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题目材料：
Because they accumulate in an ordered manner that records changes within a lake and its drainage basin, lake sediments are particularly valuable for reconstructing ancient climate conditions. Natural materials such as pollen grains, minerals, diatoms, and animal microfossils preserved in sediments can be used to infer past conditions as these materials are affected by environmental variables, Pollen grains composed of sporopollenin, for example, can be used to infer past vegetation shifts. Because both natural climate change and human agricultural practices can lead to changes in vegetation, however, differentiation of these effects using pollen alone is difficult, in many areas of Mesoamerica, for instance, changes in the relative abundance of pollen types after 1000 B.C. were highly influenced by human-induced land clearance.
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