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

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 Permeability: Relates the velocity of fluid flow through a porous medium in response to a pressure gradient Unit = darcy Permeability is related to Grain size:

Flow over small-scale topography on permeable sediments Pressure Arrows are velocity vectors Effect of the flow on a solute produced at ~8-10 cm below surface

Experiment in a flume Particle transport 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 sandsplaced 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 Flow Beads of various sizes added to flowing water Rhodamine dye: -- added to flow -- pore water at 7-9 cm and cm stained with dye

Result Arrows show direction of flow Particles Dye

Field experiment Solution containing iodine released around a central, iodine-sensitive Electrode. Time between dye release and detection of iodine at depth Below sediment surface measured Depth at which electrode was placed; I-containing solution released 1-4 cm above electrode Velocity = Distance between Release and electrode Divided by time between Release and detection

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 Sediment cores -- profiles and incubations In situ benthic flux chambers

Coring device

Results Initial profile (T0) And profiles Measured at 1-week intervals

Interpretation Time series used to determine Production rates of solutes Model initial profile: Question: Why are the results from 2 solutes so different? Transport mechanism: -- biological irrigation? (less likely) -- pressure-driven flow (more likely)

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

But are flux chambers accurate in permeable sediments? Eddy correlation flux measurements mean Mean vertical velocity = 0 Measurement rate = 25 Hz raw & smoothed data

Results 2 muddy sediment sites