Marine Geochemistry 2 Reference: Schulz and Zabel Marine Geochemistry Springer, New York pp. ISBN 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 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)

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).

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.

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

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,…

18 O / 16 O ratios The oxygen isotopic composition of seawater (  18 O w ) 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

18 O / 16 O ratios Modern  18 O w 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

18 O / 16 O ratios  18 O w 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

13 C / 12 C 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.

13 C / 12 C ratios Surface water is enriched in 13 C because photosynthesis preferentially removes 12 C from the CO 2 Deeper water masses have lower  13 C values due to decomposition of organic matter.

13 C / 12 C ratios Modern  13 C values are close to 0 o / oo Deep water mass  13 C ranges from +1.2 o / oo in North Atlantic Deep Water to +0.4 o / oo in Pacific Deep Water.

13 C / 12 C ratios  13 C 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

15 N / 14 N 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

15 N / 14 N ratios Few measurements published NO 3 - dominates the ocean pool (NO 2 -, NH 4 + )  15 N from oxygenated deep waters ranges between 3 o / oo and 7 o / oo.

34 S / 32 S 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

34 S / 32 S ratios  34 S in the modern ocean is mostly constant with a value of +20 o / oo and a standard deviation of +/ o / oo Cambrian maximum of about +30 o / oo Permian minimum of about +10 o / oo