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Difficulty: HardWeaken

Marine Geoarchaeologist: Deep sediment cores extracted from the silted ancient harbor of Portus Albus reveal a dramatic, threefold increase in coarse sand and marine shell fragments dating between 420 CE and 450 CE, accompanied by a sharp decline in ballast stones dropped by merchant ships. Because coarse sand and shell fragments are typically swept into sheltered harbors only by catastrophic marine inundations, such as tsunamis, the harbor's sudden decline as a commercial maritime center during this period must have been initiated by a major tsunami event rather than economic migration.

Which of the following statements, if true, most seriously undermine the geoarchaeologist's argument? (Select all that apply)

  1. Commercial trade records and tax ledgers indicate that merchant fleets permanently redirected their operations to a deeper neighboring port in 410 CE, well before the initial deposition of the coarse sand layer.Answer
  2. B
    Excavations at nearby coastal settlements along the same bay revealed identical strata of coarse sand and marine shells dating to the mid-fifth century.
  3. Municipal records from 425 CE document the deliberate dumping of thousands of tons of coastal sand and crushed shell aggregate to reinforce the harbor's decaying earthen seawalls.Answer
  4. D
    Routine daily tidal currents and seasonal winds regularly sweep coarse sand and large marine shell fragments into naturally sheltered harbor basins.
  5. E
    The ballast stones found in earlier sediment layers at the harbor consisted of granite rocks not native to the local coastline.

Answer

The argument is undermined both by evidence establishing that economic abandonment preceded the sediment deposition and by evidence providing an alternative explanation for the presence of the coarse sand and shell aggregate.
The argument concludes that a tsunami initiated the port's commercial decline based on coarse sediment layers and declining ballast stones. The choice establishing that merchant fleets had already permanently moved elsewhere in 410 CE breaks the timeline, proving the abandonment began prior to the sediment event. The choice documenting the deliberate dumping of sand and shell aggregate to reinforce seawalls provides an alternate explanation for the sediment layer, eliminating the need for a tsunami hypothesis.

Step-by-Step Solution

1
Analyze the argument's structure and core causal claim.
The premises state that sediment cores show increased coarse sand/shells and decreased ballast stones between 420 CE and 450 CE, and that coarse sediment enters sheltered harbors only via extreme marine events like tsunamis. The author concludes that a tsunami event, rather than economic migration, initiated the harbor's commercial decline.
Isolating the conclusion and underlying causal assumptions is required before identifying potential weaknesses.
2
Evaluate unstated assumptions and vulnerability points in the causal reasoning.
The author assumes that (1) the coarse sediment must have been deposited by a natural disaster rather than artificial human construction, and (2) the timing of the decline aligns chronologically with the sediment deposition.
Attacking either the assumed cause (providing an alternate cause) or the assumed chronological sequence weakens a causal conclusion.
3
Assess the options against the weaken criteria.
The statement showing that merchant activity ceased in 410 CE demonstrates that economic abandonment preceded the sediment event, breaking the causal timeline. The statement showing deliberate seawall reinforcement provides a plausible alternative explanation for the sediment layer.
Both statements independently undermine the author's conclusion that a tsunami initiated the commercial abandonment.

Key Concept

Undermining causal arguments via chronological inconsistency and alternative explanations
Estimated Time:2m 0s
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