Marine Geochemistry 2

1
Marine Geochemistry 2

• Reference
• Schulz and Zabel
• Marine Geochemistry Springer, New York
• 2000
• 453 pp.
• ISBN 3-540-66-453-X

2
Oxygen and Nitrate in Marine Sediments

• One of the most intensely studied topic in marine
  geology is the early diagenesis of organic
  material deposited in marine sediments
• Oxygen and nitrate are thermodynamically the most
  favorable electron acceptors in the diagenetic
  sequence of organic matter

3
Oxygen and Nitrate in Marine Sediments

• Oxygen is introduced by photosynthesis and
  exchange with the atmosphere.
• Nitrate is used as the next suitable electron
  acceptor for degradation when oxygen is limited.
• Availability of nitrate as an oxidant is limited
  since it is an important limiting nutrient for
  primary productivity.

4
Oxygen distribution

• Oxygen distribution results from
• Input from atmosphere and phytoplankton
• Surface water supersaturated
• Microbial degradation of organic matter by
  oxidation
• Depleted by bacterial respiration below the mixed
  layer (upper 1000m or lower end of the permanent
  thermocline)
• Physical transport and mixing processes in the
  ocean
• Deep water currents raise oxygen concentrations
  (EX. North Atlantic Deep Water Current)

5
Nitrate distribution

• An increase in dissolved nutrients (nitrate and
  phosphate) is observed with depth due to organic
  carbon oxidation with water depth.
• Older water masses are generally more enriched in
  nitrate (as well as phosphate).

6
Iron

• The reactivity of iron at the interface of the
  bio- and geosphere help to understand the
  interactions between living organisms and the
  solid earth.
• Bacteria and phytoplankton depend on the uptake
  of iron as a prerequisite for their cell growth.
• Some organisms conserve energy from the reduction
  of oxidized ferric iron.
• Redox-reactions cause dissolution and
  precipitation of iron bearing minerals forming
  discrete iron enriched layers which challenge
  geochemists to reconstruct environmental
  conditions of their formation.

7
Iron input to Marine Sediments

• Iron is the fourth most abundant element in the
  continental crust (4.32 wt ).
• It is transported to marine sediments by
• Fluvial processes
• Aeolian processes
• Highly efficient
• Submarine hydrothermal input

8
Iron as a Limiting Nutrient

• Detail investigations concerning its importance
  have only been possible for the last decade due
  to limitations in analytical methods.
• Virtually all microorganisms require iron for
  their respiratory pigments, proteins and many
  enzymes.

9
Iron as a Limiting Nutrient

• Dissolved iron shows similar vertical profiles to
  nitrate.
• Reduced to near 0 within the surface layer
• Increase within the oxygen minimum zone
• An increase of 2-4 in primary productivity
  results form the addition of atmospheric iron.

10
Stable Isotope Distribution in Marine Sediments

• Stable isotopic compositions of elements having
  low atomic numbers (H, C, N, O, S) vary
  considerable as a consequence of the fact that
  certain thermodynamic properties of molecules
  depend on the masses of the atoms of which they
  are composed.

11
Stable Isotope Distribution in Marine Sediments

• The partitioning of isotopes between two
  substances or two phases of the same substance is
  called isotopic fractionation.
• isotopic fractionation occurs during several
  kinds of physical processes and chemical
  reactions
• Isotope exchange reactions
• Redistribution among different molecules
• Kinetic effects
• Condensation/evaporation crystallization,
  melting,

12
18O / 16O ratios

• The oxygen isotopic composition of seawater
  (d18Ow) is controlled by fractionation effects
  due to
• evaporation and precipitation at sea surface
• freezing of ice in Polar Regions
• admixing of water masses with different ratios
  (melt water, river run-off)
• global isotopic content of the oceans

13
18O / 16O ratios

• Modern d18Ow values of seawater are close to 0
  o/oo (SMOW).
• It serves then as an excellent tracer for
  indicating the influence of freshwater input to
  the oceans

14
18O / 16O ratios

• d18Ow has been shown to vary considerably in
  geologic history.
• 1.2 o/oo for the last glacial maximum (sea level
  low stand of 100m)
• -0.8 o/oo in the ice-free world of the Cretaceous

15
13C / 12C ratios

• Controlled in seawater mainly by two processes
• Biochemical fractionation due to the formation
  and decay of organic matter.
• Physical fractionation during gas exchange at the
  air-sea boundary.

16
13C / 12C ratios

• Surface water is enriched in 13C because
  photosynthesis preferentially removes 12C from
  the CO2
• Deeper water masses have lower d13C values due to
  decomposition of organic matter.

17
13C / 12C ratios

• Modern d13C values are close to 0 o/oo
• Deep water mass d13C ranges from 1.2 o/oo in
  North Atlantic Deep Water to 0.4 o/oo in Pacific
  Deep Water.

18
13C / 12C ratios

• d13C varied considerably in geologic history due
  to
• Changes in surface water productivity
• Changes in the gas exchange rate between oceans
  and atmosphere due to changes in surface
  temperatures and ocean circulation

19
15N / 14N ratios

• New tool in the field
• Records changes in the nutrient dynamics in the
  water column like
• Utilization of different dissolved forms of
  inorganic nitrogen by phytoplankton
• Consumption of phytoplankton by grazers
• Remineralization of organic nitrogen by animals
  and bacteria
• Nitrogen fixation
• Nitrification and denitrification

20
15N / 14N ratios

• Few measurements published
• NO3- dominates the ocean pool
• (NO2-, NH4)
• d15N from oxygenated deep waters ranges between 3
  o/oo and 7 o/oo.

21
34S / 32S ratios

• Sensitive indicator for the transfer of sulfur
  between different reservoirs
• Riverine input of sulfate from sulfur-bearing
  rocks
• Precipitation of evaporites from seawater
• Biological reduction of seawater sulfate
• Formation of sedimentary pyrite
• In the marine environment occurs most commonly
• oxidized as dissolved sulfate
• precipitated as sulfate in evaporites
• reduced form as sedimentary pyrite

22
34S / 32S ratios

• d34S in the modern ocean is mostly constant with
  a value of 20 o/oo and a standard deviation of
  /- 0.12 o/oo
• Cambrian maximum of about 30 o/oo
• Permian minimum of about 10 o/oo