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Download Causes of cooling during the last 55 million years
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Climate Cooling Proxy evidence indicates an erratic cooling Over both poles and mid latitudes Roughly equal cooling in first and second half of interval Tectonic Scale Cooling Mechanism? Lower volcanic CO2 emissions Increased weathering Increased ocean heat transport 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 BLAG Hypothesis Depends of global spreading rates 55-15 mya general decrease in spreading Produce cooling 15 mya to today spreading increased Produce warming Consistent with record prior to 15 mya Inconsistent with record from 15 mya to present Cannot alone explain cooling Uplift Weathering Hypothesis 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 Elevation on Earth 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 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 Physical Weathering High Does the amount of high elevation terrain result in unusual physical weathering? Most likely given 10 fold increase of sediment to the Indian Ocean Steep terrain along southern Himalayan margin Presence of powerful South Asian monsoon Chemical Weathering 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 Need chemical indicator of hydrolysis Isotopes of strontium and osmium Strontium Isotopes 87Sr/86Sr for carbonate rocks has been measured throughout the Phanerozoic 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 Late Cenozoic increase in 87Sr due to increased rates of continental weathering by glaciation 87Sr/86Sr & Chemical Weathering Increase in 87Sr/86Sr 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 Osmium Isotopes Radiogenic Os formed from Re 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 Rivers draining the Himalayans not particularly rich in Os nor in radiogenic Os Available evidence indicates Himalayans not a source for strongly radiogenic osmium Infer Chemical Weathering Rates Tibetan-Himalayan complex very large and at high elevation Steep slopes receive lots of rainfall Heavy rains produce high suspended load Probably also provide high dissolved load BLAG or Uplift Weathering? No “proof” of either hypothesis exists BLAG explains well cooling from 55-15 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? Did Himalayan uplift balance increased CO2 from enhanced spreading? Ocean Heat Transport Although it appears “cool tropics paradox” is resolved Several important tectonic events influenced oceanic circulation Opening or closing of critical gateways Narrow passages linking major ocean basins Change heat and salt balance 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 Opening of Drake Passage Opening the gap between South America and Antarctica 25-20 mya allowed start of ACC Prior to opening, flow from north kept Antarctica warm Onset of ACC proposed to initiate glaciations on Antarctica Timing of Opening Drake Passage opened 25-20 mya Glaciations on Antarctica began 35 mya Most intense glaciation 13 mya 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 Isthmus of Panama Closure within last 10 my Complete closure 4 mya N. America glaciations 2.7 mya 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 Results of Closure 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 Did not affect atmospheric moisture However, warmed N. Atlantic and increased summer melting of snow and ice Assessment of Gateway Changes Illustrates fundamental disagreement Stopping pole-ward flow enhanced glaciations Starting pole-ward flow enhanced glaciations 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 Promote melting and ablation of glacial ice Ablation of glacial ice important Importance of Gateways 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 Closure of Isthmus of Panama Increased rate of NADW formation • Redirected dense water to north – Easier to form bottom water Brief Tectonic-Scale Change Attempt to explain erratic nature of cooling Volcanic aerosols Formation of sulfuric acid droplets or particles Sulfate aerosols block incoming solar radiation when in stratosphere Burial of organic carbon Brief intervals of enhanced burial Reduction in atmospheric CO2 Earth’s Active Volcanoes Most volcanoes associated with subduction andesitic and relatively explosive Explosive eruptions between 23.5°N and 23.5°S have most effect on climate Sulfate Aerosols Aerosols that reach stratosphere Attain maximum concentration within months of eruption Concentrations decline exponentially as particles settle Cooling effect follows concentration Documentation of Effect 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 Size of effect 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 CO2 Volcanic eruption also a source for CO2 CO2 residence time in atmosphere higher Burial of Organic Matter Changes in the rate of burial of organic matter affect atmospheric CO2 Rate of burial of marine organic matter sensitive to: Changes in rates of production Nutrient supply • Change in upwelling • Change in delivery of nutrients from land Changes in mode of preservation Bottom water oxygenation Cooling 13 mya Organic carbon-rich sediments deposited along California coast 13 mya Coincided with global cooling Strong winds enhanced upwelling • Termed the Monterey Hypothesis Timing of maximum organic carbon burial lags maximum cooling rate by 3 my Coastal deposition of organic-rich sediments May be exposed during sea-level low stand Organic matter oxidized, CO2 released Future Research Directions What is needed to better resolve mechanisms underlying tectonic-scale changes? Detailed record of atmospheric CO2 Geochemical tracer for chemical weathering Better understanding of feedbacks in climate system More detailed ocean general circulation models
