In paleolimnology, the ratio of titanium to calcium () in sediment cores is often used as a proxy for terrestrial runoff, as titanium is transported from soil erosion while calcium typically precipitates from lake water during dry periods. However, geochemist Maya Lin noted that in lakes surrounded by limestone bedrock, which is rich in calcium carbonate, intense rainfall dissolves the surrounding limestone. This dissolved calcium then enters the lake and precipitates rapidly. Consequently, in lakes with limestone catchment basins, periods of high rainfall and subsequent runoff will likely
Which choice most logically completes the text?
- Aresult in a decrease in the absolute amount of titanium deposited in the sediment cores.
- fail to produce the elevated sediment ratios that typically indicate increased runoff.Answer
- Conly occur during periods of prolonged drought when lake water calcium concentration reaches its peak.
- Dcompel researchers to rely on alternative indicators, such as boron isotopes, to measure runoff in all freshwater lakes.
Answer
The correct option is the one stating that runoff will likely fail to produce the elevated sediment ratios that typically indicate increased runoff.
The passage establishes that a high titanium-to-calcium () ratio typically indicates wet periods because titanium increases with runoff while calcium precipitates during dry periods. However, in lakes with limestone bedrock, intense rainfall causes limestone to dissolve, adding calcium to the runoff. Since both titanium and calcium are deposited during high rainfall, the ratio of titanium to calcium in these sediment cores will not rise as it normally would, meaning wet periods will not produce the expected high ratios.
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