Shallow water sediments Early diagenesis in sandy sediments

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 �ermeability is related to P 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 13 -15 cm stained with dye

Result Particles Dye Arrows show direction of flow

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 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 O 2 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 (T 0) And profiles Measured at 1 -week intervals

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

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

Taking a closer look… Gross O 2 production balances Gross CO 2 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