Passage II
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 δ18O values of groundwater taken from samples of the top 40 m of sediment at 3 sites along the same cross section. δ18O is calculated from a ratio of 2 oxygen isotopes (18O and 16O) in the groundwater. Smaller δ18O values indicate cooler average temperatures.
10. Precipitation that falls at Sites 1, 2, and 3 soaks into the soil until it reaches the groundwater table about 3 m below the surface. Based on Figure 3, and assuming no alteration of the precipitation, the δ18O value of present-day precipitation in the study area is closest to:
Your Answer is
Correct Answer is J
Explanation
Look at figure 3, when the vertical coordinate depth is about 3m, the horizontal coordinate δ18O is about -15