12.710 Intoduction to Marine Geology and Geophysics

11/1 Mid Term

Sediments, Processes, and the Sedimentary Record

11/6 (McManus) Deep-sea sediments: composition, distribution 11/8 (McManus) Biological, chemical, and physical abyssal processes

11/13 (McManus) Dating methods and the sedimentary record11/15 (McManus) Paleothermometry

11/20 (McManus) Deep water chemistry and atmospheric p(CO2)

11/22 Thanksgiving

11/27 (McManus) Ocean chemistry and continental weathering11/29 (McManus) Astronomical climate theory

12/4 (McManus) Sedimentary records of abrupt climate change

12/6 Final Exam

Abyssal sedimentary processes

I Early diagenesisPhysicalChemical Addition Removal Sed.-water interaction

II Abyssal reworkingNepheloid layersSyndepositional reworkingDeep currentsSediment drift deposits

III Bioturbation Discrete, episodic

Modeled as diffusive processVaries with depth, age, sizeEnvironmental influence

Sediment traps and bottom sedimentsComposition of settling particles not reflected on the sea floor.

Case 1. No diagenesisVariable property preserved through time at increasing depth.

Case 2. Constant diagenesisSteady state profile preserved through time at migrating depth.

Process

Compaction

Cementation

Authigenesis

Recrystallization

Inversion

Replacement

Dissolution

Bioturbation

Sequence of chemical reactions

Energy considerations yield predictable sequence.

CaCO3 more solublein the deep ocean:

Pressure effect combineswith lower [CO3=].

The (“older”) deep Pacific is more corrosive.

Pressure effect combines with lower [CO3=].

“Delta carbonate” (CO3=) is defined as difference from saturation (after Broecker).

Nepheloid layer

Turbidity increasestoward the bottom dueto resuspension.

McCave and Tucholke, 1986

Suspended load

Suspended load

Deep turbidity occurswhere western boundarycurrents provide energy.

Sediment drift deposits

Structure defined bysediment availabilityand bathymetry.

Sediment drift deposits

Large features overlying basement structure.

Sediment drift deposits

Large features overlying basement structure.

Sediment drift deposits

Large features overlyingbasement structure.

Water massesand drifts

Drift depositsfollow deepcurrents andbathymetry.

Bioturbation

Abundant, complex, benthic communitiesinfluence bottom sediments.

Bioturbation

Instantaneous event (impact)distributed throughout thesediment column.

Modeling bioturbation

Berger and Heath (1968)

Observations suggest mixing in at least thetop few centimeters, and an exponentialdecrease in concentration above boundariesand event horizons.

Suggest a simple, useful model to explore.

Instantaneous mixing---------------------------

No mixing

Mixed layer influence

Mixing is both upwardand downward,influencing the overallsediment column.

Biodiffusivity

Estimates for Db

display a strongrelationship tolocation.

Size-dependent bioturbation

Different grain sizes mixed differently, giverange of values for biodiffusivity.

Wheatcroft, 1992

Size-dependent bioturbation

Different grain sizes mixed differently, giverange of values for biodiffusivity.

Wheatcroft, 1992

Estimating Db

A range of radioactivetracers can be used.

Age-dependent mixing

Shorter-lived isotopes yield higher estimates for Db.

Subsurface maxima

Not a simple 1=D process.

Influence of bioturbation

May alter structure and timing of sedimentary signal.

Original signal Mixed record

Spectral influence of bioturbation

May alter apparentfrequency andamplitude of signal

Temporal influence of bioturbation

May smooth appearanceOf abrupt transitions.

Controls on mixing depth L

In single region, strong influence of Corg.

Trauth et al.

Controls on biodiffusivity Db

Globally, higherDb (mixing coeff.)follows highersedimentation rate.

Boudreau

Controls on mixing depth L

Net result of com-peting influences is similar L.

Boudreau