Marine magnetic anomalies arise from the combination of sea floor spreading and geomagnetic polarity reversals and thus delineate a history of global plate motions and paleofield behaviour. Just as sea surface magnetometer profiles led to a revolution in our view of the ocean basins thirty years ago, recent developments in near-bottom, high resolution magnetic studies are fundamentally changing the way we look at the structure and evolution of oceanic crust, and beginning to answer basic questions concerning geomagnetic field behavior. In response to these new developments, a Conference on the Magnetization of Oceanic Crust was held during September 21 to 24, 1996, on Orcas Island, one of the San Juan Islands in northwestern Washington State. The Conference was supported by funding from the National Science Foundation, RIDGE, and United States Science Advisory Committee (USSAC), and was attended by 47 scientists from 20 institutions in 7 countries. The format of the meeting alternated between plenary sessions (including invited talks) and Working Group sessions. The four Working Groups reflected the diverse interests of the group and consisted of (1) Source Layer and Crustal Architecture, (2) Zero-age Magnetization and Crustal Formation, (3) Short-term Variability of the Anomaly Signal (less than 1 My), and (4) Long-term Variability of the Anomaly Signal (greater than 1 My). Working Groups 2 and 3 were combined for the final day of discussion because of overlapping interests.

The major goals of the Conference were: (1) to define the outstanding hypotheses concerning the magnetization of ocean crust that should be tested, particularly those related to geomagnetic field behavior and to crustal formation/evolution; (2) to identify the essential new tools, analytical techniques and field programs needed to carry out these tests; and (3) to determine how the insights gained from these new magnetic studies and techniques could be integrated with the objectives of other scientific disciplines and programs. Although the discussion in the Working Groups and Plenary Sessions was lively and animated, there was general agreement on the high-priority areas and on the approaches to these problems, as outlined below.

The source of marine magnetic anomalies depends inherently on the crustal architecture as well as the physical and chemical processes that give rise to magnetism of the crust. It appears that all lithologies composing oceanic crust and upper mantle can acquire some degree of remanent magnetization, thus the key to identifying the source layer lies both in rock magnetic studies of the stability, the timing and carriers of the remanence, and the spatial geometry and contribution of this remanence to the observed anomalies. The internal structure of oceanic crust in the vertical and spatial dimensions is an important record of how oceanic crust formed, and how it becomes deformed and alters with age, but this architecture remains poorly understood. While seismic data now can provide high resolution images believed to correspond to the base of the extrusive layer, these studies do not provide temporal control and cannot be related to the individual volcanic sequences responsible for crustal formation. Magnetic studies provide the temporal control and proper resolution to define and ultimately understand oceanic crustal architecture.

High resolution magnetic surveys using deep-towed instruments and those mounted on submersible and autonomous vehicles can determine the depth-dependent shape of crustal isochrons, the geometry of individual volcanic units, and the timing of hydrothermal alteration associated with the accretionary process. Mapping of the magnetization distribution on near-vertical crustal exposures by submersible provides a unique view of the formation of the extrusive crust and on the variation of crustal isochrons with depth. Mapping an isochron through the deep crustal sections would have a major influence upon both models of deep crustal petrology and on the hydrothermal cooling of the accreting system (Fig. 1).

Evidence that the deeper crust, particularly the gabbroic section, may contribute significantly to the anomalies has been obtained from satellite magnetic surveys, studies of anomaly transition widths, rock magnetic studies of ophiolites and dredged rocks, and is supported by recent drilling of ODP Hole 735B in the southwest Indian Ocean. At the 735B site, correlatable and lineated magnetic anomalies are present, even where the upper extrusive layer has been completely removed to expose the gabbroic section. More complete vertical cross-sections of exposed oceanic crust are known to exist at Hess Deep, Blanco Trough, Endeavor Deep, and Pito Deep in the Pacific, and Kings Trough and Royal Trough in the Atlantic, and these sections were recommended for study. Work in the Atlantic suggests that intrusive rocks are frequently exposed at the surface at the inside corners of ridge segment offsets and transforms. Thus, the exposure of lower oceanic crust, with overlying lineated magnetic anomalies, may be a common feature of slow spreading ridges.

In addition to the well-established pattern of large-scale polarity reversals, the lineated nature of some small-scale anomalies has been well-documented, and in many cases appears to have a geomagnetic origin. Important new evidence was presented at the Conference regarding the geomagnetic contribution to short wavelength anomalies, based on a deep-tow survey of Anomaly 5 in the NE Pacific. These new data show a remarkable coherence of short wavelength variations the near bottom field between two spreading corridors that were separated by 100 km. Specific research directions identified by this Working Group as the most important are 1) determining if lineated small-scale anomaly variations are intensity fluctuations or polarity reversals, 2) identifying the origin of the Central Anomaly Magnetic High (CAMH) that overlies most spreading centers, and 3) developing fine-scale anomalies as a tool for examining crustal accretionary processes.

Resolving the origin of short wavelength anomaly variations has an importance beyond the marine magnetics community. If, for instance, these high frequency fluctuations are due to full polarity reversals, then the large number of additional polarity intervals would call into question the accepted Poisson-distributed nature of geomagnetic reversals. Alternatively, if short wavelength anomalies are related to paleointensity variations, then the relationship of these fluctuations to reversal rate is critical to understanding the temporal evolution of the geomagnetic dynamo. Additional near-bottom magnetic surveys, obtaining profiles over the same time intervals but within different spreading corridors, can distinguish geomagnetic field behavior from crustal formation processes. Companion land-based magnetostratigraphic studies, which can identify discrete short polarity intervals and provide independent estimates of intensity variations, will also be required for comparison with the seafloor record. Intervals where lineated short wavelength anomalies have already been documented, such as Anomalies 5, 24-27, and the Central Anomaly, are the highest priority for detailed study.

The CAMH is a short-wavelength anomaly that overlies almost all spreading centers and is clearly visible even in sea surface profiles. The 2-4 km wavelength of this feature in near-bottom profiles requires an abrupt decrease in the magnetization of the upper crustal layers with age, a phenomenon opposite to the effect of the increasing extrusive layer thickness with age that is inferred from seismic studies. Both low temperature alteration and paleointensity variations have been suggested as plausible causes for the magnetization contrast producing the CAMH. However, sample collections suitable for assessing the relative contributions of these two factors are presently not available, and the existing data are ambiguous. The origin of the CAMH can be addressed through near-bottom magnetic surveys and densely-spaced flowline sampling in regions where seismic data that define the thickness of Layer 2A are available. From these dredge and drill core samples, the alteration state of the crustal rocks as well as direct estimates of the paleofield intensity could be determined.

When linked with other high resolution geological data, the pattern and morphology of near-bottom magnetic anomalies can be used to better constrain crustal accretion and magmatic supply. For example, the CAMH is always associated with the youngest volcanic centers providing the temporal and spatial variation of the neovolcanic zone. Correlation of magnetic anomaly amplitudes with high-FeTi lavas suggests that geochemical variations could be remotely mapped with magnetics. Similarly, the pronounced reduction in magnetization associated with focussed hydrothermal alteration suggests that near-bottom magnetic surveys may also provide a means for delineating the extent of hydrothermal activity.

Polarity intervals vary in duration from the 37-Ma long Cretaceous Normal Superchron (CNS) to individual chrons lasting only a few thousand years. This polarity history of the geomagnetic field is systematically recorded in marine magnetic anomalies for the Cenozoic and much of the Mesozoic. However, the amplitude of sea-surface anomalies is extremely variable and not at all well understood. In general, oceanic crustal magnetization as represented by anomaly inversions and the magnetization of basalt samples decreases markedly from a maximum at the ridge crest to a minimum at 10 - 20 Ma. Crustal magnetizations then increase with age to a maximum during the Cretaceous superchron (83-120 Ma), although sea surface profiles suggest an additional maximum in the early Cenozoic. Prior to the Cretaceous superchron, oceanic crustal magnetizations have intermediate values from 120-150 Ma which decrease systematically from 150-170 Ma into the Middle Jurassic, when the amount of crust of this age to sample or survey becomes vanishingly small.

The general nature of the long-term variations in ocean crust magnetization is far from universally accepted or understood. Although present global compilations of paleointensity data do not show these same variations, other data suggest that the Cretaceous Normal Superchron is associated with high ocean crust magnetization that may be related to a high paleofield strength. Determining if the CNS has a high, low or extremely variable field intensity remains a fundamental unanswered question that requires a global sampling program that includes both continental and oceanic basalts. If the source of large-scale variations in ocean crust magnetization is the paleointensity of the field, then this should be reflected in samples from both continental and oceanic environments. If, however, the observed large-scale changes are specific only to ocean crust, then other processes, such as the inhibition of low temperature oxidation of magnetic minerals or a systematic geochemical variation of the crustal rocks, will need to be investigated.

Although there are several periods of large-scale variation of crustal magnetization, study of the Cretaceous Normal Superchron has the highest priority for this problem. As well as being the longest well-characterized polarity interval, it is a time of increased volcanic activity, proposed increases in spreading rates, and fundamental changes in global sea level and atmospheric temperature. Support for an ODP drilling transect across the Cretaceous Normal Superchron in the Pacific was expressed by the Conference participants. Data from existing ODP drill holes suggest that basement penetration of at least 100 m is necessary for the appropriate magnetic studies.

The participants of the Conference agreed that future studies in marine magnetics would require data at a much higher resolution than is currently routinely available from sea surface studies. The Working Groups identified several areas of promise for future marine magnetic studies; (1) high resolution, near-bottom surveys, including the construction of additional inexpensive deep-tow magnetometer packages, (2) the routine addition of magnetic sensors to submersibles, remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs), (3) development of analytical techniques for the full utilization of vector and gradient data obtained by these 3-component sensors, (4) development of wireline rock drills to obtain 'in situ' oriented samples from exposed bare rock areas, (5) wider use of satellite magnetometer data to define the long wavelength component of the anomaly field, and finally, (6) greater involvement of the marine magnetics community in the planning process for ODP crustal legs. Initiatives covering most of these areas are presently underway in some form, and there was considerable enthusiasm for the formation of a Working Group on Ocean Crustal Magnetization within the new Ocean Drilling Program structure.

While the need for near-bottom magnetic observations is critical to advance the next generation of oceanic magnetic studies, we also stress the continuing importance of satellite and sea surface magnetometer data. These data are essential for analyzing regional variability in oceanic crustal structure and providing overall tectonic context for the proposed high-resolution studies. The synoptic view from space allows a continuous, high-fidelity anomaly field to be constructed for the waveband of 300-3000 km, a section of the spectrum that is not available otherwise. Low-altitude satellite measurements, particularly below 300 km, will be most valuable in studies of regional variations in anomaly amplitude. Finally, participants at the Conference expressed strong concern regarding the status of magnetic anomaly data from the extensive (classified) sea surface and aeromagnetic surveys that were conducted by the U.S. Navy over the last 30 years. These data, which have full global coverage of almost all the ocean basins, are presently archived on short-lived magnetic tape. Their finite shelf life is being approached and the quality of the records may also be negatively affected by the impending retirement of Navy personnel who are familiar with the data archives. The participants in the Conference expressed strong support for the release of these data by the U.S. Navy to the world data center.

A consensus emerged from the Orcas Island meeting that the study of ocean crustal magnetization is entering into a new phase, which will yield important new information about the geomagnetic field beyond the well-utilized spectrum of polarity reversals. At the same time, near-bottom high-resolution magnetic studies will add a much-needed temporal component to studies of volcanic processes at the ridge axes, the subsequent chemical and tectonic evolution, and the overall crustal structure. A high degree of collaboration is expected between the specific magnetic studies proposed by the Conference and the larger marine geophysical community. This is particularly true for major research initiatives such as C-SEDI, where oceanic magnetic studies are expected to provide a unique perspective in efforts to understand the evolution of the geodynamo, and RIDGE, where crustal magnetization provides a valuable but largely unexploited tool for the study of the crustal accretion process. The participants at the Orcas Island meeting noted that the near-bottom surveys, the high resolution rock sampling programs, and the ODP drilling required to address the exciting new problems in marine magnetics are exactly the same tools needed by geochemists, petrologists, and structural and tectonic geologists. These scientists are working on similar problems associated with ocean crust but from a different disciplinary perspective. Those of us who study the magnetization of the ocean floor are looking forward to the new paradigms that will emerge from these collaborations.