1 / 36

Deglaciation

Deglaciation. LGM climate controlled by Ice sheets and atmospheric CO 2 Deglacial world shift Higher insolation and CO 2 Smaller ice sheets As insolation increased Ice sheets melted Influenced climate much less CO 2 had a largely secondary role in climate. Timing of Ice Sheet Melting.

Leo
Download Presentation

Deglaciation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Deglaciation • LGM climate controlled by • Ice sheets and atmospheric CO2 • Deglacial world shift • Higher insolation and CO2 • Smaller ice sheets • As insolation increased • Ice sheets melted • Influenced climate much less • CO2 had a largely secondary role in climate

  2. Timing of Ice Sheet Melting • Determined by dating organic remains formed during ice sheet retreat • Although scarce, suitable samples exist • N. American ice sheets began retreating 14,000 14C years ago • Gone by 6,000 14C years ago • Area does not yield ice volume • Thickness of ice debatable

  3. Sea Level from Coral Reefs • One meter of sea level rise = 0.4 million km3 of ice • Total global ice volume can be compared with insolation record • Coral reefs on Barbados gave sea level history • Know Barbados had minor subsidence • 14C dated sea level curve supports expectations • Rate of sea level rise maximized during maximum summer insolation • Insolation record well known • Summer insolation maximum 10,000 years ago • Barbados corals gave a 14C dated record of sea level rise

  4. 14C Age Not True Age • When 14C dated corals dated by Th/U • 14C ages were too young • Implication was that rate of 14C production from 14N • Greater during LGM • More 14C present in sample • Gives an age that is too young • Young age confirmed by tree ring studies

  5. Implications for Ice Volume • The Th/U chronology more accurate • Highest rates of sea level rise • Before maximum summer insolation • The bigger they are, the quicker they melt • Generally consistent with Milankovitch theory • Response time curve predict lag • Must be other feedbacks at work

  6. Rise in Sea Level Not Smooth • Record of sea level rise is not smooth • Rapid rise from 20K to 14K years ago • Slow from 14-12K years • More rapid rise after 12K years • Rates of sea level rise changed dramatically

  7. Rate of Sea Level Rise • Rate of sea level rise slowed significantly between 14K and 12K years ago • Two major pulses of freshwater influx to oceans • Melting glaciers • Glacier melting episodic • Flow of meltwater to oceans episodic

  8. Meltwater Pulses • Oxygen isotopic composition of planktic foraminifera • Monitor freshwater influx to ocean • Anomalously low d18O measured in foraminifera • Norwegian Sea • Barents Sea ice sheet • Gulf of Mexico • Laurentide ice sheet via the Mississippi River

  9. Meltwater Pulses – Additional Evidence • Ice-rafted debris in non-fossiliferous sediments west of Ireland • Suggest large influx of fresh water into North Sea • Sourced by massive release of ice bergs • Sediments deposited between 17K and 14.5K years ago • Coincide with first major meltwater pulse • Calving ice bergs would accelerate ice sheet melting

  10. Younger Dryas • Mid-deglacial pause in ice melting • Accompanied by brief climate cooling • Particularly in subpolar N. Atlantic Ocean • Pollen records in Europe and Scotland indicate • Cold-tolerant tundra (including the Arctic plant Dryas) • Displaced early growth of forests • Evidence of Younger Dryas also found in N. Atlantic sediments

  11. Younger Dryas • Southward re-advance of polar water in the N. Atlantic evident in faunal assemblages • Reversal towards Artic vegetation in Europe • Cold-tolerant insects in England (~7°C)

  12. Younger Dryas • Recorded in Greenland ice core • Ice sheet accumulation rates changed abruptly • Ice accumulation slow during LGM and Younger Dryas • Large changes in windblown dust • As indicated by Ca content in cores • Younger Dryas was cold, dry and windy climate

  13. Causes of Younger Dryas • Broecker called upon change in NADW formation • Meltwater diverted from Gulf of Mexico to N. Atlantic • Pulse of low-salinity meltwater cut off NADW formation • Cut off heat transfer to subpolar Atlantic from tropics

  14. Critics of Broecker • Meltwater pulses to N. Atlantic • Occurred when global rates of ice melting were a factor of 5 lower • With such low rates of meltwater influx • How could such a small diversion cause such a big change in climate? • Mechanisms causing cooling hotly debated • Cooling appears global (e.g., greenhouse gases) • Signal could be transferred quickly from N. hemisphere ice sheets

  15. Testing Broecker’s Model • If thermohaline overturn in N. Atlantic slowed • Decrease northward heat transport • Warm tropical Atlantic • If greenhouse gas reduction • Produced Younger Dryas cooling • Expect synchronous global cooling • SST measurements in tropical Atlantic • Help sort out mechanism • Greenland ice core records clearly document N. hemisphere, high latitude cooling • Due to heat released from N. Atlantic • N. Atlantic cooled

  16. Synchronous or Asynchronous Cooling? • Oxygen isotope records from GRIP and Byrd ice cores • Temperature differences between N. and S. hemispheres • Suggest asynchronous cooling • Changes in rates of NADW formation • Yet terrestrial climate records suggest synchronous cooling • Changes in greenhouse gas concentrations • Oceanic or atmospheric control

  17. Temperature Records • SST based on alkenone unsaturation in core taken near Grenada • Tropical western N. Atlantic (12°N) • d18O from coexisting planktic foraminifer • During Younger Dryas • Alkenone SST increase • GRIP temperature decreases • Asynchronous cooling

  18. Mechanisms of Change • Compare SST with benthic Cd/Ca • Cd/Ca record from Bermuda Rise • Cd indicator of phosphate • Barometer of changes in the source of deep water • Low nutrient N. Atlantic deep water • High nutrient Antarctic sources • Cd/Ca maximum in younger Dryas indicates less NADW • Slowdown in NADW formation from injection of freshwater • Cooled N. Atlantic and Greenland • Less heat transferred N. from tropics so tropics warmed

  19. Support from 10Be • Variations in production rates of atmospheric 10Be and 14C • Linked to solar activity and Earth’s magnetic field • Concentration of atmospheric 14C also affected by removal of radiocarbon • Changes in global carbon cycle • Muscheler et al. (2000 Nature, 408:567-570) used 1OBe to constrain production rates of 14C during Younger Dryas • Residual variation due to carbon cycle • Consistent with lower ventilation rates • Therefore a reduction in deep water formation during Younger Dryas

  20. Warm Eastern & Cold Western Atlantic • SST records off northwest Africa show cooling during younger Dryas • Implies southward advection of cold water • Along Canary Current • Meltwater shut down NADW formation • Reduced NADW formation caused • Tropical western and southern Atlantic warmed • Eastern and northern Atlantic cooled • Results predict asynchronous N. and S. hemisphere temperatures in ice cores • On short times scales • Consistent with Cuffy and Vineux (2001)

  21. Paradox: Freshwater Influx? • If rate of NADW formation is reduced by meltwater pulse during Younger Dryas • Why was the pre-Younger Dryas climate and presumably NADW formation seemingly unaffected • Large documented meltwater pulses? • Paradox seemingly resolved if large pulses originated from Antarctic ice sheet • Recent modeling (Clark et al. 2002 Nature, 415:863-869) • Thermohaline circulation sensitive to small changes in hydrologic cycle (~0.1 Sv) • Why no thermohaline circulation in Pacific? • Surface waters are too fresh to sink

  22. Huh? Antarctic Ice Sheet Melting • Large ice sheets melted early and melted fast • d18O data from Norwegian sea imply early melting of Barents ice sheet • Sediments in N. Sea and d18O data in Gulf of Mexico imply melting of Laurentide ice sheet • Indicate N. hemisphere ice sheet melting • Some evidence exists for early deglacial warming in Antarctica • Suggest that this acted as a trigger for melting ice sheets in north polar regions

  23. Abrupt Melting Events • Suggest feed backs in climate system accelerated ice sheet melting • Iceberg calving would increase rate of melting • Moves ice quickly into relatively warm waters • Ice sheets in marginal marine environments • Susceptible to rapid melting • Internal flow of ice sheets increased • Ice fluxed to margins along ice streams • Effectively thinning the ice sheet • Lowering volume but not aerial extent of ice

  24. Changes in Landscapes • Morphological changes accompanied deglaciations • Proglacial lakes • Flooding • When impounded water in proglacial lakes was suddenly released • Rise in sea level • Inundation of coastal regions • Submerged land connections between continents exposed during LGM

  25. Proglacial Lakes • Proglacial lakes develop in bedrock depressions left by melting ice sheets • Lake Agassiz, largest proglacial lake N. America • 200,000 km2, 100 m deep (20,000 km3) • Sudden release of large proglacial lakes caused massive floods

  26. Increased Insolation Produced Monsoons • Earth’s orbital configuration 10K years ago • Summer insolation 8% higher than today • Conducive to summer monsoon development • Model simulations supported by geologic observations • Lake levels higher in • Arabia • North Africa • Southeastern Asia

  27. Enhanced Upwelling in Arabian Sea • Strong monsoon winds blowing across Somalia and eastern Arabia • Enhanced coastal upwelling • Altering the planktic foraminifera species

  28. Climate Evidence • Evidence for wet climate range from • Large dry river valleys in deserts • Fossil evidence includes • Grass pollen in lake deposits • Variety of water-loving animals (hippopotamuses, crocodiles, turtles, rhinoceroses, etc)

  29. Timing • 14C dates for lake deposits in N. Africa • Match the 10K insolation maximum • When corrected for greater 14C production

  30. Intensity • Summer insolation 8% higher but lakes 24% larger in volume • Relationship not necessarily linear • Mismatch between models and observations • Required addition of vegetation-moisture feedback

  31. Insolation Reduced Monsoons • Decreased summer insolation expected to weakened summer monsoons • Lake levels in N. Africa match well expected patterns • Most lakes today much lower or dried out

  32. Climate Change Over Last 10K Years • Ice sheets melting (reduced influence) • Atmospheric CO2 levels stable and high • Summer insolation gradually decreasing • Expect warmer and then cooler climate

  33. Vegetation • General gradual movement of warm-adapted biomes north • Pollen records indicate spruce and oak moved north • Mid-glacial produced no-analog vegetation • Mixtures that do not exist today • Different response of a particular type of plant to changing climate

  34. Peak Deglacial Warmth • With atmospheric CO2 levels steady and high • Glacial ice largely melted • Summer insolation and vegetation changes affected temperatures • Insolation 5% higher warmed high latitudes • Displacement of high-albedo tundra by low-albedo spruce caused positive feedback • Greater warming

  35. Cooling Followed Deglacial Warming • Ample evidence for gradual cooling • Summer insolation dropped over last 10K years • Less frequent melting of ice caps • More frequent sea ice off Greenland indicated by drop in diatoms • Advances in ice caps on Arctic islands • Lower Atlantic SST • Southward shift in the boundary between spruce and tundra

  36. Future Climate • Over the next 10K years precession maximize at low latitude • Intensify summer monsoons • Tilt should minimize at high N. latitudes • Help promote further glaciations • Pattern consistent with glaciations in next few thousand years • Predictions complicated by millennial-scale oscillations and anthropogenic greenhouse gases

More Related