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Ch. 9: Ocean Biogeochemistry

NOAA photo gallery

Overview •  The Big Picture

•  Ocean Circulation

•  Seawater Composition

•  Marine NPP

•  Particle Flux: The Biological Pump

•  Carbon Cycling

•  Nutrient Cycling

•  Time Pemitting: Hydrothermal venting, Sulfur cycling, Sedimentary record, El Niño

•  Putting It All Together Slides borrowed from Aradhna Tripati

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Ocean Circulation

•  Upper Ocean is wind-driven and well mixed

•  Surface Currents deflected towards the poles by land.

•  Coriolis force deflects currents away from the wind, forming mid-ocean gyres

•  Circulation moves heat poleward

•  River influx is to surface ocean

•  Atmospheric equilibrium is with surface ocean

•  Primary productivity is in the surface ocean

Surface Currents

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Deep Ocean Circulation

*  Ice forms in the N. Atlantic and Southern Ocean, leaving behind cold, saline water which sinks

*  Oldest water is in N. Pacific

*  Distribution of dissolved gases and nutrients: N, P, CO2

•  Deep and Surface Oceans separated by density gradient caused by differences in Temperature and Salinity

•  This drives thermohaline deep circulation:

Seawater Composition •  Salinity is defined as grams of salt/kg seawater, or parts

per thousand: %o

•  Major ions are in approximately constant concentrations everywhere in the oceans

•  Salts enter in river water, and are removed by porewater burial, sea spray and evaporites (Na, Cl).

•  Calcium and Sulfate are removed in biogenic sediments

•  Magnesium is consumed in hydrothermal vents, in ionic exchange for Ca in rock.

•  Potassium adsorbs in clays.

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Major Ions in Seawater

The Two-Box Model of the Ocean

River Flow

Precipitation Evaporation

Upwelling Downwelling

Sedimentation

Particle Flux

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Residence time vs. chemistry

Marine Primary Production •  Marine NPP occurs in the top 200 meters of the ocean

•  Difficult to measure

•  Estimates range from 27-50 Pg C/year (50 x 1015g C/yr)

•  Trophic cycling: single-celled phytoplankton, zooplankton, bacteria

•  Recycling of dissolved organic material (DOM) retains nutrients in the upper ocean in particulate form

•  Some particulate organic matter sinks out of upper ocean

•  Important sink for Carbon cycle: “Biological Pump”

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Marine Primary Production

Marine Primary Production

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Decomposition/breakdown of organic C

Particle Flux

•  Marine snow: sinking particles composed of living and dead organisms, fecal pellets: a constant, slow rain

•  Bacterial decomposition (respiration) continues through the entire depth of the water-column remineralizing organic material

•  Less than 1% of NPP actually makes it to the bottom to be buried in sediments (Estimated at 0.157 Pg C/yr)

•  Diagenesis in near-shore organic sediments:

*  Sulfate reduction ⇒ Pyrite formation

*  Methanogenesis

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Carbon Sedimentation

•  Some organisms form carbonate shells or tests

•  These may be deposited as sediments in particle flux, or they may dissolve:

CaCO3 + H2O ⇒ Ca2+ + CO32-

•  Calcium carbonate dissolves better in colder water / higher pressures: deep ocean conditions.

•  CaCO3 deposits on Continental shelves, Mid-ocean ridges, island flanks

•  No Deep Ocean deposition

Carbonate Compensation Depth

•  CO2 equilibrates between surface ocean and atmosphere in accordance with Henry’s Law

•  In seawater, CO2 dissociates:

CO2 + H2O ⇔ H2CO3 ⇔ H+ + HCO3- ⇔ H+ + CO3

2-

•  Supersaturation of CO32- in upper ocean prevents CaCO3

dissolution

•  Lysocline: dissolution rate increases rapidly with depth

•  CCD: The depth below which the calcium carbonate deposition drops below about 20%

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Lysocline and CCD

Carbonate Deposition

20 %

Dep

th

5 km

4 km

CCD Lysocline

100%

Cycling of inorganic carbon (carbonate)

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SeaWiFS website: NFS/NASA

Nutrient Cycling

•  Near-surface levels of biolimiting nutrients are low: N & P

•  Increasing levels as sinking organic materials degraded

•  Remineralization increases concentration of dissolved nutrients

•  Strongly controlled by organisms: non-conservative

•  “Older” Pacific water has higher levels

•  Nutrients in upwelling water recharge surface ocean

•  Redfield Ratio in upwelling water similar to the ratio in organisms

C : N : P

106 : 16 : 1

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Depth Profiles of N and P