![]() It is now for the first time possible to correlate and date short-term Cretaceous sea-level records with a resolution appropriate for their detailed analysis. Major international efforts such as EARTHTIME, EARTHTIME-EU and GTSnext programs are improving the Cretaceous time scale to yield a resolution comparable to that of younger Earth history. Recent refinements of the geological time scale using new radiometric dates and numerical calibration of bio-zonations, carbon and strontium isotope curves, paleomagnetic reversals, and astronomically calibrated time scales have made major advances for the Cretaceous. The mechanisms for these are controversial and include brief glacial episodes, storage and release of groundwater, regional tectonism and mantle-induced processes. However, superposed shorter-term, 3rd to 4th order (kyr to 100s of kyr), sea level changes are recorded in Cretaceous sedimentary sequences. ![]() The changing rates of ocean crust production led first to long-term sea-level rise, high stands, and then decline during Cretaceous times. 1st to 2nd order cycles occurring over millions to tens of millions of years, is controlled by the internal dynamic history of the Earth. Coastal cities, such as America's New York City, New York, and Mumbai, India, must also prepare for higher sea levels.The long-term sea-level record, i.e. Island nations, such as Maldives and Comoros, are particularly at risk. Sea-level rise threatens low-lying areas around the world. This phenomenon is called sea-level rise. Melting ice sheets cause an elevation in sea level. Global warming, the current period of climate change on Earth, is causing glaciers and ice sheets to melt. At the peak of the most recent ice age, about 18,000 years ago, sea level was perhaps 100 meters (300 feet) lower than it is today. During past ice ages, sea level was much lower because the climate was colder and more water was frozen in glaciers and ice sheets. Changes in Sea Level Sea level may vary with changes in climate. Local mean sea-level measurements are a combination of sea-level variations and movement of the land. That same land bounces back (a process called postglacial rebound) if the ice retreats, or melts, and its weight is removed. For example, the tremendous weight of a glacier on land pushes the land down, closer to sea level. Just like the ocean, the elevation of land may rise and fall over time. Sea level is measured in relation to the adjacent land. It also averages out the effects of local weather and oceanographic conditions. It enables scientists to account for the long-term variations in the moon's orbit. The 19-year period is called a Metonic cycle. ![]() In the United States and its territories, local mean sea level is determined by taking hourly measurements of sea levels over a period of 19 years at various locations, and then averaging all of the measurements. So the surface of the ocean can be used as a base for measuring elevations, the concept of " local mean sea level" has been developed. However, winds, currents, river discharges, and variations in gravity and temperature prevent the sea surface from being truly level. Because the ocean is one continuous body of water, its surface tends to seek the same level throughout the world. Sea level is the base level for measuring elevation and depth on Earth. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |