Bernard Coakley

The International Bathymetric Chart of the Arctic Ocean (IBCAO) Version 3.0

The International Bathymetric Chart of the Arctic Ocean (IBCAO) released its first gridded bathymetric compilation in 1999. The IBCAO bathymetric portrayals have since supported a wide range of Arc ...

New grid of Arctic bathymetry aids scientists and mapmakers

For over two decades, Sheet 5.17 of the Fifth Edition of the General Bathymetric Chart of the Oceans (GEBCO) [Canadian Hydrographic Service, 1979] has been considered the authoritative portrayal of the sea floor north of 64 N.This sheet was constructed from publicly available bathymetric data sets, which in the late 1970s were rather sparse, consisting almost entirely of underway measurements collected from ice-breakers, drifting ice islands, and point measurements obtained along snow-mobile tracks or using air support. Data coverage tended to be fairly good at lower latitudes where ice cover was not a hindrance, but at higher latitudes, where ice was more prevalent, major features such as the Amerasian and Eurasian Basins were not well delineated. This situation posed problems not only for expedition planners but also for scientific investigators, who needed an accurate description of the sea floor to design field experiments and to link their research with processes affecting or affected by the shape of the seabed (for example, sea level change, ocean circulation, sediment transport,seafloor spreading, and Pleistocene glaciation).

Ice shelves in the Pleistocene Arctic Ocean inferred from glaciogenic deep-sea bedforms

It has been proposed that during Pleistocene glaciations, an ice cap of 1 kilometre or greater thickness covered the Arctic Ocean. This notion contrasts with the prevailing view that the Arctic Ocean was covered only by perennial sea ice with scattered icebergs. Detailed mapping of the ocean floor is the best means to resolve this issue. Although sea-floor imagery has been used to reconstruct the glacial history of the Antarctic shelf , little data have been collected in the Arctic Ocean because of operational constraints. The use of a geophysical mapping system during the submarine SCICEX expedition in 1999 provided the opportunity to perform such an investigation over a large portion of the Arctic Ocean. Here we analyse backscatter images and sub-bottom profiler records obtained during this expedition from depths as great as 1 kilometre. These records show multiple bedforms indicative of glacial scouring and moulding of sea floor, combined with large-scale erosion of submarine ridge crests. These distinct glaciogenic features demonstrate that immense, Antarctic-type ice shelves up to 1 kilometre thick and hundreds of kilometres long existed in the Arctic Ocean during Pleistocene glaciations.

Structure of the Dead Sea pull‐apart basin from gravity analyses

Analyses and modeling of gravity data in the Dead Sea pull-apart basin reveal the geometry of the basin and constrain models for its evolution. The basin is located within a valley which defines the Dead Sea transform plate boundary between Africa and Arabia. Three hundred kilometers of continuous marine gravity data, collected in a lake occupying the northern part of the basin, were integrated with land gravity data from Israel and Jordan to provide coverage to 30 km either side of the basin. Free-air and variable-density Bouguer anomaly maps, a horizontal first derivative map of the Bouguer anomaly, and gravity models of profiles across and along the basin were used with existing geological and geophysical information to infer the structure of the basin. The basin is a long (132 km), narrow (7–10 km), and deep (≤10 km) full graben which is bounded by subvertical faults along its long sides. The Bouguer anomaly along the axis of the basin decreases gradually from both the northern and southern ends, suggesting that the basin sags toward the center and is not bounded by faults at its narrow ends. The surface expression of the basin is wider at its center (<16 km) and covers the entire width of the transform valley due to the presence of shallower blocks that dip toward the basin. These blocks are interpreted to represent the widening of the basin by a passive collapse of the valley floor as the full graben deepened. The collapse was probably facilitated by movement along the normal faults that bound the transform valley. We present a model in which the geometry of the Dead Sea basin (i.e., full graben with relative along-axis symmetry) may be controlled by stretching of the entire (brittle and ductile) crust along its long axis. There is no evidence for the participation of the upper mantle in the deformation of the basin, and the Moho is not significantly elevated. The basin is probably close to being isostatically uncompensated, and thermal effects related to stretching are expected to be minimal. The amount of crustal stretching calculated from this model is 21 km and the stretching factor is 1.19. If the rate of crustal stretching is similar to the rate of relative plate motion (6 mm/yr), the basin should be ∼3.5 m.y. old, in accord with geological evidence.

The role of the sediment load in sequence stratigraphy: The influence of flexural isostasy and compaction

Simplified, physically based models are utilized in order to examine the effects of sea level, sedimentation, tectonic subsidence, isostatic compensation, and compaction on development of unconformities. Unconformities divide volumes of relatively conformable sediments defining the time and spatial extent of sedimentary sequences. The model parameters are varied both individually and in concert in order to isolate their contributions to sequence architecture. These models reveal the importance of sedimentation and subsidence rates, as well as sea level amplitude and rate, in determining the type and extent of sequence boundary formed. Sedimentation and subsidence rate can vary significantly within and between margins, producing different sequence boundaries given an identical eustatic sea level fall. It is shown that sea level amplitude plays an important role in determining which systems tracts are present and sequence boundary timing for type 1 sequences and that sea level rate controls the same attributes in type 2 sequences. Isostatic compensation and compaction, which have been described as secondary effects, are shown to have considerable influence on sequence architecture. These two processes, due to isostatic response to sediment loading and sediment self-loading, initiate feedback by enhancing and partitioning accommodation space. These additional effects invalidate application of the equilibrium point as a guide to where and how all accommodation space is created. The manner in which flexure distributes accommodation space is a function of lithospheric rigidities: higher rigidities partition space laterally, producing wide shelves which favor type 1 sequence boundaries, whereas lower rigidities partition space vertically forming narrow shelves which favor type 2 sequence boundaries. Compaction generates a net landward shift of the shelf edge and favors type 2 sequence development Both isostatic compensation and compaction introduce time delays in sequence development. One consequence of compaction is to rotate sedimentary packages, generating growth and leakage of anticlinal structures as fluids are driven from the underlying section. This process has important implications for hydrocarbon migration in compaction-dominated systems, such as the Gulf of Mexico margin.

New Gravity Field for the Arctic

The study of the Arctic Ocean has been hampered by incomplete basic knowledge of the basin and its structure. An improved gravity anomaly map (Figure 1) and grid have been developed that complement the new International Chart of the Arctic Ocean [Jakobsson et al., 2008]. This article announces the availability of the Arctic Gravity Project (ArcGP) grid version 2.0 and discusses the genesis of the project.

Far-field tilting of Laurentia during the Ordovician and constraints on the evolution of a slab under an ancient continent

During a brief period of 10 to 15 million years in the Middle-Ordovician, the Michigan Basin departed from its bulls-eye subsidence pattern and tilted toward the east, opening to the Appalachian basin. This tilting is observed in maps of tectonic subsidence estimated for the Black River and Trenton Formations and extends over 300 km across the Michigan Basin and into eastern Wisconsin. Contours of constant tectonic subsidence rate are approximately parallel to the inferred position of the Laurentian-Iapetus convergent margin. The distance between the inferred position of the subduction zone to the limit of tilting is approximately 1000 km. Three alternative models for the tilting are tested, two relying on the rigidity of the continental lithosphere and a third on the viscous flow generated by a subducted slab. In the first elastic model we assume the edge of the elastic plate is simply loaded from above (by a fold and thrust sheet, volcanic pile, etc.). This model, however, cannot simultaneously satisfy the space of tectonic subsidence and subsidence rate, even for lithospheres which have a rigidity of 10^28 Nm. In the second elastic model, the Laurentian continental margin descends into a trench of an eastward dipping slab, that is, a slab descending under the Iapetus ocean. This process cannot generate any significant far field displacements, even for extremely rigid plates, and must be rejected. For the third model we use finite element solutions of a negatively buoyant slab in a viscous medium with a faulted lithosphere. Such slabs can easily generate not only realistic trenches on the under thrusting plate but also significantly tilt the lithosphere as much as 1000 km from the plate margin. The magnitude and distribution of far-field displacements depend on the age, length, and dip angle of the slab. In contrast to the elastic models, penetration of a west-dipping slab beneath the continent can reproduce both the extent, magnitude, and rate of tectonic subsidence observed in the Trenton and Black River Formations. The observed data are best fit by an old slab (140 Ma), which initially descended at a moderate dip (20°–30°) for 10–15 m.y., which then steepened as the slab penetrated deeper into the mantle. At the position of the previous Ordovician plate margin, there are narrow, block faulted basins which underwent rapid subsidence ∼10 m.y. before the Michigan Basin tilted toward the east. We propose that these earlier subsidence events were caused by the initial descent of the slab under the preexisting Cambro-Ordovician passive margin. The time lag of ∼10 m.y. may be due to the time it takes the slab to penetrate the upper mantle. This result is important for understanding the time evolution of mantle convection and mechanisms for the initiation of subduction.

Morphology and structure of the Lomonosov Ridge, Arctic Ocean

The Lomonosov Ridge is a band of continental crust that stretches across the Arctic Ocean and separates the Mesozoic Amerasian Basin from the Cenozoic Eurasian Basin. From about 87°N north of Greenland across the Pole to about 86°N, the Lomonosov Ridge is a single highstanding blocky ridge with minimum depths of ∼950–1400 m. South of 86°N on the Siberian side, the ridge breaks up into a series of ridges spread over a width of about 200 km. In this region a highstanding blocky ridge with minimum depths of ∼650–1400 m bounds the Eurasian Basin and continues to the Siberian continental margin. This ridge is continuous with the single ridge making up the Lomonosov Ridge toward North America and is the former outermost continental shelf of Eurasia bounding the Amerasian Basin. The Eurasian Basin margin of the Lomonosov Ridge consists of a series of rotated fault blocks stepping down to the basin that result from nearly orthogonal rifting to form the Eurasian Basin. No rotated fault blocks are observed on the Amerasian Basin margin of the Lomonosov Ridge. On the Amerasian Basin side, Marvin Spur, a linear ridge separated from Lomonosov Ridge by a deep basin, parallels Lomonosov Ridge on the North American side of the pole. At the bend in the Lomonosov Ridge near the North Pole, Marvin Spur continues along strike across the Makarov Basin. South of 86°N toward Siberia, a continuous outer ridge makes up the Amerasian Basin edge of the Lomonosov complex with a series of basins and ridges between it and the former Eurasian shelf. The outer ridge marks an abrupt boundary between the Lomonosov Ridge complex and the apparently oceanic crust of the Makarov Basin. The outer ridge and Marvin Spur very closely follow small circles about a pole located on the Mackenzie delta. The observed structure on the Amerasian Basin side of the Lomonosov Ridge is analogous to that observed at well-studied shear margins and supports rotational models for the development of the Amerasian Basin.

SCICEX Investigations of the Arctic Ocean System

Abstract In 1993 the United States Navy and the marine research community embarked on an ambitious program to study the Arctic Ocean using nuclear-powered submarines. The program, termed SC ience IC e EX ercise (SCICEX), was designed to simultaneously sample and map the ice canopy, physical, chemical, and biological water properties, seafloor and seabed subsurface. The small size of the Arctic Basin relative to Earths other oceans and the unique capabilities of the nuclear submarines, high speed coupled with the ability to operate independently of the sea ice cover, combined to allow the first holistic investigation of an entire ocean basin. The data acquired during eight submarine cruises helped refine hypotheses and models for the broad spectrum of subdisciplines that comprise arctic science and, perhaps more importantly, illuminated the linkages between the various components of the Arctic Ocean system. This paper presents an overview of the SCICEX program, summarizing the results published to date and briefly describing each submarine deployment and the instruments used to acquire various datasets, to demonstrate the important contribution of this collaborative venture to arctic science.

Airborne gravimetry from a small twin engine aircraft over the Long Island Sound

In January 1990, a test of the feasibility of airborne gravimetry from a small geophysical survey aircraft, a Cessna 404, was conducted over the Long Island Sound using a Bell Aerospace BGM-3 sea gravity meter. Gravity has been measured from large aircraft and specially modified de Havilland Twin Otters but never from small, standard survey aircraft. The gravity field of the Long Island Sound is dominated by an asymmetric positive 30 rnGal anomaly which is well constrained by both marine and land gravity measurements. Using a Trimble 4000 GPS receiver to record the aircrafts horizontal position and radar altimeter elevations to recover the vertical accelerations, gravity anomalies along a total of 65 km were successfully measured. The root mean square (rms) difference between the airborne results and marine measurements within 2 km of the flight path was 2.6 mGaI for 15 measured values. The anomalies recovered from airborne gravimetry can also be compared with the gridded regional free air gravity field calculated using all available marine and land gravity measurements. The rms difference between 458 airborne gravity measurements and the regional gravity field is 2.7 mGai. This preliminary experiment demonstrates that gravity anomalies, with wavelengthsas short as 5 km, can be measured from small aircraft with accuracies of 2.7 mGal or better. The gravity measurements could be improved by higher quality vertical and horizontal positioning and tuning the gravimeters stabilized platform for aircraft use.