Transcript Marine Geochemistry

Marine Geochemistry 1
Reference:
Schulz and Zabel
Marine Geochemistry
Springer, New York
2000
453 pp.
ISBN 3-540-66-453-X
The Organic Carbon Cycle
Divided into two parts :
1. Biological cycle
2. Geological cycle
Biological cycle
Photosynthesis in surface waters of
oceans or lakes

organic matter from carbon dioxide

organic matter from bicarbonate
Ends with metabolic or chemical oxidation
of decayed biomass to carbon dioxide
Geological cycle
Incorporation of biogenic organic matter
into sediments and soils
Leads to the formation of natural gas,
petroleum and coal or metamorphic
forms of carbon
Organic matter
accumulation in sediments
In the fossil record:

Dark colored sediments


periods of time favorable to organic matter
accumulation
White or red colored sediments or rocks

devoid of organic matter
Causes leading to deposition of
massive organic-matter rocks
Good Preservation

Sluggish circulation in the deep ocean

Shallow epicontinental seas accompained
by water column stratification
Good Productivity

High primary productivity in a dynamic
system
Primary Production
Photosynthetic plankton

produce 20 to 30 billions tons/year of
carbon

fixation is not evenly distributed on
the oceans but display zones of:

Higher activity on continental margins

Lower activity within the central ocean
gyres
Export to the Ocean Bottom
Of the total biomass formed only a very
small portion reaches the underlying
sea floor and is ultimately buried a
sediment
Most of the organic matter enters the
biological food web and it is respired or
used for new biomass production
Sedimentation Rate
vs.
Organic Matter Burial
Oxic open-ocean conditions:

2X increase in organic carbon content for
every 10X increase in sedimentation rate in
marine sediments
Anoxic conditions:

no change in organic carbon content over a
wide range of sedimentation rates
Organic Carbon Content
of Marine Sediments
Mean organic carbon content :

0.3% with a median value of 0.1%

(data from deep sea drilling)
Varies over several hundreds of
magnitude
Organic Carbon Content
of Marine Sediments
Depends on:

extend of supply of organic matter

preservation conditions

dilution by mineral matter
Chemical Composition
of Biomass
Chemical nature of biomass is commonly
described by its elemental composition
Marine phytoplankton

Redfield et al. (1963) ratio
C:N:P = 106:16:1
Ratio changes drastically :

food chain processes

early digenetic processes
Chemical Composition
of Biomass
Chemical composition can also be confined
to a limited number of compound classes
Their proportions will vary in the different
groups of organisms (Romankevitch, 1984)
Principle of Selective Preservation
Organic compounds and compound classes:
differ in their potential to be preserved in
sediments
differ in their potential survive early
diagenesis
Principle of Selective Preservation
Low Preservation Potential
= easily hydrolyzed

Water-soluble organic compounds

Organic macromolecules
High Preservation Potential
= low solubility in water

Lipids

Hydrolysis resistant molecules
Biological Markers
Molecules with high degree of structural
complexity provide the possibility of relating a
certain product to a specific precursor
EXAMPLE:

24-methylenecholesterol and dinoserol are preferentially
biosynthesized by diatoms and dinoflagellates (Volkman
et al., 1998)
Marine vs. Terrigenous
Organic Matter
Variations in marine and terrigenous organic
matter proportions important for:

paleoclimatic studies

paleoceanographic studies
Parameters used to assess
the organic matter sources
Carbon / Nitrogen Ratio

10 in marine / 20 in terrigenous
Hydrogen Indices (mg HC/g TOC)

150 in marine / 300-800 in terrigenous
Stable Carbon Isotope Rations

d13C = -27o/oo in marine / - 7o/oo in terrigenous
Molecular Paleo-Seawater
Temperature and Climate Indicators
Biosynthesis of Long-Chain Alkenones
in the microalgae Class Haptophyceae
depends on the water temperature
during growth
Coccolithoophorids belong to this class !