Presentation on theme: "Paleoclimate Review Sanpisa Sritrairat January 26, 2007."— Presentation transcript:
Paleoclimate Review Sanpisa Sritrairat January 26, 2007
Topics to be covered Important climatic states of the earth Evidences Why did “climate change”?
Paleo-Climate Climate is the mean state of the environment, long-term average of daily variations “Climate change” depends on the resolution of the proxies and the length of the “mean state” in consideration
Climate Change Events Tectonic scale (Millions of years ago) Orbital Scale, when Milankovich started showing up (3 Ma) Deglacial and Millennial Scale Historical climate changes
Time line Ma: Snowball Earth (Neoproterozoic) 300 Ma-5Ma: Hot house world (Mesozoic/Cenozoic ) 3 Myr-present: Orbital-scale variability: series of glaciation and retreat 20 Kyr: Last glacial maximum (LGM) ~13 Kyr:Bolling/Allerod warming ~12 Kyr: Younger Dryas (YD) Heinrich events and D-O cycles; BP: Medieval Warm Period BP: Little Ice Age
How to study paleoclimate? Marine Ocean sediment cores (more regional) Terrestrial (more local) Lakes and wetlands cores Tree ring/Coral (growth response) Ice cores Speleothem Sedimentary rocks/uplifted sediments
Proxies: Lithology, sediment composition Black: –high organic, anoxic, high productivity Sharp pebbles, unsorted: –Glacial deposit High silica: upwelling, high productivity Terrigenous: high weathering, river flow Calcareous: Warm, high productivity Tephra: volcanic eruption
Proxies: plant and animal remains Pollens, forams Molecular techniques (transformation of molecules at a specific condition, or specific remains of group of living organism).i.e. alkenones, lignin Each species has a specific range of habitat (precip, T, soil type, nutrients, salinity) i.e. found foram in freshwater wetland cores: must have been saltier, Tropic pollen in the arctic = warmer
Proxies:Stable Isotopes If relative ratios of the selected pair changes systematically according to climatic parameters (T, precip, pH, etc) Mg/Ca: T δ 13 C: ocean circulation, productivity, C cycle δ 18 O:Temperature/Salinity/Sea level –More ice on land: ocean δ 18 O becomes heavier
Rayleigh Distillation (Precipiation Fraction)
Chronology Radiometric –C-14, U/Th, Ar/Ar, etc. Paleomagnetic Wiggle match (cross dated): matching the same features
Understanding the Past Climate Many explanation are theoretical without consensus A lot of underlying hypothesis has to do with the equilibrium of earth cycling process: weathering, precipitation Think in term of feedbacks
Snowball Earth (~ Ma) Evidences Glacial deposits sandwiched between “cap carbornates” every continents (including those at paleo-equator) Banded iron in the glacial deposit anoxia (ice- covered ocean can’t circulate O 2 down.) Several paleomagnetic reversals within each glacial layer millions of years
Cycles of cold/hot period
What trigger the Snowball Earth Young Faint Sun? (70% irradiance 4Ga) (Took time for H/He fusion to heat up to the surface)
Faint Young Sun Paradox The Sun's luminosity has increased through geologic time due to a nuclear reaction in the Sun's interior that fuses nuclei of hydrogen together to form helium. This nuclear reaction has caused the Sun to expand and become brighter. Consequently, the early Sun shone 25-30% less brightly than it does today. This raises a paradox. At such a low solar luminosity, we would expect all water in Earth to have been frozen. Yet, sedimentary rocks provide evidence of running water at least 4 billion years ago. Some mechanism must have kept Earth warm. Yet, wouldn't the same mechanism cause the Earth to be intolerably hot today? It has been hypothesized that the solution to the faint young sun problem is that outgassing from volcanoes was high due to vigorous seafloor spreading. At the same time, weathering was very low due to a dearth of continents. Thus, atmospheric CO2 was much higher than today, providing a healthy greenhouse effect to keep the early Earth warm.
High obliquity hypothesis? May be from asteroid that hit the earth and made the moon But.. Very high seasonality variability in the tropic
Land mass distribution Higher tropical albedo (Land albedo > ocean)
Hot House World (300-5 Ma) Mesozoic/Cenozoic) Thermal max: 55 Ma (Cenozoic) Clues: –Marine T proxies –Tropical plants fossils, alligator, & pollens up in the arctic –Organic-rich deposit (anoxia) –Much lighter in δ 18 O (-40‰ vs. 0 of ocean, and vs. -25 ‰ if melt all ice now. Must have additional T effects.) –No glaciomarine deposit
Causes of the hot house? Tectonic block circum polar current can’t form arctic ice sheet? Can’t form deep water since there is no strong T gradient (est. 12ºC vs. ~0 now) CH 4 Clathrate release>> positive greenhouse feedback –supported by lighter δ 13 C
BLAG hypothesis Rate of plate movement influences global climate by controlling atmospheric CO 2 concentrations Support: faster seafloor spreading rate 100 Ma than now Weathering: CaSiO 3 + CO 2 --> CaCO 3 + SiO 2
Uplift weathering hypothesis Uplift accelerates chemical weathering, drawing down CO 2, and cooling the global climate.
Support of the Uplift Hypothesis Tibetan Paleau (also high monsoon strength which encourages weathering), Colorado Paleau, the Andes uplifted about that time. High 87/86 Sr ratio = higher weathering
Factors that control chemical weathering 1. Temperature- chemical weathering increases with increased temperatures 2. Precipitation- increased precipitation raises the level of groundwater in soils, promoting the production of carbonic acid 3. Vegetation- plants extract CO2 from the atmosphere and deliver it to soils, where it combines with groundwater to make carbonic acid
Long-term carbon cycle Carbon added to atmosphere through metamorphic outgassing and outgassing of volcanoes and mid-ocean ridges Hydrolysis-weathering of silicate minerals in continental crust: CaSiO3 + H2CO3 >> CaCO3 + SiO2 + H2O The products of continental weathering are transported to the oceans by rivers, where they are used to make CaCO3 and SiO2 shells of marine organisms. When these organisms die, many of them are deposited and buried on the seafloor. The carbon cycle is completed upon subduction and melting of these sediments. The melt may rise as magma, providing volcanoes and MORs with a source of recycled CO2. Important flows of carbon on 100,000 year time scales
Long-term carbon cycle Chemical weathering can also occur through a process called dissolution, the chemical weathering of carbonate sediments (CaCO3) (limestone, for example). Dissolution can be described by the following reaction: CaCO3 + H2CO3 >> CaCO3 + H2O + CO2 Note, however, that the net removal of atmospheric CO2 is 0. CO2 is taken from the atmosphere to make carbonic acid, but is released to the atmosphere during the creating of CaCO3 shells.
Summary: influence of plate tectonics on climate 1. Location of continents 2. Mountain building- alters atmospheric flow 3. Open/close ocean gateways 4. Sea-level change- modifies ratio of land to ocean 5. Altering weathering rates- linked to concentration of CO 2 in atmosphere 6. Altering rates of outgassing- linked to concentration of CO 2 in atmosphere
Ice house world: (Eocene onset, 34Ma- present) Started to have polar ice cap –35 Ma: Australian Gateway: initial Antarctic glaciation –15 Ma: Drake Passage opened: more Antarctic glaciation Current glaciation cycles: 3 Ma-present
What caused the onset? Cane: NW drift of Halmahera (N. Australian) block PAC and form warm pool before 3 Ma (permanent El nino)? Characteristic of this period: Insolation variability hypothesis Summer insolation controls North Hemisphere ice sheet growth. Ice growth occurs during times when summer insolation is low in high northern latitude.
Orbital forcing: Milankovitch Theory Obliquity: 41, 000 yr cycle
Orbital forcing: Milankovitch Theory Eccentricity: 100,000 years
Orbital forcing: Milankovitch Theory Precession: 19,000-23,000 years The major axis of each planet's elliptical orbit also precesses within its orbital plane, in response to perturbations in the form of the changing gravitational forces exerted by other planets. This is called perihelion precession. It is generally understood that the gravitational pulls of the sun and the moon cause the precession of the equinoxes on Earth which operate on cycles of 23,000 and 19,000 years.
Orbital scale insolation change Strong 23 ky (precession) and 40 k cycles (Obliquity) 100 ky cycle is not obvious
Northern Hemisphere Ice sheet History 41 and 23 kyr cycles from ~.7-3 Ma But why 100 ky cycles dominate climate records of the last 700 ky?
Orbital monsoon hypothesis Changing seasonal insolation will change the strength of the monsoons. Stronger summer radiation will strengthen the summer monsoon. Weaker winter radiation will strengthen the winter monsoon. It turns out that the African monsoon is very sensitive to insolation variations. The African monsoon is responsible for precipitation over northern Africa. Today, the summer solstice occurs at aphelion. So, the summer insolation is near its minimum. As a consequence, northern Africa summer monsoon is weak. Although the strength of the winter monsoon also varies, it has less impact on the African environment because the winter monsoon has little affect on precipitation over Africa.
Evidence for an orbitally-controlled monsoon 1. Lake levels across North Africa 2. Mediterranean circulation and deposition of marine sediments 3. Freshwater diatoms (small plant plankton) in the tropical Atlantic 4. Upwelling in the equatorial Atlantic Relationship between summer radiation and African monsoon (from Earth's Climate Past and Future by W.F. Ruddiman).
Millenial Scale Climate Change Last glacial maximum (LGM): ~21kya Bolling/Allerod warming-> Younger Dryas cooling:~ kya Heinrich events Dansgaard-Oeschger events
Last glacial maximum (~20 Ky) Cold, dry and windy Continent-sized ice sheets (Laurentide ice sheet over North America) 110m lower sea level than present
Tropical debate over LGM cooling Small tropical cooling (~2°C ) : CLIMAP reconstruction based on the changes in planktic fauna and flora in the low-latitude oceans. Other evidences: biochemical composition of plankton shells (double bonds of alkenones), δ 18 O measurements on the CaCO 3 shells of plankton. Large tropical cooling (~5°C ): Mountain glacial ice line change, noble gases dissolved in glacial-age groundwater. GCMs can only get level of ice sheet and tropical glacier growth with ~5ºC shift in tropical temperature
Abrupt climate change Heinrich events: ice-rafted debris & terrigenous material found in deep-sea cores, corresponding to Greenland ice core low δ 18 O. Dansgaard-Oeschger cycle: A series of warm-cold oscillation punctuated the last glaciation from 15 to 110 Kyr BP. The D-O cycles have been marked by abrupt terminations, and often by abrupt onsets.
Heinrich and D-O events
Antarctic Record v. Greenland An absence of D-O events in Antarctica
The Younger Dryas was first detected from layers in north European bog peat, and named for the alpine/tundra plant Dryas octopetala. It was a brief (approximately /- 70year ) cold climate period following the Bölling/Allerød interstadial at the end of the Pleistocene, and preceding the Preboreal of the early Holocene. It is dated approximately 12,900-11,500 BP calibrated, or 11,000-10,000 BP uncalibrated, but dating is difficult because it occurs during a radiocarbon plateau It is dated approximately 12,900-11,500 BP calibrated, or 11,000-10,000 BP uncalibrated, but dating is difficult because it occurs during a radiocarbon plateau
Younger Dryas The prevailing theory holds that the Younger Dryas was caused by a significant reduction or shutdown of the North Atlantic thermohaline circulation in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America. The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the north Atlantic Ocean. This theory does not explain why South America cooled first.
Younger Dryas A problem with this hypothesis is the timing of meltwater pulses that are supposed to have triggered the THC shutdown: it was found that a second meltwater pulse, albeit slightly smaller than the first one, occurred at the end of the YD (Fairbanks, 1989): why didn't it also trigger a similar chain of consequences in the climate system? An alternate explanation (Clement et al., 2001) invokes the abrupt cessation in the El Nino -Southern Oscillation in response to changes in the orbital parameters of the Earth, although how such a change would impact regions away from the Tropics remains to be explained. For further discussion, see: Broecker, WS., Does the trigger for abrupt climate change reside in the oceans or in the atmosphere? Science 300 (5625): JUN
Medieval Warming 10th century-14th century in Europe; May recent finding in North America Coincided with a peak in solar activity
Little Ice Age A period of cooling from approx. 14th-19th century, occurs after the medieval warming, though there seems to be little global agreement on the timing. Most evidence in Europe and north America Hypotheses of the cause include decreased sunspot activity (Maunder minimum) and increased volcanic activity, others claim it had to do with a decrease in population resulting from the black death and thus a decrease in agricultural activity
Global Warming?: The Hockey Stick The infamous Mike Mann’s“Hockey Stick” graph – The temperature is rising rapidly
Greenland Ice Core
Coral Reef Hypothesis –More reef production when sea level rises (warm), increases CO 2 : Positive feedback –Ca HCO 3 CaCO 3 + CO 2 Fe-fertilization: increased dust during glaciation may increase productivity and drawdown CO 2 Biosphere Why CO 2 increased during interglacials (and why drop at glacial max.)
Atmospheric CO2 through Earth history How to explain long-term changes in CO 2 ? According to Berner (1994): 1. increase in solar radiation has caused gradual drop in atmospheric CO2 2. high CO2 during Mesozoic and decrease in Cenozoic are due to high Mesozoic relief and Cenozoic mountain uplift combined with decreasing metamorphic/volcanic degassing of CO2 during Cenozoic 3. variable degassing, due to changes in seafloor spreading was not a major control on CO2