Shallow -water sediments: Early diagenesis in sandy sediments

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