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Difficulty: HardProcedural Steps

To reconstruct past climates using tree rings, researchers follow a meticulous multi-step preparation and analysis protocol. First, core samples are extracted from living trees using an increment borer. Before mounting the fragile cores, they must be slowly air-dried in protective paper straws; attempting to mount wet cores leads to structural warping. Once dry, the cores are glued into wooden support channels, ensuring the tracheid cells run vertically to optimize the surface for sanding. The mounted cores are then sanded using progressively finer sandpaper grits—typically starting at 120-grit to level the surface, transitioning to 220-grit, and finishing with 400-grit to polish the cell walls. Prior to cellular measurement under a microscope, any residual sawdust must be cleared from the ring boundaries using compressed air. Measuring uncleaned rings leads to significant errors, as dust particles can mimic cell walls. Subsequently, researchers utilize skeleton plotting to cross-date the samples against a master chronology, aligning narrow rings that represent dry years. Only after cross-dating is complete can researchers confidently input the raw ring-width measurements into standardization software to remove age-related growth trends.

According to the passage, at what point in the preparation and analysis process must researchers clear residual sawdust from the ring boundaries using compressed air?

  1. A
    Immediately after the dried cores are glued into the wooden support channels but before sanding begins
  2. B
    Prior to mounting the dried cores in the wooden support channels but after they are extracted
  3. After the wood cores have been sanded with the finest grit but before the cells are measured under a microscopeAnswer
  4. D
    After the ring widths are measured under a microscope but before they are cross-dated against a master chronology

Answer

The clearing of residual sawdust must take place after the wood cores have been sanded with the finest grit but before the cells are measured under a microscope.
The passage states that prior to cellular measurement under a microscope, any residual sawdust must be cleared from the ring boundaries. This step immediately follows the sanding process, which concludes with 400-grit sandpaper (the finest grit mentioned). Therefore, clearing the sawdust must occur after the wood cores have been sanded with the finest grit but before the cells are measured under a microscope.

Step-by-Step Solution

1
Locate where the step of clearing residual sawdust is mentioned in the passage.
The passage states that 'Prior to cellular measurement under a microscope, any residual sawdust must be cleared from the ring boundaries using compressed air.'
This establishes that the clearing step must occur before cellular measurement under the microscope.
2
Identify the step that immediately precedes the clearing of sawdust.
The passage describes that the cores are 'sanded using progressively finer sandpaper grits—typically starting at 120-grit... and finishing with 400-grit to polish the cell walls' immediately before mentioning the clearing of residual sawdust.
This establishes that the clearing of sawdust occurs after the sanding process is completed with the finest grit (400-grit).
3
Synthesize the two findings to determine the correct window for the clearing step.
Clearing the sawdust must occur after the wood cores have been sanded with the finest grit but before the cells are measured under a microscope.
This matches the sequence of operations outlined in the passage.

Key Concept

Identifying the correct chronological sequence and prerequisites of steps within a complex technical process.
Estimated Time:2m 0s
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Procedural Steps

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