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/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 Permeability is related to Grain size:

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

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

Result:

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

Flume experiment: Particle and solute transport Huettel 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

Result: photographs of embedded core

Result: velocities Particles Arrows show direction of flow Dye

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

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

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 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

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

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

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

Coring device

Collected pore water data

… and did whole-core incubations

… and extrapolated results to SAB

But are flux chambers accurate in permeable sediments But are flux chambers accurate in permeable sediments? Eddy correlation flux measurements Berg et al., 2003

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

Results 2 muddy sediment sites