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This assessment question was developed at Columbia University for use in an introductory level course for majors in Geology and Environmental Sciences, called Earth's Environmental Systems: Solid Earth. The course is one of series of three courses, which collectively cover the Atmosphere/Hydrosphere, the Lithosphere, and the Biosphere.
We used this as one of three question on the midterm (a closed-book, 75 minute in-class exam). The question would also work well as a homework assignment, on a take-home exam or open-book exam, or as an in-class small group discussion activity.
This question is supposed to stimulate the student to pull together a lot of previously-unconnected information and concepts developed in lecture, reading, labs and the class field trip. To answer the question completely, the student needs to know and understand the following:
Basic familiarity with ocean sediments and their environments of deposition, sufficient to recognize "foram-nanno ooze" as a calcareous biogenic open ocean sediment, and to recognize deep sea drilling (Joides Resolution) as a tool for recovering ocean sediments. We did this through one lecture on ocean sediments, readings in the textbook, and one lab built around a simulation of an expedition aboard the Joides Resolution.
Law of superposition: under most circumstances sediments record first-occurring events at the bottom, overlain by record of more-recently occurring events. This idea recurs several times throughout the course.
In clastic sediments, grain size is related to transportability. Thus coarse grain (sand or gravel) usually means you are closer to the sediment source, whereas fine grain size (mud or silt) in the same depositional system usually means you are farther from the sediment source. We discussed this point in a lecture on erosion. In addition, the course field trip included a transect across the Triassic sediments of the Newark Basin (a fault-bounded extensional basin), which exhibit a grain-size gradient with distance away from the bounding Ramapo fault.
Knowledge of the existence of the Calcite Compensation Depth (CCD). This was covered in the lecture on Ocean Sediments. In addition, most of the students had previously taken Earth's Environmental Systems: Climate, so they were experienced thinking about vertical and horizontal variability in oceanic water mass properties.
Basic familiarity with plate tectonics, sufficient to understand that new seafloor is formed at mid-ocean ridge spreading centers, and that seafloor subsides as it ages and moves away from its parent spreading center. We accomplished this through a lecture on the manifestations of plate tectonics, and through readings in the USGS This Dynamic Earth booklet, which we use as a supplementary textbook.
Knowledge that basalt is an extrusive igneous rock, and that the top layer of oceanic crust is made out of basalt.
Knowledge that subduction is usually accompanied by vulcanism, and that such vulcanism is often explosive, and that explosive vulcanism results in wind-carried ash.
Basic knowledge of hydrothermal venting processes at mid-ocean ridges, sufficient to recognize that hydrothermal vents dump sulfide-rich particles onto the newly-formed basaltic seafloor near spreading centers. We covered this in the lecture on manifestations of plate tectonics, and in This Dynamic Earth reading.
Basic knowledge of world bathymetry, sufficient to recognize that in 7500m of water depth, you must be drilling in a subduction zone trench. We did this through a lab in which students located plate boundaries from data sets of bathymetry/topography, earthquake locations, and volcano locations.
Basic understanding of scientific method, including some experience developing hypotheses, building chains of reasoning from observation to interpretation, and designing experiments to test hypotheses. We stress this repeatedly in our course.
The idea for this assessment question came from a classic paper:
Berger, W. H. and E. L. Winterer (1974). Plate Stratigraphy and the Fluctuating Carbonate Line. Pelagic Sediments: On Land and Under the Sea. K. J. Hsu and H. C. Jenkyns. Oxford, International Association of Sedimentologists: 11-98.
We gave 33 points maximum for this question.
Part A (maximum 27 points): "Develop a hypothesis…."
A well-documented hypothesis would have the following aspects or attributes:
Demonstrate understanding of the law of superposition; i.e. understand that the geological story begins at the bottom of the sequence of lithologies.
Unit 5: basalt results from volcanic eruption. Most likely setting for basalt found under a thick pile of marine sediments was at a spreading center. Hypothesis begins with formation of this piece of seafloor at mid-ocean ridge.
Unit 4 combines open marine biogenic sediment with sulfide deposits. A likely setting for this combination is near a mid-ocean ridge spreading center where hydrothermal vents are spewing sulfide-laden "black smoke" onto the seafloor.
Unit 3: This piece of seafloor then gradually moved away from its parent spreading center, and out of reach of hydrothermal deposits. On the upper flanks of the mid-ocean ridge, sedimentation was dominated by remains of calcareous planktonic organisms settling down through the water column to form foram-nanno ooze.
The calcareous sediments of unit 3 gave way to clay of unit 2 clay. This could reflect the subsidence of this piece of seafloor beneath the calcite compensation depth (CCD). Beneath the CCD, calcareous fossils tended to dissolve away before they could be incorporated into the sedimentary record, and all that was left was the very slowly depositing clay particles. Some of the clay in unit 2 could also indicate that the site had moved into range of the most distal reach of a terrigeneous clastic depositional system.
Volcanic ash layers of Unit 1, combined with the site's location in a bathymetric trench (7500m water depth) suggest that during the time of Unit 1 deposition, the site has moved closed to a subduction zone, where ash is being blown in from adjoining arc volcanoes.
The Unit 1 clastic sediments (the silts and muds and occasional sandy layers) are being shed into the trench from an adjoining continent, likely the overriding plate of a continent/ocean subduction zone.
This part of the question was scored to give 4 points for each of the bullet points above, up to a maximum of 27 points. Note that you didn't need to get every aspect of every bullet point correct to get full credit (the 8 bullet points could in theory have yielded 32 points of credit, but that level of interpretation was beyond what we were looking for. )
Other variants were possible, other than the exact story above. Credit was given for other sequences of events, provided that the events suggested were connected by a plausible and well-articulated process to the sediment types observed.
Part B (maximum 6 points): Design an experiment to test this hypothesis.
Whenever you see "design an experiment to test a hypothesis," a productive way to think about an answer is in three parts:
what predictions are made by this hypothesis?
what observations can I make that will support or refute one of these predictions?
if the hypothesis is true what should I see when I make those observations, and conversely if the hypothesis is false what might I see when I make those same observations?
The hypothesis from part 1 makes dozens of predictions. You only need to test one prediction in your experiment. Examples:
The hypothesis says that the transition between units 3 and 2 occurred because this piece of seafloor subsided beneath the calcite compensation depth. Look at the degree of preservation and dissolution of the microfossils and nannofossils within unit 3. If subsidence beneath the CCD were the reason why biogenic ooze stopped being the dominant sediment type at this site, then we would expect to see that the fossils were increasingly dissolved away and poorly preserved upsection approaching the contact between units 3 and 2. If this were not the reason, then we would expect that the fossils would be equally well-preserved (equally undissolved) throughout unit 3.
In scoring this part of the question, 2 points were given for a plausible plan of action, 2 points for detailing what observations you would make when you carried out the plan, and 2 points for explaining how the proposed observations would support or contradict the hypothesis.
DLESE resource created by Kim Kastens, Lamont-Doherty Earth Observatory, Palisades, NY 10964; firstname.lastname@example.org; October 2000.