Marine Geochemistry 1

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Feb 21, 2014 (3 years and 6 months ago)

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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


d
13
C =
-
27
o
/
oo

in marine /
-

7
o
/
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 !