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Climate Variability on Millennial Time Scales. Introduction Dansgaard-Oeschger events Heinrich events Younger Dryas event Deglacial meltwater Meridional overturning circulation. Introduction.
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Climate Variability on Millennial Time Scales • Introduction • Dansgaard-Oeschger events • Heinrich events • Younger Dryas event • Deglacial meltwater • Meridional overturning circulation
Introduction • Some proxy records provide evidence of substantial climate variability at time scales that are considerable shorter than those of orbital forcing. • Typical time scales are O(103) years • Only certain natural archives have sufficient temporal resolution to record such variability.
Ice Core Paleoclimatology • As snow falls on very cold glaciers or ice sheets and gradually is converted to ice, air is trapped in bubbles. • This “fossil air” can be chemically analyzed to determine past atmospheric composition. • Other paleoclimatic proxies (isotopes, dust, acidity) can also be determined from the ice, providing information about temperature, sulfate aerosols, precipitation.
Dansgaard-Oeschger Events • Analysis of rapidly accumulating ice cores in Greenland yielded evidence of “rapid” shifts in isotopic composition. • Source: Dansgaard et al. (1982)
Glacials, Interglacials, Stadials and Interstadials • Glacials: Cold phases of 100-kyr cycles • Interglacials: Warm phases of 100-kyr cycles. • Stadials: Relatively cold periods during glacials. • Interstadials: Relatively warm periods during glacials.
stadial interstadial Glacials, Interglacials, Stadials and Interstadials Interglacial Glacial Interglacial
Methane Variations DuringD-O Events • Methane is regarded as an index of tropical wetland variations. • Methane covaries with isotopes in Greenland ice cores. • Source: Brook et al. (2000)
Global Extent? • Millennial-scale climate variations have been found in a number of records. • Most are in or near the North Atlantic region, but there is some evidence elsewhere.
Heinrich Events • Ice-rafted material appears in marine sediments in North Atlantic every several thousand years. • Events appear to be correlated with D-O events in Greenland ice cores. • Source: Bond and Lotti (1995)
A Heinrich Event Sediment Core • In this image, a Heinrich event is represented by the light-colored sediment in the bottom half of this core segment. • The black patches within the light-colored section is probably due to bioturbation, the mixing of sediments by living organisms such as deep sea worms.
Evidence of Heinrich Events Site withice-rafteddebris Site withoutice-rafteddebris
Possible effects of Heinrich events have been found outside of the region of ice-rafted debris. (Bard et al. 2000)
The relative magnitude of Heinrichand Dansgaard-Oeschger eventsvaries with location. Only Heinrich events are evidentin Fe/Ca (continental runoff proxy)from Brazilian Margin.
The Younger Dryas • During the last deglaciation, a dramatic climate “flip-flop” occurred in which the deglacial warming was interrupted by a return to near-glacial conditions. • This “flip-flop” in known as the Younger Dryas, deriving its name from a cold-loving plant species whose pollen reappeared during this interval.
Younger Dryas Climate Records Proxy for NorthAtlantic DeepWater formation
Cariaco Basin Bathymetry • Water exchange with the open Caribbean Sea is restricted • Intense seasonal productivity and high sedimentation rate • Anoxic below 300 m • Limited bioturbation (post-depositional mixing of sediments by marine life)
Deglacial Meltwater • As the Pleistocene ice sheets melted, meltwater collected in large postglacial lakes, such as Glacial Lake Agassiz. • As crustal rebound occurred, these lakes discahrged into the ocean.
Meridional Overturning Streamfunction Units of ψare Sverdrups; 1 Sv = 106 m3s-1
Temperature-Salinity Diagram • At low temperatures, salinity has a large effect on the density of sea water. • Higher salinity → more dense. • Lower salinity → less dense.
Younger Dryas Climate Records Meltwater RoutingHudson R./St. Lawrence R.Hudson Strait