Question and Answer Set 3
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1: A delta progrades into water that is 80 m deep. The sediment supply is a mixture of sand and mud. The water is open marine and subject to storms that can suspend sediment over the entire range of water depths. On top of the delta is a meandering river with a channel depth of 2 m. Although much of the delta top is vegetated, the climate is arid and there are occasional areas where windblown sand is deposited. Sketch a vertical profile of grain size that you would expect in a core taken in this system.
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2. In class we discussed two mechanisms that have been proposed for dolomitization: the reflux mechanism and the dorag (mixing) mechanism. Which of these would be favored on a greenhouse earth, and which on an icehouse earth, and why?
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3. It has been proposed that the ocean of the early earth was much richer in Mg than the present ocean, allowing precipitation of dolomite as a primary carbonate phase. How would this affect carbonate diagenesis? What if the oceans had the same chemistry they do now, but with no Mg?
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4. What would happen to the main oceanic tides if there were no Moon? No Sun?
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5. It has been estimated from studies of coral growth rings that in Ordovician time the earth's rotation was faster, so that there were about 420 days in a year, and that the Moon was closer to the earth but orbited the earth with the same period (about 12 times per year, so about 35 Ordovician days). How would this affect the tides, and what evidence of these effects could you look for in the stratigraphic record?
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6. Are the effects or variables in the following pairs positively correlated (if one increases the other increases), negatively correlated (if one increases the other decreases), or uncorrelated (changes in one are unrelated to changes in the other)?
percent dolomite in carbonates - time
wave energy - frequency of deltaic avulsion
CCD - pH of oceans
period of neap-spring cycle - CCD
presence of black (high organic C) shale - sea level
percent aragonite in initial sediment - percent secondary porosity
sea level - CCD
CCD - ACD
tempestite frequency - sea level
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7. If we do eventually manage to add enough CO2 to the atmosphere to cause significant warming and melting of the ice caps, how would this be recorded in open-marine sediments?
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8. You put a sample of typical modern carbonate sand in a beaker of distilled water and wait for a very long time. What happens to the sand?
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9. You put a sample of well sorted quartz sand containing 5% coral fragments in a beaker of distilled water. What happens?
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10. Sketch the vertical sedimentary sequence deposited on a section of Pacific crust that starts at the East Pacific Rise (spreading center) and is carried east to the Peru-Chile Trench adjacent to the Andes Mountains. Note: the EPR is above the local CCD but the Trench is below it.
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A 1: The sketch should show overall upward coarsening over a distance equal to the water depth (80 m). Superimposed on this would be small scale upward fining sequences associated with storm resuspension and deposition (tempestites). These should range in thickness from a few cm to several dm and thicken upwards. At the top the deposit would show upward fining over a distance of the order of 2 m, the depth of the meandering channel. The overbank (top) of this would show mm-cm scale coarsening upward (inversely graded) sets associated with eolian ripples. The grain size profile would show packaging over some five orders of magnitude in vertical scale (10-3 m to 102 m), with two entirely separate mechanisms of upward fining (tempestites and fluvial point bar migration) and two equally separate mechanisms of upward coarsening (deltaic progradation and eolian ripple migration).
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A 2: The key requirements for reflux are arid climate and a low-lying storm or tidal mudflat near the sea, to enable evaporite precipitation in the surface sediments. These conditions could occur in either climate scenario, but should be favored under greenhouse conditions, when the earth's average temperature is higher and low-lying continental areas are flooded. On the other hand, the greenhouse earth would not be favorable for mixing dolomitization: since the dolomite forms only in a narrow mixing zone between fresh and salt water, this model requires fluctuations in sea level to promote dolomitization over significant rock thicknesses. By far the best mechanism for producing frequent large changes in sea level is advance and retreat of continental ice sheets, which does not occur during greenhouse phases since there are no ice sheets. Hence although mixing would still occur on a greenhouse earth, significant dolomitization by this mechanism probably would not.
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A 3: The main diagenetic reactions that occur in present-day carbonates are all driven by the basic fact that much marine carbonate is formed as aragonite or magnesian calcite, both of which are unstable and prone to conversion to low-magnesian calcite in fresh water (i.e. once the Mg is removed from the system). Since dolomite is the most stable of the carbonate minerals, a carbonate deposit in which dolomite was the main primary mineral would already exist in its most stable phase. The main diagenetic reaction we would expect in this case would be cementation, as is the case for siliciclastic sediments.
Removing Mg from the ocean would produce a comparable though less extreme effect. Without Mg, there would be no reason to form aragonite or high-Mg calcite, so low-Mg calcite would be the main carbonate phase. Again, this would lead to primary carbonate deposits dominated by a relatively stable mineral, and the extent of diagenesis would be greatly reduced.
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A 4: Eliminating the Moon would not eliminate the tide, as you might be tempted to think. There would still be a tide due to the Sun, but of much smaller amplitude (range). Of equal importance, there would be no interference between the Moon and Sun, and so there would be no fortnightly (neap-spring) cycle. The tidal amplitude would remain nearly constant from day to day. The period of the semi-diurnal (twice-daily) tide would not change since the earth's rotation speed would remain the same. Eliminating the Sun would not reduce the average tidal amplitude, but it would eliminate the fortnightly cycle.
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A 5: The first point is that with the Moon closer to the earth, the amplitude of the basic (semi-diurnal) tide would be larger than on the modern earth. One might expect tidally dominated sediments to be more common in older rocks than younger ones. The tides would still occur twice per (Ordovician) day, but the period of the neap-spring cycle would be about 17 days rather than the present 14 days. This change in neap-spring period might be recognizable by counting the number of cycles in a thick-thin-thick sequence of dune foresets such as we saw in class.
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A 6:
percent dolomite in carbonates - age of rocks POSITIVE (Empirically, the percentage of dolomite as a fraction of all carbonates increases with geologic age.)
wave energy - frequency of deltaic avulsion NEGATIVE (As wave energy increases the delta front is smoothed and flattened, reducing the driving mechanism of deltaic avulsion.)
CCD - pH of oceans POSITIVE (A decrease in pH means an increase in acidity and dissolution of carbonate, which raises the CCD, i.e. reduces the depth.) period of neap-spring cycle - CCD UNCORRELATED (These two things are unrelated to one another.)
presence of black (high organic C) shale - sea level POSITIVE (Black carbon-rich shales form when sluggish marine circulation allows complete consumption of oxygen in the deep water by respiration, which is favored during greenhouse intervals when sea level was high. The connection is real but indirect.)
percent aragonite in initial sediment - percent secondary porosity POSITIVE (Secondary porosity usually results from aragonite dissolution.)
sea level - CCD NEGATIVE (Sea level is highest during greenhouse intervals when the CCD is shallow.)
CCD - ACD POSITIVE (The ACD is generally above the CCD but they are controlled by the same relationships and tend to move together.)
tempestite frequency - sea level NEGATIVE (Sea level is highest during greenhouse intervals when storm frequency is reduced by the absence of strong temperature gradients.)
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A 7: Without the ice caps, there would be no mechanism for cooling surface
water near the poles and causing it to sink. Hence the power source for the
deep-ocean circulation would be shut off. Sluggish deep-water circulation
would allow CO2 to build up in the water and would make it more acidic. The
CCD would rise and of the carbonate produced in surface water, less would
ultimately be preserved in marine sediments. (This would provide a positive
feedback on the atmospheric CO2 level, but note that there are other
aspects of global warming that would have the opposite effect.)
The marine sed record would record this rise (shallowing) of the CCD
particularly well in regions whose depth was between the old and new
values. There the record would show an upward transition from carbonate
muds to brown pelagic muds. Note that the previously formed carbonate
deposits would not be destroyed. The change in CCD would simply prevent new
carbonate sediments from depositing. Below the old CCD there would be no
change; brown mud would continue to accumulate. Likewise, above the new,
higher CCD carbonates would continue to accumulate.
Some parts of the deep ocean would become anoxic if the deep circulation
were shut off. This would be reflected in the deposition of black,
organic-rich shales.
Though this would not be on an exam because we haven't talked about it,
another major way the change would be recorded is via sediments in areas
directly influenced by marine currrents. Areas such as the outer parts of
the US continental margin south of Cape Hatteras, where the Gulf Stream is
quite strong, would begin depositing sediment that is now carried off the
shelf by the current. Sedimentation on a number of deep 'drift' deposits
fed by deep, cold currents would cease.
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A 8: The main point here is that by definition distilled water (an extreme case of fresh water) is undersaturated with respect to all carbonate
phases, since it doesn't have any Ca in it. Moreover, equilibration with
atmospheric CO2 would make the water somewhat acidic. From the problem
statement we assume that the sand would contain the usual mixture of
carbonate phases: high-Mg calcite, low-Mg calcite, and aragonite. Of these,
low-Mg calcite is the most stable. Initially all would dissolve, with
aragonite and high-Mg calcite going the fastest. Eventually the water would
reach a point where it was supersaturated with respect to pure (low-Mg)
calcite but still undersaturated with respect to high-Mg calcite and
aragonite. At this point the latter two would begin converting to low-Mg
calcite, which would continue until there was no more high-Mg calcite or
aragonite left. As we discussed in class, the high-Mg calcite might be able
to convert without wholesale dissolution of the grains, while aragonite
grains would more likely be dissolved out and replaced by calcite spar.
Some of the calcite would also precipitate as cement. The sample would go
in as a soft, young sediment and emerge as a rock.
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A 9: The background here is the same as for above. The coral fragments are aragonitic and so would dissolve readily in the water. Once the water
reached saturation with respect to calcite, calcite would begin to
precipitate as cement on the quartz grains. You'd end up with a
calcite-cemented quartz arenite.
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A 10: Since the rise is above the CCD, the first sediment deposited would be pelagic carbonate. Sinking of the crust as it is carried east by plate
motion would drop it below the CCD, leading to deposition of pelagic brown
clay on top of the carbonate (note that already-deposited carbonates do not
dissolve if they are then placed below the CCD). As the crustal section we
are following approaches the west coast of South America, it would begin to
receive sediments derived from the Andes. The initial deposits would
represent the most distal material, so would be finest. Moving the crust
closer would lead to upward coarsening. The sediments would be basin-plain
turbidites followed (going up) by more proximal fan turbidites and perhaps
by slope and shelf deposits, depending on exactly where the subduction zone
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