18O isotope ratios as Climate Proxy
Fractionation mechanisms Fractionation ratios and temperature Ice cores analysis Minerals and sediments Isotope distribution in Corals and trees gradually accumulating arctic snow layers, ice, ocean sediments, fossils, rocks and minerals!
http://www.gerhardriessbeck.de Rayleigh processes A particular important case of isotope fractionation processes is the change of isotopic composition in a reservoir because of the removal of an increasing fraction of its content. Two compounds are formed, A and B. This would be the case of partial or sequential precipitation processes from the initial cloud with B being the rain and A the remaining cloud system. This process is described by the Rayleigh distillation law in terms of the fractionation factor and a factor f, which corresponds to the remaining 1 1 fraction of the reservoir. RB Rinitial f RA Rinitial f
1 Rcloud Rinitial f R: reservoir 18 18 evaporation O O 1 E: extract 16 16 f O cloud O initial cloud initial 18 18 1000 1 ln f O O
1 Rrain Rinitial f 18 18 O O 1 16 16 f precipitation O rain O initial rain initial 18 18 1000ln 1000 1 ln f O O A cloud migrates over land masses to higher latitudes gradually loosing rain. The fractionation factor is =1.008! Assuming the initial fractionation to be
0=-8 in the cloud water vapor after the evaporation process from ocean water.
The 18O isotopic abundance decreases with increasing distance to coast. 18O enriched water rains down first while slowly depleting the cloud of its 18O isotope content.
rain initial 18 18 1000ln 1000 1 ln f O O rain 0 18 f 0.7 8 1000ln1.008 1000 1.008 1 ln 0.7 3 O 00 rain 0 18 f 0.25 88 1000ln1.008 1000 1.008 1 ln 0.25 11 O 00
cloud initial initial 18 18 1000 1 ln f 18 0 O O O cloud 0 18 f 0.7) 8 1000(1.008 1)ln 0.7 11 O 00 cloud 0 18 f 0.25 8 1000(1.008 1)ln 0.25 19 O 00 The isotope cycle of water The temperature dependence of fractionation processes can be utilized to study the water cycle and its impact on the global fractionation distribution. The global water cycle relies on water exchange between the four reservoirs, ocean, cryosphere, fresh water, and atmosphere. A volume of ~1/2 million km3 of water is cycled annually by evaporation and precipitation processes.
Considering a cloud moving northwards gradually changing the 16O/18O abundance by precipitation. The cloud (and the rain) become enriched in light 16O isotopes with distance according to the Rayleigh distillation process but by moving north, the cooling of the environment enhances the distillation because of the temperature dependence of the fractionation factor ! The fractionation factor increases with decreasing temperature which cloud initial 18 18 1000 1ln f enhances the fractionation effect. O O 11.63 C2 0.03 C C1 cloud initial T 2 T 18 18 1000 e 1 ln f ln C1 e O O T for T in Kelvin : C 0.03 C 11.63 rain initial 1 2 18 18 1000ln 1000 1ln f O O 1.02 1.018 rain initial 11.63 1.016 18 18 1000 0.03 O O 1.014 T 1.012 1.01 11.63 1.008 0.03 T 1.006 1000e 1ln f Fractionation factor factor Fractionation -40 -20 0 20 40 60 Temperature oC 0
-10
-20
Direct correlation between the
O -30 cloud T=20C 18
18 O fraction and temperature!
rain T=20C -40 cloud T=20C->-20C
-50 rain T=20C->-20C
-60 0 0.5 1 1-f Correlation 18O to temperature
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Temperature dependence of 18O in snow and rain is clearly demonstrated by numerous measurements at different locations worldwide, as lower the temperature as lower the 18O content in snow. This introduces a perfect tool for determining temperature by the analysis of accumulated compressed snow layers in Arctic, Antarctic, or glacier environments. 18 O 0.675T 0 15 C Ice core drilling GRIP: European GReenland Ice-core Project GISP; US Greenland Ice-Sheet Project
Ice Cores Annual variations of 18O over a 30 year period of relatively constant global temperatures (medieval warm period). The Winter snowfall is consistently 18 o O depleted (-34‰, TW=-38 C) compared to the summer o snowfall (-26‰, TS=-32 C).
-38.5oC
-38.7oC Temperature record for more than 120,000 years
The observed 18O fluctuations represent the major climate (temperature) variations earth has experienced beginning with the onset of the last ice age period through the Holocene period of human evolution. Corresponding fluctuations were observed in the GISP 2 ice core samples Going back in time, ocean sediments, fossils, and rocks
On longer paleoclimate timescales ocean sediment analysis, the analysis of lithified rocks from former ocean sediments and the analysis of fossils in terms of elemental and isotope distribution in sediments of fossil molecules such as CaCO3 or other carbonates, SiO2 or other oxides. For fossil material biological processes may have changed 18O fractionation during the uptake. This has to be corrected to determine 18O ratio in former ocean water.
Rock with layers of iron oxide that Mineralized fossil layer in sedimentary was formed in Precambrian more rock. Dating occurs through the analysis than 500M years ago. of long-lived radioactivity in the fossil material. Paleothermometer CaCO3 Temperature 18O fractionation relation in carbonates is expressed by a thermometric relation developed by Harold Urey and Brian Epstein:
2 18O 18O 18O 18O T0 16.5 4.3 CO H O 0.13 CO H O C 3 2 3 2 18O simplified version is : T0 16.5 4.3 C CO3
18O 18O 16 16 18 O O O CO2 ,CO3 CO2 standard CO 1000 3 18O 16 O CO2 standard The accuracy of 18O measurements is 0.1‰, that translates into an uncertainty for the temperature of : 18 T 4.3 O 0.4 0C CO3 Jurassic belemnite, based on data it lived for 3.5 years; it was born in fall and died in spring (Urey 1951)
Paleothermometer SiO2
18 Measuring O for SiO2 requires to correct for systematic variations with the type of mineral and the type of rock the minerals belong to since they have formed at different high temperature conditions in the Pre-Cambrian phase, where the temperature dependence of the fractionation factor needs to be considered. C A ln B T T 2 The difference between two minerals M1 and M2 is: A106 B 1000ln M 1,M 2 M1 M 2 T 2
M 1,M 2 M 1,water M 2,water Example The 18O values for the minerals of a metamorphic rock are for quartz 18O=+14.8, magnetite 18O=+5. What is the temperature at which this rock was formed?
A106 B M 1,M 2 M 1 M 2 T 2 5.57 106 14.8 5 9.8 quartz,magnetite T 2 5.57 106 T 7540C 9.8 18O in corals
18O in coral skeletal reflects combination of surface temperature and salinity. In ocean regions where salinity is constant changes in coral skeletal 18O reflects change in sea surface temperature. Therefore primarily applicable to tropical ocean regions. In regions with strong rainfall or river run-off 18O can be used for salinity measures. Analysis of multiple isotope (or element) ratios can provide detailed information about the ocean environment history!
X-ray analysis of the banding pattern establishes the chronology scale of the environment history of the coral. Analysis of coral growth characteristics gives time line. Bright lines under UV radiation indicate fresh water inflow and therefore reduced salinity conditions.
Isotope analysis of corals from the Great Barrier Reef in Australia indicate a higher sea surface temperature and/or higher salinity of the pacific ocean in the 18th century than it is today. Climate history of Pacific and Indian Ocean environments
Variation of 18O in different coral reefs over the last 400 years. The black arrows indicate sudden changes in temperature or salinity which is frequently correlated with the El Nino effect. The cooler time period after 1800 may be related to volcano activity (Tambora on the Sunda Islands in Indonesia) El Nino reflection in 14C
The 14C enrichment in ocean surface water occurred after nuclear test program 1946-1969 due to ocean atmosphere carbon exchange.
Radiocarbon is enriched in Pacific Ocean surface water but reduced by upwelling of Humboldt current. If upwelling is prevented by ENSO effect, enhanced 14C ratios are observed. This was used to investigate strength and frequency of past El Nino events from 14C analysis of skeletal carbonate of corals from the Galapagos Islands. Dendrochronology
Dendrochronology or tree-ring dating is the dating of past events through study of tree ring growth.
Trees showing sensitive rings are affected by slope gradient, poor soils, or too little moisture and climate conditions.
Trees showing complacent rings have generally constant climatic conditions such as high water table, good soil, or protected locations. Dating back in time The oldest Trees Bristlecone pine Dendrochronologygroves are found drill atset elevations up to 11,000 feet (3352m).
"Methuselah" was found to be 4,723 years old and remains today the world's oldest known living tree.
Tree-ring dating ~7000 years backwards The world map of tree ring recordings
Tree ring analysis can be coupled 18O isotope fractionation analysis for climate studies and with 14C dating techniques to establish or verify the chronology! 18O in tree rings
5 year study of 18O uptake
Clear correlation of 18O and Dendrochronology has emerged as a climate in tree ring analysis! major tool for calibrating 14C dating results and plays an increasing role in climate analysis during the historic period but also for the entire Holocene! Climate correlation with human history and development
The recorded climate history of Earth
The curve records a sequence of cold periods of glaciation and warm periods of sometimes significantly higher temperatures than observed during the Holocene period of Human evolution to the present time.