Fe-Mn Nodule Field Indicated by GLORIA

Fe-Mn Nodule Field Indicated by GLORIA, North of the Puerto Rico Trench

Kathryn M. Scanlon¹ and Douglas G. Masson²

¹U.S. Geological Survey
Woods Hole, MA 02543

²Institute of Oceanographic Sciences
Wormley, U.K. GU8 5UB

modified from Geo-Marine Letters, 1992. v. 12, p. 208-213

ABSTRACT
A 2500 km² area of seafloor on the southeastern flank of the Greater Antilles Outer Ridge north of the Puerto Rico Trench displays anomalously high acoustic backscattering properties on GLORIA long-range sidescan-sonar data. Previously collected dredges, bottom photographs, and sediment cores indicate the presence of Fe-Mn nodules within the area of high backscatter. We were able to map the extent of the inferred nodule field on the basis of acoustic property contrast between the nodule-covered sediment and the surrounding nodule-free sediment.

INTRODUCTION
A change in the intensity of acoustic backscatter in GLORIA long-range sidescan-sonar images from the region north of the Puerto Rico Trench can be attributed to the presence of nodules on the sea floor. Using GLORIA, the 2500 km² area of nodule occurrence was outlined in three days of ship time. Previously collected bottom photographs and samples are used to support the interpretation of the sonographs.

The study area lies approximately 150-350 km north of the island of Puerto Rico, north of the Puerto Rico Trench (Fig. 1). Fe-Mn nodule occurrences have been reported from this region (e.g., Rawson and Ryan, 1978), but the extent of the nodule field was not previously known.

DATA
Data for the present study were collected on the research vessel FARNELLA in late 1985 and consist of GLORIA long-range sidescan-sonar imagery, 80-cubic-inch airgun seismic-reflection profiles, and 3.5 kilohertz (kHz) and 10 kHz echo-sounder profiles. The GLORIA mosaics and airgun profiles have been published in an atlas (EEZ-SCAN 85 Scientific Staff, 1987). All data were collected simultaneously along the FARNELLA tracklines shown in Figure 2. Trackline orientations were chosen on the basis of several factors, including orientation of major geologic features, bathymetry, weather conditions, and sea-state.

GLORIA is a long-range sidescan-sonar system designed and built at Great Britain's Institute of Oceanographic Sciences for deep-ocean reconnaissance mapping. Technical details on the GLORIA system can be found in Somers and others (1978). The GLORIA images presented here have been processed and digitally mosaicked using the USGS Mini Image Processing System (MIPS). Processing included a correction for slant-range geometry, removal of distortions due to changes in the ship's speed, and spectral corrections for noise and variations in signal strength. The data were then mosaicked and appropriate spectral stretches were applied to enhance the images. Further details regarding GLORIA processing procedures can be found in Chavez (1986) and EEZ-SCAN 85 Scientific Staff (1987).

Pixels in the processed images correspond to approximately 50 m by 50 m areas of the sea floor. Resolution, however, varies with slant range and, in the near range, is about 125 m in the along-track direction and 50 m in the across-track direction because pixels are duplicated to correct for the ship's speed. This means that, in general, a seafloor feature must have one dimension of at least two or three hundred meters to be discernible in a GLORIA sonograph. A feature only a few tens of meters high, however, can be detected if it has sufficient length.

Note that areas of high acoustic backscatter show as bright areas on GLORIA images, whereas areas with low backscatter are dark. This is opposite to the convention used with most shorter-range sidescan-sonar systems. The intensity of backscatter is affected by several factors including water depth, sea floor morphology, bottom type (i.e., rock or sediment), and in the case of sedimentary sea floors, sediment composition and degree of consolidation. In addition, the presence of fields of bedforms and other types of seafloor roughness that, individually, are well below the resolution of the GLORIA system can often be inferred from changes in the intensity of backscatter over a large area (e.g., Kidd and others, 1985; Huggett and Somers, 1988).

To aid in the interpretation of the GLORIA mosaic, additional data from the archives of the Woods Hole Oceanographic Institution and Lamont-Doherty Geological Observatory were used. These include core, grab sample and dredge descriptions, and bottom photographs (Fig. 2).

GEOLOGIC SETTING
North of the Puerto Rico Trench the sea floor rises abruptly from 8000 m to depths between 5000 m and 6000 m. North-northeast trending basement ridges (bright features in the GLORIA mosaic, Fig. 2) can be seen cropping through the sediment cover in this area. These basement ridges are parallel to the magnetic anomaly pattern in this region (Klitgord and Shouten, 1986) and are presumed to be related to seafloor spreading at the mid-Atlantic Ridge (Scanlon and others, 1988; Masson and Scanlon, 1991). Depressions in the sediment adjacent to some of the basement ridges (Fig. 3) probably result from current scour.

Two distinct seismic facies are seen in the sediment cover: a stratified layer resting on acoustic basement and an overlying acoustically transparent layer (Fig. 3). The stratified layer appears to fill low areas between basement ridges. It consists of Middle Cretaceous through early Tertiary (middle Eocene) interbedded limestone, chert, and consolidated and unconsolidated silt, clay, and ooze and is separated from the overlying transparent layer by the middle Eocene Horizon A (Tucholke and Ewing, 1974). The transparent layer is composed of terrigenous sediment deposited by the Western Boundary Undercurrent (flowing south and east) where it interacts with the Antarctic Bottom Water (flowing north and west). The resulting sediment drift, the Greater Antilles Outer Ridge (GAOR), crosses and extends beyond the study area in an approximately 100-km-wide band for more than 300 km to the northwest and also several hundred kilometers eastward (Tucholke and Ewing, 1974). Within the study area, the GAOR is thickest in the north and thins southward (Fig. 3). The depositional history of the Greater Antilles Outer Ridge has been discussed in detail elsewhere (Tucholke and others, 1973; Tucholke, 1974; Tucholke and Ewing, 1974; Tucholke, 1975).

INTERPRETATION
The GLORIA mosaic shows an irregularly shaped patch of anomalously high acoustic backscatter on the southern flank of the Greater Antilles Outer Ridge. It covers an area of approximately 2500 km² and is centered at about 20 degrees 30 minutes N and 66 degrees 30 minutes W (Fig. 2). The patch is significantly brighter than the other sediment-covered areas, but displays lower backscatter of more uniform intensity than basement outcrops. We attribute this anomalous backscatter to the presence of Fe-Mn nodules on the seafloor. Previously collected bottom photographs, dredge samples, and even one sediment core from the bright area reveal the presence of nodules but show none in the surrounding low backscatter areas (Figs. 2, 4). Several cores taken within the bright area did not contain nodules, but considering the small diameter of a core sample (7.5 or 10 cm) and the probable density of nodule coverage (< 25% in photographs), this is not surprising. It is more significant that one core did contain nodules.

The difference in acoustic reflectivity between pelagic sediments and Fe-Mn nodules has been shown by Huggett and Somers (1988) to be great enough to create a mappable difference in backscatter in GLORIA data. Their study is based on the acoustic scattering effects of a 10% coverage of the seafloor by nodules. We have bottom photographs from only three sites within the bright area; these show nodule coverage of between 5% and 25%. Compressional wave velocity in the transparent layer sediment (1.50 to 1.77 km/s according to Savit and others, 1964) is comparable to the velocity used by Huggett and Somers (1988) (1.52 km/s). It is therefore reasonable to apply Huggett and Somers' (1988) conclusions to our study area. The presence of Fe-Mn nodules is consistent with the area's low sedimentation rate as inferred from the thinness of the transparent layer.

DISCUSSION AND CONCLUSIONS
Fe-Mn nodules are present on the seafloor in the area of high backscatter and have not been reported to occur in the low backscatter parts of our study area. It has been shown by Huggett and Somers (1988) that a nodule field with characteristics similar to those of the nodule field seen in our study area is capable of producing a mappable difference in acoustic backscatter in GLORIA images. Although this suggests a cause and effect relation between the nodules and the high backscatter, other possibilities should be considered.

First, we considered possible topographic effects. The area of high backscatter lies on the flank of the Greater Antilles Outer Ridge. However, it produces the same level of backscatter whether insonified from upslope or downslope, indicating that the slope of the sea floor is not causing the difference in backscatter.

Second, we considered whether a strong seismic reflector, such as Horizon A, were exposed at the sea floor, and evaluated whether GLORIA could be penetrating the sea floor and recording backscatter from a buried horizon. Horizon A is exposed (or lies very near the surface) in the southern part of the bright area. However, the transparent layer covers Horizon A in the northern part of the bright area (figure 3), eliminating the possibility that the high backscatter could be caused by exposure of Horizon A or the deeper stratified layer. Although the GLORIA signal can penetrate a thin layer of sediment, the transparent layer is over 50 m thick in parts of the bright area (figure 3). Huggett and Somers (1988) calculated that 10 m of pelagic ooze would effectively mask an underlying horizon. We therefore feel confident that GLORIA is not penetrating the transparent layer and that the bright area is not caused by a buried reflector such as Horizon A or the stratified layer.

Third, there is the possibility that a covariant, some characteristic of the seafloor that exists in the bright area with the nodules, may cause the difference in backscatter. Because the Greater Antilles Outer Ridge is a current-deposited sediment body, and because Fe-Mn nodules form in areas of low sedimentation rates or current scour, the most likely covariants would be current-related surface features such as sandwaves or winnowed sediment. Bottom photographs from two of the three locations shown in Figure 2, however, do not show evidence of currents. One shows evidence of biological activity (burrows and trails), which would not be expected to persist in an area swept by currents. We therefore suggest that a covariant is not likely to cause the difference in backscatter.

The samples and photographs used to support our interpretation of the GLORIA data were collected by others for other purposes and are not ideally suited to prove our hypothesis. They do, however, provide circumstantial evidence that a field of Fe-Mn nodules is causing the anomalously high backscatter in the study area. Deep-towed camera transects across the boundary between high and low backscatter would provide more conclusive evidence. A good place for such a transect would be in the vicinity of the seismic profile shown in Figure 3, where the boundary is particularly sharp.

Our data suggest that the extent of a Fe-Mn nodule field can be mapped in a relatively short time using GLORIA long-range sidescan. This type of mapping with GLORIA is not limited to Fe-Mn nodules, but can be applied to any sediment facies on the deep sea floor that provides sufficient acoustic contrast with the surrounding sea floor (e.g., Kidd and others, 1985). Although additional data, such as bottom photographs, are needed to verify the cause of backscatter differences and higher resolution sidescan and sampling studies are necessary to more accurately determine the economic value of a deposit, this study indicates that a reconnaissance program using GLORIA data could greatly reduce the time needed for such studies.

Acknowledgments
We thank the officers and crew of the research vessel FARNELLA for a successful cruise. W. Dunkle and M. Johnson (Woods Hole Oceanographic Institution) and L. Sullivan (Lamont-Doherty Geological Observatory) kindly made available archived data. We also thank D. O'Leary, L. Poppe, D. Stanley, and an anonymous reviewer for their comments and criticisms.

REFERENCES

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FIGURE CAPTIONS

Figure 1. Location map showing major geographic features and the outline of the area shown in Figure 2.
Figure 2. GLORIA mosaic (A), interpretation (B), and bathymetry (C) of the study area, north of the Puerto Rico Trench (see Fig. 1 for location). NE-SW trending basement ridges create the rough-looking areas of highest backscatter. The dark areas are sediment-covered sea floor. Where Fe-Mn nodules are inferred to occur on the sea floor, relatively high backscatter of fairly uniform intensity is seen. GLORIA, 3.5 kHz, and airgun data were collected simultaneously along the Farnella tracklines indicated in the interpretation. Locations of previously collected cores, dredges, grab samples, and bottom photographs, used to aid interpretation, are shown. Location of the airgun profile in Figure 3 is identified by a bold line. The double bold line indicates the location of the 3.5 kHz profile (Fig. 3). Bathymetry (C) is based on Tucholke and Ewing (1974).
Figure 3. Airgun and 3.5 kHz echo-sounder profiles taken along the same trackline north of the Puerto Rico Trench across a boundary between the area of high backscatter and an area of low backscatter in the GLORIA mosaic (see Fig. 2). The transparent layer, which overlies the stratified layer, thins southward toward the trench, exposing Horizon A at the southeast end of the profile.
Figure 4. Photograph (Cruise 11, R/V Chain, 1960) showing Fe-Mn nodules on the seafloor north of the Puerto Rico Trench (see Fig. 2 for location). Although the individual nodules are far below the resolution of the GLORIA system, they create a seafloor roughness that we suggest increases the overall backscatter of the region, allowing the areal extent of the nodule field to be inferred (Fig. 2). An exact scale for this photograph is not known, but the nodules are probably 2-4 cm in diameter. Water depth is 5339 m (Tucholke, 1974). (Photo courtesy of Woods Hole Oceanographic Institution Data Archives.)