Alberto Malinverno

Walter Pitman, magnetic anomalies, seafloor spreading, and the geologic time scale, by Alberto Malinverno

“It hit me like a hammer.” This is how Walter recalled his realization of what the Eltanin-19 profile meant [1, p. 203]. That hammer hit the final nail in the coffin of a long-held static view of the solid Earth, provided the decisive confirmation of seafloor spreading at mid-ocean ridges, and prompted the development of plate tectonics. This note highlights Walter’s seminal work on magnetic anomalies and connects it to the developments that followed. More detail can be found in historical overviews of the plate tectonic revolution [2, 3, 4]. Walter wrote a summary of his recollections of this time [5], and audio and video interviews are also available [6].

Eltanin-19

Starting in the early 1960s, progress in the radioisotopic dating of volcanic rocks, the discovery of rocks that were magnetized in the opposite direction of the present magnetic field, and the realization that lava flows of the same magnetic polarity occurred at the same time on different continents demonstrated that the Earth’s magnetic field reversed itself several times in the last few million years (Myr). This work led to the development of an initial magnetic time scale with about ten intervals of alternating normal and reversed polarity between the present and 3.4 Myr ago [2, Parts I and II]. The same reversal pattern was also observed in the magnetization of sediments deposited on the ocean floor [7].

Around the same time, it was observed that anomalies of the magnetic field (highs and lows that remain once the large-scale field trend is subtracted) followed an ordered pattern in ocean regions. A prominent anomaly high along mid-ocean ridge crests was surrounded by alternating lows and highs arranged in symmetric stripes parallel to the ridge crest. Combining these observations with the idea that mid-ocean ridges are created by seafloor spreading, Vine and Matthews [8] (and independently Morley [10]) proposed that lavas that solidified at ridge axes acted as a tape recorder, creating a series of crustal blocks on ridge flanks that were magnetized in opposite directions and generated the observed symmetric magnetic anomaly stripes. This came to be known as the Vine-Matthews-Morley hypothesis [2, Chapter 7], and was originally formulated without a close comparison to measured anomalies. In 1965, Vine and Wilson [9] compared the magnetic anomalies predicted from the existing magnetic reversal sequence (0-3.4 Myr ago) with a profile crossing the Juan de Fuca Ridge in the northwest Pacific. The measured profile displayed some symmetry, and the comparison between predicted and measured anomalies was close, although the fit implied a variable rate of spreading in the last 2.5 Myr.

The evidence for the Vine-Matthews-Morley hypothesis was not enough to convince all, and several authors claimed that it did not fully explain the observed magnetic anomaly patters. In hindsight, a smoking gun was needed: a data set of magnetic anomalies that were indisputably symmetric about the ridge axis and that recorded a long sequence of alternating field polarities. That was Eltanin-19.

Walter and Ellen Herron sailed on Leg 20 of the Eltanin, a military ship converted for oceanographic work around Antarctica. In this voyage, Walter “paid his dues” at sea and gained access to measurements 1 from from the Eltanin Legs 19, 20, and 21 [5, p. 88]. Back at Lamont, Walter processed the magnetic data, which were digitized and formatted so that they could be readily available for any future study. Maurice Ewing established this far-sighted policy that made Lamont the data bank for plate tectonics ([2, Chapter 7]; [4, p. 469]). Once the magnetic anomaly profiles were plotted, the Eltanin-19 profile showed strikingly symmetric patterns on both flanks of the Pacific-Antarctic ridge (Figure 1). The Eltanin-19 data confirmed the Vine-Matthews-Morley hypothesis to an unprecedented degree: the symmetry of the anomalies was absolutely clear out to nearly 500 km from the ridge axis (compared to about 100 km in Vine and Wilson [9]), and the anomaly patterns correlated between profiles from Eltanin Legs 19, 20, and 21 that were hundreds of kilometers apart.

The 1968 polarity time scale of Heirtzler, Pitman et al. In March 1968, three summary papers based on the Lamont data bank demonstrated that magnetic anomalies in the Indian, Pacific, and South Atlantic Oceans displayed the same symmetric pattern out to more than 1,000 km from the ridge axis [15, 16, 17]. A fourth paper by Heirtzler, Pitman and collaborators summarized these studies and established a geomagnetic polarity time scale that covered the last 80 Myr or so of Earth’s history [18].

As previously done by Pitman and Heirtzler for the last 10 Myr, the time scale was obtained assuming a constant spreading rate and a single date of 3.35 Myr for the beginning of the Gauss normal polarity interval [19]. In principle, any one of the Indian, North Pacific, South Pacific, and South Atlantic profiles could have been used to generate the time scale. The Indian Ocean data were excluded because they did not record the oldest anomalies, whereas spreading rates in the South and North Pacific seemed to have varied with time when compared to data from other oceans. Hence, the time scale was based on the magnetic anomaly pattern in the South Atlantic.

This time scale allowed for dating large portions of the ocean floor, suggesting that roughly half of the global ocean had to be younger than 80 Myr and that the Earth’s oceans were young compared to the age of the continents. Heirtzler et al. also explored the implications of the ocean floor age pattern on the breakup of Gondwanaland, the large Mesozoic supercontinent in the southern hemisphere.

The Heirtzler et al. 1968 time scale also enjoyed a spectacular confirmation. During Leg 3 of the Glomar Challenger, a number of holes were drilled at different distances from the axis of the South Atlantic ridge. These drill holes reached the top of the volcanic ocean crust, and the “paleontological ages” of the oldest sediments determined from their fossil content were found to match the “magnetic age” predicted by the time scale within a few Myr [20, Table 6]. During its South Atlantic operations, the Glomar Challenger coincidentally came to be a few miles from the Lamont ship Vema, with Walter on board (luck struck again). He was invited to go on the Glomar Challenger to celebrate, and came back to the Vema with a gift of frozen steaks [21].

Progress in the geomagnetic polarity time scale

Heirtzler et al. recognized that assuming a constant spreading rate was a fundamental limitation of their 1968 time scale: “The possible error inherent in such an extrapolation cannot be overemphasized. By adopting the South Atlantic time scale we are assuming that the ocean floor has been spreading at a constant rate [. . . ] for the last 80 [Myr]” [18, p. 2122]. For example, the South and North Pacific seemed to have variable spreading rates when compared to the South Atlantic; but if spreading rate changed on other mid-ocean ridges, why should it be constant just in the South Atlantic?

To improve the time scale reliability and accuracy, spreading rate variations had to be taken into account. As more radioisotopic dates tied to magnetic stratigraphy became available, it became clear that South Atlantic spreading rates could not have been not been absolutely constant. The 1992 geomagnetic polarity time scale of Cande and Kent [22] accounted for this variation by smoothly interpolating between nine 3 radioisotopic dates the distances to magnetic anomalies measured in a South Atlantic traverse near that used by Heirtzler et al. (Cande and Kent also added details from faster-spreading ridge flanks in the Pacific and Indian Oceans).

A shortcoming of the Cande and Kent GPTS is that while spreading rates change smoothly with time in the South Atlantic, they vary considerably from a polarity interval to the next in other spreading centers, for example in the well mapped North Pacific [22, Figure 42]. Intuitively, rapid fluctuations of spreading rate seem unrealistic as they would require accelerating or decelerating large tectonic plates. A rigorous analysis of the maximum rate of change in the forces that affect plate motion (mainly subduction and orogeny at active margins) shows that spreading rate changes of 10-20% can only take place over at least 1 Myr [23].

Even if the time scale were absolutely accurate, the spreading rates estimated over any polarity interval in a profile would be affected by uncertainties in the distances between anomalies. These uncertainties are due to many sources, such as navigation errors, noise in the magnetic field measurements, and imperfections of the mid-ocean ridge tape recorder. Therefore, there is no reason to expect that estimated spreading rates would vary smoothly over a single mid-ocean ridge while being erratic everywhere else; the most accurate time scale would instead minimize the fluctuations of computed spreading rates over as many mid-ocean ridges as possible [24]. An updated GPTS that minimizes global spreading rate variations has been obtained recently for the interval between 84 and 33 Myr ago [25]. This time scale is based on 154 ship tracks that measured magnetic anomalies in the Southern and Northern Pacific, the Southern Atlantic, and the Indian Ocean (Figure 2).

Conclusion

Eltanin-19 was “Pitman’s Magic Profile” [13, p. 1181], a chance discovery that settled the case for seafloor spreading. As more and more data were acquired, studies progressively used a growing, global magnetic anomaly data set to inform the magnetic polarity time scale. In a broad view, the field went from a heroic phase of exploration and revolution to a protracted time of “normal science” focused on the refinement of the key revolutionary ideas [26]. The data acquired in the early stages are still relevant: a ship track segment from the Eltanin Leg 19 is one of those that were used in the most recent update of the polarity time scale (Figure 2). Maurice Ewing originally insisted on recording all the data all the time and making them easily available for studies that were not even imaginable when data were acquired. This enlightened vision is the precursor of the many open access data repositories that today sustain progress in studies of the Earth.

Walter’s discoveries tied together ideas and observations from different fields: marine magnetic anomalies, paleomagnetic measurements in continental lava flows and marine sediments, and radioisotopic dating. His pioneering work on the geomagnetic polarity time scale led to an unprecedented advance in dating the ocean floors. Moreover, the polarity time scale provided an absolute dating reference for stratigraphic studies, giving ages that constrain the timing of environmental changes recorded in sediment sequences. With his usual modesty, Walter often remarked on the serendipity of his findings and on his fortune of being in the right place at the right time. We were all immensely lucky to have him.

References

[1] Wertenbaker, W. (1974). The floor of the sea: Maurice Ewing and the search to understand the Earth. Boston: Little, Brown.

[2] Glen, W. (1982). The Road to Jaramillo. Stanford, CA: Stanford University Press.

[3] Oreskes, N. (2001). Plate tectonics: an insider’s history of the modern theory of the Earth. Boulder, Colorado: Westview Press.

[4] Frankel, H. R. (2012). The Continental Drift Controversy: Volume 4: Evolution into Plate Tectonics. Cambridge: Cambridge University Press. https://doi.org/10.1017/CBO9781139095938

[5] Pitman, W. (2001). On board the Eltanin-19. In N. Oreskes, Plate tectonics: an insider’s history of the modern theory of the Earth (pp. 86-94). Boulder, Colorado: Westview Press.

[6] Recordings of interviews with Walter are available at https://dlc.library.columbia.edu/catalog/cul:rjdfn2z52b https://www.ldeo.columbia.edu/news-events/walter-pitman-smoking-gun-plate-tectonics https://soundcloud.com/user-306819187/plate-tectonics-ldeo

[7] Opdyke, N. D., Glass, B., Hays, J. D., & Foster, J. (1966). Paleomagnetic Study of Antarctic Deep-Sea Cores. Science, 154(3747), 349-357.

[8] Vine, F. J., & Matthews, D. H. (1963). Magnetic anomalies over oceanic ridges. Nature, 199(4897), 947-949.

[9] Vine, F. J., & Wilson, J. T. (1965). Magnetic Anomalies over a Young Oceanic Ridge off Vancouver Island. Science, 150(3695), 485-489.

[10] Morley, L. W. (2001). The Zebra Pattern. In N. Oreskes, Plate tectonics: an insider’s history of the modern theory of the Earth (pp. 67-85). Boulder, Colorado: Westview Press.

[11] Sykes, L. R. (2019). Plate tectonics and great earthquakes: 50 years of earth-shaking events. New York: Columbia University Press. 5

[12] Opdyke, N. D., & Pitman, W. (1997). Pitman receives Ewing Medal. Eos, Transactions American Geophysical Union, 78(11), 119-122. https://doi.org/10.1029/97EO00076

[13] Opdyke, N. D. (1985). Reversals of the Earth?s magnetic field and the acceptance of crustal mobility in North America: A view from the trenches. Eos, Transactions American Geophysical Union, 66(47), 1177-1182. https://doi.org/10.1029/EO066i047p01177-05 [14] Pitman, W. C., & Heirtzler, J. R. (1966). Magnetic anomalies over the Pacific-Antarctic Ridge. Science, 154, 1164-1171.

[15] Pitman, W. C., Herron, E. M., & Heirtzler, J. R. (1968). Magnetic anomalies in the Pacific and sea floor spreading. Journal of Geophysical Research, 73(6), 2069-2085. https://doi.org/10.1029/JB073i006p02069 [16] Dickson, G. O., Pitman, W. C., & Heirtzler, J. R. (1968). Magnetic anomalies in the South Atlantic and ocean floor spreading. Journal of Geophysical Research, 73(6), 2087-2100. https://doi.org/10.1029/JB073i006p02087

[17] Le Pichon, X., & Heirtzler, J. R. (1968). Magnetic anomalies in the Indian Ocean and sea-floor spreading. Journal of Geophysical Research, 73(6), 2101-2117. https://doi.org/10.1029/JB073i006p02101

[18] Heirtzler, J. R., Dickson, G. O., Herron, E. M., Pitman, W. C., & Pichon, X. L. (1968). Marine magnetic anomalies, geomagnetic field reversals, and motions of the ocean floor and continents. Journal of Geophysical Research, 73(6), 2119-2136. https://doi.org/10.1029/JB073i006p02119

[19] Doell, R. R., Dalrymple, G. B., & Cox, A. (1966). Geomagnetic Polarity Epochs: Sierra Nevada Data, 3. Journal of Geophysical Research, 71(2), 531-541. https://doi.org/10.1029/JZ071i002p00531

[20] Maxwell, A. E., Herzen, R. P. V., Hsu, K. J., Andrews, J. E., Saito, T., Percival, S. F., et al. (1970). Deep Sea ¨ Drilling in the South Atlantic. Science, 168(3935), 1047-1059. https://doi.org/10.1126/science.168.3935.1047

[21] Minute 12:35 of the recording at https://soundcloud.com/user-306819187/plate-tectonics-ldeo

[22] Cande, S. C., & Kent, D. V. (1992). A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic. J. Geophys. Res., 97(B10), 13,917-13,951.

[23] Iaffaldano, G. (2014). A geodynamical view on the steadiness of geodetically derived rigid plate motions over geological time. Geochemistry, Geophysics, Geosystems, 15(1), 238-254. https://doi.org/10.1002/2013GC005088

[24] Huestis, S. P., & Acton, G. D. (1997). On the construction of geomagnetic timescales from non-prejudicial treatment of magnetic anomaly data from multiple ridges. Geophys. J. Int., 129, 176-182.

[25] Malinverno, A., Quigley, K. W., Staro, A., & Dyment, J. (2020). A Late Cretaceous-Eocene Geomagnetic Polarity Time Scale (MQSD20) that steadies spreading rates on multiple mid-ocean ridge flanks. Earth and Space Science Open Archive. https://doi.org/10.1002/essoar.10502925.1

[26] Kuhn, T. S. (1962). The structure of scientific revolutions. Chicago: University of Chicago Press.