Stratigraphy of climate change Lecture 19

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

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

Barbados

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

Overview Introduction to Pleistocene Climate Sources of climatic data Pleistocene Climate Cycles –Glacial Cycles –Heinrich Events –Dansgaard-Oeschger Events Possible causes of rapid climate shifts

Factors that influence climate Atmosphere – 1-10 years Solar Activity – years Oceans – years Orbital Forcing – 10, ,000 years Tectonics – Millions to hundreds of millions of years

Marine Sediment Cores Available in widespread locations, including low latitudes Record lots of information about both biological and non-biological variables Requirements: –High Sedimentation (Bermuda Rise) –Low Bioturbation (Anoxic conditions)

Ice Sheet Cores Record yearly snowfall – annual resolution for last kyr, but good data for several hundred kyrs Include many useful climate proxies, mostly related to atmospheric circulation Only available in certain places (Greenland, Antarctica)

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

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

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

Source of Heinrich events Black areas are regions with large carbonate deposits Sediment must have been ice- rafted

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

Heinrich Events in California? Phillips, 1996 Sierran Glacial advances seem to correspond to Heinrich events 1,2,3,5 well

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

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 Dansgaard-Oeschger Events (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

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

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 Even modern agricultural processes probably couldn’t overcome such variability Beginning of Agriculture End of Agriculture???

A few references… Bradley, Raymond S. Paleoclimatology. Harcourt Press, Siedov et al., Ed. The Oceans and rapid climate change. AGU, Hesse, R. and Khodabakhsh, S. Depositional Facies of Late Pleistocene Heinrich Events I nthe Labrador Sea. Geology 26: , Dansgaard, W et al. Evidence for general instability of past climate from a 250kyr ice- core record. Nature 364, 15 July Bond, G. et al. Evidence for massive discharges of icebergs into the North Atlantic Ocean during the last glacial period. Nature 360, 19 Nov Sarnthein, M. et al. Exploring Late Pleistocene Climate Variations. Eos. 81: Bond, G. et al. Correlations between climate records from North Atlantic sediments and Greenland ice. Nature 365, 9 Sept Bard, E. Climate Shock: Abrupt changes over Millennial time scales. Physics Today Dec 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, Phillips, FM. Climatic and hydrologic oscillations in the Owens Lake basin and adjacent Sierra Nevada, California. Science 274:5288, Wang, YJ. A High-Resolution Absolute-Dated Late Pleistocene Monsoon Record from Hulu Cave, China. 294:5550, 2001.