12.710 Intoduction to Marine Geology and Geophysics
11/1 Mid Term
Sediments, Processes, and the Sedimentary Record
11/6 (McManus)
11/8 (McManus)
Deep-sea sediments: composition, distribution
Biological, chemical, and physical abyssal processes
11/13 (McManus)
11/15 (McManus)
11/20 (McManus)
11/22 Thanksgiving
Dating methods and the sedimentary record
Paleothermometry
Deep water chemistry and atmospheric p(CO
2
)
11/27
11/29
(McManus)
(McManus)
12/4 (McManus)
12/6 Final Exam
Ocean chemistry and continental weathering
Astronomical climate theory
Sedimentary records of abrupt climate change

I
Early diagenesis
Physical
Chemical
Addition
Removal
Sed.-water interaction
II
Abyssal reworking
Nepheloid layers
Syndepositional reworking
Deep currents
Sediment drift deposits
III
Bioturbation
Discrete, episodic
Modeled as diffusive process
Varies with depth, age, size
Environmental influence

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

Case 1. No diagenesis
Variable property preserved through time at increasing depth.

Case 2. Constant diagenesis
Steady 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.

CaCO 3 more soluble in the deep ocean:
Pressure effect combines with lower [CO 3 = ].

The (“older”) deep Pacific is more corrosive.
Pressure effect combines with lower [CO 3 = ].
“Delta carbonate” (
 CO 3 = ) is defined as difference from saturation (after Broecker).

Nepheloid layer
Turbidity increases toward the bottom due to resuspension.

McCave and Tucholke, 1986
Suspended load

Suspended load
Deep turbidity occurs where western boundary currents provide energy.

Sediment drift deposits
Structure defined by sediment availability and bathymetry.

Sediment drift deposits
Large features overlying basement structure.

Sediment drift deposits
Large features overlying basement structure.

Sediment drift deposits
Large features overlying basement structure.

Water masses and drifts
Drift deposits follow deep currents and bathymetry.

Bioturbation
Abundant, complex, benthic communities influence bottom sediments.

Bioturbation
Instantaneous event (impact) distributed throughout the sediment column.

Modeling bioturbation
Berger and Heath (1968)
Observations suggest mixing in at least the top few centimeters, and an exponential decrease in concentration above boundaries and event horizons.
Suggest a simple, useful model to explore.

Instantaneous mixing
---------------------------
No mixing

Mixed layer influence
Mixing is both upward and downward, influencing the overall sediment column.

Biodiffusivity
Estimates for D b display a strong relationship to location.

Size-dependent bioturbation
Different grain sizes mixed differently, give range of values for biodiffusivity.
Wheatcroft, 1992

Size-dependent bioturbation
Different grain sizes mixed differently, give range of values for biodiffusivity.
Wheatcroft, 1992

Estimating D b
A range of radioactive tracers can be used.

Age-dependent mixing
Shorter-lived isotopes yield higher estimates for D b
.

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 apparent frequency and amplitude of signal

Temporal influence of bioturbation
May smooth appearance
Of abrupt transitions.

Trauth et al.
Controls on mixing depth L
In single region, strong influence of C org .

Boudreau
Controls on biodiffusivity D b
Globally, higher
D b
(mixing coeff.) follows higher sedimentation rate.

Boudreau
Controls on mixing depth L
Net result of competing influences is similar L.