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)