Discovery of Ice Age Climate Cycles
Arrhenius carbonate cycles seen in two cores (#45 on the left and #60 on the right) from the eastern Pacific, graphed as depth versus carbonate concentration (given as percent). Note the correlation in the calcium carbonate patterns despite the fact that the sediment cores came from different locations. An approxiamate age scale is located on the far left.
Arrhenius Cycles & Parker Cycles
In hindsight, perhaps the single most important discovery of the Swedish Deep-Sea Expedition was the observation of the cyclic nature of patterns of color, chemistry, and fossil content of the sedimentary sequences in the Albatross cores. The best-known example of this type of evidence was the cycles of carbonate minerals buried in the equatorial Pacific. These were discovered by Gustaf Arrhenius, member of the expedition and later a faculty member of Scripps Institution of Oceanography, who used carbonate chemistry and observations in the microfossils (the diatoms and foraminifers) to successfully argue for a cyclic pattern to glacial-to-interglacial changes on the productivity in the eastern tropical Pacific. Arrhenius proposed that the changing strength of trade winds resulting from cooling and warming of the regions outside the tropics caused corresponding changes in the strength of equatorial upwelling and vertical ocean mixing. In turn, the fluctuations in vertical mixing were responsible for variations in the rate of nutrient supply to the sunlit layer, hence changing the rate of growth of diatoms and other plankton.
Other scientists continued to investigate the Albatross cores. Eric Olausson and his collaborators conducted the geochemical and micropaleontological studies in Gothenburg, Sweden for many years after the expedition. They extended their findings to all ocean basins, adding fundamental concepts concerning changes in deep circulation in response to climate change. Frances Parker was the first to provide a systematic and
quantitative representation of faunal cycles for the Mediterranean, based on planktonic foraminifers in the Albatross cores. Her careful work demonstrated and established a new kind of statistical paleontology, which subsequently provided the basis for semi-quantitative reconstruction of the environment of growth of the organisms whose remains make up the fossils counted.
Parker cycles of ocean circulation from Mediterranean Sea cores. Note Parkerís original age estimates on the right and the revised age estimates from your instructors.
The most fruitful work on cyclic sedimentation in the deep-sea record was done by the Italian-American paleontologist and isotope chemist Cesare Emiliani, then at the University of Chicago. At the suggestion of Harold Clayton Urey (1893-1981), his distinguished mentor, Emiliani analyzed the oxygen-isotope composition of planktonic foraminifers within the sediment sequences. Urey had already established, with his collaborators, that the ratio between two different isotopes of oxygen atoms (O-16 and O-18) within carbonate shells is a measure of the temperature during which the shell-forming organism grew (Recall Section 7.0 and application of oxygen isotopes).
Urey was delighted to see that Emiliani's measurements established regular oxygen-isotope cycles in the deep-sea sediments, and especially that these cycles could be correlated over large distances from one region to another. This fact emerged when Emiliani studied additional cores from other expeditions that had been launched by the newly founded Lamont Geological Observatory of Columbia University, in Palisades, New York (now called the Lamont-Doherty Earth Observatory).
However, when the first results of Emiliani's isotope measurements were read from a mass spectrometer the University of Chicago, chemists immediately realized that these data could not be simply interpreted in terms of ocean temperature alone. Measurements on snow and rain had already shown that within the precipitation in high latitudes the lighter isotope, O-16, was more abundant relative to what is found in seawater. This meant that when ice sheets grew, during the glacial periods, O-16 would be preferentially extracted with the water leaving the ocean, and glacial-time seawater would become enriched in O-18. Emiliani reasoned that the great North American ice sheet would have had roughly the same composition, in terms of the O-16 to O-18 ratio, as the snow falling in Chicago during present winters. From this convenient guess he calculated the change of the ratio in the ocean for conditions with and without great continental ice sheets. As it turned out, his calculation was off by a factor of two (better values emerged as the Greenland ice sheet and the Antarctic ice were drilled), but the principle of his idea was correct. The next question was: What was driving these cyclic patterns in deep-sea sediments?