Dissolution of calcite in sediments -- metabolic dissolution

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Dissolution of calcite in sediments --metabolic
dissolution
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CaCO3 accumulation in marine sediments

• CaCO3 is the predominant biogenic sediment
  constituent over much of the
• ocean
• Changes in the marine alkalinity budget
• can drive changes in atmospheric PCO2
• involve changes in the accumulation rate of
  CaCO3 in marine sediment
• gt The study of climate change is linked to the
  study of CaCO3 in marine sediments

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CaCO3 dissolution
50 of calcite dissolution in the ocean occurs
at the sea floor Up to 50 of seafloor calcite
dissolution may occur in sediments that lie
above the calcite saturation horizon what
drives this dissolution?
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Metabolic calcite dissolution
Oxic respiration results in the release of acids
to solution
Acids are neutralized by
(and similar reactions for neutralizing H)
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Sediments vs water column
In the water column, A very large reservoir of
dissolved CO32-, B(OH)4- result of
respiration lower pH and CO32- in deep water
over its 1000 yr trip from NAtl to
NEPac How are sediments different? spatial
scale 8 cm mixed layer, w1cm/ky res time
8000 y \ supply of dissolved CO32-, B(OH)4-
limited by diffusion from BW Abundant CaCO3 (s)
in sediments gt Dissolution is favored in
sediments if pore waters are driven to
undersaturation
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Theoretical consideration of metabolic dissolution
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Predicted CO32- vs. depth
General
A specific case, supersaturated bottom water
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Evidence -- Solid phase
Measurements Of CaCO3 in NA sediments
Calcite sat. horizon
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Evidence -- Pore water
Organic matter oxidation by O2 With no
dissolution
With dissolution, add to this reaction
Now
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Review oxic metabolism and calcite dissolution
Corg
CaCO3
Net Alk flux out
No net Alk flux
equilibration
dissolution
Fluxes of solids solid lines Fluxes of solutes
dashed lines
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Pore waters from Atlantic Both above and below CSH
Lines predicted, with dissolution
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Evidence - Pore waterIn situ microelectrode
profiling
measure pore water pH and O2 at high
resolution interpret profiles using a model
of coupled oxic metabolism / calcite
dissolution the combination allows (1)
qualitative demonstration of metabolic
dissolution (2) quantification of dissolution
rate
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Metabolic dissolution models
For each solute
Assume acid-base equilibrium in solution !
For CaCO3
For mass
Where
Assume no calcite precipitation !
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Supersaturated bw
Undersat.
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In situ MEPArcher et al. (1989) GCA 53, 2831-2845
Data from Station 12 -- above CSH Station 13
-- below CSH
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pH, modeled to give Calcite dissolution rate
O2, modeled to give rate of Corg ox vs depth
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Curves dif. Corg rain rates
Dash diff. calcite sat.
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In situ MEPNorth AtlanticHales et al., 1994,
DSR 41, 695-719
From 2 sites 5 -- 2159m 7 -- 4236m
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With NO dissolution
Clear demonstration that dissolution must occur
if model is correct! Quantification of
dissolution by fits of model output to pH data
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Evidence -- Pore waterin situ whole-core
squeezingMartin and Sayles, 1996, GCA 60, 243-263
In situ measurement of pore water TCO2, Alk, Ca2
demonstration of dissolution without
model Either shipboard or in situ measurement of
NO3-, shipboard O2 quantification of ox. vs
depth Model of coupled Corg ox. and calcite
dissolution to quantify dissolution rate
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Ceara Rise, w. tropical Atlantic Site A just
above CSH
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Ceara Rise, Site A
In all lines are fits of model to data
to constrain fluxes and parameter values
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2nd in situ wcs result --Cape Verde Plateau, E.
tropical Atlanticwell above CSH
Lines fits of model to data to quantify
dissolution rate
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Pore water evidence -- carbon isotopesMcCorkle
The ?13C of pore water TCO2
Organic carbon -20
CaCO3 0
dissolution
respiration
Pore water DIC its ?13C reflects the
proportions of the 2 processes
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Note compare shipboard and in situ sampling
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SummaryMcCorkle isotopic data
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SummaryPore water pH, Alk, TCO2 data
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Counter evidence?In situ benthic flux
chambersJahnke Jahnke, 2004, GCA 68, 47-59
Above CSH
Below CSH
Alk
Alk
Ca
Ca
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SummaryBenthic flux chamber data
? No dissolution in sediments with CaCO3 gt
30? -- ? precipitation at interface? -- ?
Incorrect speciation model -- calcite surface
chemistry -- ? Or are chambers just not
sensitive enough
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One more approach --230Th activity changes near
swiMartin, 2004
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Best place to observe this effectsites with
very high CaCO3 Ontong-Java Plateau
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Bottom water saturation stateCaCO3
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If metabolic dissolution occurs,how important is
it?
2 competing processes diffusion across
sediment-water interface CaCO3
dissolution Balance between them depends
on Ratio, Corg rain CaCO3 rain -- today,
constant at 0.8 depth distribution of Corg
oxidation in sediments saturation state of
bottom water
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Depth distribution of Corg oxidation in
sedimentsin the deep seaMartin and Sayles
(2006) DSR II, 53, 771-792
Then e-folding depth is 1/p1
If
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How does metabolic dissolution vary with depth
distribution of oxidation and saturation state?
Model test Keep Corg and CaCO3 rain rates
constant, vary sat. state of bw and e-fold depth
of oxidation
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What is a best guess of metabolic
dissolution-- rain rate ratio constant at
0.8-- observed Corg ox. Distribution in sediments
Dissolution rate increases with Increasing rain
at given saturation
CaCO3 burial efficiency is constant At given
sat., but decreases with Decreasing bw. saturation
mde constant at given sat., But increases as
sat. decreases