Acoustic environment of the Hatteras and Nares Abyssal Plains, western North Atlantic Ocean, determined from velocities and physical properties of sediment cores

Author Posting. © Acoustical Society of America, 1980. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 68 (1980): 1376-1390, doi:10.1121/1.385105.Seventeen piston cores up to 13 m long were recovered from representative acoustic and lithologic environments of the Hatteras and Nares Abyssal Plains in the western North Atlantic. Compressional-wave velocities (corrected to in situ conditions) and bulk physical properties measured on the cores are used to characterize the acoustic framework of these areas. For correlation with conventional seismic data, whole-core averages of properties are a better index to the acoustic nature of abyssal plain sediments than properties of the upper few centimeters of the seafloor because (1) strong changes in lithofacies (and acoustic properties) occur over depth scales of tens of centimeters to meters in the sediment column, and (2) conventional seismic frequencies of 3.5 kHz or less sample these variations to subbottom depths of tens of meters and more. Whole-core properties are a function of the thickness and distribution of high-velocity silt and sand layers in the core; they vary in a complex fashion with proximity to the source of turbidity currents, distance from axial paths of turbidity-current flows, local and regional basin geometry, and seafloor slope. Thus strongly reflective seabed regions with numerous high-velocity layers are not restricted simply to near-source areas nor are weakly reflective seabed regions (clay sediments only) limited to ''distal'' areas. Whole-core properties show a good qualitative correlation to variations in 3.5-kHz reflection profiles, and 3.5-kHz echo character therefore provides a useful means of mapping general acoustic properties over large regions of abyssal plains.Data collection and much of the analysis were supported by ONR Contract N00014-75-C-0210 to Lamont- Doherty Geological Observatory (Columbia University) during my residence there. At Woods Hole, support by ONR Contract N00014-79-C-0071, NR083-004 is gratefully acknowledged

The history of sedimentation and abyssal circulation on the Greater Antilles outer ridge

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August, 1973The Greater Antilles Outer Ridge is an 1800 km long, submarine sedimentary ridge which lies below 5000 m in the southwestern North Atlantic Ocean. Seismic reflection profiles and core data indicate that the ridge is composed of more than 6 x 104 km3 of acoustically transparent sediment which has accumulated above a sequence of acoustically stratified sediments deposited before late Eocene time. The sediments consist of low-carbonate, homogeneous, terrigenous lutites which have accumulated at rates of up to 30 cm/1000 yr since the middle Eocene. Clay-mineral analyses indicate that the chlorite-enriched sediment is derived from the northeastern continental margin of North America. Abyssal contour-following currents which flow around the Greater Antilles Outer Ridge are interpreted as an extension of the Western Boundary Undercurrent (WBUC) found along the continental rise of eastern North America. This current system is proposed to be the agent which has transported sediment southward for more than 2500 km and deposited it on the Greater Antilles Outer Ridge. Sediment is presently carried in concentrations up to 65 ug/liter in the currents flowing around the outer ridge, and mineral analyses show that the suspended sediment has a northern provenance; it is similar in composition to the bottom sediment and is interpreted as the source of sediment deposited on the Greater Antilles Outer Ridge. The Puerto Rico Trench began to form in middle Eocene time, and it cut off direct downslope sedimentation to the Greater Antilles Outer Ridge. At the same time, the newly formed WBUC interacted with existing sea-floor topography and the Antarctic Bottom Water (AABW) flowing in from the South Atlantic, and it began to deposit acoustically transparent sediment on the eastern outer ridge. This depositional pattern persisted until the middle or late Miocene, when increased AABW flow diverted the WBUC to the northwest and initiated deposition of the western sector of the Greater Antilles Outer Ridge. Shortly thereafter, decreased AABW flow and lower current speeds allowed rapid deposition of sediment on the Greater Antilles Outer Ridge and on the Caicos Outer Ridge to the west. The bottom topography has controlled the abyssal current pattern, and current-controlled deposition has continued to construct the Greater Antilles Outer Ridge since early Pliocene time.Prepared under National Science Foundation Grant GA-24872, the Office of Naval Research Contract N00014-66-C0241; NR 083-004

The western boundary undercurrent as a turbidity maximum over the Puerto Rico Trench

Author Posting. © American Geophysical Union, 1974. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 79 (1974): 4115–4118, doi:10.1029/JC079i027p04115.Nephelometer measurements in the Puerto Rico trench record a midwater light scattering maximum at the depth of the near-bottom nepheloid layer found in the deep Atlantic basin to the northwest. This midwater maximum is best developed near the south slope of the trench and is interpreted as a southeasterly continuation of the western boundary undercurrent, which has been documented along the continental rise of eastern North America. The eastward-advecting core of the flow overrides clearer colder antarctic bottom water that enters the trench from the east. A near-bottom nepheloid layer, best developed in the eastern part of the trench, appears to be associated with the westward-flowing antarctic bottom current.The nephelometer program at Lamont has been supported by the National Science Foundation under grant GA 41657 and GA 27281 and the Office of Naval Research under contract NOOOI4-67-A-0108-0004. One of us (B.E.T.) was supported by a Lamont-Doherty PostDoctoral Fellowship during this research

Multiscale spectral analysis of bathymetry on the flank of the Mid-Atlantic Ridge : modification of the seafloor by mass wasting and sedimentation

Author Posting. © American Geophysical Union, 1997. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 102, no. B7 (1997): 15447–15462, doi:10.1029/97JB00723.The results of a multiscale spectral analysis of bathymetric data on the flank of the Mid-Atlantic Ridge are described. Data were collected during two cruises using Hydrosweep multibeam (tens of kilometers to ∼0.2 km scale range) and Mesotech scanning pencil-beam sonar attached to remotely operated vehicle Jason (∼1 km to ∼0.5 m scale range). These data are augmented by visual data which enabled us to identify bathymetric profiles which are over unsedimented or thinly sedimented crust. Our analysis, therefore, is focused primarily on statistical characterization of basement morphology. Work is concentrated at two sites: site B on ∼24 Ma crust in an outside-corner setting, and site D on ∼3 Ma crust in an inside-corner setting. At site B we find that an anisotropic, band-limited fractal model (i.e., the “von Kármán” model proposed for abyssal hill morphology by Goff and Jordan [1988]) is not sufficient to describe the full range of scales observed in this study. Our observations differ from this model in two ways: (1) strike and cross-strike (dip) spectral properties converge for wavelengths smaller than ∼300 m, and (2) in both strike and dip directions the fractal dimension changes at ∼10 m wavelength, from ∼1.27 at larger scales to ∼1.0 at smaller scales. The convergence of strike and dip spectral properties appears to be associated with destruction of ridge-parallel fault scarps by mass wasting, which develops canyon-like incisions that cross scarps at high angles. The change in fractal dimension at ∼10 m scale appears to be related to a minimum spacing of significant slope breaks associated with scarps which are created by faulting and mass wasting. At site D, although there is no significant abyssal hill anisotropy, the spectral properties at all scales are consistent with the von Kármán model. The fractal dimension at this site (∼1.15) is less than at site B. This difference may be reflect different morphology related to crustal formation at inside-corner versus outside-corner position or, more likely, differences in the degree of mass wasting. The smoothing of seafloor morphology by sediments is evident in Hydrosweep periodograms where, relative to basement roughness, spectral power decreases progressively with decreasing wavelength.This work was supported under ONR grants N00014-94-1-0197 and N00014-96-1-0462 (J.A.G.) and N00014-90-J-1621 and N00014-94-1-0466 (B.E.T.)

Spatial and temporal variations in crustal production at the Mid-Atlantic Ridge, 25°N–27°30′N and 0–27 Ma

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 120 (2015): 2119–2142, doi:10.1002/2014JB011501.We use high-resolution multibeam bathymetry, shipboard gravity, side-scan sonar images, and magnetic anomaly data collected on conjugate flanks of the Mid-Atlantic Ridge at 25°N–27°30′N and out to ~27 Ma crust to investigate the crustal evolution of the ridge. Substantial variations in crustal structure and thickness are observed both along and across isochrons. Along isochrons within spreading segments, there are distinct differences in seafloor morphology and gravity-derived crustal thickness between inside and outside corners. Inside corners are associated with shallow depths, thin crust, and enhanced normal faulting while outside corners have greater depths, thicker crust, and more limited faulting. Across-isochrons, systematic variations in crustal thickness are observed at two different timescales, one at ~2–3 Myr and another at >10 Myr, and these are attributed to temporal changes in melt supply at the ridge axis. The shorter-term variations mostly are in-phase between conjugate ridge flanks, although the actual crustal thickness can be significantly different on the two flanks at any given time. We observe no correlation between crustal thickness and spreading rate. Thus, during periods of low melt supply, tectonic extension must increase to accommodate the full plate separation rate. This extension commonly is concentrated in long-lived faults on only one side of the axial valley, resulting in strong across-axis asymmetries in crustal thickness and seafloor morphology. The thin-crust flank has few volcanic features and exhibits elevated, blocky topography with large-offset, often irregular faults, while the conjugate thicker-crust flank shows shorter-offset, regular faulting, and common volcanic features. The variations in melt supply at the ridge axis most likely are caused either by episodic convection in the subaxial mantle or by variable melting of chemically heterogeneous mantle.This study was funded by Chinese Natural Science Foundation grant 41206034 and Chinese Postdoc Scholarship award 2012M511130 (T.W.), by Ministry of Science and Technology 973 Project award 2012CB417303, and by the WHOI Henry Bryant Bigelow Chair (J.L.). ARSRP and MAREAST data acquisition was funded by Office of Naval Research grant N00014-90-J-6121 and by U.S. National Science Foundation grant OCE-9503561, respectively.2015-10-2

Continental crust beneath the Agulhas Plateau, southwest Indian Ocean

Author Posting. © American Geophysical Union, 1981. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 86, no. B5 (1981): 3791–3806, doi:10.1029/JB086iB05p03791.The Agulhas Plateau lies 500 km off the Cape of Good Hope in the southwestern Indian Ocean. Acoustic basement beneath the northern one third of this large, aseismic structural high has rugged morphology, but basement in the south is anomalously smooth, excepting a 30- to 90-km-wide zone with irregular relief that trends south-southwest through the center of the plateau. Seismic refraction profiles across the southern plateau indicate that the zone of irregular acoustic basement overlies thickened oceanic crust and that continental crust, locally thinned and intruded by basalts, underlies several regions of smooth acoustic basement. Recovery of quartzo-feldspathic gneisses in dredge hauls confirms the presence of continental crust. The smoothness of acoustic basement probably results from erosion (perhaps initially subaerial) of topographic highs with redeposition and cementation of debris in ponds to form high-velocity beds. Basalt flows and sills also may contribute locally to form smooth basement. The rugged basement of the northern plateau appears to be of oceanic origin. A plate reconstruction to the time of initial opening of the South Atlantic places the continental part of the southern plateau adjacent to the southern edge of the Falkland Plateau, and both abut the western Mozambique Ridge. Both the Agulhas and Falkland plateaus were displaced westward during initial rifting in the Early Cretaceous. Formation of an RRR triple junction at the northern edge of the Agulhas continental fragment during middle Cretaceous time may explain the origin of the rugged, thickened oceanic crust beneath the northern plateau as well as the apparent extension of the continental crust and intrusion of basaltic magmas beneath the southern plateau.Our field program (R/V Vema cruise 34-11) and subsequent analyses were supported by NSF grant OCE76-21782 to Lamont-Doherty Geological Observatory. Many aspects of this work benefitted from use of existing seismic and other geophysical data acquired with the support of ONR contract N00014-75C-0210 and National Science Foundation grants GA-27281 and OCE-76-18049

Cemented mounds and hydrothermal sediments on the detachment surface at Kane Megamullion : a new manifestation of hydrothermal venting

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 14 (2013): 3352–3378, doi:10.1002/ggge.20186.Long-lived detachment faults are now known to be important in tectonic evolution of slow-spreading mid-ocean ridges, and there is increasing evidence that fluid flow plays a critical role in development of detachment systems. Here we document a new manifestation of low-temperature hydrothermal venting associated with the detachment fault that formed Kane Megamullion ∼3.3–2.1 m.y. ago in the western rift-valley wall of the Mid-Atlantic Ridge. Hydrothermal effects on the detachment surface include (1) cemented mounds of igneous rock and chalk debris containing hydrothermal Mn oxides and Fe oxyhydroxides, and (2) layered deposits of similar Fe-Mn minerals ± interbedded chalks. Mounds are roughly conical, ∼1–10 m high, and contain primarily basalts with lesser gabbro, serpentinite, and polymict breccia. The layered Fe-Mn-rich sediments are flat-bedded to contorted and locally are buckled into low-relief linear or polygonal ridges. We propose that the mounds formed where hydrothermal fluids discharged through the detachment hanging wall near the active fault trace. Hydrothermal precipitates cemented hanging-wall debris and welded it to the footwall, and this debris persisted as mounds as the footwall was exhumed and surrounding unconsolidated material sloughed off the sloping detachment surface. Some of the layered Fe-Mn-rich deposits may have precipitated from fluids discharging from the hanging-wall vents, but they also precipitated from low-temperature fluids venting from the exposed footwall through overlying chalks. Observed natural disturbance and abnormally thin hydrogenous Fe-Mn crusts on some contorted, hydrothermal Fe-Mn-rich chalks on ∼2.7 Ma crust suggest diffuse venting that is geologically recent. Results of this study imply that there are significant fluid pathways through all parts of detachment systems and that low-temperature venting through fractured detachment footwalls may continue for several million years off-axis.NSF grant 0118445 supported data acquisition and processing for Knorr Cruise 180- 2. The Deep Ocean Exploration Institute at Woods Hole Oceanographic Institution supported research and analytical costs for this study.2014-03-0

Problematic plate reconstruction

Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Geoscience 5 (2012): 676-677, doi:10.1038/ngeo1596.As has been previously proposed, Bronner et al. suggest that opening of the rift between Newfoundland and Iberia involved exhumation of mantle rocks until 112 million years ago, subsequent seafloor spreading, and crustal thickening along the high-amplitude J magnetic anomaly by magma that propagated from the Southeast Newfoundland Ridge area. Conventionally, the anomalous magnetism and basement ridges associated with the J anomaly north of the Newfoundland-Gibraltar Fracture Zone are thought to have formed about 125 million years ago at chron M0 (Fig. 1a), although the crust probably experienced some later magmatic overprinting. The M0 age would make their formation simultaneous with that of the similar J anomaly and basement ridges (the J Anomaly Ridge and Madeira Tore Rise) along the Mid-Atlantic Ridge to the south and place them within a zone of exhumed mantle in the Newfoundland-Iberia rift. In contrast, Bronner et al. propose that the J anomaly and associated basement ridges were formed by later magmatism (about 112 million years ago) that marked the end of mantle exhumation in the rift. We argue here that constraints from plate tectonic reconstructions render this possibility untenable.2013-04-0

Benthic storms, nepheloid layers, and linkage with upper ocean dynamics in the western North Atlantic

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Marine Geology 385 (2017): 304–327, doi:10.1016/j.margeo.2016.12.012.Benthic storms are episodic periods of strong abyssal currents and intense, benthic nepheloid (turbid) layer development. In order to interpret the driving forces that create and sustain these storms, we synthesize measurements of deep ocean currents, nephelometer-based particulate matter (PM) concentrations, and seafloor time-series photographs collected during several science programs that spanned two decades in the western North Atlantic. Benthic storms occurred in areas with high sea-surface eddy kinetic energy, and they most frequently occurred beneath the meandering Gulf Stream or its associated rings, which generate deep cyclones, anticyclones, and/or topographic waves; these create currents with sufficient bed-shear stress to erode and resuspend sediment, thus initiating or enhancing benthic storms. Occasionally, strong currents do not correspond with large increases in PM concentrations, suggesting that easily erodible sediment was previously swept away. Periods of moderate to low currents associated with high PM concentrations are also observed; these are interpreted as advection of PM delivered as storm tails from distal storm events. Outside of areas with high surface and deep eddy kinetic energy, benthic nepheloid layers are weak to non-existent, indicating that benthic storms are necessary to create and maintain strong nepheloid layers. Origins and intensities of benthic storms are best identified using a combination of time-series measurements of bottom currents, PM concentration, and bottom photographs, and these should be coupled with water-column and surface-circulation data to better interpret the specific relations between shallow and deep circulation patterns. Understanding the generation of benthic nepheloid layers is necessary in order to properly interpret PM distribution and its influence on global biogeochemistry.Funding for construction of the Bottom Ocean Monitor was provided by Lamont-Doherty Geological Observatory (now Lamont-Doherty Earth Observatory). BOM and mooring deployments and data analysis were funded by the Office of Naval Research (contracts N00014-75-C-0210 and N00014-80-C-0098 to Biscaye and Gardner at Lamont-Doherty; Contracts N00014-79-C-0071 and N00014-82-C-0019 at Woods Hole Oceanographic Institution and ONR Contracts N00014-75-C-0210 and N00014-80-C-0098 at Lamont-Doherty Geological Observatory to Tucholke), Sandia National Laboratories (contract SL-16-5279 to Gardner), the National Science Foundation (contract OCE 1536565 to Gardner and Richardson), Earl F. Cook Professorship (Gardner), and the Department of Energy (contract DE-FG02-87ER-60555 to Biscaye)

Quantitative analysis of abyssal hills in the Atlantic Ocean : a correlation between inferred crustal thickness and extensional faulting

Author Posting. © American Geophysical Union, 1995. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 100, no. B11 (1995): 22509–22522, doi:10.1029/95JB02510.A recent cruise to the Office of Naval Research Atlantic Natural Laboratory obtained ∼100% Hydrosweep bathymetrie coverage, >200% Hawaii MRl (HMRl) side scan coverage, gravity and magnetics over an area spanning three ridge segments along axis (∼25°25′N to ∼27°10′N), and crustal ages from 0 to 26–30 Ma (∼400 km) on the west flank of the Mid-Atlantic Ridge. This data set represents a first opportunity for an extensive regional analysis of abyssal hill morphology created at a slow spreading ridge. The primary purpose of this work is to investigate the relationship between abyssal hill morphology and the properties of the ridge crest at which they were formed. We apply the method of Goff and Jordan [1988] for the estimation of two-dimensional statistical properties of abyssal hill morphology from the gridded Hydrosweep bathymetry. Important abyssal hill parameters derived from this analysis include root-mean-square (rms) height, characteristic width, and plan view aspect ratio. The analysis is partitioned into two substudies: (1) analysis of near-axis (< 7 Ma) abyssal hills for each of the three segments and (2) analysis of temporal variations (∼2–29 Ma) in abyssal hill morphology along the run of the south segment. The results of this analysis are compared and correlated with analysis of the gravity data and preliminary determination of faulting characteristics based on HMRl side scan data. Principal results of this study are: (1) Abyssal hill morphology within the study region is strongly influenced by the inside-outside corner geometry of the mid-ocean ridge segments; abyssal hills originating at inside corners have larger rms height and characteristic width and smaller plan view aspect ratio than those originating at outside corners. (2) The residual mantle Bouguer gravity anomaly is positively correlated with intersegment and along-flow-line variations in rms height and characteristic width, and it is negatively correlated with plan view aspect ratio. From this result, we infer that lower-relief, narrower, and more elongated abyssal hills are produced when the crust being generated is thicker. (3) Intersegment variations in near-axis rms height negatively correlate with average fault density as determined from analysis of HMRl side scan imagery.This research was supported by the Office of Naval Research under grants N00014-92-J-1214, N00014-94-I-0197, N0014-90-J-1621, and N0014-94-1-0466. G.E.J. was supported by ONR AASERT grant N00014-93-I-1153, and additional support to J.L. was provided by NSF grant OCE93-00708