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Difficulty: Very hardLogical Inferences

Foraminifera shell boron-to-calcium (B/CaB/Ca) ratios are widely used to reconstruct ancient ocean carbonate ion concentrations ([CO32−][CO_3^{2-}]). However, recent experiments show that boron incorporation is also influenced by dissolved inorganic carbon (DICDIC) levels, which affect calcification rates. When [CO32−][CO_3^{2-}] is constant, higher DICDIC accelerates calcification and increases shell B/CaB/Ca, while lower DICDIC suppresses calcification and decreases B/CaB/Ca. Thus, using B/CaB/Ca ratios to estimate historical [CO32−][CO_3^{2-}] without adjusting for past DICDIC variations could lead to errors because ______

Which choice most logically completes the text?

  1. A
    historical [CO32−][CO_3^{2-}] concentrations were consistently higher in regions of the ancient ocean where DICDIC levels were lowest.
  2. changes in paleoclimatic DICDIC can alter shell B/CaB/Ca ratios in a manner that leads researchers to misidentify the corresponding [CO32−][CO_3^{2-}] concentrations.Answer
  3. C
    decreases in shell B/CaB/Ca ratios are directly caused by increases in the rate of calcification rather than by variations in DICDIC levels.
  4. D
    calcification rates of benthic foraminifera accelerate when [CO32−][CO_3^{2-}] is held constant while DICDIC is increased.

Answer

changes in paleoclimatic DICDIC can alter shell B/CaB/Ca ratios in a manner that leads researchers to misidentify the corresponding [CO32−][CO_3^{2-}] concentrations.
The correct option is supported because the passage explains that dissolved inorganic carbon (DICDIC) levels influence calcification rates and shell B/CaB/Ca ratios even when carbonate ion concentrations ([CO32−][CO_3^{2-}]) are constant. Consequently, an observer who assumes that shell B/CaB/Ca is determined solely by [CO32−][CO_3^{2-}] would misattribute DICDIC-driven shifts in B/CaB/Ca to changes in [CO32−][CO_3^{2-}].

Step-by-Step Solution

1
Analyze the established proxy relationship in the text.
The text states that shell B/CaB/Ca ratios are used as a proxy to estimate ancient ocean [CO32−][CO_3^{2-}] concentrations.
This establishes the basic assumption of the reconstructive models before introducing the complicating variable.
2
Identify the confounding variable and its effect.
Dissolved inorganic carbon (DICDIC) levels affect calcification rates, which in turn affect shell B/CaB/Ca ratios even when [CO32−][CO_3^{2-}] is held constant.
This shows that shell B/CaB/Ca ratios can change due to DICDIC independent of [CO32−][CO_3^{2-}].
3
Determine the consequence of failing to account for this confounding variable.
If reconstructive models do not adjust for DICDIC variations, they might misinterpret a change in shell B/CaB/Ca (caused by DICDIC) as a change in [CO32−][CO_3^{2-}].
This provides the logical link to explain why ignoring DICDIC variations leads to errors in reconstructing historical [CO32−][CO_3^{2-}].

Key Concept

Logical Inferences
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Logical Inferences

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