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.
7. According to Figure 3, at Sites 1, 2, and 3, the smallest δ18O value of the groundwater in the lake clay was recorded at a depth between:
Your Answer is
Correct Answer is C
Explanation
Look at figure 3. In the three pictures, δ18O has the smallest value on the left, and gradually increases to the right. The vertical coordinate depth corresponding to the smallest value is about 20 m-30 m.< /p>