Micro-magnetic field measurements near the ocean floor.
The study of marine magnetic anomalies played a major role in the discovery and understanding of plate tectonics. In the early 1960s Earth scientists found through dating and paleomagnetic studies of terrestrial lavas that Earth's magnetic field, which is created by the circulation of core materials, had reversed polarity frequently and regularly in the past at intervals of about half a million years, with each reversal probably taking only a few thousand years. (During periods of "normal" magnetization, the north-seeking end of a compass needle would behave as it does now; during periods of reversed magnetism, the "north-seeking" end of the needle would point south.)
Meanwhile, marine scientists noticed that the newly discovered mid-ocean ridges were marked by significant magnetic anomalies that persist away from the ridge crests in a systematic pattern of "magnetic stripes." These two observations led Frederick Vine and Drummond Matthews (Cambridge University) to hypothesize in 1963 that the ocean crust acts as a sort of tape recorder that preserves Earth's magnetic field through time. The Vine & Matthews hypothesis begins with the magnetization of newly formed mid-ocean ridge crust as it cools. This crust then moves out of the formation zone through the process of seafloor spreading to become part of an oceanic lithospheric plate. The positive magnetic stripes form during normal polarity periods and the negative stripes during reversed periods.
This pivotal idea provided confirmation of the theories of plate tectonics and continental drift. Unlike on land, the marine magnetic reversal anomalies provide a virtually continuous record of Earth's magnetic field for the past 200 million years. Correlating the sparse but well-dated terrestrial magnetic-reversal time scale with the continuous marine magnetic-reversal record produced the Geomagnetic Polarity Time scale, which allows scientists not only to accurately date the ocean basins and unravel their tectonic history, but also to obtain a detailed record of Earth's magnetic-field behavior.
Although marine magnetic anomalies are now an indispensable tool in marine geophysics, important issues remain unresolved. For example, the crustal source region of these anomalies is a subject of continued controversy. Some models define the uppermost volcanic extrusive layer (about 500 to 1,000 meters thick) as the source layer, whereas other models suggest a significant contribution from the deeper intrusive dike and gabbro layers that compose the remainder of oceanic crust. Another fundamental question that remains to be answered is the source of Earth's magnetic field itself. No convincing models of the geodynamo that satisfy all of the observations have yet been demonstrated.
Ships Measure Broad-Scale Magnetic Anomalies from the Surface
Typically, a marine magnetic survey involves a ship towing a sea-surface magnetometer at speeds between 4 and 10 knots. The spatial resolution of the magnetic signal is approximately equal to the water depth, typically about 3 or 4 kilometers. For seafloor created at a medium spreading rate of 30 kilometers per million years, a 3-kilometer wavelength is equivalent to a time interval of about 0.1 million years. Since the average reversal rate is approximately 0.3 million years, sea-surface measurements provide an adequate measure of Earth's magnetic-polarity reversal history. Spectral analysis of marine magnetic anomalies also shows that the magnetic signal due to seafloor topography contributes significantly to the magnetic field at wavelengths less than 2 kilometers. Thus, the sea-surface data are relatively free from the effects of topography. It is clear, however, that short polarity events (on the order of 0.1 million years or less) are severely filtered when measured at the sea-surface. This missing signal is important to understanding the source of the geomagnetic field. For example, are these short-wavelength magnetic anomalies crustal in origin, reflecting the processes of crustal construction and evolution? Or, are they geomagnetic, reflecting rapid reversals or intensity variations of Earth's magnetic field? If these short-wavelength anomalies are geomagnetic, they provide another constraint on geodynamo models for the generation of Earth's magnetic field.
Thus while sea-surface magnetic surveys are fast, easy to accomplish, and provide a good first-order understanding of the history of Earth's magnetic field, we need more detailed information to resolve short-wavelength magnetic anomalies and, ultimately, the sources of these anomalies.
Submersibles and Towed Vehicles Survey Near-Bottom Magnetics
Near-bottom surveys provide an opportunity to improve the resolution of marine magnetic measurements by measuring the anomalies closer to their sources. To date, this kind of measurement has been accomplished using either deep-towed vehicles or manned submersibles. There are a number of advantages to both of these methods but also several logistical disadvantages.
The advantage of getting closer to the magnetic source is that the spatial filtering effect is reduced to hundreds of meters instead of 3 or 4 kilometers (or several thousands of years compared to 0.1 million years at a 30-kilometers-per-million-years spreading rate). This finer resolution provides a more accurate record of the polarity reversal history than is possible with sea-surface data. Being closer to the source also means stronger signals, which is more important in equatorial regions where diurnal magnetic variations (daily magnetic variations due to the sun) can be of the same magnitude as the crustal magnetic signal. Although the magnetic signal is improved, a greater percentage of this signal is "contaminated" with topographic and shallow crustal magnetic variations. Significant post-processing is required to analyze these near-bottom data sets and separate the various signals. Furthermore, the two-dimensional assumptions used in analyzing the sea-surface data must be used with caution in the near-bottom environment where topography becomes more three dimensional. Logistically, deep-towed surveys are complicated operations. The long cable lengths, cable drag, slow tow speed (1 to 2 knots), depth control, navigation, and difficult maneuvering around turns make deep-towed operations a challenge. Without a dynamically positioned ship and tow-fish system, near-bottom surveys over rough topography or along scarp faces are virtually impossible to achieve.
Though limited in range and expensive to operate, manned submersible surveys offer the ability to negotiate such terrain. In addition, submersible-mounted magnetometers offer the opportunity to link seafloor observations such as rock type and tectonic structure directly to magnetic-field measurements.
Recently, the capabilities of the US deep submersible Alvin, operated by the Woods Hole Oceanographic Institution, and the French deep submersible Nautile, operated by the Institut Francaise pour Recherche et Exploitation de la Mer, provided the opportunity to conduct two different types of surveys. The 1990 Alvin survey focused on the actively venting TAG (Trans-Atlantic Geotraverse) hydrothermal mound on the Mid-Atlantic Ridge at 26 |degrees~ 08'N, 44 |degrees~ 49'W. This survey was designed to define the effects of hydrothermal alteration on the basaltic rocks of the upper crust by carrying out a gridlike survey over the entire mound. The 1991 Nautile survey consisted of vertical magnetic traverses up a fracture-zone wall in order to map the vertical magnetic structure of the ocean crust. Both surveys used a three-axis fluxgate magnetometer mounted to the sample basket of the submersible. The magnetometers were calibrated on each submersible by having Alvin and Nautile actively spin on descent and ascent, so the magnetic effects of the submersibles themselves could be determined and necessary corrections applied. We used new data-analysis techniques that promise to expand the use of submersible and near-bottom magnetic surveys in the future.
The Alvin TAG Magnetic Survey. Hydrothermal mid-ocean-ridge vent systems first discovered in 1977 represent a significant source of mass and heat exchange between the oceanic crust and the surrounding ocean. Many of the world's major metallic ore deposits appear to have formed in this kind of environment. The initial magnetization of oceanic crust can be virtually destroyed by thermal and chemical processes at work in such vent systems. At the TAG site, for example, a significant hydrothermal vent system appears to be coincident with a sea-surface magnetic anomaly that has a clearly three-dimensional morphology. The resolution limitations of the sea-surface data can only resolve the size of the source body to an area smaller than 4 by 4 kilometers. The 3,760-meter-deep central vent region consists of a 50-meter-high, 200-meter-diameter mound. Alvin carried out a detailed, near-bottom magnetic survey at an altitude of 20 meters over the actively venting mound on grid lines that were 40 meters apart and 400 meters long.
The magnetic results suggest that a zone of reduced magnetization exists beneath the mound. This 100-meter diameter magnetization low is consistent with the highly altered upflow zone of a hydrothermal vent system that feeds the actively venting mound. This low is not sufficient to account for the anomaly observed at the sea surface, which suggests that there is also a deeper and broader zone of low magnetization beneath the mound (too broad to be detected by the Alvin survey) that is the source of the sea-surface anomaly. The overall model of crustal magnetization at a hydrothermal field with discrete zones of demagnetization in the upper crust and a broader zone of demagnetization at depth is consistent with studies of hydrothermal systems in ophiolites (remnants of ocean crust that are now exposed to land). These studies show narrow upper-crust alteration pipes that feed the seafloor vent deposits and pervasive alteration at depth, both of which are commonly associated with late-stage intrusive bodies.
Nautile Survey of Blanco Scarp. The Vertical Magnetic Profiling project (VMAG) was designed to determine the vertical magnetic structure of ocean crust using newly developed survey methods and analysis techniques. In a modification of conventional near-bottom magnetic survey methodology, Nautile surveyed a vertical cross section of ocean crust on the Blanco Scarp at the western end of the Blanco Transform (part of the Juan de Fuca Ridge, in the northeastern Pacific). This scarp had previously been mapped with high-resolution sidescan sonar on a 1987 cruise with John Delaney (University of Washington) as chief scientist. The 1991 Nautile dive was led by Thierry Juteau (Universite de Bretagne Occidentale, Brest, France) aboard Nautile's mother ship, Nadir.
Out of about 24 Nautile dives, 13 were focused on the scarp face itself. Magnetic data were collected on 14 dives, and 242 rock samples were recovered. Navigation was provided by a transponder system, and the 1987 sidescan map provided an excellent "base map" for planning dives and navigating the submarine. The geological and petrological observations of the dive program divide the scarp into:
* a lower intrusive dike section, from the base of the scarp to 3,400-meters depth,
* a rubble slope where some highly fractured pillow-lava flows outcrop, and
* an upper basaltic pillow-lava section, from 3,300 meters to the top of the scarp at 2,200 meters.
These studies provide an excellent geological framework for interpreting the magnetic data collected. The magnetic data show a remarkably coherent picture. The figure below shows the total magnetic field recorded during three vertical traverses up the Blanco Scarp face. The most outstanding feature of these profiles is the dramatic increase in the magnetic field at approximately 3,400 meters depth. This magnetic anomaly coincides perfectly with a transition from dikes to pillow-lava that was observed from the submersible, and the magnetic character is consistent with models of a highly magnetic pillow-lava section and a relatively low-intensity intrusive-dike section, and with data collected from the surface. Preliminary analysis indicates, then, that a majority of the source of the sea-surface magnetic anomaly pattern lies with the extrusive section, at least for 1-million-year-old crust.
We conclude from these two case studies that near-bottom magnetic field studies utilizing a submersible platform can provide new information on the nature and structure of oceanic crust that is not currently obtainable any other way. The results of the Alvin magnetic survey show the presence of a narrow, pipelike body of demagnetized crust directly beneath the mound, inferred to reflect the upflow zone that channels the hydrothermal fluids to the surface there. The Nautile magnetic survey shows the occurrence of a distinct anomaly at the pillow-dike transition on the scarp, and provides new constraints on the source of the "magnetic stripes." Both surveys demonstrate the value of measuring the near-bottom magnetic field during submersible dives on oceanic crust to complement conventional observations from the sea surface.
Editor's Note: As we went to press, author Tivey was participating in another set of Nautile dives to measure the magnetism of deep oceanic crustal layers (gabbros) exposed in the Kane Fracture Zone on the Mid-Atlantic Ridge.
Maurice Tivey, the son of a 22-year veteran of the British Royal Navy, was told by his father not to choose a life at sea--it was an arduous life. Armed with this advice he opted to study geology at Dalhousie University in Nova Scotia and rock magnetism at the University of Washington in Seattle. Little did he realize that this course of study would lead to 18 major research cruises and a position on the staff as Assistant Scientist at the Woods Hole Oceanographic Institution.
Hans Schouten was educated in Holland and received his marine geophysical training aboard Dutch freight ships carrying Heineken with great urgency and speed (16 knots) to Caribbean isles and South American republics and dictatorships. He left this frantic pace for a more sedentary science in the US where he began applying fast Fourier transforms to the magnetics and roughness of the seafloor. Since then, he has quit smoking, collected 10 children, and has become obsessed with rapidly rotating microplates.
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|Author:||Tivey, Maurice I.; Schouten, Hans|
|Date:||Dec 22, 1992|
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