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(10 3 ) 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 During D-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 with ice-rafted debris Site without ice-rafted debris

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 Heinrich and Dansgaard-Oeschger events varies with location. Only Heinrich events are evident in 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 North Atlantic Deep Water 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 Dryas in the Amazon Basin

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 Units of ψ are Sverdrups; 1 Sv = 10 6 m 3 s -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 Routing Hudson R./St. Lawrence R. Hudson Strait

Meltwater Routing Oscillator Clark et al. (2001)