Microfossils from deep-sea are crucial elements for our understanding of past and present oceans. Their skeletons take up chemical signals from the sea water, in particular isotopes of oxygen and carbon. Over millions of years, these skeletons accumulate in the deep ocean to become a major component of biogenic deep-sea sediments.
The importance of microfossils as tool for paleoclimate reconstruction was recognized early in the history of oceanography. John Murray, naturalist of the CHALLENGER Expedition (1872-1876) found that differences in species composition of planktonic foraminifera from ocean sediments contains clues about the temperatures in which they lived.
Following this pioneering work, Schott working on sediments of the METEOR Expedition (1925-1927) introduced quantitative counting of species within the fossil assemblages on the sea floor and realized that surface water temperature changed as the climate fluctuated between glacial and interglacial conditions.
In 1955, Emiliani, who was then a student of Harold Urey at the University of Chicago, published a paper entitled “Pleistocene temperatures” where introduced isotope stratigraphy to paleoceanography. He used the density of a heavy oxygen isotope in planktonic foraminifera from deep sea cores to outline oxygen isotope stages for the Quaternary, believing these would reflect surface temperature changes and the ice volume changes. He concluded that the last glacial cycled had ended about 16,000 years ago, and found that temperature increased steadily between that time and about 6000 years ago. Many of Emiliani’s findings are still valid today, however in 1970 several improvements to Emiliani’s work were made, such as a revision of the temperature scale.
Oxygen isotope records have also been obtained from well-preserved microfossil materials in the Late Cretaceous when bottom waters appear to have been much warmer than at present.
This concepts of paleotemperature reconstruction, as first developed for planktic foraminifera, apply to other groups of microfossils. Diatoms and radiolarians are susceptible to different set of dissolution parameters than calcareous fossils, resulting in a different distribution pattern at the sea floor and have been used for temperature estimates in the Pacific and in the Antartic Oceans, especially where calcareous fossils are less abundant. Diatom assemblage are also used in reconstructions of paleoproductivity.
The calcareous nannoplankton represents good proxy for the sea-level fluctuations. The group exhibit a clear latitudinal distribution pattern, for instance, the presence of mixed nannofloral assemblages (taxa of low-middle latitudes together with high ones) are indicative of the sea-level rise, while endemic assemblages characterize periods of low sea-level.
By studying cores from those ocean sediments, its possible determine the ages of the rocks, the ocean environment and some atmospheric conditions using the information provided by the microfossils present in that core, as well as stable isotope analysis and magnetic stratigraphy.
Each layer of the core recorded the geological history of the ocean basins, changing climates, evolving biota and the events that could altered the course of Earth history.
Armstrong, Howard A. and Martin D. Brasier. Microfossils. Blackwell Publishing, 2005.
Berger, W. H., Sea level in the late Quaternary: patterns of variation and implications, Int J Earth Sci (Geol Rundsch) (2008) 97:1143–1150