| Popis: |
The geologic record provides constraints on the rates, amplitudes, and mechanisms controlling globally averaged (eustatic) and relative (eustatic plus subsidence/uplift) changes of sea level on various time scales. On geological time scales, global sea level changes are tied primarily to long-term (107–108-year scale) tectonism and short-term (103–106-year scale) changes in continental ice volume, though recent studies also illustrate the importance of tectonism on 106-year time scales. The history of 106-year scale eustatic changes has been controversial; the most widely used sea level curves agree with independently derived estimates with regard to the ages of sea level falls, but depart significantly from many more recent studies with regard to amplitudes. We present a 180-million-year history of sea level changes. A global sea level rise of 120 m followed the last glacial maximum, with rates that exceeded 10 times the modern rate of rise (> 40 mm yr-1 versus ~ 3 mm yr-1). The “ice ages” of the past 2.6 million years were due to growth/decay of large Northern Hemisphere ice sheets. Those of the past 780,000 years caused sea level changes that were large (> 100 m) and paced primarily by the ~100,000 year eccentricity cycle; smaller changes (typically < 60 m) prior to this time were paced primarily by the 41,000 tilt cycle. The growth and decay of a continental-scale ice sheet in Antarctica caused 50–60-m variations on the 106-year scale beginning ~ 33.5 million years ago. Prior to this time, Earth had been a warm, high-CO2 “greenhouse” world that was largely ice-free back to 260 million years ago, though recent evidence suggests that 15–25-m sea level changes may have been caused by the growth and decay of small, ephemeral continental ice sheets. |
