GMAT Reading Comprehension

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Source: Princeton

Level: 3

The Mediterranean Sea sedimentary record reveals intermittent black sediment layers that seem to represent environmental changes over time. These layers, darkened by organic matter, rare in most ocean sediments, indicate the presence of a strong, physical control. Some geologists credit "reverse circulation," wherein waters flowing inward from the Atlantic Ocean are flushed back out in a reverse flow by the constant influx of fresh water from the several rivers that feed the sea. As surface waters evaporate, the heavily salted remnant, containing less dissolved oxygen, sinks. During intensely wet periods, soil nutrients fertilize the surface, phytoplankton production increases, and evaporation dissipates, creating an organically rich environment.

However, sedimentary records show bottom-dwellers coexisting with the dark layers were oxygen-stressed, so, reverse circulation could not have existed. Milankovitch Cycles, associated with earth-sun positional relationships, which change the dates of the equinoxes/solstices in 20,000 year cycles, are more likely. Climatic effects include intense rainy seasons depending on the earth's tilt, orbital eccentricity, and distance from the sun. Additionally, the 20,000 year cycles match the spacing of dark layers in the Mediterranean record.

Question List: 1 2 3

Which of the following can be inferred from the passage?

  • A Milankovitch cycles are affected by the size of the earth's orbit
  • B dark layers of sediment could indicate climate changes in history
  • C dark sedimentary layers are caused by the presence of inorganic matter
  • D Milankovitch cycles cause rainy seasons
  • E phytoplankton are present in the ocean only during wet weather cycles

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