Ocean Carbonates: Global Budgets and Models Michael Schulz (Research Center Ocean Margins, Bremen) Version Nov. 25, 2003

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 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 Weathering feedback probably stabilizes atmospheric pCO2 at timescales ≥ 106 years Ruddiman (2001)

CaCO3 Compensation Today: P = 4 × R D = 3 × R  B = R 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. River Input R (Ca2+, HCO3-) Production P Dissolution D Burial B Today: P = 4 × R D = 3 × R  B = R 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.

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

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. PA: Practical alkalinity

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

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

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. PA: Practical alkalinity

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

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

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)

Biogenic Calcium Carbonate Production Raises Dissolved CO2 Concentration pH Reaction: (1) Biogenic carbonate uptake Some marine organisms form shells of CaCO3 (Coccolith., Foram) (2) More bicarbonate dissociates (3) More CO2 is formed

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

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 Note poor coverage in Indian and Southern Ocean To obtain global coverage  Extrapolation via regional %CaCO3-depth relationships Catubig et al. (1998)

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)

Regional Modern CaCO3 Mass-Accumulations Rates Corrected total = Total / 0.67 Err[Corrected Total] = Err[Total] / 0.67 (follows from error propagation) Global Burial Rate: 8.6 ± 0.5 × 1012 mol CaCO3/yr Catubig et al. (1998)

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) Position Global: 24 × 1012 mol CaCO3/yr Note poor data coverage Isolines based on primary production contours (Berger, 1989) Milliman (1993); Milliman & Droxler (1996)

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 Lee (2001)

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

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 Chung et al. (2003)

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

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 92 96 45 38 (92-45-9) 9 Modified after Milliman et al. (1999)

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 OM = Organic Matter Jahnke et al. (1997 GBC)

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) 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

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) Total Production: ~ 25 × 1012 mol CaCO3 / yr Total Accumulation: ~ 15 × 1012 mol CaCO3 / yr Milliman and Droxler (1996)

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

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 Ridgwell (2001, Thesis)

Modeling Deep-Sea Sediments Ridgwell (2001, Thesis)

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.

Modeled and Observed Modern CaCO3 Content of Deep-Sea Sediments Observations  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. Heinze et al. (1999)

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

Modeled vs. Reconstructed Glacial-Interglacial Lysocline Variations South Atlantic South Pacific 60 % (Farrell & Prell, 1989) Glac. Deposition Reduced Glac. Deposition Enhanced   Model-Forcing: Prescribed changes in NADW formation, terrestrial carbon storage, neritic CaCO3 storage, among others (Ridgwell, 2001)

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

Modelled Lysocline Response to Closing of the Panama Gateway Reduced CaCO3 Preservation × Shallowing Deepening Change in Lysocline Depth [m] Heinze, C. and T. J. Crowley, 1997: Sedimentary response to ocean gateway circulation changes. Paleoceanography, 12, 742-754.

Reconstructed Lysocline Response in the Easter Equatorial Pacific Enhanced (!) CaCO3 Preservation During Closure of Panama Gateway Farrell, J. W. and W. L. Prell, 1991: Pacific CaCO3 preservation and d18O since 4 Ma: Paleoceanic and paleoclimatic implications. Paleoceanography, 6, 485-498.

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.