Holocene Interglacial Learning Materials

HOLOCENE INTERGLACIAL Perhaps one of the most dramatic climate changes in geological history occurred during the current interglacial period known as the Holocene epoch. After about 100,000 years of gradual accumulation of snow and ice up to 2 miles and thicker, glaciers melt in just a few thousand years, retreating at a rate of several hundred feet per year. About a third of the ice melted between 16,000 and 12,000 years ago, when average global temperatures increased by about 5 degrees Celsius to almost current levels. The renewal of the deep-sea circulation system, which was shut down or very weak during the Ice Age, may have thawed the planet from deep freezing. A giant ice dam on the border between Idaho and Montana holds up a huge lake hundreds of miles wide and up to 2,000 feet deep. About 13,000 years ago, a sudden dam explosion sent water pouring into the Pacific Ocean.Along the way, floodwaters carved out one of the most bizarre landscapes known, called the Channel Scablands (Figure 195). Lake Agassiz, a large reservoir of meltwater formed in bedrock depressions carved by glaciers, sits on the edge of a retreating ice sheet in southern Manitoba, Canada. As the North American ice sheet retreats, its melt flows down the Mississippi River into the Gulf of Mexico.After the ice sheet subsides beyond the Great Lakes, the melt takes an alternative route down the St. Lawrence River.Cold water enters the North Atlantic Ocean. Simultaneously, Niagara River Falls began cutting through its canyon and has traversed more than 5 miles to the north since the ice sheet melted. The rapid melting of glaciers culminated in the extinction of microscopic organisms called foraminifera (Fig. 196). They met their end when bursts of meltwater and icebergs spilled into the North Atlantic.Major floods formed a cold freshwater cap in the oceans that significantly changed the salinity of seawater. Cold water also blocks warm currents flowing to the poles from the tropics, causing land temperatures to drop to near ice age levels. The ice sheet appears to have stopped mid-step between 13,000 and 11,500 years ago. The period was called the Younger Dryas, named after the Arctic wildflower, a cold-tolerant plant that grows in Europe.The climate returned to near-glacial conditions amid the transition from the Ice Age to the interglacial. After that, the warm current returns. The warming remains permanent, prompting a second episode of melting that caused the current volume of ice about 6,000 years ago. After the ice sheet receded, plants and animals began to return to northern latitudes. As the ice sheet melted, massive flooding raged across the country as water gushed from reservoirs trapped beneath the glacier. As it flows beneath the ice, water surges in vast turbulent sheets that explore deep grooves in the Earth's crust, forming steep ridges carved out of solid bedrock. Each flood continues until the weight of the ice sheet shuts off the outlet of the reservoir. When the water pressure accumulates again, a huge wave of meltwater gushes from under the glacier and rushes towards the sea. A huge torrent of sediment-laden meltwater surged along the Mississippi River toward the Gulf of Mexico, expanding the channel to several times its current size. Many other rivers extend beyond their banks to carve out new floodplains. This period of unusually warm and wet conditions began 6,000 years ago and lasted for 2,000 years. When Climate Optimum opened, many regions of the world warmed by an average of about 5 degrees Celsius. Melting ice sheets released massive floodwaters into the ocean, raising sea levels 300 feet higher than when the Holocene began. The inland sea is filled with sediment. The subsequent uplift expelled the water, leaving behind a salt lake. Utah's Great Salt Lake today is just the remains of a vast inland sea. During a long wet period between 12,000 and 6,000 years ago, it expanded to several times its current size and flooded the nearby salt flats.