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The maximum glaciation of the late Wisconsinan stage of the Pleistocene Epochoccurredabout20,000–22,000yearsago.Astheseicesheetsmelted, sea level in the Gulf of Mexico and throughout the world began to rise from a low of about -130 m relative to its present position (Fig. 4-1).The rate and direction of sea-level change from initiation of the rise 18,000 years ago until the present was quite complicated. In fact, the position of the maximum lowstand of sea level was different in various parts of the world. Some data show that in the Gulf of Mexico sea level was not as low at this time as it was in much of the rest of the world.This is likely due to the combination of subsidence caused by the mass of the ice on North America and the compaction of the sediments associated with the Mississippi Embayment. It is appropriate at this point in the discussion to consider the time scales. The time over which sea level has risen in the Gulf of Mexico from this all-time low to its present level began before people migrated into North America. In fact, this time of low sea level provided the conditions under which it was possible for people to cross from Asia to Alaska about 15,000 years ago across the Beringia land bridge. They eventually migrated down across the North American continent and probably first reached the Gulf of Mexico about 12,000 years ago. The rapid sea-level rise that took place at the end of the last glaciation caused the shoreline to move rather quickly across the continental shelf. The early portion of this period of sea-level rise was the latest Pleistocene and the rest occurred in the Holocene Epoch.The Holocene is a time period namedbypaleontologists,scientistswhostudyfossils.Theyhavedesignated the period from 10,000 years to the present as Holocene because all remains of organisms older than that are considered fossils.This is an entirely artificial designation and has no significance in terms of earth processes or the genesis of geologic features.This explains why this trend in sea-level rise is continuous without regard for the Pleistocene-Holocene boundary. 4 Melting Ice Sheets and Sea-Level Rise 66 CHAPTER 4 Figure 4-1. Diagrammatic map of the reconstruction of the surface of the continental shelf at the beginning of the last period of sea level rise about 18,000 years ago. From U.S. Environmental Protection Agency, There are thousands of radiometric dates, primarily carbon-14 (C14 ), from a wide variety of samples and locations across the continental shelf of the Gulf of Mexico. Typically these dates are from salt marsh and mangrove peats, oysters, the coquina clam (Donax), and beachrock. All were originally at or very near sea level when they were alive.These data provide informationthatcanbeplottedastimeanddepthparameterstoconstructa sea-level curve. Obviously each curve, regardless of where it is applied, has a scatter of data points.This scatter is partly due to location during the time the biogenic material was alive, the fact that it might have been moved a bit, the date determined has some error, and there might have been some contamination of the material. Nevertheless, it is possible to construct a curve that accurately shows the trend in sea-level rise as the ice sheets melted. We can make a general summary. There was an overall rapid rise of nearly a centimeter per year from 18,000 years ago until about 6000– 7000 years ago. There were numerous perturbations during this time including short periods of sea-level fall. The overall average rate of sea-level rise then slowed to about 2 mm per year until about 3500 years ago. From that time until the present there are multiple possibilities and they may change from one coastal region to another: 1) sea level may have reached its present position by ~3500 years ago and remained essentially stable; 67 MELTING ICE SHEETS Figure 4-2. Sea level curve that shows the three distinct periods and rates of rise since the time that extensive glacial melting began. From Toscano, M. A. and I. G. MacIntyre, 2003. Corrected western Atlantic sea-level curve for the last 11,000 years based on calibrated dates 14 C dates from Acropora palmata framework and intertidal mangrove peat. Coral Reefs, 22:257–70. 2) sea level may have been rising very slowly from 3500 years ago to the present with few significant changes in rate or direction...


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