Early diagenesis in marine sediments
Why study this part of the ocean?
Particle flux to the
sea floor
ocean
surface sediments
Biogeochemical
“early diagenesis
layer”
reactions
Why study this part of the ocean?
Particle flux to the
sea floor
Solute exchange :
effects on ocean
chemistry
Biogeochemical
“early diagenesis
layer”
reactions
Why study this part of the ocean?
Particle flux to
the sea floor
“early diagenesis
layer”
Biogeochemical
reactions
Accumulating sediments:
composition differs from
particle inputs
Marine Sediments
1) The flux of particles to the sea floor
2) Preservation rates of biogenic components of
the flux
3) Consequences of early diagenesis
4) Specifics:
For each of : Organic matter, CaCO3, Biogenic
SiO2 :
a) mechanism for decomposition / dissolution
b) how do we know?
The composition of the particulate rain to the sea floor -- Examples
Flux: mg/m2/day
Composition: wt. %
70
Eastern Equatorial Atlantic
200
150
40
100
50
20
0
0
70
200
Percent
Equatorial Pacific
150
100
50
0
60
Equatorial Pacific
40
20
0
70
200
60
150
North Atlantic
-
Lithogenic
Opal
Org Matter
CaCO3
0
Total
Lithogenic
Opal
Org Matter
20
CaCO3
0
North Atlantic
40
100
50
Total
Flux: mg/m2/day
Eastern Equatorial Atlantic
60
By weight
Figure by MIT OCW.
Composition of the particulate rain to the sea floor -- the Southern Ocean
-
Figure by MIT OCW.
A specific example: Balance of fluxes in the central equatorial Pacific
Flux (mmol m-2 d-1)
Berelson et al., 1997, DSR II 2251-2282
Corg
0.80
CaCO3
Opal
0.60
0.40
-
0.20
0.00
Burial +
Remineral.
Budgets of the biogenic constituents in the EqPac region, corrected
and adjusted as describeded in the text.
Burial
Remineral.
Rain
Figure by MIT OCW.
Burial : measured accumulation rates
Remineralization : in situ benthic flux chamber determinations
Rain : sediment traps
Benthic Fluxes >> Burial Rates:
Does it matter?
Goal : To learn about changes in rain rates to the sea floor over
time from measurements of sediment accumulation rates
*Assume steady state. For a given component of particle flux:
Rain rate to
sea floor
=
Accumulation
rate
(preserved)
+
Diagenetic reaction
rate
(lost to reaction)
In symbols
R = A+D
⎡
⎤
⎢ 1 ⎥
∗A
R=⎢
⎥
D
⎢⎣1 − R ⎥⎦
Case 1: D << R : R ~A
===>
Case 2: D/R constant : R α A
Case 3: D~R, D/R variable:
small ∆(D/R) --> large ∆R
In picture form:
⎡
⎤
⎢ 1 ⎥
∗A
R=⎢
⎥
D
⎢⎣1 − R ⎥⎦
Some consequences of early diagenesis
A. Low and variable preservation rates of biogenic components
and the interpretation of the sedimentary record
B. Early diagenesis and atmospheric oxygen… (long time scales)
C. In the contemporary ocean…
1. Deep-water oxygen consumption
~ 50% of O2 consumption below 1000m occurs
in sediments
2. Denitrification…
>~ 50% of dentrification in the modern ocean occurs
in sediments…
Continental margin sediments: O2 --> 0 near the sediment-water interface !
Continental Margin
Continental Slope
Sea-level
100
200
300
400 500
-
600
800
900
Vertical
Exaggeration x
Shelf Break
1
2
3
4
Continental Shelf
5
0
100
200
300
Continental Rise
400 500
600
Abyssal Plain
700
800
Distance From Shore (km)
900
1000 1100
0
O2 penetration depth (mm)
10
20
30
40
50
60
70
0
1000 1100
Sea-level
0
Depth (km)
700
Vertical Exaggeration x
Water Column Depth (m)
0
5
0
1000
2000
3000
4000
5000
The depth where O2
0
in continental margin sediments
Northwest Atlantic (Lohse et al., 1998)
Northeast Atlantic (Martin and Sayles, 2004)
Northeast Pacific (Reimers et al., 1994)
Figure by MIT OCW.
How sedimentary processes differ from water
column processes
Particles! Surface sediments ~ 40-70% particles by weight
Time :
Particles fall through the water column:
τ ≤
Mixed layer ~
“reaction layer”
τ~
3500m
≈ 70 days
50 m
day
8cm
0.001 cm
~ 8000yr →
y
(pelagic)
swi
20cm
~ 200yr
0.1 cm
y
(coastal)
Reactions that are too slow to occur in the water column
can happen in surface sediments
How sedimentary processes differ from water
column processes
In sediments,
reactants are supplied from above:
Particles from
Water column
solutes diffuse from
water column
Particles from
Sediment surface
particle mixing
React with
solutes diffusing
from above
First order approximation*:
sediments have a layered structure
Mechanism for organic matter oxidation
Familiar Processes:
Organic matter
e- acceptor
products
Mechanism: more complex than overall rxn written!
Order : decreasing ∆G
Evidence : Benthic fluxes and
pore water profiles
of reactants and / or products
reduced eacceptor
Pore water profiles :O2
all done by in situ microelectrode profiling
0
50
O2 (µmol/l)
100
150
200
0
250
O2 (µmol/l)
100
200
0
50
O2 (µmol/l)
100
150
200
-1
0
0
1
0
3
4
1
2
5
6
Ceara Rise
3200 m
3
(Hales et al., 1996)
4
7
Total
Corg ox.
Rate
(µmol/cm2/y)
616m
JSL II Dive 2949-a
JSL II Diev 2949 -c
JSL II Dive 2949 -d
Fit
Range of x*
14
Depth (cm)
Depth (cm)
Depth (cm)
2
1
BBay August 2003
dep_22
dep_12
dep_13
2
45
350
Interpretation of pore water profiles : 1. Qualitative interpretation
Assume: ** steady state ** + ~ constant porosity & diffusivity, negligible advection
0
Depth (cm)
2
CA
xA
Concentration of Solute II
0.6
0.7
0.8
0.9
CB
xB
PRODUCTION between XA and XB:
A straight line shows what the
8 profile would be with NO net production
between A and B. At each depth, the
concentration is GREATER than that on
a straight line between A &B
10
1.0
CA
A
2
4
6
CB
0
B
Depth (cm)
0.0
Concentration-Solute I
0.1
0.2
0.3
0.4
xA
A
4
6
8
10
xB
B
CONSUMPTION between A and B.
The profile between A and B would
be a straight line if no reaction were
occurring. Conc. is smaller at each depth
than the no-reaction case ==> the
solute is being consumed between A and B
NO3-
ZONE 1
2+
5.
ZONE 1
Ft
ZONE 1
2
d [NO3-]
=0
dz2
2
2+
3. d [Mn ] > 0
dz2
2
2+
d
[Mn
]
4.
<0
dz2
6.
ZONE 1
DEPTH
ZONE 1
Mn2+
CHARACTERISTIC REACTION
2
d [MO3-]
1.
10
<0
dz2
2.
ZONE 1
O2
ZONE 1
CONCENTRATION
7.
2
d [NO3-]
dz2
d [Fe2+]
dz2
>0
Diffusion
14
11
12
2
d [Fe2+]
>0
2
dz2
<0
13
Schematic representation of trend in pore water profiles. Depth and concentration axes
in arbitrary units. The zones, characteristic curvature OF the gradients and reaction
numbers are discussed in the text.
Figure by MIT OCW.
Interpretation of profile shapes : quantitative
Steady-state mass balance in a sediment
layer: Rate of reaction within the layer = net
flux out of the layer
R = Fout − Fin
Diffusive flux :
NO-3 μmol/l
0
10
0
oxic
20
30
1
2
10
denitrification
Depth (cm)
20
40
F = −φD sed
dC
dx
Flux at pt. 1 (x=0) : gives total, net NO3
Production in sediment column
Flux at pt. 2 : gives rate of NO3 consump.
By denitrification
30
Sum of absolute values of
Flux at 1 + Flux at 2:
Gives rate of NO3 production by oxic
Decomposition of organic matter
40
50
60
CVP - Site M
NO3 data
Fit: A&S stoich.
Example: pore water data
450m water depth, NW Atlantic
NO-3 (mmol/1)
O2 (mmol/1)
0
100
0
200
-1
3
4
Depth (cm)
1
-
460m
Dive JSL II 2941
Dive JSL II 2944
Fit
Range of X*
2
3
Depth where
O2
O
4
5
wcs
460m
NH4
0
40
5
10
80
460m
Data and best fit
0
Depth (cm)
Depth (cm)
2
20
0
0
1
10
NO3
O
N. W. Atlantic
continental Margin
15
A
Figure by MIT OCW.
Example : pore water data
Coastal site
Corg ox. Rate ~ 850 µmol C / cm2/y
2+
2+
(µmol/l)
25
Fe
0
H2S (µmol/l)
0
1000
(µmol/l)
400
800
0
0
5
5
5
10
15
20
25
15
20
25
2-
12
SO4 (mmol/l)
16
20
24
0
5
10
15
20
10
Buzzards Bay Aug 04
Hingham Bay Oct 02
dsbw
Hingham Bay
2001
January
June
October
Depth (cm)
0
Depth (cm)
Depth (cm)
Mn
0
10
15
20
25
Which electron acceptors are used the most in sediments for organic matter oxidation?
Electron Acceptors in Pelagic Sediment(1)
-
Site
MANOP H
MANOP C
E. Eq. Atlantic
Region
E. Eq. Pacific
Central Eq. Pac
o
0-3 N, 6-16 W
O
Corg ox. rate
(mmol/cm2/y)
12.0
20.4
12.4
O2
NO3-
Mn(IV)
99.2
0.8
0.4
98.1
93.8
1.6
4.4
0.4
0.1
FE(III)
SO42-
1.8
Electron Acceptors in Continental Margin Sediments
% of organic C oxidation by different
election acceptors
Location
Water depths
Total Corg ox
(mmol/cm2/y)
O2
NO3-
Mn
Fe
SO4
N.E. Atlantic(1)
N.W. Atlantic(2)
208-4500
36-158
67-97
1-8.5
0-2.1
0-1.7
1-20
260-2510
36-52
74-90
1.8-6.0
780-1440
1900-4070
66-75
5.0-46
41-69
36-74
69-75
11-18
N.E. Pac: O2<50 mM(3)
(3)
N.E. Pac: O2 = 73-145
8-20
0.1
0.7-1.3
5.7-25
0.1-6.9 0.3-0.7
5.6-18
(1) Lohse et al., 1998; (2) Martin and Sayles, 2004; (3) Reimers et al., 1992
Figure by MIT OCW.
Which electron acceptors are used the most in sediments for organic matter oxidation?
ORGANIC CARBON BURIAL IN MARINE SEDIMENTS
Organic Carbon Burial Rates (and percentages)In Different Ocean Regimes
Sediment type
Deltaic
Shelf
Slope
Pelagic
Total
Data from Gershanovich et at. (1974)
All sediment types
0 (0)
23 (10)
195 (88)
5 (2)
223
S = 223
Data from Berner(1989)
Terrigenous deltaic-shelf sediments
Biogenous sediments (high-productivity zones)
Shallow-water carbonates
Pelagic sediments (low-productivity zones)
Anoxic basins (e.g. Black sea)
104 (82)
0
0
0
0
0
0
6 (5)
0
1 (1)
0
7 (6)
0
0
0
0
3 (2)
0
5 (4)
0
104
10
6
5
1
S = 126
Recalculation of data from Berner (1989)a
Deltaic sediments
Shelves and upper slopes
Biogenous sediments (high-productivity zones)
Shallow-water carbonates
Pelagic sediments (low-productivity zones)
Anoxic basins (e.g. Black Sea)
70 (44)
0
0
0
0
0
0
68 (42)
0
6 (4)
0
1 (0.5)
0
0
7 (4)
0
0
0
3 (2)
0
5 (3)
0
70
68
10
6
5
1
S = 160
-
12
Units are 10 g C yr-1 (parenthetical units = % of total burial)
a Deltaic-shelf sediments were reapportioned assuming that 33% of the sediment discharge from rivers is deposited either along nondelatic shelves or upper slopes, and assuming that those deposits have total loadings of 1.5% organic carbon rather then 0.7% as in
delatic regions. Estimates for all other regions remain the same.
Figure by MIT OCW.
The distribution of organic
matter in marine sediments :
What determines the observed
pattern?
….local productivity?
….variable preservation?
Image removed due to copyright restrictions.
Organic carbon preservation
So:
A correspondence between regions of high 1° productivity and
high % Corg in sediments,
And:
And:
These regions of high %Corg are ALSO regions of low bottom
water O2 in many cases,
It has been shown that some naturally occurring organic
molecules REQUIRE O2 for decomposition
… Does sedimentary %Corg (Corg accumulation rate, really) depend on:
productivity?
preservation? (bw O2)
both?
“Oxygen Exposure Time”
Hartnett et al. (1998) Nature 391, 572-574
Studied 2 areas:
-
1)
squares: Washington margin: higher productivity, less intense O2 min
2)
Circles: Mexico margin: lower productivity, intense O2 min.
500
1000
1500
2000
0
100
200
300
0
0
2
4
6
8 10 12
2
Depth in Sediments (cm)
Water Column Depth (m)
0
2500
3000
Oxygen Concentration (µ mol 1-1)
4
6
8
10
12
14
16
18
20
Organic Carbon (wt%)
Figure by MIT OCW.
“Oxygen Exposure Time”
Hartnett et al. (1998) Nature 391, 572-574
They defined “oxygen exposure time”:
And examined its effect on Corg “burial efficiency” (= burial rate / rain rate)
“Oxygen Exposure Time”
60
OC burtal efficiency (%)
Oraganic carbon burial efficlency (%)
( Linear scale )
Hartnett et al. (1998) Nature 391, 572-574
50
40
60
50
40
30
20
10
0
0
-
30
200
400
600
800
Oxygen exposer time (yr)
20
10
0
0.01
0.1
1
10
100
1000
Oxygen Exposure Time (yr)
(Log Scale)
Organic carbon burial efficiency as a function of oxygen exposure time
Promising idea, Limited data
Note: Provides feedback between atmospheric O2 + Corg burial rate
Figure by MIT OCW.