题目材料
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.
The author is primarily concerned with
- Aanalyzing the source of a geological controversy
- Bdescribing a geological phenomenon
- Ccriticizing a scientific theory
- Dproposing a new theory to explain existing scientific evidence
- Epointing out the flaws in a geological study
显示答案
正确答案: D