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

Feb 25, 2016

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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. - PowerPoint PPT Presentation
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Page 1: Shallow -water sediments: Early diagenesis in sandy sediments

Shallow -water sediments:Early diagenesis in sandy sediments

Results from:

Experimentslaboratoryfield

Field measurements

Page 2: Shallow -water sediments: Early diagenesis in sandy sediments

An early observationBacon et al., 1994

“Expected” based on:

atmospheric input+ local accumulation

~ 25 dpm/cm2

Page 3: Shallow -water sediments: Early diagenesis in sandy sediments

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?

Page 4: Shallow -water sediments: Early diagenesis in sandy sediments

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

Page 5: Shallow -water sediments: Early diagenesis in sandy sediments

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

Page 6: Shallow -water sediments: Early diagenesis in sandy sediments

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

Page 7: Shallow -water sediments: Early diagenesis in sandy sediments

Result:

Page 8: Shallow -water sediments: Early diagenesis in sandy sediments

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

Page 9: Shallow -water sediments: Early diagenesis in sandy sediments

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

Page 10: Shallow -water sediments: Early diagenesis in sandy sediments

Result:photographs of embedded core

Page 11: Shallow -water sediments: Early diagenesis in sandy sediments

Result:velocities

Particles

Dye

Arrows show directionof flow

Page 12: Shallow -water sediments: Early diagenesis in sandy sediments

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

Page 13: Shallow -water sediments: Early diagenesis in sandy sediments

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

Page 14: Shallow -water sediments: Early diagenesis in sandy sediments

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

Page 15: Shallow -water sediments: Early diagenesis in sandy sediments

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

Page 16: Shallow -water sediments: Early diagenesis in sandy sediments

In situ benthic flux chambers

Use 2 chambers, deployed side-by-side: one transparent, one opaque

!!

Page 17: Shallow -water sediments: Early diagenesis in sandy sediments

Taking a closer look…

Gross O2 production balancesGross CO2 consumption

-- Benthic primary production

Page 18: Shallow -water sediments: Early diagenesis in sandy sediments

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

Page 19: Shallow -water sediments: Early diagenesis in sandy sediments

Coring device

Page 20: Shallow -water sediments: Early diagenesis in sandy sediments

Collected pore water data

Page 21: Shallow -water sediments: Early diagenesis in sandy sediments

… and did whole-core incubations

Page 22: Shallow -water sediments: Early diagenesis in sandy sediments

… and extrapolated results to SAB

Page 23: Shallow -water sediments: Early diagenesis in sandy sediments

But are flux chambers accurate inpermeable sediments?

Eddy correlation flux measurementsBerg et al., 2003

Page 24: Shallow -water sediments: Early diagenesis in sandy sediments

Eddy correlation flux measurements:data

mean

Mean vertical velocity = 0

Measurement rate = 25 Hz raw & smoothed data

Page 25: Shallow -water sediments: Early diagenesis in sandy sediments

Results2 muddy sediment sites