History and Nature of Paleoceanography

A simple definition of "paleoceanography" is "the study of the development of ocean systems In a larger context, it involves the study of the interconnectedness of Earth systems. That interconnectedness is reflected in the history of paleoceanography, for it demonstrates how a multitude of human endeavors, including detailed scientific descriptions by individuals and by teams of researchers, brilliant syntheses by individuals and groups, visionary leadership by scientists and politicians, wartime technologies, and large-scale international scientific cooperation, all contributed to revolutionizing our understanding of the oceans. This knowledge base and recognition of the interconnectedness of Earth systems will be crucial to development of philosophies, in the 21st century, for local, regional and global management of Earth resources for future generations of both humans and other inhabitants of the planet.

The history of the oceans is recorded in the rocks and sediments of the ocean basins and margins. Deciphering that history has involved observations, research and discoveries in fields as diverse as geography, paleontology, petrology, structural geology, engineering, geophysics, sedimentology, geochemistry, and biological and physical oceanography. Kennett concluded that the rapid progress in Cenozoic paleoceanography has resulted from technical and conceptual breakthroughs in four major areas:

Stratigraphic Time Frames

Analysis of deep-sea cores and samples ranges from time-honored fossil identification and sediment grain-size analysis to use of the most sophisticated geophysical and geochemical tools. Data from high technology procedures are by no means more valuable than basic fossil and sedimentological evidence. In fact, fossils and sediments are the direct records of oceanographic processes; geochemical data can be irrevocably modified by diagenesis or can be misinterpreted because the biogeochemical processes that influenced a particular geochemical record might be poorly known or misunderstood.

The most important kinds of data from sediment cores are relative age dates that allow cores to be compared with one another. There are many ways to do this and usually several methods are used. Biostratigraphic correlations , based upon the makeup and changes in assemblages of planktic foraminifera, coccoliths, radiolaria and/or diatoms, are the basic means of comparing cores. Because these groups of microorganisms have different ecologic requirements and because their remains tend to be preserved under quite different deep-sea conditions, ocean-wide correlations require use of all of these groups. Because remains of silicious and calcareous microorganisms are scarce to absent in deep-sea clays, analyses of fish debris (ichthyoliths), spores and pollen are required to correlate those sediments. Evolutionary changes in plants and animals are unidirectional, so assemblages for any biostratigraphic zone are unique to that zone, which represents a relative time unit. Sedimentological, geophysical and geochemical data, with a few exceptions, provide records of abrupt fluctuations or gradual changes that have occurred numerous times in Earth history. Such fluctuations can often be correlated and may provide greater resolution than microfossils, but require microfossil data to accurately place within the relative time frame.

Paleomagnetic measurements are still among the most important geophysical data collected from deep sea cores. Most marine sediments contain little material of use in radiometric dating, which is the closest thing to "absolute" age dating available in geologic research. Thus, absolute age dates are often assigned by a three or more step process. Microfossils are used to determine the relative age of a sample, whose paleomagnetic signature is also determined. The known paleomagnetic episode from a deep sea core is correlated with its counterpart from a terrestrial volcanic event whose rocks have been dated radiometrically. That is how paleoceanographers estimate that a particular event occurred , for example, 36.5 million years ago.

Emiliani proposed that stable isotope signatures in fossiliferous sediments would provide high resolution stratigraphy, and that has occurred with technological advances in mass spectrometry. The highest resolution schemes are based upon the integrated use of biostratigraphy, magnetostratigraphy, and isotope stratigraphy. High-resolution isotope sequences are often interpreted in the context of Milankovitch cycles of 22,000, 41,000 and 96,000 years.

Plate Tectonics as a Context for Interpretation

Paleogeographic reconstructions based upon the theory of plate tectonics provide a context for interpreting the paleoceanographic record. For example, sequences of tropical limestones in the Emperor Seamounts in the subpolar northwest Pacific are readily interpretable if the seafloor/conveyor belt, upon which these seamounts sit, has moved from low to high latitudes over the past 100 million years.

Equally important is understanding the depth history of the ocean floor. There is a relatively simple relationship between age of the sea floor and its depth, caused by crustal cooling with distance from the spreading center. Sea floor depth (D) can be estimated as
D = 2,500 + 350 t0.5
where t is age of the floor in millions of years. This depth history provides a context for interpreting a sediment sequence of, for example, basalt crust overlain by calcareous ooze grading upward into siliceous ooze and finally into brown clay, that might be seen in a deep-sea sediment core taken in the northwest Pacific, where subsidence has occurred.

Perhaps most importantly, recognition of oceanic plate creation and subduction, and accompanying breakup and movement of continental plates, provides a context for interpreting major changes in oceanic circulation and global climatic patterns that have been recognized and will be summarized in the final section of this chapter. On the other hand, paleoceanographic evidence, including changes in sediment composition and texture, biogenic constituents and geochemical characteristics, have contributed immeasurable to interpreting the timing and effects of key tectonic events.

Tools and Techniques for Paleoceanographic Interpretations

The history of the oceans is recorded in the sediments and rocks of the deep sea and ocean margins. We read that history by studying the characteristics of those sediments and rocks by seismic profiling, sediment sampling and ocean drilling. Those basic techniques have been discussed in previous chapters.

The most basic data from a sediment core is the lithologic sequence, i.e., what are the types of sediments and rocks recovered in the core and in what order. As indicated in the chapter on deep sea sediments, the composition of the sediments has been influenced by the climatic conditions under which the terrigenous sediments eroded and in which the organisms that produced the biogenic sediments lived. Physical and geochemical conditions of the bottom-water masses and interstitial waters determined whether the sediment particles that reached the sea floor were preserved or altered.

There are three important long-term, unidirectional phenomenon that influence Earth history and the paleoceanographic record.

  1. as the Sun has aged, its solar output has increased about 40%
  2. the segregation of the Earth into the heaviest components in the core and the lightest components in the atmosphere, driven by the escape of heat from the Earth's interior and the gradual cooling of the Earth
  3. the evolution of life, which has profoundly changed the Earth's atmosphere from a CO2-rich, reducing atmosphere to a CO2-poor, oxidizing atmosphere.