributions to Oceanic CaCO3 Budget (Sediment-Trap Data; Schiebel, 2002 GBC)



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


Neritic Carbonate Budget

  • Estimation of CaCO3 production similar to reefs (Milliman, 1993)



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)


A Global Marine CaCO3 Budget



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



Structure of a Global Biogeochemical Model





A Modeled Sediment Stack in the North Atlantic



Modeled and Observed Modern CaCO3 Content of Deep-Sea Sediments



Case Study I: Glacial-Interglacial Variations in Pacific Lysocline Depth



Modeled vs. Reconstructed Glacial-Interglacial Lysocline Variations



Evolution of Ocean Gateways Since the Eocene



Modelled Lysocline Response to Closing of the Panama Gateway



Reconstructed Lysocline Response in the Easter Equatorial Pacific



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 CaCO3 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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