It is interesting to consider how the MG&G community was so uniquely able to capitalize on the ability to make key observations even on non-MG&G cruises and to rapidly store the information in computer-aided archives. Whereas marine bathymetry, magnetics, and gravity could be collected while underway without interfering in whatever other science was to be accomplished on the trip, marine chemists, biologists, and physical oceanographers needed to stop to lower their instruments and collect their samples. Whereas the pertinent information on depth, magnetic field, and gravity field could be reduced to a simple series of numbers, this was not the case for water and biological samples. Even sediment cores collected by oceanographic institutions or by the drilling program were carefully cataloged, subsampled, and archived in a systematic way unduplicated for samples of interest in the other oceanographic disciplines. The ease with which key measurements could be acquired and shared, with help from NSF funding, helped propel U.S. researchers in MG&G to the forefront in two of the most important revolutions in science.
The saga of the plate tectonic revolution has been so oft cited that I will not take the time to repeat it here. It is the archetypal scientific revolution that had its roots back in Wegner's theory of continental drift in the 1920s. But plate tectonics was a concept that was poorly represented on the continents, and therefore there was little hope of getting the story straight before the post-World War II era of ocean exploration.
The decade of the 1950s was marked by a total lack of consensus on Earth history. Was the Earth expanding? Contracting? Did continents drift? Remain fixed? In 1959, Americans Harry Hess (from Princeton), Bill Menard, and Maurice Ewing were joined by the Canadian Tuzo Wilson and the British Sir Edward Bullard at an international oceanographic congress in New York City right after the end of the International Geophysical Year. All believed that the mid-ocean ridges were the source of some wholesale motion of Earth' s crust in a manner not compatible with continental drift. The data collected by Ewing and others showed that the mid-ocean ridges were clearly the youngest part of the seafloor. Wilson thought that Earth was expanding along the mid-ocean ridge system, whereas Ewing, Bullard, and Hess believed the ridges to be the rising limbs of thermal convection cells. Hess balanced the expansion with contraction at the trenches and mountain belts. Menard kept the continents in place while the seafloor recycled. After the congress, Hess and Robert Dietz wrote papers revising the notion of continental drift to include spreading seafloor. Most others were skeptical, citing the inability of rising and descending limbs of thermal convection to explain the fact that Antarctica is nearly entirely circled with mid-ocean ridges.
In 1963 came the breakthrough that would allow the concept of seafloor spreading to take a firm hold. Fred Vine and Drummond Matthews of Cambridge University became the first to publish the hypothesis that the puzzling magnetic anomalies in the ocean basins were the result of seafloor spreading combined with aperiodic reversals of Earth's magnetic field. In reaching this conclusion, they relied heavily on evidence just published by Allen Cox, Richard Doell, and Brent Dalrymple (Cox et al., 1964) for reversals of Earth's magnetic field globally recorded in volcanic rocks. This is one clear example of how advances in terrestrial Earth science research helped fuel a great discovery in MG&G. For the most part, however, it was an advantage not to have been too indoctrinated by the theories of terrestrial geologists in order to embrace the new paradigm.
Despite the attractiveness of the Vine-Matthews hypothesis, most Americans were still skeptical. George Backus published a paper in Nature in 1964 that proposed an elegant test of the Vine-Matthews hypothesis. He reasoned that the rate of seafloor spreading should increase from north to south in the Atlantic as a consequence of plate motion on a sphere. It should be simple enough to determine whether the pattern of magnetic stripes in the South Atlantic repeated that already found off Iceland, except with greater thickness to the stripes. His NSF proposal to fund just such an expedition was declined by a panel of his peers as being ''too speculative." NSF would soon prove the validity of the plate tectonic hypothesis, but not through deliberate forethought.
In 1965, J. Tuzo Wilson published a new explanation for the offset of the magnetic lineations across fracture zones. The lineations were offset because the ridge itself was offset (Figure 1). Earthquakes occurred only along the segment of the fracture zone between the two ridges where he predicted, based on seafloor spreading, that crust was moving in opposite directions. Later, Lynn Sykes at Lamont would go on to prove Wilson's hypothesis by showing that the first motions of earthquakes were consistent with this theory.
Lithospheric plate motion in three dimensions shows plate generation along the mid-ocean ridges, transform motion associated with ridge offsets, and sinking of the plate at the ocean trenches. Reprinted from Isacks et al. (1968), with permission from (more...)
The tide turned in favor of the acceptance of seafloor spreading with the publication of the Eltanin-19 profile (Figure 2). The Eltanin was a southern ocean research ship owned by the National Science Foundation and operated by Lamont until she was retired in 1973. Walter Pitman, a student at Lamont, was the first, in December 1965, to take a careful look at that profile across the South Pacific and note the nearly perfect symmetry in the magnetic lineations. Eltanin -19 was fortuitous; it was collected in the Southern Ocean near the magnetic pole such that the magnetic anomalies were large and barely skewed. The seafloor spreading history had been steady to first order, with no major plate reorganizations back to 80 million years. Pitman began numbering the magnetic anomalies on a paper record, beginning at the left edge. By the time he got to the mid-ocean ridge, the numbers were large. He quickly realized that this would not do, erased his numbers, and began counting anew from the ridge outward. This original profile now hangs on the wall in John Mutter's office at Lamont. By this time, Cox et al. (1964) had firmed up the magnetic reversal time scale for the first few million years, and the correspondence with the spacing of the anomalies on the Eltanin profile was staggering. By February 1966, Pitman's colleagues at Lamont quickly embraced Vine-Matthews and the other tenets of the new theory. The institution with more than half of the existing magnetic and profiler records from the oceans and 80 percent of the deep-sea cores would from then on be working to help establish the evidence for seafloor spreading.
Comparison of magnetic anomaly profiles from the South Atlantic (A), and North Pacific (B) with the Elianin profile (C) from the South Pacific. Correlations of individual anomalies are indicated with dashed lines. |Shaded boxes are the magnetic reversal (more...)
The conversion of Lamont came just before a National Aeronautics and Space Administration (NASA) conference at Columbia in 1966 on the "History of the Earth's Crust." The papers ultimately presented there bore in many cases little resemblance to the abstracts submitted months earlier. The field was moving too fast. At this meeting, Heirtzler presented the results of the Eltanin surveys. After his talk, Pitman recalls:
Menard from Scripps, who had opposed [continental drift] sat and looked at Eltanin-19, didn't say anything, just looked and looked and looked. Next, Lynn Sykes delivered the one-two punch by showing that earthquake focal mechanisms on transform faults were consistent with J. Tuzo Wilson's theory of ridge offset. Menard returned to Scripps a complete convert.
Although the battle for acceptance of plate tectonics was quickly waged and won in the mid-1960s, there were still a number of details to be filled in, much of which was done under the sponsorship of NSF. The present-day plate kinematics were to be sorted out using the azimuths of transforms and the width of the near-ridge magnetic anomalies. The history of plate motions and reorganizations needed to be worked out, a problem often requiring targeted expeditions funded by NSF to key areas where there were gaps or complexities in the magnetic records. Second-order effects, such as the existence of propagating ridges and microplates, were observed from detailed surveys and found to be important mechanisms for accommodating changes in the direction of relative plate motion.
The vertical motion of the seafloor was predicted from conductive cooling relations and compared with the depth data. The archives of heat flow observations were compared with what was predicted based on the thermal cooling model that fit the subsidence of the seafloor away from the ridges, but were found lacking. The conductive heat flow was less than predicted near the ridges and on the flanks, leading to the proposal that hydrothermal circulation was appreciable in young crust. Later expeditions funded by NSF, notably the RISE (Rivera Submersible Experiments) Expedition to the East Pacific Rise in 1979, found the "smoking gun" for hydrothermal circulation near mid-ocean ridges in the form of hot vents and the completely unexpected chemosynthetic food chain associated with them. Thus, even the crowning achievement in the field of marine biology can be claimed by MG&G.
Hotspots, although not a natural component of the plate tectonic paradigm, proved to be a useful indicator of the direction and speed of absolute plate motion. Observations of the flexure of the lithosphere beneath the weight of the hotspot islands and seamounts, and seaward of subduction zones, were used to calibrate the strength of the oceanic plates. These studies, funded mostly by NSF, led to unprecedented abilities to predict the horizontal and vertical history of seafloor in all of the world's oceans.
I recall the first time I heard about the theory in 1972. I was an undergraduate at Colorado College majoring in physics, soon to graduate. One of my physics professors gave me an article from Scientific American written by John Dewey describing the new theory. After the geology courses I had taken that spoke of geosynclines deformed under unknown forces, plate tectonics seemed so simple and elegant. Soon after, J. Tuzo Wilson came to speak at the college. I was hooked. I had already applied to graduate school in physical oceanography, but quickly decided that geophysics was what I really wanted to study—nothing like getting in on the first decade of a major paradigm shift. On my first oceanographic expedition, there was no one more senior than the graduate students, including the two co-chief scientists, Peter Lonsdale and Kim Klitgord. Everything had to be discovered anew and reinterpreted in terms of the new model, and who better to do it than the graduate students who had no stake in any previous ideas?
It is impossible to understate the importance of plate tectonics. It grandly explained the distribution of earthquakes and volcanic eruptions. It exactly predicted evolutionary patterns and distributions of related species. It predicted the history of possible pathways for ocean circulation, trends in ocean volume that controls sea level, and alteration of seawater chemistry via fluid circulation at ridges and trenches. In the chemosynthetic colonies in the hot vents, it might even explain the origin of life.
Reconstruction of Earth's Paleoclimates
The impact on society of the use of MG&G observations to reconstruct paleoclimates has been no less important and followed fast on the heels of the plate tectonic revolution. Whereas the time scales for plate tectonics are measured in millions of years, the deep sea record from sediment cores has taught us that Earth's climate vascillates on thou-sand-year time scales, and possibly much less. No great revolution sparked the acceptance of the climate proxies from the deep sea, as was the case in plate tectonics, but the impact on mankind could be much greater. We doubt that plate tectonics will render Earth uninhabitable for mankind on a human time scale, but there is every reason to believe that natural climate cycles enhanced by man's degradation of air, water, and land could result in an Earth unable to support the present population in a matter of centuries or less.
The climate story is also one of fortuitous gathering of samples, specifically the deep-sea cores, before their significance was established. A large number of researchers labored long and hard to work out the biostratigraphy of the cores using the carbonate and siliceous shells of microscopic marine animals. These cores demonstrated that the carbonate compensation depth in the oceans had varied over time, for not completely understood reasons, as had sea level. Furthermore, the microfossils indicated that there had been sudden swings of climate from warm-loving to cold-loving marine planktonic microfossils and back again at rates too fast to have been caused by plates drifting into different climate zones. But the resolution in the biostratigraphy was too poor to work out the rates of climate shift and to establish absolute global synchronicity. Here again the pioneering work of Cox et al. (1964) proved useful, in that the reversal of Earth's magnetic field at the beginning of the Bruhnes epoch, about 700,000 years ago, was often faithfully preserved in the paleomagnetic field of the core, such that it provided at least one absolute calibration point for estimating average rates of sediment accumulation.
Nick Shackleton, a British marine geologist, was the foremost figure in promoting another proxy for climate change, stable isotopes. Working in England, he used a high-resolution mass spectrometer to analyze the down-core oscillations in the ratio of the heavy oxygen isotope, 18O, to the light oxygen isotope, 16O. Based on the correlation with the biostratigraphy, these variations were clearly correlated with changing climate, but it was unclear whether the isotopic variations were caused by changes in ocean temperature or in terrestrial ice volume. With the encouragement of NSF, Shakleton became the first international corresponding member of NSF's CLIMAP program, which sought to decipher Earth's paleoclimate during the last glacial maximum. U.S. researchers were intrigued by Shakleton's stable isotope work, and Shakleton badly needed better samples on which to work. He had been using samples collected 100 years earlier by the HMS Challenger! Under CLIMAP sponsorship, Shakleton came to the United States and worked on core V28-238, a high-resolution core in the Lamont data bank collected by the Vema from the Ontong Java Plateau (Figure 3). This core contained well-preserved benthic and planktonic foraminifera, which showed the same oxygen isotopic signal. The argument was that whereas surface waters are very prone to temperature changes, the deep sea is roughly isothermal. Therefore, the fact that the signal was the same in the surface waters as the deep sea argued that the ultimate cause was climate-related changes in ice volume, not temperature directly.
Oxygen isotope records of planktonic and benthic foraminifera. Reprinted from Shackleton and Opdyke (1973), with permission from Academic Press, Inc.
The impact of the development of the stable isotope proxy on paleoceanography was substantial. On the assumption that sedimentation rates were constant throughout the entire Bruhnes epoch, the oscillations in the stable isotopes became the paleoclimate equivalent of the magnetic reversals for plate tectonics. The pattern could be used for global correlation. But unlike the magnetic reversal signal, which defies prediction and is likely an excellent example of chaos, there was a pattern to the variations in the oxygen isotopes. In 1976, Hays at Lamont, working with Imbrie at Brown and Shackleton, applied spectral techniques to the signals from cores that were thought to be fairly well dated such that the isotopic signal as a function of depth could be accurately converted to a time series. The result was the identification of spectral peaks that matched the predictions of the Milankovitch hypothesis (Figure 4). According to this theory, variations in Earth' s orbital parameters (eccentricity, tilt, and precession of the equinoxes) caused variations in solar insolation that resulted in changes in climate.
Spectra of climate variations in sub-antarctic piston cores as inferred from variations in oxygen isotopes. Prominent spectral peaks, labeled a, b, and c, correspond to the predicted periods of eccentricity, obliquity, and precession of the Earth's orbit. (more...)
Although there was some cause to question how well core depth had been converted to time, the strength of the spectral peaks and the repeatability of the pattern won many converts—so much so that now cores with poor age control are assigned dates by assuming that the isotopic peaks and troughs should correspond in time to what is predicted by the Milankovitch hypothesis ("orbital tuning"). Not all is completely understood, however. For example, northern and southern hemispheres would be predicted tc be out of phase for the precession period, but they are not. Overall, phase relationships demonstrate that regional insolation is not important. The net effect on the whole globe ,with its unequal distribution of continents and oceans must be taken into account. In addition, the strength of the spectral peaks is not consistent with the hypothesis that it is variations in solar insolation that leads to ice volume variations, and the spectral amplitudes are not stationary in time. Despite these remaining questions, the deep sea has provided a well-calibrated record of Earth's natural climate changes that can be used to help assess the future impact of man's activities.
The National Science Foundation was by far the greatest supporter of climate research, including the very successful CLIMAP project (Figure 5). A large amount of the paleoclimate work was supported and continues to be supported by NSF-MG&G. However, the Division of Atmospheric Sciences and the Ocean Drilling Program were also major players. MG&G has benefited greatly from broader NSF initiatives in global change that support paleoceanographic research beyond what the MG&G program could afford.
Sea-surface temperatures for northern hemisphere summer 18,000 years ago as determined by climate proxies mapped by the CLIMAP project. Contour intervals are 1°C for isotherms. Black dots show the locations of cores used to determine paleoclimate. (more...)
Across multiple Departments within the School, marine geologists explore the intersection of water, sediments, and life in the deepest regions of Earth’s oceans. Students and faculty pursue research topics ranging from microbial chemical transformations in our environment, to remote sensing mapping of the seafloor. Staff seek to understand the processes responsible for the creation and destruction of important natural resources, seafloor mud volcanism, and tectonic subduction. Using ships, remote sensing systems, and innovative analytical methods, ongoing research in SOEST aims to place new constraints on the chemical evolution of our oceans and atmosphere, and the underlying physical processes that govern this history.
- Dunn, Robert A.
- Edwards, Margo
- Fletcher, Chip
- Fryer, Patricia
- Hey, Richard N.
- Johnson, Kevin
- Martinez, Fernando
- McMurtry, Gary M.
- Rognstad, Mark R.
- Shor, Alexander (Sandy)
- Sinton, John M.
- Taylor, Brian
- Wessel, Paul