EKSAMEN i GEO-3121, Marin geologi. Du kan bruke linjal, kalkulator og ordbok! You are allowed to use a ruler, a calculator, and a dictionary!

Transcription

1 EKSAMEN i GEO-3121, Marin geologi Tirsdag 13. desember, 2005, klokka Asg5rdveien 9 Du kan bruke linjal, kalkulator og ordbok! You are allowed to use a ruler, a calculator, and a dictionary! f-' Du har fire oppgaver & lsse og for hver oppgave kan du f& 20 poeng, totalt 80 poeng. You have four major tasks and for each task you can reach 20 points, and a total of 80 points. Ved eventuelle sporsmil ringer varamann Stefan Buenz tel ($5266 Lykke ti]! Juergen Mienert

2 Task 1 : Sea floor spreading Sea floor spreading is one component of the theory of plate tectonics. According to this concept an ocean basin can develop. Procedures: A map of Iceland and a portion of the Norwegian-Greenland sea region are provided (Fig.1). On either side of the ocean ridge are stripes of oceanic crust labelled from chron 0 (ridge) to chron 24 (margin). The distance between Point A and Point B, which has approx. the width of Iceland is ca. 400 kilometres (km). A: Determine the ages (polarity boundary from black to white) in millions of years using the geomagnetic polarity time scale (Fig. 2) Chron 5: Age- Chron6: Age- Chron 24: Age- Ma Ma Ma B. Calculate the half-rate of sea floor spreading, the velocity at which one strip of this rock has spread away from the ocean ridge for Chron 6. (Note: Rate of spreading = distance of chron from ocean ridge crest divided by time of polarity change) Chron 6 (Ma): half spreading rate = krn/ma and cmlyear C. Calculate the total rate of seafloor spreading, and answer whether this is a fast or a slow spreading ridge. Chron 6 (Ma): total spreading rate = - kmma and, cmlyear D. What are the geological periods and the age during which (i) the Nonvegian- Greenland Sea and (ii) Iceland began to form? Use the geomagnetic polarity time scale (Fig. 2) and Fig. 1.

3 Figure 1: TOP figure - Age of spreading shown along line A-B indicated by magnetic isochrones. 'Ihe approximate distance between point A and B is 400 km. BOTTOM figure - Sketch of the mid-ocean ridge showing Iceland an the direction of the NWh American and Eurasian Plate movement.

4 The magnetic reversal timescale *8 Ml MI 81, Yn ilii Uf9 M11 MI, MU MI* ma0 111 YII Mi3 U14 ltl Yt7 )te the long riod of irmal polarity in the Cretaceous: from -83 to 135 Ma you have no reversals to work with!: a: Figure 2: Geomagnetic polarity timescale. Note: the age of chron 5 is -1 1 Ma.

5 Task 2: Ocean basins, mid-ocean ridges and sealevel You will deduce that if sea floor is created at mid-ocean ridges, it must be destroyed elsewhere, or the Earth would be expanding. A digital map showing isochrones of ocean basins (Figure 3) allows you to identify areas of subduction and creation of sea floor. A: Does the Atlantic or the Pacific Ocean show major sea floor subduction along its plate boundaries? Mark the subduction zones on the map in Figure 3. B: Would the sea level rise or fall if all ocean ridges today would become fast spreading ridges? Explain your answer. C: Is the seafloor spreading rate increasing or decreasing towards the pole of rotation? Where is the pole of rotation for the Nansen basin in the Arctic Ocean (Figure 4). Mark it on the map in Fig. 4. D: Which of the ocean ridges in Figure 5 are fast and which are slow spreading ridges? Explain your answer.

6 I Figure 5: Ocean ridges showing various spreading rates

7 Task 3: Ocean water mass circulation A: What drives the global circulation of surface and deep-water masses? How much water moves the Gulfstram poleward on a daily scale if you assume a flux of 50 Sv, and how much water exchange has a fjord on a daily scale if you assume 0.2Sv inflow? Note that 1Sv = 10 m s B: How can the density of surface water and thus water mass sinking processes in the oceans to be increased in (a) the Greenland Sea and (h) the Mediterranean Seas? C: Draw a schematic diagram for a fjord circulation and the Mediterranean circulation and explain the major differences. D: Explain the effect of cyclonic and anticyclonic winds in the northern and southern hemisphere on a) the shape of the sea-surface and the depth of the thermocline, and h) ocean biogenic productivity.

8 Figure 6: Surface ocean circulation showing current directions and major gyres. Figure 7: Siliceous and calcareous zooplankton and phytoplankton.

9 Task 4: Sediments in ocean basins A. What controls the location of high biogenic productivity along the ocean margins? Make a sketch of wind systems and water mass movements for the direction of the Ekman transport off NW and SW Africa. Indicate areas of high siliceous concentrations in Figure 6 and identify the siliceous phytoplankton and zooplankton in figure 7. B. What controls the sedimentation rate and distribution of terrigenous and biogenic sediments in the Norwegian-Greenland Sea during glacial and interglacial times? Indicate the areas with increased silica and carbonate concentrations for the Norwegian-Greenland Sea during interglacial times in Figure 6. C. Explain - using a sketch -the solubility of carbonate and silica changes with depth and temperature? Do you expect to find higher carbonate concentrations in water depth shallower or deeper than 4000 m? Use Figure 8 to explain and to determine the depth of the carbonate compensation depth (CCD)! D. A sedimentary sequence drilled in the eastern Mediterranean basin shows fkom bottom to top: carbonates, anhydrite, gypsum, halite, terrigenous sediments, carbonate, anhydrite, gypsum, and halite. Explain the conditions of basin water mass inflow and climate using the sedimentary sequence.

10 carbonate ion concefltration [GO:-] (mat m-3} * I I C Atlantic E - t8" S, 3i0W 5 n w carbonate ion concentration [co~;] (lo6 moi T') Figure 8: Laboratory derived saturation curves of ~03~- concentration versus depth for calcite and aragonite (solid lines). Dots present a profile of measured and calculated values for a station in the Atlantic.