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Shallow -water sediments: Early diagenesis in sandy sediments

Shallow -water sediments: Early diagenesis in sandy sediments. Results from: Experiments laboratory field Field measurements. An early observation Bacon et al., 1994. “Expected” based on: atmospheric input + local accumulation ~ 25 dpm/cm 2. Some observations.

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Shallow -water sediments: Early diagenesis in sandy sediments

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  1. Shallow -water sediments:Early diagenesis in sandy sediments Results from: Experiments laboratory field Field measurements

  2. An early observationBacon et al., 1994 “Expected” based on: atmospheric input + local accumulation ~ 25 dpm/cm2

  3. Some observations Shallow water environments are ~ 10% of ocean area, but account for ~ 30% of marine primary production ~ 50% of PP on continental shelves settles to sea floor BUT ~ 70% of continental shelves are relict sands… how is organic matter recycled at the sea floor? how do low-density particles settle to the sediments in these high-energy environments?

  4. Advective flow through sediments : theory Permeability: Relates the velocity of fluid flow through a porous medium to the pressure gradiennt Unit = darcy Permeability is related to Grain size:

  5. Flow over small-scale topography on permeable Sediments : theory Pressure Arrows are velocity vectors Effect of the flow on a solute produced at ~8-10 cm below surface From Huettel et al., 1998

  6. Experiment in a flumeParticle transportHuettel and Rusch, 2000 Flume: 200 cm long x 35 cm wide Sandy sediments placed in flume A ridge built: 3 cm high x 11 cm wide A suspension of algae added to flowing water

  7. Result:

  8. Field Experiment 2 box cores containing sorted sands placed in intertidal bay 3 µm diam. Fluorescent beads placed in a ring around them Left for 10 hours, then subcored Measure: penetration of beads and microalgae

  9. Flume experiment:Particle and solute transportHuettel et al., 1996 Flow Beads of various sizes added to flowing water 1µm: black ; 10µm: blue ;  = 1.45-1.48 g/cm3 Rhodamine dye: -- added to flow -- and pore water at 7-9 cm and 13-15 cm stained with dye

  10. Result:photographs of embedded core

  11. Result:velocities Particles Arrows show direction of flow Dye

  12. Field experimentReimers et al., 2004 Solution containing iodine released around a central, iodine-sensitive Electrode. Time between dye release and detection of iodine at depth Below sediment surface measured

  13. Result: tracer transport Velocity = Distance between Release and electrode Divided by time between Release and detection Depth at which electrode was placed; I-containing solution released 1-4 cm above electrode

  14. Does flow at these speeds affect the rate of decomposition of organic matter in the sediments? Take sediment cores -- seal top and bottom -- flow through the cores -- measure O2 at inlet and outlet

  15. Measurements in the southern Mid-Atlantic BightJahnke et al., 2000, 2005 Non-accumulating, relict sands %Corg = 0.06% High permeability In situ benthic flux chambers 2. Pore water profiles and sediment incubations

  16. In situ benthic flux chambers Use 2 chambers, deployed side-by-side: one transparent, one opaque !!

  17. Taking a closer look… Gross O2 production balances Gross CO2 consumption -- Benthic primary production

  18. Generalizing the results Relate measured production to Pigment concentration in surface Layer of sediment And Light level at sediment surface … and extrapolate -- Benthic PP may occur over ~ 70% of SAB area, And may equal ~ 60% of water column production

  19. Coring device

  20. Collected pore water data

  21. … and did whole-core incubations

  22. … and extrapolated results to SAB

  23. But are flux chambers accurate inpermeable sediments?Eddy correlation flux measurementsBerg et al., 2003

  24. Eddy correlation flux measurements:data Measurement rate = 25 Hz raw & smoothed data mean Mean vertical velocity = 0

  25. Results2 muddy sediment sites

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