Woo-Yeol Jung

Predicting bathymetry from Geosat-ERM and shipborne profiles in the South Atlantic Ocean

Abstract Portions of two Geosat-ERM altimeter tracks and corresponding suborbital shipboard gravity and bathymetry profiles in the South Atlantic Ocean were analyzed: one across the Walvis Ridge (about 1100 km long) and the other in the Brazil Basin (about 2300 km long). Together, these profiles sample those types of sea-floor topography which dominate the gravity signature at wavelengths of 20 to 300 km. The Walvis Ridge is a massive aseismic ridge and the Brazil Basin profile crosses both an old seamount (emplaced at the time the crust was young) and a very young mid-plate volcano. Both profiles cross fracture zones. After the gravity and bathymetry profiles were split into subprofiles, various cross-spectral characteristics could be determined by FFT techniques. Analysis showed that observed admittance is not well constrained by either an Airy-type or flexural compensation models across the Walvis Ridge, but those over the Brazil Basin can be readily explained by an Airy-type model with a mean crustal thickness of about 20 km. A theoretical filter was then designed, based on a priori geological knowledge, and used to predict bathymetry from the high-passed gravity/geoid anomalies. Not surprisingly, the predicted bathymetry shows more detailed and correct short-wavelength (20–300 km) features than those predicted from the historical data base, as represented e.g., by the DBDB5 gridded bathymetric model. For areas where historical shipboard bathymetry measurements are widely spaced (longer than about 10 km for single-beam data) but where some regional geologic information is available (such as the relative ages of mid-plate volcanoes and crust), bathymetry predicted from altimetric data can be used to upgrade regional bathymetrie data bases, on which regional geologic/geophysical understanding depends.

A gravity and magnetic anomaly study of the extinct Aegir Ridge, Norwegian Sea

In 1990 the U.S. Naval Research Laboratory mapped the extinct Aegir Ridge in the Norwegian Sea with SeaMARC II side-scan sonar, Hydrosweep multibeam, seismics, gravity, and magnetics onboard R/V Maurice Ewing. We analyzed the 92 center beam bathymetry, gravity, and magnetic profiles in this study to examine slow spreading ridge processes and cessation of spreading about 25 Ma. In bathymetric expression the rift valley deepens and narrows southward, while the rift mountain summits become higher. South of ∼67°N, the western rift mountains and flanks stand higher than the eastern mountains and flanks. Free-air anomalies along the ridge axis show undulating intermediate wavelength (∼100 km) anomalies superimposed on a regional decrease of the rift valley negative from ∼0 mGal in the north to −55 mGal in the south. After removal of sediment effects, results from three-dimensional gravity modeling imply a variation of crustal thickness. A new rotation pole for the movement of the Jan Mayen microplate relative to the Eurasia plate from ∼50 Ma to 25 Ma was found at 64°34′N, 13°W, based on magnetic lineations and inferred discontinuity traces. A plot of angular separation between significant anomaly peaks versus absolute age suggests an opening rate of ∼2.0°/m.y. (>36 Ma) followed by gradual deceleration to 0 at 25 Ma. Full spreading rate between chrons C13n and C21n ranged from ∼13 mm/yr in the north to 8 mm/yr in the south.

Effects of bottom water warming and sea level rise on Holocene hydrate dissociation and mass wasting along the Norwegian‐Barents Continental Margin

[1] Gas hydrate (GH) stability modeling results explain why some major Holocene submarine landslides along the Norwegian-Barents margin could have been triggered by GH dissociation during the early to middle Holocene, not during the lowest sea levels of the Last Glacial Maximum (LGM). Our model results show that subbottom depths of 170-260 m below the pre-slide continental slope (ca. 350-475 m present water depth) must have passed out of gas hydrate stability zone (GHSZ) by 8.15 ka as the effect of warm bottom water inflow at 11 ka penetrated into the subbottom, overcoming the effects of pressure increase due to sea level rise (SLR). The component of local SLR due to the isostatic response to Fennoscandian deglaciation is shown to be relatively insignificant, particularly for the part of the upper continental slope where the slide probably began. The stability relations show that GH could have formed under the ice sheet before deglaciation, and below deeper shelf areas after sea levels began to rise, but before significant warming near the GHSZ base. To the extent water deeper than 800 m has remained cold (-1° to 0°C) since LGM times, the GHSZ continued to thicken in deep water and GH dissociation could not have triggered Holocene failure in that regime. The present distribution of GH stability is limited to water depths greater than about 400 m in the Storegga slide area, and the thickness of the GHSZ increases with water depth.

Satellite radar altimetry aids seafloor mapping

The connection between gravity and oceanic water depth was realized more than a century ago. In 1859, J. H. Pratt derived a formula relating anomalous vertical deflection and sea level height on the coast of India to the depth of the adjacent Indian Ocean. Because the predicted deflections (5′–20′ for a mean depth of 4.8 km) were not measurable, Pratt later (in 1871) inferred an excess of mass below the ocean basins, thus prefiguring the concept of isostasy in marine geology. With the accurate measurement of marine gravity on submarines by F. A. Vening Meinesz in the 1920s and 1930s, isostasy was verified and later [e.g., Watts and Ribe, 1984] related to the thickness and flexural rigidity of mechanical lithosphere.

GOMaP: A matchless resolution to start the new millennium

The worlds ocean floor, which is almost equal in area to two moons plus two Mars-sized planets, is one of the most poorly mapped terrestrial surfaces in our solar system (Figure 1) [Vogtand Tucholke, 1986]. We propose a multiyear international effort to map the entire ocean floor using hull-mounted or towed sidescan/swath bathymetric systems.The Global Ocean Mapping Project (GOMaP) would produce a seafloor backscatter image whose lowest spatial resolution, in the deep trenches, would be at least 100 m, comparable to that returned by the Magellan radar mission to Venus or Clementines optical imaging of Earths moon. GOMaP would simultaneously recover the bathymetry, a tight grid of water depths, as the second kind of ocean floor image but at slightly lower spatial resolution than the backscatter image. A GOMaP mission would collect numerous additional piggy-back data, from seismic reflection profiles of the subbottom to whale counts, at little extra cost.

Arctic margin gravity highs remain puzzling

Many passive continental margins, particularly in the Arctic (Figures 1 and 2), are festooned by chains of elongate positive gravity anomalies overlying, in part, marginal oceanic crust. To this day, no simple unifying explanation has emerged to account for these highs. Variously modeled by mantle doming, crustal thinning, dense crust, basement rises, postbreakup sediment depocenters, and topographic effects, these anomalies epitomize nonuniqueness in geophysical inversion. It seems unlikely that all the highs share a common origin, but it seems even more unlikely that many disparate processes produce gravity anomalies so similar to each other. Upon consideration of several hypotheses, a phase change explanation seems the most plausible for Arctic margin gravity highs (AMGHs). The better studied highs are spatially correlated with sediment depocenters (Figure 2), which however by themselves account for only a fraction of the gravity anomalies (for example, Figure 3). Thus, a reason has to be found as to why dense or thinned oceanic crust (or dense mantle) is found underneath sediment loads. Essentially, the phase change explanation relies on the added lithostatic pressure under sediment piles to “densify” some of the buried oceanic crust, for example, by conversion to garnet granulite or even eclogite [Neugebauer and Spohn, 1981 ]. More well-designed seismic experiments are needed, however, to delineate the velocity (and hence density) structure of the underlying crust to verify the depocentermascon association and test the phase change hypothesis and others discussed below.

Paired basement ridges: Spreading axis migration across mantle heterogeneities?

The origin of off–Mid-Oceanic-Ridge (MOR)-axis paired (conjugate) basement ridges and other conjugate structures is examined, with a focus on the North Atlantic. Paired-ridge morphologies are found at volcanic edifice scales (influencing ~25to 75-km spreading boundary) and at 200to 1000-km scales (where structures may be V-shaped, suggesting propagation along the axis at rates from a few to 200 mm/yr). Both scales modulate the longest-scale along-axis MOR topographic anomalies (~3100 km for the Azores and ~3800 km for Iceland). At the short scale, MOR axial magma centers with off-axis “split-volcano” pairs suggest magmatic episodicity at 0.1to 1-m.y. intervals, erratic along-strike displacements (1–10 km) between episodes, and no fixity in a hotspot frame. However, along-strike axis motion (calculated from the Gripp and Gordon, 2002, model) seems to inhibit formation of organized, long-lived axial volcanoes. Intermediate-scale ridge pairs have been attributed to “blobs” and temporal variability of mantle plumes, but some may have formed by passive tapping of anomalous mantle patches as the MOR migrates across them. A great variety of such passive “conjugate ridges” is geometrically possible as a function of MOR velocity over the mantle, the spreading rate and its asymmetry, the plan-view shape of the MOR axis (with transforms, normal, and oblique spreading), and the mantle source, whose shape (outline) could be constrained if the model is shown correct. Among observed ridge pairs (e.g., Morris Jesup Rise/Yermak Plateau; Reykjanes Ridge V-shaped ridges; East and West Thulean Rise; Flores and Faial Ridges; J-Anomaly and Madeira ridges; Ceara and Sierra Leone ridges), some have sharp older and/or younger edges, implying that the anomalous mantle sources, whether fixed or not, also have sharp boundaries, some of which are diachronous, implying along-strike propagation. The short and long scales appear nearly symmetrical alongstrike, but the intermediate scales, including geochemical and isotope anomalies in axial basalts, suggest preferential southward propagation, perhaps reflecting the southward asthenosphere motion predicted by counterflow models. Discrimination between the “passive heterogeneity” and “active plume” concepts is discussed.

Seafloor reconnaissance and classification of the Storegga Slide headwall region, Norwegian Sea, using side-scan sonar, video, and photographs acquired on submarine NR-1

The U.S. Navy nuclear research submarine NR-1 was used to investigate the Storegga Slide (Norwegian Sea) shelf break, headwall, and upper debris-flow fields to a maximum water depth of 630 m (2070 ft). About 275 km (170 mi) of seafloor was traversed in 1 week, collecting 150-kHz side-scan sonar, current speed and direction, bathymetry, optic imagery, and visual observations. Side-scan imagery was used to identify four provinces, some corresponding to distinct optic-scale characteristics. We found the Storegga outer shelf streaked and locally incised by iceberg plow marks or otherwise lineated in the side-scan imagery. We attribute the streaking to the strong Norwegian-Atlantic Current and perhaps the Norwegian Coastal Current. Despite our short current-sampling snapshot, we found good agreement between measured current directions and current-generated seafloor features. Along the headwall, just seaward of the iceberg plow marks, are deep-water, coral (Lophelia pertusa) reefs. Recent instability (post-8.15 ka) along part of the headwall region is indicated by cobble/boulder fields devoid of sessile biota (such as sponges). No obvious fluid expulsion or extensional features were discovered in the small portion of the Storegga Slide that was investigated.

The “Azores Geosyndrome” and Plate Tectonics: Research History, Synthesis, and Unsolved Puzzles

The Azores volcanic archipelago, the Azores Plateau (AP) and the Azores triple junction (ATJ) between the Eurasia, North America and Nubia plates occupy the summit of a regional feature we refer to as the ‘Azores Geosyndrome’. Included are anomalies in crustal thickness, rock composition, basement depth, plate boundary morphology, seismicity, gravity and geoid, and upper mantle seismic velocity structure, and there are many similarities between the Azores and Iceland geosyndromes. The location of the Azores in the central North Atlantic, technological advances in marine geophysics as well as logistic, geomilitary and geopolitical motivations and advanced research of island geology/volcanology have contributed to make the ATJ the most studied oceanic triple plate junction. However, a unified understanding of the Azores Geosyndrome awaits future deep crustal boreholes (particularly on the AP) and regional sea-floor seismometer arrays to resolve the seismic velocity structure below the AP down to the middle and perhaps lower mantle. Whereas a deep mantle plume appears unlikely to exist below the Azores, it cannot yet be excluded (see O’Neill and Sigloch, Chapter “ Crust and Mantle Structure Beneath the Azores Hotspot—Evidence from Geophysics”, and Moreira et al., Chapter “ Noble Gas Constraints on the Origin of the Azores Hotspot”). What is already clear is that the development and evolution of the Azores Geosyndrome has involved dynamic interactions among the North America-Nubia-Eurasia plates and at least the uppermost mantle below those plates—even far from the ATJ area. The plate boundary reorganization that resulted in the triple plate junction jumping from the end of King’s Trough south to create the ATJ was largely complete by Chron 6C (23 Ma) and coincided within dating uncertainties with the jump of the spreading plate boundary from the Norway Basin to the new Kolbeinsey Ridge just north of Iceland. Major geological changes in the Pyrenees and Alpine Tethys region at that time have long been known. In fact, the Palaeogene-Neogene boundary, a time of global change in planktonic biogeography, is placed at 23.0 Ma, in the upper part of C6C. Why the ATJ developed where it did and not elsewhere along the MAR suggests the lithosphere and subjacent mantle had already created a region of plate weakness. The subsequent development of the AP, largely via Mid-Atlantic Ridge spreading, produced a thick crust and more fertile mantle lithosphere-particularly from ca. 12 to 8 Ma. This mantle lithosphere was and continues to be relatively weak and fertile, favouring transtensional fissuring, formation of central volcanoes, as well as oblique hyperslow spreading along the Terceira Rift—particularly in the last 1.5 Ma.

Seismic reflectivity effects from seasonal seafloor temperature variation

The effects of seasonal temperature variation on sound speed contrasts at the seafloor are usually considered negligible in the analysis of seismic data but may be significant at large incidence angles (offsets) important for inversion of sediment elastic properties, or long-range acoustic transmission. In coastal areas, the maximum annual seafloor temperature variation can be several degrees Celsius or more, corresponding to a sound speed variation of 30 m/s or more. Thermal pulses propagate via conduction several meters into the seafloor resulting in a damped quasi-sinusoidal temperature profile with predictable wave number characteristics. The oscillating seasonal and spatial character of this signal creates a time- and frequency-dependent effect on the elastic seafloor reflectivity. Results of numerical simulations show that the expected temperature profile for most sediment types and porosities will have the strongest affect on frequencies between about 60 and 600 Hz, at incidence angles greater than about 50°.