Early diagenesis in marine sediments

Why study this part of the ocean?

Particle flux to the sea floor

“early diagenesislayer”

Biogeochemical

reactions

ocean

surface sediments

Why study this part of the ocean?

Particle flux to the sea floor

“early diagenesislayer”

Biogeochemical

reactions

Solute exchange :effects on ocean

chemistry

Why study this part of the ocean?Particle flux to the sea floor

“early diagenesislayer”

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 / dissolutionb) how do we know?

The composition of the particulate rain to the sea floor -- Examples

200150100500 0

200150100500

200150100500

70

60

40

20

0

70

60

40

20

0

70

60

40

20

Tota

l

CaC

O3

Org

Mat

ter

Opa

l

Lith

ogen

ic

Tota

l

CaC

O3

Org

Mat

ter

Opa

l

Lith

ogen

ic

Eastern Equatorial Atlantic

Equatorial Pacific

North Atlantic

Eastern Equatorial Atlantic

Equatorial Pacific

North Atlantic

Flux: mg/m2/dayFl

ux: m

g/m

2 /day

Composition: wt. %

By weight

Perc

ent

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Figure by MIT OCW.

Composition of the particulate rain to the sea floor -- the Southern Ocean

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Figure by MIT OCW.

A specific example: Balance of fluxes in the central equatorial PacificBerelson et al., 1997, DSR II 2251-2282

Burial : measured accumulation rates

Remineralization : in situ benthic flux chamber determinations

Rain : sediment traps

CorgCaCO3Opal

0.80

0.60

0.40

0.20

0.00

Flux

(mm

ol m

-2 d

-1)

Burial Remineral. Rain Burial +Remineral.

Budgets of the biogenic constituents in the EqPac region, correctedand adjusted as describeded in the text.

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Figure by MIT OCW.

Benthic Fluxes >> Burial Rates:Does it matter?

Goal : To learn about changes in rain rates to the sea floor overtime from measurements of sediment accumulation rates

Rain rate to sea floor

*Assume steady state. For a given component of particle flux:

= Accumulationrate

(preserved)

+Diagenetic reaction

rate(lost to reaction)

In symbols

R = A + D

R = 1

1 − DR

⎡

⎣

⎢ ⎢ ⎢

⎤

⎦

⎥ ⎥ ⎥ ∗ A

===>

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

R = 1

1 − DR

⎡

⎣

⎢ ⎢ ⎢

⎤

⎦

⎥ ⎥ ⎥ ∗ A

In picture form:

Some consequences of early diagenesis

A. Low and variable preservation rates of biogenic componentsand 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 occursin sediments…

Continental margin sediments: O2 --> 0 near the sediment-water interface !

Distance From Shore (km)

Vertical Exaggeration x

Vertical Exaggeration x

Sea-level

Sea-level

Continental Margin

Continental Slope O2 penetration depth (mm)

The depth where O2 0in continental margin sediments

Northeast Atlantic (Martin and Sayles, 2004)Northwest Atlantic (Lohse et al., 1998)

Northeast Pacific (Reimers et al., 1994)W

ater

Col

umn

Dep

th (m

)

0 100 200 300 400 500 600 700 800 900 1000

1000

2000

3000

4000

50001100

0 100 200 300 400 500 600 700 800 900 1000 1100

0

0

1

2

3

4

5

50

010 20 30 40 50 60 70

Shelf Break

Continental Shelf Continental Rise Abyssal Plain

Dep

th (k

m)

-

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 ≤ 3500m

50mday

≈ 70 days

swi

Mixed layer ~

“reaction layer”

τ ~ 8cm

0.001cmy

~ 8000yr → 20cm

0.1cmy

~ 200yr

(pelagic) (coastal)

Reactions that are too slow to occur in the water columncan happen in surface sediments

How sedimentary processes differ from water column processes

In sediments, reactants are supplied from above:

Particles fromWater column

solutes diffuse from

water column

particle mixing

Particles fromSediment surface

solutes diffusingfrom aboveReact with

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 profilesof reactants and / or products

reduced e-

acceptor

Pore water profiles :O2all done by in situ microelectrode profiling

2

1

0

Dep

th (

cm)

200150100500O2 (µmol/l)

BBay August 2003 dep_22 dep_12 dep_134

3

2

1

0

-1

Dep

th (c

m)

2001000O2 (µmol/l)

616m JSL II Dive 2949-a JSL II Diev 2949 -c JSL II Dive 2949 -d Fit Range of x*

7

6

5

4

3

2

1

0

Dep

th (

cm)

250200150100500O2 (µmol/l)

Ceara Rise3200 m

(Hales et al., 1996)

TotalCorg ox.Rate(µmol/cm2/y)

14 45 350

10

8

6

4

2

0

Dep

th (

cm)

0.40.30.20.10.0Concentration-Solute I

A

BPRODUCTION between XA and XB:A straight line shows what theprofile would be with NO net productionbetween A and B. At each depth, the concentration is GREATER than that on a straight line between A &B

xB

xA

CA CB

10

8

6

4

2

0

Dep

th (

cm)

1.00.90.80.70.6Concentration of Solute II

A

B CONSUMPTION between A and B.The profile between A and B wouldbe a straight line if no reaction wereoccurring. Conc. is smaller at each depththan the no-reaction case ==> the solute is being consumed between A and B

CACB

xA

xB

Interpretation of pore water profiles : 1. Qualitative interpretation

Assume: ** steady state ** + ~ constant porosity & diffusivity, negligible advection

Ft2+

O2 NO3

Mn2+

-

DEP

TH

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.

ZON

E 1

ZON

E 1

ZON

E 1

ZON

E 1

ZON

E 1

ZON

E 1

ZON

E 1

CONCENTRATION CHARACTERISTIC REACTION

< 0dz2

d2[MO3-]

= 0dz2

d2[NO3-]

> 0dz2

d2[NO3-]

> 0dz2

d2[Mn2+]

< 0dz2

d2[Mn2+]

> 0dz2

d2[Fe2+]

< 0dz2

d2[Fe2+]

Diffusion

10

14

11

12

13

1.

2.

3.

4.

5.

6.

7.

Figure by MIT OCW.

60

50

40

30

20

10

0

Dep

th (

cm)

403020100

NO3- μmol/l

CVP - Site M NO3 data Fit: A&S stoich.

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 :

F = −φDseddCdx

oxic

denitrification

12

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

Sum of absolute values of Flux at 1 + Flux at 2:Gives rate of NO3 production by oxicDecomposition of organic matter

Example: pore water data450m water depth, NW Atlantic

460mData and best fit

Dep

th (c

m)

Dep

th (c

m)

Dep

th (c

m)

NH4

0

0

0

0

40 80

5

10

15

A

10 20

1

2

3

4

5

wcs460m

100 2000

0

-1

2

3

4

1

460mDive JSL II 2941

Dive JSL II 2944Fit

Range of X*

O2 (µmol/1) NO3 (µmol/1)

Depth whereO2

NO3 O

O

-

N. W. Atlanticcontinental Margin

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Figure by MIT OCW.

Example : pore water dataCoastal site

Corg ox. Rate ~ 850 µmol C / cm2/y

25

20

15

10

5

0

Dep

th (

cm)

8004000Fe

2+ (µmol/l)

Hingham Bay2001 January JuneOctober

25

20

15

10

5

0

Dep

th (

cm)

250Mn

2+ (µmol/l)

25

20

15

10

5

0

Dep

th (

cm)

10000H2S (µmol/l)

20

15

10

5

0

24201612SO4

2- (mmol/l)

Buzzards Bay Aug 04 Hingham Bay Oct 02 dsbw

Which electron acceptors are used the most in sediments for organic matter oxidation?

Site Region

% of organic C oxidation by differentelection acceptors

Corg ox. rate(µmol/cm2/y)

MANOP HMANOP C

E. Eq. Atlantic

E. Eq. PacificCentral Eq. Pac

0-3oN, 6-16O W

12.020.412.4

99.2 0.8 0.498.1 1.6 0.493.8 4.4 0.1

208-4500260-2510780-1440

1900-4070

36-15836-52

66-7536-74

67-9774-905.0-4669-75

1-8.51.8-6.041-6911-18

0-2.1

0.10.1-6.9

0-1.78-20

0.7-1.30.3-0.7

1-20

5.7-25

5.6-18

Location Water depthsTotal Corg ox(µmol/cm2/y)

O2

O2

NO3-

NO3-

2-Mn(IV)

Mn

FE(III) SO4

SO4

1.8

Fe

N.E. Atlantic(1)

N.W. Atlantic(2)

N.E. Pac: O2<50 µΜ(3)

N.E. Pac: O2 = 73-145(3)

Electron Acceptors in Continental Margin Sediments

Electron Acceptors in Pelagic Sediment(1)

(1) Lohse et al., 1998; (2) Martin and Sayles, 2004; (3) Reimers et al., 1992

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Figure by MIT OCW.

Which electron acceptors are used the most in sediments for organic matter oxidation?

Sediment type Deltaic Shelf Slope Pelagic Total

0 (0) 23 (10) 195 (88) 5 (2) 223Σ = 223

104 (82)0000

000

0

000

00

0

0

0

0

000 0

0

000000

0

6 (5)7 (6) 3 (2)

5 (4)1 (1)

Σ = 126

10410651

706810651Σ = 160

70 (44)68 (42)

6 (4)

1 (0.5)

7 (4) 3 (2)

5 (3)

Data from Berner(1989)

Recalculation of data from Berner (1989)a

Data from Gershanovich et at. (1974)All sediment types

Terrigenous deltaic-shelf sedimentsBiogenous sediments (high-productivity zones)Shallow-water carbonatesPelagic sediments (low-productivity zones)Anoxic basins (e.g. Black sea)

Deltaic sedimentsShelves and upper slopesBiogenous sediments (high-productivity zones)Shallow-water carbonatesPelagic sediments (low-productivity zones)Anoxic basins (e.g. Black Sea)

Units are 1012 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 non-delatic 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.

Organic Carbon Burial Rates (and percentages)In Different Ocean RegimesORGANIC CARBON BURIAL IN MARINE SEDIMENTS

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Figure by MIT OCW.

The distribution of organicmatter in marine sediments :

What determines the observedpattern?

….local productivity?….variable preservation?

Image removed due to copyright restrictions.

Organic carbon preservationSo:

A correspondence between regions of high 1° productivity andhigh % Corg in sediments,

And:These regions of high %Corg are ALSO regions of low bottomwater O2 in many cases,

And:It has been shown that some naturally occurring organicmolecules 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.

00

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2000

2500

3000

100 200 300

20

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12

10

10

8

8

6

6

4

4

2

20

0

Oxygen Concentration (µ mol 1-1) Organic Carbon (wt%)

Dep

th in

Sed

imen

ts (c

m)

Wat

er C

olum

n D

epth

(m)

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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”Hartnett et al. (1998) Nature 391, 572-574

60

50

40

30

20

10

00.01 0.1 1 10 100 1000

00 200 400 600 800

1020

30

405060

Oxygen exposer time (yr)

OC

bur

tal e

ffici

ency

(%)

Ora

gani

c ca

rbon

bur

ial e

fficl

ency

(%)

( Lin

ear s

cale

)

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

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Figure by MIT OCW.