Lec12

Information about Lec12

Published on October 22, 2007

Author: Gulkund

Source: authorstream.com

Content

Tectonics and climate:  Tectonics and climate The last 55 million years Ruddiman Chapter 7 Evidence from ice and vegetation:  Evidence from ice and vegetation First ice appeared on Antarctica at 35 Myr ago - IRD in sediment Greenland ice developed 7 to 3 Myr ago N. American ice sheet appeared 2.7 Myr ago Oxygen isotope data:  Oxygen isotope data Deep sea 18O record shows a cooling trend starting 55 Myr ago 55 - 35 Ma - cooling of ~ 6oC 35 Myr - present - cooling of 7oC plus 1‰ change due to ice sheet growth Why did global climate cool over the last 55 Myr?:  Why did global climate cool over the last 55 Myr? Three hypotheses: The spreading rate hypothesis (BLAG) The uplift weathering hypothesis The ocean heat transport hypothesis The organic burial hypothesis (The Monterey hypothesis) The spreading rate hypothesis Berner, Lasaga, and Garrels:  The spreading rate hypothesis Berner, Lasaga, and Garrels Climate change is driven by the rate of CO2 input into the atmosphere CO2 is generated by plate tectonic processes at mid ocean ridges and subduction zones The CO2 input is therefore controlled by seafloor spreading rate Negative feedback mechanism involving chemical weathering:  Negative feedback mechanism involving chemical weathering Carbon cycling according to BLAG:  Carbon cycling according to BLAG Chemical weathering: CaSiO3 + CO2 -> CaCO3 + SiO2 Subduction zone metamorphism CaCO3 + SiO2 -> CaSiO3 + CO2 Evaluating BLAG:  Evaluating BLAG Ocean floor spreading rate has slowed, leading to slower rates of CO2 input into the atmosphere, hence global cooling. But spreading rate is controversial Production rate of oceanic crust :  Production rate of oceanic crust According to the recent estimate by D. Rowley, oceanic crust production rate has not varied significantly from 180 Myr to present This is in contrast to most estimates in which the rate has decreased 50% since 100 Myr ago 2. Uplift weathering hypothesis :  2. Uplift weathering hypothesis Tectonic uplift in the last 55 Myr (e. g. the Himalayas, the Andes) in the last 55 Myr caused increased physical breakdown of rocks. The consequent increase in chemical weathering results in draw down of atmospheric CO2 leading to cooling. High terrains on Earth:  High terrains on Earth Tibet uplift:  Tibet uplift India collided with Asia about 55 Myr ago Uplift of Himalayas has continured since then Tibet and the Monsoon:  Tibet and the Monsoon Tibet Plateau draws moist air from Indian ocean and enhances monsoon Monsoonal rainfall could increase the rate of chemical weathering which decreases atm CO2 causing cooling 3. Ocean heat transport hypothesis:  3. Ocean heat transport hypothesis Ocean currents transport heat poleward and cools the tropics. This accounts for the greenhouse climate 100 Myr ago Loss of ocean heat contributes to the cooling of the last 55 Myr Case 1 - Opening of passageway around Antarctica:  Case 1 - Opening of passageway around Antarctica Drake Passage between S. America and Antarctica (25-10 Myr ago) allowed Circumpolar Current to flow around Antarctica disrupting the warm current toward the pole Problems: wrong timing and not supported by general circulation model Case 2 - Closing of the the Central American seaway (Isthmus of Panama):  Case 2 - Closing of the the Central American seaway (Isthmus of Panama) Closing of the Panama Isthmus (10-4 Myr ago) directed warm salty water to the N. Atlantic. Warm salty water would suppress sea ice formation but supply moisture to form the N. hemisphere ice sheet. But warm temperature could increase summer melting of snow and therefore would not enhance ice sheet formation. 4. The organic carbon burial hypothesis:  4. The organic carbon burial hypothesis Increased upwelling in the coastal ocean and increased nutrient delivery could enhance the rate of organic carbon production CO2 + H2O <-oxidationorganic C burial> CH2O + O2 Increase rate of the organic carbon burial in the sediment means loss of the total amount of carbon in the ocean-atm system and decreased level of CO2 in the atm Is 87Sr/86Sr an index of chemical weathering?:  Is 87Sr/86Sr an index of chemical weathering? Geochemistry of Rb Alkali element Incorporated in K-bearing minerals such as K-feldspars, micas Because of its large ionic radius (1.48 A), during fractional crystallization of magma, Rb is concentrated in the residual liquid (melt). Rb is concentrated in shale but is relative low in carbonates. Isotopic composition of Rb:  Isotopic composition of Rb Isotope % Atomic abundance 3785Rb 72.1654% 3787Rb 27.8346 % 87Rb decays to 87Sr by emission of  particle 3787Rb -> 3887Sr +  The half-life of 3787Rb is 4.88 x 1010 years Geochemistry of Sr :  Geochemistry of Sr Sr is an alkali earth element. Its ionic radius is 1.13 A, slightly larger than that of Ca (0.99 A). Sr can replace Ca in many minerals such as plagioclase, apatite and calcite and aragonite. During fractional crystallization of magma, Sr is concentrated in plagioclase, whereas Rb remains in the liquid phase. Consequently, Rb/Sr ratio of the residual magma increases with increasing degree of differentiation. That is why felsic rocks have higher Rb/Sr ratios than mafic rocks. Isotopic composition of Sr:  Isotopic composition of Sr Isotope % atomic abundance 3888Sr 82.53 3887Sr 7.04 3886Sr 9.87 3884Sr 0.56 Because 87Sr is the radiogenic daughter of 87Rb, the 87Sr/86Sr ratio in rocks are variable, depending on the Rb/Sr ratio and the age of the rocks. 87Sr/86Sr as an indicator of geological reservoirs:  87Sr/86Sr as an indicator of geological reservoirs Because the age and Rb/Sr of different rocks from different geological reservoirs are different, their 87Sr/86Sr ratios are different. Hence 87Sr/86Sr is a provenance tracer. For example: Oceanic crust- (87Sr/86Sr ~ 0.7023 – 0.7035) Continental crust - 87Sr/86Sr ~ 0.712 or higher Sources of Sr in seawater:  Sources of Sr in seawater The rivers Rivers 87Sr/86Sr Major lithology Amazon 0.7109 varied Mississippi 0.7109 varied Japan rivers 0.7076 arc volcanics Xingu 0.7292 Brazilian shield Ganges 0.7257 Himalayan Brahmaputra 0.7217 Himalayan 2. Hydrothermal fluids circulating through mid-ocean ridges (Graphics source: Press et al., Understanding Earth):  2. Hydrothermal fluids circulating through mid-ocean ridges (Graphics source: Press et al., Understanding Earth) Hot springs resulting from seawater circulating through hot ocean crust: 87Sr/86Sr = 0.703-0.704 3. Carbonates 87Sr/86Sr = 0.708, like seawater Origin of submarine hydrothermal fluids (Graphics source Press et al. Understanding Earth):  Origin of submarine hydrothermal fluids (Graphics source Press et al. Understanding Earth) 87Sr/86Sr of seawater depends on the flux and isotope composition of the three input (h, r, and c are fractional fluxes of hydrothermal solutions, rivers and carbonates):  87Sr/86Sr of seawater depends on the flux and isotope composition of the three input (h, r, and c are fractional fluxes of hydrothermal solutions, rivers and carbonates) Secular variation of seawater 87Sr/86Sr :  Secular variation of seawater 87Sr/86Sr Sr has a long residence time in the ocean (3-5 Myr) and is well mixed, so the 87Sr/86Sr is everywhere the same in the world’s oceans at a given time 87Sr/86Sr of seawater is recorded in marine carbonates Slide28:  Changing of the proportion of the three inputs can lead to variation of (87Sr/86Sr)sw with time. Carbonate weathering has negligible influence on the seawater ratio (87Sr/86Sr)sw mainly reflects the balance between the input non-radiogenic Sr from hydrothermal activity and the input of radiogenic Sr from continental weathering Variation of seawater 87Sr/86Sr during the Phanerozoic time (Burke et al., 1982):  Variation of seawater 87Sr/86Sr during the Phanerozoic time (Burke et al., 1982) Variation of 87Sr/86Sr with time:  Variation of 87Sr/86Sr with time Cambrian high (0.709) - Pan African orogeny Jurassic low (0.7067) - opening of the Atlantic ocean Steep rise since Eocene to 0.7092 - Himalayan uplift? Rapid increase of seawater 87Sr/86Sr over the last 40 Mys (Eocene to present) (Hess et al., 1986):  Rapid increase of seawater 87Sr/86Sr over the last 40 Mys (Eocene to present) (Hess et al., 1986) Slide32:  Seafloor spreading rates have not changed much over the last 40 Myr, so the rise in 87Sr/86Sr could not be due to the decrease in hydrothermal flux, but could be the result of increased radiogenic Sr flux from the continents. What caused the rapid rise of seawater 87Sr/86Sr?:  What caused the rapid rise of seawater 87Sr/86Sr? Greater river flux of Sr (with no change in the 87Sr/86Sr ratio) Increase in the 87Sr/86Sr carried by the rivers (but with no increase in total Sr influx) The first explanation requires increased chemical weathering which could affect climate, but the second does not. What caused the rapid rise of seawater 87Sr/86Sr?:  What caused the rapid rise of seawater 87Sr/86Sr? Intense weathering of silicate rocks (granites and gneisses) increased the Sr flux Redistribution of radiogenic Sr into carbonates Exposure of metamorphic rocks to weathering The weathering uplift hypothesis (Raymo and Ruddiman, 1992):  The weathering uplift hypothesis (Raymo and Ruddiman, 1992) The post-Eocene rise in 87Sr/86Sr is coincidental with the Himalayan uplift Raymo and Ruddiman (1992) proposed that the uplift of Himalaya and Tibet plateau is a major driving force of the Cenozoic climate The extensive high terrain can affect atmospheric circulation on a hemispheric scale thus affecting temperature and precipitation (Regionally it creates monsoonal rain). Slide36:  The high terrain also promotes physical fragmentation of rocks and the fragmentation and high erosion rates expose fresh rock surface causing unusually high rates of chemical weathering. Thus Himalayan uplift results in higher chemical weathering rates, causing a drawdown of atmospheric CO2 and global cooling. The rapid rise in seawater 87Sr/86Sr is related to higher weathering rate of silicate rocks associated with the Himalayan uplift Exposure of metamorphic rock to weathering (Edmond, 1992):  Exposure of metamorphic rock to weathering (Edmond, 1992) Himalayan rivers have unusually high 87Sr/86Sr (0.8) Radiogenic Sr comes from the metamorphic core complex (consisting of granites and gneiss) of the Himalayas that was deeply buried and later unroofed by uplift. High grade metamorphism and partial melting of the rocks during the Tertiary collision of India with Asia mobilized radiogenic Sr out of more resistant mineral phases into phases that weather more rapidly (Ca and Na rich phases). Only Himalayan type collision that generates large elevated terrains can induce global perturbation in climate Carbonate versus silicate weathering:  Carbonate versus silicate weathering A study of a water shed in the High Himalayan Crystalline Series suggests that the ion flux is primarily due to dissolution of carbonates (Blum et al., 1998) The metasedimetary carbonates have high 87Sr/86Sr due to equilibration with silicate phases Carbonate chemistry of Himalayan river waters also indicate carbonate weathering contributes most of the dissolved Sr (Jacobson et al., 2002) Dissolution of carbonates have no net effect on atmospheric CO2 while silicate weathering consumes CO2 Models that assumes radiogenic Sr is derived from silicates may overestimate the consumption of CO2 due to silicate weathering Other processes (besides Himalayan uplift) that could affect the fluvial input:  Other processes (besides Himalayan uplift) that could affect the fluvial input Continental glaciation Eustatic sea level Atmospheric CO2 Mountain building (for example the Andes)

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