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MIT 12 740 - Oxygen isotope paleothermometry

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Oxygen isotope paleothermometry: the early applicationsEmiliani (1954, Am. J. Sci. 252:149) temperature calibration from Gulf of Mexico core tops• Emiliani (1955 and other papers) then analyzed foraminifera from piston cores from the deep sea, and made temperature estimaOxygen isotope paleothermometry: the early applicationsRayleigh distillation of oxygen isotopesCumulative Rayleigh Isotope Distillation as a function of temperatureObserved 18O - surface temperature relationshipOxygen isotopes of precipitation over North AmericaEmiliani’s ice volume correctionBecause they flow, glaciers are filled from their summits:How can we know the isotopic composition of ice sheets from the last glacial maximum?Pore water chlorinity and oxygen isotopesCaution: this method requires some knowledge of past water mass distributions in the ocean (salinity and 18O variations)Effect of glaciation on the oxygen isotope composition of the oceanSea-level estimates from drilling submerged coral terraces Shackleton benthic foram 18O Shackleton benthic foram 18OOxygen isotope stratigraphyV19-29 (Eastern Tropical Pacific) Shackleton’s first high resolution 18O recordV28-238 (Western Pacific) paleomagnetic stratigraphy: the (more) correct 18O chronologyLife gets complicated: Duplessy et al. (1970) find species-dependent 18OEventually, Shackleton reports the same thing: core M12392 (northeast tropical Atlantic)Oxygen isotopes, phases 1 & 2, the plateauPart 2: supplementary referencesMIT OpenCourseWare http://ocw.mit.edu 12.740 Paleoceanography Spring 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.Oxygen isotope paleothermometry: the early applications• 1950's: Emiliani• A. Using Urey's mass spectrometer, which required about 5 mg of calcium carbonate (about 100-200 individuals of foraminifera), Emiliani analyzed a variety of species of planktonic foraminifera from the Caribbean and found an apparent depth stratification, with some species (G. sacculifer, G. ruber) recording isotopic temperatures close to that of surface seawater; others recording colder temperatures.• B. Emiliani analyzed downcore records of apparent surface dwellers throughout the Atlantic; making a correction for changes in the isotopic composition of seawater (more about this later), he calculated a 6-8°decrease in tropical ocean surface temperatures during glacial periods (i.e. about 1.5‰ δ18O increase). He found evidence for many glacial/interglacial cycles over the last half million years; he coined the isotope stage stratigraphy system (now commonly referred to as "MIS" (Marine Isotope Stage); and he argued that the data supported the Milankovitch mechanism of climate change.Emiliani (1954, Am. J. Sci. 252:149) temperature calibration from Gulf of Mexico core topsSpecies Isotopic TG. rubra 27.2G. sacculifera 25.1G. conglobata 23.2P. obliquiloculata 21.6G. menardii 19.8O. universa 19.8G truncatulinoides 16.1• Emiliani (1955 and other papers) then analyzed foraminifera from piston cores from the deep sea, and made temperature estimates:• He found multiple cycles of cold and warm periods during the past ~500,000 years.2. This work created quite a stir, and quickly was criticized on several grounds:a. It violated the prevailing 4-ice-age theory from continental geology.b. Meteorologists though that the tropical temperature change seemed excessive.c. Micropaleotologists thought that their micropaleontological work (G. menardiistratigraphy) contradictedwith O-18 record.d. Biologists (e.g. Bé) argued that foraminiferal ecological shifts may have altered the depth habitat of organisms(and hence temperatures).e. The time scale (based on 230Th/231Pa) was criticized.f. Various statistical errors were pointed out.Image removed due to copyright restrictions.FIG.--Core A179-4: percentages of the fraction larger than 74 µ and isotopic temperatures obtained from Globigerinoides rubra (a)Globigerinoides sacculifera (b), Glabigerina dubia (c), and Gluborolalia menardii (d).Oxygen isotope paleothermometry: the early applications• Despite all of this criticism, with 1996 hindsight we can say that Emiliani was right about most of these points, with the major exception being the time scale and the amplitude of tropical cooling.• The problem of the time scale: Emiliani derived his time scale from a core which had been dated by the "231Pa/230Th" method. This method assumes that the initial value of the ratio at zero age is the production rate from the 235U and 234U in seawater. We now know that this assumption is incorrect: Thand Pa are fractionated by their differential particle reactivity in the ocean.Rayleigh distillation of oxygen isotopesliquidvaporVapor pressure = f(T)(Clausius-Clapeyron equation, exponential with increasing T)At 25°C, the vapor pressure of H216O is 0.9% higher than H218OImagine a 50-50 mixture of liquid H216O and H218O, equilibrated with the vapor phase at 25°C. Separate the vapor from the liquid:δ18O = -9‰ T=25°C1009H216O1000H218OCool the vapor to 20°C; allow liquid to condense from vapor:δ18O = -11‰ T=20°C745H216O737H216O264 H216O 263 H218ORR0= fα−1Rayleigh equation:R0= initial isotope ratioR = isotope ratio after coolingf = fraction of water condensedα = isotope fractionation factorCumulative Rayleigh Isotope Distillation as a function of temperature Fraction Remaining Vaporδ 18O in cloud vapor and condensate plotted as a function of the remaining vapor in the cloud for a Rayleighprocess. The temperature of the cloud in degrees Celsius is shown on the lower axis. δ 18O values are relativeto SMOW. The increase in fractionation with decreasing temperature is taken into account. After Dansgaard (1964).1.0 0.750.500.25 0Condensate-5O (SMOW)-1018-15 δVapor-20-25-3020 15 10 0 -20Cloud Temperature OC9%11%Figure by MIT OpenCourseWare.Adapted from source: Broecker (1974)Chemical Oceanography.Observed δ18O - surface temperature relationshipNote: this line is not the relationship predicted by the Rayleigh distillation curve. It includes many other effects: evaporation-precipitation cycles, cloud-T / surface-T relationships; multiple sources of water vapor at different temperatures, etc. O (SMOW)18 δMean Annual Air Temperature OCMean annual δ 18O of precipitation as a function of the mean annual air temperature atthe earth's surface. Note that δ 18O values are progressively lighter as the mean


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