6/3/13 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 1 6/3/13 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 2 6/3/13 Deep Ocean Circulation • Deep and Surface Oceans 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. • Potassium adsorbs in clays. 3 6/3/13 Major Ions in Seawater The Two-Box Model of the Ocean Precipitation Evaporation River Flow Upwelling Downwelling Particle Flux Sedimentation 4 6/3/13 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” 5 6/3/13 Marine Primary Production Marine Primary Production 6 6/3/13 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 7 6/3/13 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% 8 6/3/13 Lysocline and CCD Carbonate Deposition 20 % 100% 4 km Depth Lysocline CCD 5 km Cycling of inorganic carbon (carbonate) 9 6/3/13 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 10 6/3/13 Depth Profiles of N and P 11 .