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.
GISP2 Drilling Project
Extracting An Ice Core
Annual Layers In Ice Core
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.
Glacials, Interglacials, Stadials and Interstadials
stadial
interstadial
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
Heinrich Event
Chronology
• Possible effects of Heinrich events have been found outside of the region of ice-rafted debris. (Bard et al. 2000)
Heinrich Events in Florida?
The relative magnitude of Heinrichand Dansgaard-Oeschger eventsvaries with location.
Only Heinrich events are evidentin Fe/Ca (continental runoff proxy)from Brazilian Margin.
A Pervasive 1500-Year Climate Cycle?
Source: Bond et al. (1997)
Source: deMenocal et al. (2000)
Solar Forcing of 1500-Year Cycle?
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.
Schematic Deglaciation History
Younger Dryas Climate Records
Greenland Accumulation Rates
Younger Dryas Climate Records
Proxy for NorthAtlantic DeepWater formation
Cariaco Basin (Venezuela)
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)
The Younger Dryas in the
Cariaco Basin
The Younger Dryasin the AmazonBasin
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.
Meltwater Discharge Paths
Barbados Sea Level Changes
Schematic of Meridional Overturning Circulation
Meridional Overturning Streamfunction
meanzonal
zv
yw
Units of ψ are 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.
Meltwater Discharge Paths
Younger Dryas Climate Records
Meltwater RoutingHudson R./St. Lawrence R.Hudson Strait
Meltwater Routing OscillatorClark et al. (2001)