Organic Carbon Preservation
Large-scale data compilations -Do Corg Concentration and Accumulation Rate reflect
overlying water productivity?
preservation rate variations -- bottom water O2?
Evidence from the deep-sea: turbidites
The importance of continental margins
Mechanisms of organic carbon preservation in margin sediments
Mineralogical control
The role of oxygen
%organic carbon
Bottom water oxygen
Primary productivity
Note that there is a large-scale correspondence
Between %Corg and Corg accumulation rate
No difference between sites with
Low and high BW O2
Burial efficiency =

Corg accumulation rate
Corg rain rate
Open shape = Corg ox rate by O2
Symbols = Corg ox rate by SO4
Same comparison: euxinc and
Semi-euxinic sites
Result ….
Does Oxygen matter?
Evidence from turbidites
Wilson et al.,1985
GCA 49, 811-822
Pore water concentration data
Turbidite
No turbidite
Interpretation of pore water data
Consumption of contemporary Corg
Consumption of Corg in turbidite
Almost all Corg burial in the ocean occurs on
continental margins
Hedges and Keil, 1995 Mar Chem 49, 81-115
A relationship between sedimentary surface area and
%Corg
Mayer (1994) GCA 58, 1271
Observation
Interpretation
sorption protects Corg from enzymatic
attack
Generalization to other margin areas
Lines = relationship from >70m Gulf of Maine
0.57 mgOC/m2
2 additions to Mayer study:
size separation
study source of om vs fate
Confirmation of “monolayer equivalent concentration”
observation
S = sand sized
L = silt
C = clay
After:
1) Subtracting discrete org particles
2) Correcting for interlamellar surface
A conundrum…
Using N:C and dC-13 as source markers…
There appears to be nearly complete replacement of adsorbed terrestrial
Material by adsorbed marine organic matter…
An experiment: does adsorption protect sedimentary
organic matter from oxidation?
In situ: Corg and SA closely related
Experiment: desorb and
Innoculate with SO bacteria
Sorted by age inferred from depth”
A<50yr, B~160, C~300, D~470
Extent of degradation depends on age
Rate after desorption >> in situ degradation rate
Exceptions to the “monolayer coverage” rule
Deltaic seds
Low flux, high O2
Central Eq. Pac.
High flux,
Low O2
Protection by sorption
matters, but other factors
…O2? … do too
Oxic decomposition: an experimental approach
Hulthe et al., 1998 GCA 62, 1319-1328
Sediments from shallow-water,
margin setting
Incubate in the presence or absence
of O2
Incubate sediments from surface
samples (oxic layer) and deep
samples (anoxic)
Result
Experimental data
Result
Oxygen and water-column decomposition
Devol and Hartnett (2001) L&O 46, 1684-1690
Use sedimentary data to
Infer Corg flux vs. water depth
… Does water column [O2]
matter?
The use of sedimentary data
Result… water column [O2] matters
And sedimentary data agree with trap results
Data: low-O2 Mexico margin
Dotted line: J. Martin result higher [O2], greater attenuation
with water depth
A refinement of “Oxygen matters” -oxygen exposure time
Hartnett et al. (1998) Nature 391, 572-574
Another Washington/
Mexico margin study
Washington:
higher O2, productivity
Mexico:
low O2, lower productivity
Result
Organic C preservation
3 important factors:
Overlying water productivity
Protection by sorption to mineral surfaces
link between continental erosion rates and Corg preservation
Oxygen exposure
link between atmospheric O2 and Corg burial