Post on 26-Jun-2020
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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