Presentation on theme: "Basic concepts (Early Diagenesis, chapters 2-3) Transport and Physical properties Sedimentation without diagenesis (reactions that alter solid composition."— Presentation transcript:
Basic concepts (Early Diagenesis, chapters 2-3) Transport and Physical properties Sedimentation without diagenesis (reactions that alter solid composition and pore water composition) Sedimentation with diagenesis Sampling methods 14 C based accumulation rate estimates
For sediment: Advection given by sedimentation rate w, length / time (cm/ky, m/my) (if no compaction or benthic reactions, w = deposition rate at sediment water interface) More generally, sedimentation rate needs to be evaluated: -At some time (or over some interval of time) since it is not necessarily constant -At some depth (since compaction can change the linear sedimentation rate in a sediment column even with constant deposition) Diffusion is a common parameterization of particle mixing (bioturbation) D, length 2 / time (cm 2 / yr or /ky)
For porewater: Advection frequently slow relative to diffusion (but this is by no means always the case) -Water advection due to sediment compaction in pelagic sediments, by simple vertical compression of sediments due to weight of overlying sediments - Imposed water advection convection due to thermal instability when underlying crust or intruded rock is hot, and sediment cover is relatively thin flow due to compression of sediment column due to tectonics (e.g., plate collision) bottom currents flowing over bedforms hydraulic connection to onshore aquifers Over long length scales, small advective velocities tend to become more important relative to diffusion.
Diffusion in porewater Solute diffusion in porous medium (mathematical treatment analogous to heat conduction: e.g. Crank, "Mathematics of Diffusion", and Carslaw and Jeager, "Conduction of Heat in Solids") “Irrigation” Pumped flow through animal burrows
Porosity - total connected water volume as fraction of bulk sediment volume Sand Clay
Initial (uncompacted) porosity increases as grain size decreases, and as sorting decreases.
Compaction - vertical compression due to weight of overlying sediments Drives water flow out of sediment; lowers linear accumulation rate
Sedimentation without diagenesis Input sets layer composition, which then remains constant as additional layers are added. Berner, Early Diagenesis
input Sediment record Sedimentation without diagenesis Downcore variations imply changing inputs
input Sediment record But, if major component fluxes are not constant, downcore variations in minor phase can be misleading.
No diagenesis, constant fluxes 0 5 10 15 ky
No diagenesis, flux of component B varies 0 5 10 15 ky This highlights the importance of working in terms of accumulation rate (flux), (mass / area-time).
A no-diagenesis signal? “Heinrich event” layers of ice-rafted debris (IRD) in the North Atlantic reflect episodes of high IRD input.
Constant sedimentation with steady-state diagenesis Reactions alter layer composition (and pore water composition), but concentration vs. depth remains constant through time.
In the steady state case, downcore variation in the zone of active diagenesis reflects ongoing reactions, not input variations. In the real world, neither inputs nor reaction rates are necessarily constant. Downcore variations in solid phase concentrations may reflect changes in inputs, or in diagenetic efficiency (e.g., OM decomposition, carbonate dissolution), or both.
Ceara Rise (WEqAtl) sediment composition; Martin et al., 2000
Non-steady-state (non 1-D) processes 3280 m 4675 m
Diagenetic reactions will, in general, lead to changes in pore water solute concentrations, which in turn drive solute fluxes out of or into the sediments. At steady state, pore water concentrations don’t change with time, and fluxes into or out of the sediments balance net diagenetic consumption or production of solutes.
Solid phase + pore water sample collection methods: Coring Box corer Multicorer Separate pore water from sediment (glove bag, cold van) using centrifugation or shipboard whole-core squeezeing. (nutrients, metals, carbonate system) Measure pore water chemistry in a sub-core using shipboard micro- or mini-electrode profiling. (O2, pH, resistivity) Shipboard incubations, to measure change in overlying water chemistry, or in replicate sediment-packed tubes sampled over time.
In situ (sea-floor) sampling Separate pore water from sediment (by suction or squeezing) to avoid artifacts due to changes in pressure or temperature, or due to mechanical disturbance, during core recovery. Measure pore water chemistry in situ using sea-floor microelectrode profilers. Use benthic flux chambers to isolate a small volume of water over a known area of sediment, and collect time series overlying water samples.