Earth Climate Rhythm-Deciphering from shells of sea creatures

The local weather changed seasonally within one year, notablely, transition between summer and winter as most people has experienced. The Earth climate also experience similar rhythm of climate change between icehouse and warmhouse at the long time scale during the past 500 thousand years. This finding was first demonstrated by Dr.Emiliani, the pioneer scientist in the field of oceangraphy.

He found this rhythm using the shells of some sea creatures called foraminifera. These creatures live in the ocean surface water and usually produce Shells to protect them. The shells are made of carbonate from which the blackboard chalk is made. After the living foraminifera died, they would sink into to the ocean floor and preserve layer by layer with time.

Left picture: A living planktonic foraminifera in the subtropical waters of the Atlantic Ocean. (Credit: Howard Spero, UC-Davis)

right picture: A Scanning Electron Microscope image of planktonic foraminifera,Shells (Credit: Image courtesy of Cardiff University)

The Earth scientists analyzed the specific chemical composition of these shells to extract the sea surface temperature in the past. This chemical composition is the oxygen isotopic ratios. The values of oxygen ratios are thought to be dependent on the ambient seawater temperature. the larger of oxygen ratios ,the cooler the sea surface temperature , vice verse, the lower of oxygen ratios,the warmer the sea surface temperature.

These pioneer workers found systematic periodic changes in the oxygen ratios and they thought that the Earth climate has experience the warm and cold transition in the past 900 thousand years. This finding opened up a growing field-paleooceangraphy and nutrient generations of Earth scienctist to study the Earth climate in geological history.

Figure1:(Source:http://portale.ingv.it/research-areas/climate-oceans-environments/reconstruction-of-paleoclimatic-variations) Oxygen has three stable isotopes 16O, 17O and 18O. If the two substances that participates the oxygen isotope are isotopic equilibrium, the magnitude of isotope fractionation (difference of isotopic ratios of 18O/16O) between these two phases is a function of temperature proposed by Urey (1947). An empirical equation relating oxygen isotope ratios(carbonate-water) to water temperature was calibrated in one classical paper by Epstein et al., 1953. Thus, Temperature can be estimated at which the isotopic equilibrium achieved if the isotopic ratios (18O/16O) of involved materials could be measured or constrained. The 18O/16O of planktonic foraminifera shells is controlled by the ancient seawater tempeture and also on the size of the continental ice volume .After correction for the ice-volume effect, the sea surface temperature can be traced. The graph shows the transitions between glacial – interglacial climate in the past 900ky.

Reference:

Michael Berglund and Michael E. Wieser, 2011,Isotopic compositions of the elements 2009Pure Appl. Chem., Vol. 83, No. 2, pp. 397–410, 2011.doi:10.1351/PAC-REP-10-06-02

H. C. Urey (1947). "The Thermodynamic Properties of Isotopic Substances". J. Chem. Soc.: 562–581

Epstein et al., 1953 S. Epstein, R. Buchsbaum, H. Lowenstam and H.C. Urey, Revised carbonate-water isotopic temperature scale, Bull. Geol. Soc. Am. 64 (1953), pp. 1315–1326

Emiliani, D. (1954) Depth habitats of some species of pelagic foraminifera as indicated by oxygen isotope ratios, Am. J. Sci. 252:149-158.

Emiliani, C. (1955) Pleistocene temperatures, J. Geol. 63: 538.