Larry A. Mayer

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 ...

Shallow-water imaging multibeam sonars: A new tool for investigating seafloor processes in the coastal zone and on the continental shelf

Hydrographic quality bathymetry and quantitative acoustic backscatter data are now being acquired in shallow water on a routine basis using high frequency multibeam sonars. The data provided by these systems produce hitherto unobtainable information about geomorphology and seafloor geologic processes in the coastal zone and on the continental shelf.Before one can use the multibeam data for hydrography or quantitative acoustic backscatter studies, however, it is essential to be able to correct for systematic errors in the data. For bathymetric data, artifacts common to deep-water systems (roll, refraction, positioning) need to be corrected. In addition, the potentially far greater effects of tides, heave, vessel lift/squat, antenna motion and internal time delays become of increasing importance in shallower water. Such artifacts now cause greater errors in hydrographic data quality than bottom detection. Many of these artifacts are a result of imperfect motion sensing, however, new methods such as differential GPS hold great potential for resolving such limitations. For backscatter data, while the system response is well characterised, significant post processing is required to remove residual effects of imaging geometry, gain adjustments and water column effects. With the removal of these system artifacts and the establishment of a calibrated test site in intertidal regions (where the seabed may be intimately examined by eye) one can build up a sediment classification scheme for routine regional seafloor identification.When properly processed, high frequency multibeam sonar data can provide a view of seafloor geology and geomorphology at resolutions of as little as a few decimetres. Specific applications include quantitative estimation of sediment transport rates in large-scale sediment waves, volume effects of iceberg scouring, extent and style of seafloor mass-wasting and delineation of structural trends in bedrock. In addition, the imagery potentially provides a means of quantitative classification of seafloor lithology, allowing sedimentologists the ability to examine spatial distributions of seabed sediment type without resorting to subjective estimation or prohibitively expensive bottom-sampling programs. Using Simrad EM100 and EM1000 sonars as an example, this paper illustrates the nature and scale of possible artifacts, the necessary post-processing steps and shows specific applications of these sonars.

The early Miocene onset of a ventilated circulation regime in the Arctic Ocean.

Deep-water formation in the northern North Atlantic Ocean and the Arctic Ocean is a key driver of the global thermohaline circulation and hence also of global climate. Deciphering the history of the circulation regime in the Arctic Ocean has long been prevented by the lack of data from cores of Cenozoic sediments from the Arctic’s deep-sea floor. Similarly, the timing of the opening of a connection between the northern North Atlantic and the Arctic Ocean, permitting deep-water exchange, has been poorly constrained. This situation changed when the first drill cores were recovered from the central Arctic Ocean. Here we use these cores to show that the transition from poorly oxygenated to fully oxygenated (‘ventilated’) conditions in the Arctic Ocean occurred during the later part of early Miocene times. We attribute this pronounced change in ventilation regime to the opening of the Fram Strait. A palaeo-geographic and palaeo-bathymetric reconstruction of the Arctic Ocean, together with a physical oceanographic analysis of the evolving strait and sill conditions in the Fram Strait, suggests that the Arctic Ocean went from an oxygen-poor ‘lake stage’, to a transitional ‘estuarine sea’ phase with variable ventilation, and finally to the fully ventilated ‘ocean’ phase 17.5 Myr ago. The timing of this palaeo-oceanographic change coincides with the onset of the middle Miocene climatic optimum, although it remains unclear if there is a causal relationship between these two events.

HIGH-RESOLUTION SWATH SONAR INVESTIGATION OF SAND RIDGE, DUNE AND RIBBON MORPHOLOGY IN THE OFFSHORE ENVIRONMENT OF THE NEW JERSEY MARGIN

Abstract Sand ridges on the northeast US Atlantic shelf form in the near-shore environment, most likely in response to storm-driven flows. As the Holocene transgression has continued, the ridges have been transferred to an offshore hydrodynamic regime, where currents are not constrained by the coast and storms do not influence bottom currents as frequently or as strongly. Here, we qualitatively and quantitatively investigate the morphology of offshore sand ridges and smaller-scale features in an effort to place constraints on bedform development in these deeper waters. A recent high-resolution swath sonar survey mapped a portion of the New Jersey shelf from ∼20 m water depth to the shelf break (∼120 m), imaging both sand ridges and smaller-scaled dunes and ribbons in far greater detail than has been previously possible. Using a robust statistical analysis, we find that the gross morphology of ridges (height, width, length) does not change with depth beyond ∼20 m water depth, and changes in ridge orientation generally mirror changes in regional contour orientation. Hence, ridges have not continued to grow since transgression has brought them into the offshore hydrodynamic regime. However, on the inner shelf (∼20 m water depth to the Mid-Shelf shore), we do find evidence in the ridge shape, which has an asymmetry opposite to that seen near shore, and in the complex backscatter response that some important modifications to ridges are taking place at these water depths. In contrast, on the mid-shelf (from the Mid-Shelf shore to the Franklin shore), ridges tend to have higher backscatter at the crests, implying that these are largely winnowed, relict features. Lineated, smaller-scale (∼100–500 m wavelength,

Protocols for calibrating multibeam sonar

Development of protocols for calibrating multibeam sonar by means of the standard-target method is documented. Particular systems used in the development work included three that provide the water-column signals, namely the SIMRAD SM2000/90- and 200-kHz sonars and RESON SeaBat 8101 sonar, with operating frequency of 240 kHz. Two facilities were instrumented specifically for the work: a sea well at the Woods Hole Oceanographic Institution and a large, indoor freshwater tank at the University of New Hampshire. Methods for measuring the transfer characteristics of each sonar, with transducers attached, are described and illustrated with measurement results. The principal results, however, are the protocols themselves. These are elaborated for positioning the target, choosing the receiver gain function, quantifying the system stability, mapping the directionality in the plane of the receiving array and in the plane normal to the central axis, measuring the directionality of individual beams, and measuring the nearfield response. General preparations for calibrating multibeam sonars and a method for measuring the receiver response electronically are outlined. Advantages of multibeam sonar calibration and outstanding problems, such as that of validation of the performance of multibeam sonars as configured for use, are mentioned.

Marine sediment classification using the chirp sonar

The chirp sonar is a calibrated wideband digital FM sonar that provides quantitative, high‐resolution, low‐noise subbottom data. In addition, it generates an acoustic pulse with special frequency domain weighting that provides nearly constant resolution with depth. The chirp sonar was developed with the objective of remote acoustic classification of seafloor sediments. In addition to producing high‐resolution images, the calibrated digitally recorded data are processed to estimate surficial reflection coefficients as well as a complete sediment acoustic impulse profile. In this paper, surficial sediments in Narragansett Bay, RI are used to provide ground truth for an acoustic model. Quantitative acoustic returns from the chirp sonar are used to estimate surficial acoustic impedance and to predict sediment properties. A robust acoustic sediment classification model that uses core samples to account for the local depositional environment has been developed. The model uses an estimate of acoustic impedance t...

Chirp subbottom profiler for quantitative sediment analysis

A wide‐band, frequency‐modulated, subbottom profiling system (the chirp sonar) can remotely determine the acoustic attenuation of ocean sediments and produce artifact‐free sediment profiles in real time. The chirp sonar is controlled by a minicomputer which performs analog‐to‐digital and digital‐to‐analog conversion, correlation processing, and attenuation estimation in real time. The minicomputer generates an FM pulse that is phase‐ and amplitude‐compensated to correct for the sonar system response. Such precise waveform control helps suppress correlation noise and source ringing. The chirp sonar, which has an effective bandwidth of 5 kHz, can generate chirp (Klauder) wavelets with a tuning thickness (Rayleigh’s criterion for resolution) of approximately 0.1 ms. After each return is correlated, a computationally fast algorithm estimates the attenuation of subbottom reflections by waveform matching with a theoretically attenuated waveform. This algorithm obtains an attenuation estimate by minimizing the m...

Geophysical insights into the Transition fault debate: Propagating strike slip in response to stalling Yakutat block subduction in the Gulf of Alaska

On the basis of faulting mapped on seismic reflection and bathymetric data, seismicity, current plate motions, and evidence that the Yakutat block may be anomalously thick, we propose a tectonic model for Yakutat-Pacific interactions, including the often-debated Transition fault. To the east, deformation associated with the Queen Charlotte–Fairweather fault system is extending offshore, facilitating westward propagation of strike-slip motion along the eastern segment of the Transition fault. To the west, the oblique-slip Pamplona zone and Transition faults merge at an embayment in the continental margin, where a north-south dextral strike-slip fault within the Pacific plate, illuminated by the 1987–1992 earthquake swarm, intersects the Pacific-Yakutat tectonic boundary. These fault patterns are consistent with modern plate motions and reflect a plate boundary reorganization that may be caused by resistance to subduction by the Yakutat block, a possible moderate-sized oceanic plateau.

Dense biological communities at 3850 m on the Laurentian Fan and their relationship to the deposits of the 1929 Grand Banks earthquake

During Alvin dives on the Laurentian Fan aimed at exploring the nature of the deposit of the 1929 Grand Banks earthquake and turbidity current, large, dense communities of living vesicomyid and thyasirid clams, gastropods, and other epifaunal taxa similar to those found in hydrothermal and cold seep environments were unexpectedly discovered. The communities are at 3800–3900 m in a passive margin setting, with no apparent mechanism for enhanced fluid flow. The communities occur near the crests of ‘gravel waves’, depositional bedforms created during the passage of the turbidity current, and on the slope and crest of a steep (20–30°) scarp of outcropping valley floor material. We speculate that these communities have established themselves since 1929 and that they are sustained by chemosynthetic processes. The reduced compounds fueling the chemosynthesis presumably are derived from older, organic-rich fan valley floor sediments that were exposed by the 1929 event.

Great Lakes paleohydrology: Complex interplay of glacial meltwater, lake levels, and sill depths

The oxygen isotope record of ostracode and clam shells recovered from Great Lakes cores of known age allows definition of times when meltwaters from the Laurentide ice sheet were important components of lake water in the several lake basins since 12 ka. We find that the lowstands in Lake Huron and Georgian Bay are characterized by isotopically light waters (δ 18 O values of -20‰ to -22‰ relative to SMOW [standard mean ocean water]) and the highstands by isotopically heavy waters of more local origin. These data can be used to determine the degree of hydraulic separation among the early Holocene lakes. Southern Lake Michigan, for instance, may mix with northern-source waters only during times of rising and high water levels. Generally it is characterized by waters of local origin.