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ted between 11,700 and 9,500 years (see Figure 1). The lake was formed ammed several rivers. Groundwater cial sediments provides information e time the sediments were deposited. s section of the sediments (lake clay drock in the area. Figure 3 shows the water taken from samples of the top sites along the same cross section. a ratio of 2 oxygen isotopes ( 18 O water. Smaller δ 18 Lake Agassiz existed between 11,700 and 9,500 years ago in North America (see Figure 1). The lake was formed when a large glacier dammed several rivers. Groundwater trapped in lake and glacial sediments provides information about the climate at the time the sediments were deposited. Figure 2 shows a cross section of the sediments (lake clay and glacial till) and bedrock in the area. Figure 3 shows the δ Manitoba maximum extent of Lake Agassiz Site 1 • Site 3 • • O values indicate North • tures. Dakota Grand Forks 18 O values of groundwater taken from samples of the top 40 m of sediment at 3 sites along the same cross section. δ 18 O is calculated from a ratio of 2 oxygen isotopes ( 18 O and 16 O) in the groundwater. Smaller δ 18 O values indicate cooler average temperatures. surface Site 1 Site 2 elevation (m above sea level) Site 3 Winnipeg, Grand Forks, N Manitoba 200 North Dakota S ACT-64E-PRACTICE 250 150 Figure 2 44 surface 250 200 150 Figure 1 Hudson Bay Winnipeg Site 2 Great Lakes Key sediment/rock Figure 2 44 lake clay glacial till bedrock Site 1 Site 3 North Dakota Winnipeg, Grand Forks, N Manitoba North Dakota S surface Site 1 Site 2 Site 3 surface Manitoba 4 maximum extent of Lake Agassiz 250 200 150 • • • Grand Forks GO ON TO THE NEXT PAGE. Figure 1 Hudson Bay Winnipeg Site 2 Great Lakes Key sediment/rock lake clay glacial till bedrock GO ON TO THE NEXT PAGE.
4 4 δ –26 (surface) 0 –22 –18 –14 18 O δ –26 (surface) 0 –22 –18 –14 –26 (surface) 0 –22 –18 –14 18 O δ 18 O smaller larger smaller larger smaller larger depth (m) 6. According to Figure 2, the lake clay deposit is thinnest 9. According to Figure 2, which of the following graphs 6. According at which of to the Figure following 2, the cities lake or clay sites? deposit is thinnest at which best represents of the following the elevations, cities in m or above sites? sea level, of F. Winnipeg the top of the glacial till layer at Sites 1, 2, and 3 ? G. Site 11 H. Site 2 H. J. Grand Site 2 Forks J. Grand Forks A. 225 200 C. 225 200 Answer: F. In7. Figure According 2, to the Figure key 3, at shows Sites 1, 2, that and 3, lake the smallest clay is indicated with light gray. The vertical axis δ of the figure is labeled “elevation (m above sea level). This tells us, for example, that the Winnipeg site has lake clay between 230 m and 240 m above sea level. In other words, the lake clay is 240-‐230 = 10 m “thick” at the Winnipeg site. Looking at the figure, we can see that the lake clay (light gray area) is thinnest at the Winnipeg site. 18 O value of the groundwater in the lake clay was B. 225 D. 225 recorded at a depth between: A. 0 m and 10 m. 200 200 B. 10 m and 20 m. C. 20 m and 30 m. D. 30 m and 40 m. 175 175 7. According to Figure 3, at Sites 1, 2, and 3, the smallest δ 18 Ο value of the groundwater in the lake clay was recorded at a depth between: A. 0 m and 10 m. 10. Precipitation that falls at Sites 1, 2, and 3 soaks into the soil until it reaches the groundwater table about 8. B. According 10mand20m. to Figure 2, as the thickness of the lake clay deposit increases from Grand Forks to Site 3, the C. thickness 20mand30m. of the glacial till beneath it: 3 m below the surface. Based on Figure 3, and assuming no alteration of the precipitation, the δ D. F. increases. 30mand40m. G. remains the same. H. first increases and then decreases. J. decreases. 18 150 1 2 3 150 1 2 3 Site Site O value of present-day precipitation in the study area is closest to: F. −26. G. −23. H. −20. J. −15. Answer: C. 10 20 30 40 Site 1 lake clay glacial till depth (m) 10 20 30 40 Figure 3 Figures adapted from V. H. Remenda, J. A. Cherry, and T. W. D. Edwards, “Isotopic Composition of Old Ground Water from Lake Agassiz: Implications for Late Pleistocene Climate.” ©1994 by the American Association for the Advancement of Science. In Figure 3, the smallest δ18O values are farthest to the left and have values between -‐26 and -‐14. At Site 1, the smallest δ18O value recorded is a little less than -‐25 at a depth of around 27 m. At Site 2, the smallest δ18O value recorded is around -‐25 at a depth of around 27 m or 28 m. At Site 3, the smallest δ18 ACT-64E-PRACTICE GO ON TO THE NEXT PAGE. 45 O value recorded is a little less than -‐25 at a depth of around 28 m or 29 m. So the answer is C. 8. According to Figure 2, as the thickness of the lake clay deposit increases from Grand Forks to Site 3, the thickness of the glacial till beneath it: Site 2 lake clay glacial till elevation (m above sea level) elevation (m above sea level) 175 150 1 depth (m) Note: δ 18 O = – 1 × 1,000 18 O/ 16 O of groundwater sample ___________________________ 18 O/ 16 O of standard water sample 2 Site 3 10 20 30 40 elevation (m above sea level) elevation (m above sea level) Site 3 175 150 lake clay 1 2 Site 3
Page 1 and 2: SCIENCE TEST This document contains
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Page 5: C. nonzero, and the same as the amp
Page 9 and 10: G. −23. H. −20. J. −15. Answe
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