Shallow -water sediments:Early diagenesis in sandy sediments
Results from:
Experimentslaboratoryfield
Field measurements
An early observationBacon et al., 1994
“Expected” based on:
atmospheric input+ local accumulation
~ 25 dpm/cm2
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?
Advective flow through sediments : theory
Permeability: Relates the velocity of fluid flow through a porous medium to the pressure gradiennt
Unit = darcy
€
u = kϕμ
∇p− ρg( )
Permeability is related toGrain size:
€
k =5.6×10−3φds
2
1−φ( )2
Flow over small-scale topography on permeableSediments : theory
Pressure
Arrows are velocity vectors
Effect of the flow on a solute produced at ~8-10 cm belowsurface
From Huettel et al., 1998
Experiment in a flumeParticle transportHuettel and Rusch, 2000
Flume: 200 cm long x 35 cm wide
Sandy sediments placed in flumeA ridge built: 3 cm high x 11 cm wideA suspension of algae added to flowing water
Result:
Field Experiment
2 box cores containing sorted sands placed in intertidal bay3 µm diam. Fluorescent beads placed in a ring around them Left for 10 hours, then subcored
Measure: penetration of beads and microalgae
Flume experiment:Particle and solute transport
Huettel et al., 1996
Flow
Beads of various sizes added to flowing water1µm: 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
Result:photographs of embedded core
Result:velocities
Particles
Dye
Arrows show directionof flow
Field experimentReimers et al., 2004
Solution containing iodine released around a central, iodine-sensitiveElectrode. Time between dye release and detection of iodine at depthBelow sediment surface measured
Result: tracer transport
Velocity =Distance betweenRelease and electrodeDivided by time betweenRelease and detection
Depth at which electrodewas placed;I-containing solution released1-4 cm above electrode
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
Measurements in the southern Mid-Atlantic BightJahnke et al., 2000, 2005
1. In situ benthic flux chambers
2. Pore water profiles and sediment incubations
Non-accumulating, relict sands%Corg = 0.06%High permeability
In situ benthic flux chambers
Use 2 chambers, deployed side-by-side: one transparent, one opaque
!!
Taking a closer look…
Gross O2 production balancesGross CO2 consumption
-- Benthic primary production
Generalizing the results
Relate measured production toPigment concentration in surfaceLayer 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
Coring device
Collected pore water data
… and did whole-core incubations
… and extrapolated results to SAB
But are flux chambers accurate inpermeable sediments?
Eddy correlation flux measurementsBerg et al., 2003
Eddy correlation flux measurements:data
mean
Mean vertical velocity = 0
Measurement rate = 25 Hz raw & smoothed data
Results2 muddy sediment sites