Lecture 19

Stratigraphy of climate change

The predominant power in this spectrum is at about 100,000, 41,000 and 19 23,000 years

from Alley, 2000 The Milankovitch hypothesis: climate change results from changes in Earth’s orbital parameters

Barbados

Today we’ll look at examples of climate changes seen in the stratigraphic record that are NOT controlled by orbital parameters

Orbital Forcing – Ice Cores

• Ice core  18 O records temperature • Orbital frequencies are clearly dominant, but higher frequencies are present

• Discovered in 1988 in marine sediment cores • Recognized as distinct layers with significant increase in lithic fragments, and large clasts in some areas

Heinrich Events

Heinrich Layer Isopachs • Double maxima in isopachs • Layers thicken to NW into Labrador Sea

• Black areas are regions with large carbonate deposits • Sediment must have been ice rafted

Source of Heinrich events

Heinrich Layer 4 (~40,000 years BP)

 18 O in polar planktonic foraminifera Modeling shows about 250-year duration and 2-m rise in sea level Nature, Roche et al., 2004

Modeling shows about 250-year duration and 2-m rise in sea level Nature, Roche et al., 2004

Heinrich Events in context

• Occur at times of coldest weather in N. Atlantic • Followed by a sharp warming • No clear periodicity Bond 1993 figure

Summary of Heinrich events

• Effects are global – signature of Heinrich events has been found around the world • Massive discharge of ice into N. Atlantic from the Laurentide ice sheet is well established • No clear explanation for such dynamics in the ice sheet

Dansgaard-Oeschger Events

• Characterized by rapid warming in the N. Atlantic, followed by slower cooling • Quasi-Periodic, with a timescale of ~1400 years • Recorded by diverse climate proxies • Evidence for global climatic effects (Data From ftp://ftp.ngdc.noaa.gov/paleo/icecore/greenland/summit/grip/isotopes/gripd18o.txt)

Greenland and D-O events

• Within the Greenland ice cores, several independent variables all show D-O events prominently   18 O – Temperature – Ca/Dust concentrations – varying weather in Asia?

– Na/Cl concentrations – increased storminess in N. Atlantic – Etc…

Other Evidence of D-O events

• Sediment cores from the Santa Barbara Basin (Hendy and Kennet, 1999)

Other Evidence of D-O events

• Stalagmites from Eastern China (Wang et al., 2001)

Global Map of D-O records

http://www2.ocean.washington.edu/oc540/lec01-31/

Theories for rapid climate change

• Heinrich, D-O periods are too rapid for orbital frequencies • Some combination of the ocean/atmosphere/cryosphere must be responsible • Need a source with enough power to affect global climate

Atlantic Circulation

•Deep Water is formed at the Northern and Southern extents of the Atlantic Ocean •This deep circulation has an overturning timescale of ~10 3 years •Surface currents strongly influence climate in many areas, as in the N. Atlantic Deep water formation Deep water formation

Stratigraphic Evidence

• Recent work (April 2004) has investigated a proxy for Atlantic circulation using a marine sediment core from the Bermuda Rise • Th settles out of water faster than Pa, so the ratio between the two can provide information about the strength of flow away from source • Result – Atlantic circulation essentially shut down during Heinrich events

Summary

• There is still no clear trigger for Heinrich or Dansgaard-Oeschger events, nor an explanation for their periods • However, changes in Atlantic circulation seem to account for many of the side effects of both processes • More stratigraphic records = more clues

Why it all matters

• Late Pleistocene was not simply cold – it was totally chaotic

Beginning of Agriculture

• Even modern agricultural processes probably couldn’t overcome such variability

End of Agriculture???

A few references…

• • • • • • • • • • • Bradley, Raymond S. Paleoclimatology. Harcourt Press, 1999.

Siedov et al., Ed. The Oceans and rapid climate change. AGU, 2001. Hesse, R. and Khodabakhsh, S. Depositional Facies of Late Pleistocene Heinrich Events I nthe Labrador Sea.

Geology

26:2 103-106, 1998.

Dansgaard, W et al. Evidence for general instability of past climate from a 250kyr ice core record.

Nature

364, 15 July 1993.

Bond, G. et al. Evidence for massive discharges of icebergs into the North Atlantic Ocean during the last glacial period.

Nature

360, 19 Nov. 1992. Sarnthein, M. et al. Exploring Late Pleistocene Climate Variations.

Eos.

81:51 2000. Bond, G. et al. Correlations between climate records from North Atlantic sediments and Greenland ice.

Nature

365, 9 Sept. 1993.

Bard, E. Climate Shock: Abrupt changes over Millennial time scales.

Physics Today

Dec. 2002. Hendy and Kennett. Dansgaard-Oeschger cycles and the California Current System: Planktonic foraminiferal response to rapid climate change in Santa Barbara Basin, Ocean Drilling Program hole 893A.

Paleoceanography

, 15:1, 2000. Phillips, FM. Climatic and hydrologic oscillations in the Owens Lake basin and adjacent Sierra Nevada, California.

Science

274:5288, 1996. Wang, YJ. A High-Resolution Absolute-Dated Late Pleistocene Monsoon Record from Hulu Cave, China. 294:5550, 2001.