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Home , Marine snow, Redfield ratio

6/3/13

Ch. 9: Biogeochemistry

NOAA photo gallery

Overview

• The Big Picture • Ocean Circulation • Composition • Marine NPP • Flux: The • Cycling • Nutrient Cycling • Time Pemitting: Hydrothermal venting, 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. • 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 is in the surface ocean

Surface Currents

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Deep Ocean Circulation • Deep and Surface separated by density gradient caused by differences in Temperature and Salinity • This drives thermohaline deep 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

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. • adsorbs in clays.

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Major Ions in Seawater

The Two-Box Model of the Ocean

Precipitation Evaporation River Flow

Upwelling

Particle Flux

Sedimentation

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Residence time vs. chemistry

Marine

• 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 , , • Recycling of dissolved organic material (DOM) retains nutrients in the upper ocean in particulate form • Some particulate sinks out of upper ocean • Important sink for : “Biological Pump”

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Marine Primary Production

Marine Primary Production

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Decomposition/breakdown of organic C

Particle Flux

• : sinking particles composed of living and dead , 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

*

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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:

2+ 2- CaCO3 + H2O ⇒ Ca + CO3

• 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:

+ - + 2- CO2 + H2O ⇔ H2CO3 ⇔ H + HCO3 ⇔ H + CO3

2- • Supersaturation of CO3 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 % 100%

4 km

Depth Depth Lysocline CCD 5 km

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 water recharge surface ocean • 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

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