Marine Geochemistry 2

Reference: Schulz and ZabelMarine Geochemistry Springer, New York2000453 pp.ISBN 3-540-66-453-X

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

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.

Oxygen distributionOxygen 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)

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).

IronThe 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.

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 processesHighly efficient

Submarine hydrothermal input

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.

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.

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.

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,…

18O / 16O ratios

The oxygen isotopic composition of seawater (18Ow) is controlled by fractionation effects due to:evaporation and precipitation at sea surface freezing of ice in Polar Regionsadmixing of water masses with different ratios

(melt water, river run-off) global isotopic content of the oceans

18O / 16O ratios

Modern 18Ow 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

18O / 16O ratios

18Ow 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

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.

13C / 12C ratios

Surface water is enriched in 13C because photosynthesis preferentially removes 12C from the CO2

Deeper water masses have lower 13C values due to decomposition of organic matter.

13C / 12C ratios

Modern 13C values are close to 0 o/oo

Deep water mass 13C ranges from +1.2 o/oo in North Atlantic Deep Water to +0.4 o/oo in Pacific Deep Water.

13C / 12C ratios

13C varied considerably in geologic history due to:Changes in surface water productivityChanges in the gas exchange rate

between oceans and atmosphere due to changes in surface temperatures and ocean circulation

15N / 14N ratiosNew 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

15N / 14N ratios

Few measurements published

NO3- dominates the ocean pool

(NO2-, NH4

+)

15N from oxygenated deep waters ranges between 3 o/oo and 7 o/oo.

34S / 32S ratiosSensitive 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

34S / 32S ratios

34S 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