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Climate Cooling Proxy evidence indicates an erratic cooling Proxy evidence indicates an erratic cooling  Over both poles and mid latitudes  Roughly equal.

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Presentation on theme: "Climate Cooling Proxy evidence indicates an erratic cooling Proxy evidence indicates an erratic cooling  Over both poles and mid latitudes  Roughly equal."— Presentation transcript:

1 Climate Cooling Proxy evidence indicates an erratic cooling Proxy evidence indicates an erratic cooling  Over both poles and mid latitudes  Roughly equal cooling in first and second half of interval

2 Tectonic Scale Cooling Mechanism? Lower volcanic CO 2 emissions Lower volcanic CO 2 emissions Increased weathering Increased weathering Increased ocean heat transport Increased ocean heat transport Tectonic changes Tectonic changes  Atlantic widened and Pacific narrowed  India and Australia separated from Antarctica  India and Australia moved to lower latitudes  India collided with Eurasia  Key oceanic gateways open and closed

3 BLAG Hypothesis Depends of global spreading rates Depends of global spreading rates mya general decrease in spreading mya general decrease in spreading  Produce cooling 15 mya to today spreading increased 15 mya to today spreading increased  Produce warming Consistent with record prior to 15 mya Consistent with record prior to 15 mya Inconsistent with record from 15 mya to present Inconsistent with record from 15 mya to present Cannot alone explain cooling Cannot alone explain cooling

4 Uplift Weathering Hypothesis To explain cooling, 3 criteria must be met To explain cooling, 3 criteria must be met  High elevation terrain today must be unusually large  High terrain must cause unusual amount of rock fragmentation  Fragmentation and exposure must enhance chemical weathering

5 Elevation on Earth Most high elevation caused by subduction of oceanic crust and volcanism Most high elevation caused by subduction of oceanic crust and volcanism  Mountain ranges associates with subduction common throughout geologic time  Deep-seated heating and volcanism  East African plateau Mechanism of uplift not unique to last 55 myMechanism of uplift not unique to last 55 my  Existence of uplifted terrains like the Tibetan Plateau  Not common through geologic time  Conclude – amount of high elevation terrain is unusually large during last 55 my

6 Physical Weathering High Does the amount of high elevation terrain result in unusual physical weathering? Does the amount of high elevation terrain result in unusual physical weathering? Most likely given 10 fold increase of sediment to the Indian Ocean Most likely given 10 fold increase of sediment to the Indian Ocean  Steep terrain along southern Himalayan margin  Presence of powerful South Asian monsoon

7 Chemical Weathering Global chemical weathering rates difficult to determine Global chemical weathering rates difficult to determine  Dissolved ions in rivers clue  Today concentration modified by human activity  Difficult to distinguish ions from hydrolysis and dissolution  Only hydrolysis important on long term  Lots of rivers contribute ions to ocean Chemical weathering rates in past very difficult to quantify Chemical weathering rates in past very difficult to quantify  Need chemical indicator of hydrolysis  Isotopes of strontium and osmium

8 Strontium Isotopes 87 Sr/ 86 Sr for carbonate rocks has been measured throughout the Phanerozoic 87 Sr/ 86 Sr for carbonate rocks has been measured throughout the Phanerozoic Curve reflects relative contributions of Sr to the ocean Curve reflects relative contributions of Sr to the ocean  Continental weathering  Hydrothermal activity along mid-oceanic ridges General decrease in Early Phanerozoic due to increasing activity along mid-ocean ridges General decrease in Early Phanerozoic due to increasing activity along mid-ocean ridges Late Cenozoic increase in 87 Sr due to increased rates of continental weathering by glaciation Late Cenozoic increase in 87 Sr due to increased rates of continental weathering by glaciation

9 87 Sr/ 86 Sr & Chemical Weathering Increase in 87 Sr/ 86 Sr in Cenozoic could be Increase in 87 Sr/ 86 Sr in Cenozoic could be  Increase in chemical weathering  Delivers more Sr and more radiogenic Sr to ocean  Rock type being weathered is more radiogenic  No change in rate of chemical weathering No unique solution No unique solution

10 Osmium Isotopes Radiogenic Os formed from Re Radiogenic Os formed from Re Re enriched in certain phases Re enriched in certain phases  Organic-rich shales  Weathering of organic-rich shales  Certain minerals in granitic rocks  Hydrolysis reactions! Os residence time in ocean short Os residence time in ocean short Rivers draining the Himalayans not particularly rich in Os nor in radiogenic Os Rivers draining the Himalayans not particularly rich in Os nor in radiogenic Os  Available evidence indicates Himalayans not a source for strongly radiogenic osmium

11 Infer Chemical Weathering Rates Tibetan-Himalayan complex very large and at high elevation Tibetan-Himalayan complex very large and at high elevation Steep slopes receive lots of rainfall Steep slopes receive lots of rainfall Heavy rains produce high suspended load Heavy rains produce high suspended load  Probably also provide high dissolved load

12 BLAG or Uplift Weathering? No “proof” of either hypothesis exists No “proof” of either hypothesis exists  BLAG explains well cooling from mya  Uplift weathering supported by conditions in Tibetan-Himalayan Complex Would a combination of the two hypotheses explain best global cooling over last 55 my? Would a combination of the two hypotheses explain best global cooling over last 55 my?  Did Himalayan uplift balance increased CO 2 from enhanced spreading?

13 Ocean Heat Transport Although it appears “cool tropics paradox” is resolved Although it appears “cool tropics paradox” is resolved  Several important tectonic events influenced oceanic circulation Opening or closing of critical gateways Opening or closing of critical gateways  Narrow passages linking major ocean basins  Change heat and salt balance Two critical gateways Two critical gateways  Opening of Drake Passage producing the Antarctic Circumpolar Current  Appearance of the Isthmus of Panama stopped equatorial flow between Atlantic and Pacific

14 Opening of Drake Passage Opening the gap between South America and Antarctica mya allowed start of ACC Opening the gap between South America and Antarctica mya allowed start of ACC  Prior to opening, flow from north kept Antarctica warm  Onset of ACC proposed to initiate glaciations on Antarctica

15 Timing of Opening Drake Passage opened mya Drake Passage opened mya  Glaciations on Antarctica began 35 mya  Most intense glaciation 13 mya Ocean GCM models Ocean GCM models  Indicate that opening of Drake Passage had no effect on ocean/atmosphere temperatures  Antarctica cold with or without ACC  Models crude  Smaller grid  Affect of ACC on other deep currents

16 Isthmus of Panama Closure within last 10 my Closure within last 10 my  Complete closure 4 mya  N. America glaciations 2.7 mya Stopped westward flow of warm salty water Stopped westward flow of warm salty water  Redirecting flow in Atlantic into Gulf Stream  Northward flow of salty water slow sea ice formation  Reduced sea-ice cover made more moisture available on land  Triggered growth of ice sheets

17 Results of Closure Ocean GCM model results Ocean GCM model results  Agree with redirection of west flowing warm saline water into Gulf Stream  Also stops return flow of low salinity water into Atlantic from Pacific  Further increase salinity of Gulf Stream GCM model predicts reduction in sea ice in N. Atlantic GCM model predicts reduction in sea ice in N. Atlantic  Did not affect atmospheric moisture  However, warmed N. Atlantic and increased summer melting of snow and ice

18 Assessment of Gateway Changes Illustrates fundamental disagreement Illustrates fundamental disagreement  Stopping pole-ward flow enhanced glaciations  Starting pole-ward flow enhanced glaciations Argument centers about role of latent heat Argument centers about role of latent heat  Warmer ocean releases more latent heat to atmosphere  Supply moisture in atmosphere for ice sheet growth Appears that more sensible heat transferred Appears that more sensible heat transferred  Promote melting and ablation of glacial ice  Ablation of glacial ice important

19 Importance of Gateways Not satisfactory explanation for long-term global temperature changes Not satisfactory explanation for long-term global temperature changes  Discrete events that affected circulation  One-time events cannot explain well long-term temperature changes Clearly affect circulation patterns Clearly affect circulation patterns  Closure of Isthmus of Panama  Increased rate of NADW formation Redirected dense water to northRedirected dense water to north –Easier to form bottom water

20 Brief Tectonic-Scale Change Attempt to explain erratic nature of cooling Attempt to explain erratic nature of cooling Volcanic aerosols Volcanic aerosols  Formation of sulfuric acid droplets or particles  Sulfate aerosols block incoming solar radiation when in stratosphere Burial of organic carbon Burial of organic carbon  Brief intervals of enhanced burial  Reduction in atmospheric CO 2

21 Earth’s Active Volcanoes Most volcanoes associated with subduction andesitic and relatively explosive Most volcanoes associated with subduction andesitic and relatively explosive Explosive eruptions between 23.5°N and 23.5°S have most effect on climate Explosive eruptions between 23.5°N and 23.5°S have most effect on climate

22 Sulfate Aerosols Aerosols that reach stratosphere Aerosols that reach stratosphere  Attain maximum concentration within months of eruption  Concentrations decline exponentially as particles settle  Cooling effect follows concentration

23 Documentation of Effect Effect of sulfate aerosols difficult to detect in geologic record Effect of sulfate aerosols difficult to detect in geologic record  Crater size  Volume of volcanic ash deposits  Geographic area of ash fall deposit  Caveat is that sulfur content  ash content  Ice core records show ash deposits and a record of sulfuric acid  Limited time resolution

24 Size of effect Even massive eruptions that send sulfate aerosols into stratosphere Even massive eruptions that send sulfate aerosols into stratosphere  Produce cooling for only a few years  Multiple eruptions required for significant long-term cooling  Multiple large eruptions unlikely Global cooling countered by increased CO 2 Global cooling countered by increased CO 2  Volcanic eruption also a source for CO 2  CO 2 residence time in atmosphere higher

25 Burial of Organic Matter Changes in the rate of burial of organic matter affect atmospheric CO 2 Changes in the rate of burial of organic matter affect atmospheric CO 2 Rate of burial of marine organic matter sensitive to: Rate of burial of marine organic matter sensitive to:  Changes in rates of production  Nutrient supply Change in upwellingChange in upwelling Change in delivery of nutrients from landChange in delivery of nutrients from land  Changes in mode of preservation  Bottom water oxygenation

26 Cooling 13 mya Organic carbon-rich sediments deposited along California coast 13 mya Organic carbon-rich sediments deposited along California coast 13 mya  Coincided with global cooling  Strong winds enhanced upwelling Termed the Monterey HypothesisTermed the Monterey Hypothesis  Timing of maximum organic carbon burial lags maximum cooling rate by 3 my Coastal deposition of organic-rich sediments Coastal deposition of organic-rich sediments  May be exposed during sea-level low stand  Organic matter oxidized, CO 2 released

27 Future Research Directions What is needed to better resolve mechanisms underlying tectonic-scale changes? What is needed to better resolve mechanisms underlying tectonic-scale changes?  Detailed record of atmospheric CO 2  Geochemical tracer for chemical weathering  Better understanding of feedbacks in climate system  More detailed ocean general circulation models


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