Transcript Ocean Carbonates: Global Budgets and Models

Ocean Carbonates:
Global Budgets and Models
Michael Schulz
(Research Center Ocean Margins, Bremen)
9:15 - 10:45
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
10:45 - 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
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.
Today:
P=4×R
D=3×R
River Input R
(Ca2+, HCO3-)
Production P
B=R
Dissolution D
Example: (P = const.)
Burial B
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:
[Ca2 ]sw  [CO32 ]sw
W=
k sp
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.
CaCO3 Solubility and Saturation State of Seawater
Zeebe and Wolf-Gladrow (2001)
Oceanic Carbonate Buffering System
Average surfaceWater composition
CO2
0.5 %
HCO3- 89.0 %
CO32- 10.5 %
pH Reaction:
CO2 + H2 O + CO32-  2 HCO3Open Univ. “Seawater”
The Concept of Alkalinity
• Chemical definition: Total Alkalinity (TA) measures the charges of the ions of
weak acids:
TA  PA = [HCO3 ] + 2[CO32 ]+ [B(OH)4 ] + [OH ]  [H+ ]
• Physical definition (based on principle of electroneutrality): Alkalinity =
charge difference between conservative anions and cations:
TA (ec) = [Na+ ] + 2 [Mg2+ ] + 2[Ca2+ ] + [K + ] + ...  [Cl ]  2[SO24 ]  [NO3 ]  ... = PA
• 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
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
TA
SCO2
[HCO3- ] + 2[CO32- ]
3
23
[HCO ] + [CO ]
 [CO32- ]
TA - SCO2
 From measurements of TA and SCO2 the CaCO3
saturation state can be inferred
Biogeochemical Effects on Alkalinity
TA (ec) = [Na+ ] + 2 [Mg2+ ] + 2[Ca2+ ] + [K + ] + ...  [Cl ]  2[SO24 ]  [NO3 ]  ... = PA
• 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:
CO2 + H2 O + CO
23
 2 HCO
3
(1) Biogenic carbonate uptake
(2) More bicarbonate
dissociates
(3) More CO2 is formed
The Calcium Carbonate Pump
CO2
Atmosphere
CO2
Biogenic CaCO3
Formation
3
Lysocline
Ocean
CaCO3 Dissolution
CO32-
Fig. courtesy of A. Körtzinger
Carbonate Concentration and CO2
CO2 + H2 O + CO32-  2 HCO3• CaCO3 dissolution  [CO32-] ↑  reacts with
CO2 to form HCO3-  [CO2] ↓
• CaCO3 precipitation  [CO32-] ↓  HCO3dissociates  [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:
%CaCO3
Calcite MAR
Calcite MAR


Non-Calcite MAR (Total MAR) - (Calcite MAR) 100% - %CaCO3
• Non-calcite MAR required to calculated calcite MAR; usually
not known for each record  use regional estimate instead
Calcite MAR = Regional Non-Calcite MAR 
%CaCO3
100% - %CaCO3
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: nonCaCO3 MAR uncorrelated with %CaCO3 in specified
regions (otherwise violation of constant-dilution assumption)
Catubig et al. (1998)
Regional Modern CaCO3
Mass-Accumulations Rates
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)
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)
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 alkalinitybased 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
0.010.03
(1-3 %)
0.340.84
(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:
Total Accumulation:
~ 25 × 1012 mol CaCO3 / yr
~ 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
Model
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 GlacialInterglacial 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.