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

2. An early observation Bacon 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 flume Particle transport Huettel 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 transport Huettel et al., 1996
Flow
Beads of various sizes added to flowing water
1µm: black ; 10µm: blue ;  = g/cm3
Rhodamine dye:
-- added to flow
-- and pore water at 7-9 cm and cm stained with dye

10. Result: photographs of embedded core

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

12. Field experiment Reimers 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 Bight Jahnke et al
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 in permeable sediments
But are flux chambers accurate in permeable sediments? Eddy correlation flux measurements Berg et al., 2003

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

25. Results 2 muddy sediment sites