Seafloor Surfaces Research Articles

New insights from multi-proxy data from the West Antarctic continental rise: Implications for dating and interpreting Late Quaternary palaeoenvironmental records

The Antarctic Peninsula’s Pacific margin is one of the best studied sectors of the Antarctic continental margin. Since the 1990s, several research cruises have targeted the continental rise with geophysical surveys, conventional coring and deep-sea drilling. The previous studies highlighted the potential of large sediment drifts on the rise as high-resolution palaeoenvironmental archives. However, these studies also suffered from chronological difficulties arising from the lack of calcareous microfossils, with initial results from geomagnetic relative palaeointensity (RPI) dating promising a possible solution.This paper presents data from new sediment cores recovered on cruise JR298 from seven continental rise sites west of the Antarctic Peninsula and in the Bellingshausen Sea with the objectives to (i) seek calcareous foraminifera, especially at shallow drift sites, to constrain RPI-based age models, and (ii) investigate the depositional history at these locations. We present the results of chronological and multi-proxy analyses on these cores and two cores previously collected from the study area. We establish new age models for the JR298 records and compare them with published RPI-based age models. In addition, we evaluate the reliability of different palaeoproductivity proxies and infer depositional processes.Planktic foraminifera are present in various core intervals. Although their stable oxygen isotope (δ18O) ratios, tephrochronological constraints and glacial-interglacial changes in sediment composition provide age models largely consistent with the RPI chronologies, we also observe distinct differences, predominantly in the Bellingshausen Sea cores. Enrichments of solid-phase manganese together with evidence for “burn-down” of organic carbon in late glacial and peak interglacial sediments document non-steady-state diagenesis that may have altered magnetic mineralogy and, thus, RPI proxies. This process may explain discrepancies between RPI-based age models and those derived from δ18O data combined with tephrochronology. The data also indicate that organic carbon is a much less reliable productivity proxy than biogenic barium or organically-associated bromine in the investigated sediments.In agreement with previous studies, sediment facies indicate a strong control of deposition on the rise by bottom currents that interacted with detritus supplied by meltwater plumes, gravitational down-slope transport processes and pelagic settling of iceberg-rafted debris (IRD) and planktic microfossils. Bottom-current velocities underwent only minor changes over glacial-interglacial cycles at the drift crests, with down-slope deposition only rarely affecting these shallow locations. Maximum concentrations of coarse IRD at the seafloor surfaces of the shallow sites result predominantly from upward pumping caused by extensive bioturbation. This process has to be taken into account when past changes in IRD deposition are inferred from quantifying clasts >1 mm in size.

A Multiresolution Grid Structure Applied to Seafloor Shape Modeling

This paper proposes a method of creating a multiresolution depth grid containing bathymetric data describing a stretch of sea floor. The included literature review presents current solutions in the area of the creation of digital terrain models (DTMs) focusing on methods employing regular grids, with a discussion on the strong and weak points of such an approach. As a basis for the investigations, some important recommendations from the International Hydrographic Organization are provided and are related to the accuracy of created models. The authors propose a novel method of storing DTM data, involving multiresolution depth grids. The paper presents the characteristics of this method, numerical algorithms of a conversion between a regular grid and the multiresolution one, and experiments on typical seafloor surfaces. The results are discussed, focusing on the data reduction rate and the variable resolution of the grid structure. The proposed method can be applied in Geographical Information Systems, especially for the purposes of solving sea survey problems.

Formal methods and finite element analysis of hurricane storm surge: A case study in software verification

Used to predict the effects of hurricane storm surge, ocean circulation models are essential tools for evacuation planning, vulnerability assessment, and infrastructure design. Implemented as numerical solvers that operate on large-scale datasets, these models determine the geographic extent and severity of coastal floods and other impacts. In this study, we look at an ocean circulation model used in production and an extension made to it that offers substantial performance gains. To explore implementation choices and ensure soundness of the extension, we make use of Alloy, a declarative modeling language with tool support and an automatic form of analysis performed within a bounded scope using a SAT solver. Abstractions for relevant parts of the ocean circulation model are presented, including the physical representation of land and seafloor surfaces as a finite element mesh, and an algorithm operating on it that allows for the propagation of overland flows. The approach allows us to draw useful conclusions about implementation choices and guarantees about the extension, in particular that it is equivalence preserving. • An application of formal methods in the context of scientific computing and large-scale physical simulation. • Development of models in Alloy for analyzing relevant parts of an ocean circulation model used in production. • Guarantees about soundness of an extension supported by the approach through experimentation and analysis.

Automated artifact-free seafloor surface reconstruction with two-step ODETLAP

We present an updated artifact-free seafloor surface reconstruction scheme which preserves more terrain features than our previous attempt using overdetermined Laplacian Partial Differential Equation (ODETLAP) and automates the adjustment of smoothing parameter. The high resolution version of such a surface fitting problem remains a challenge since we are still confined to extremely unevenly distributed depth samples collected along and near the ships, in which case numerous generic reconstruction algorithms generate unacceptable surfaces featuring abnormal depth fluctuations which are correlated with the trackline locations. Previously we reported the use a modified ODETLAP scheme, which integrates data-density-dependent smoothing into the reconstruction process, to generate surfaces which are free from such acquisition footprint. However, that scheme still suffers from terrain feature loss due to smoothing, and the reliance of human to decide appropriate smoothing factor. This paper aims to fix these two problems with a two-step ODETLAP procedure. The procedure first applies an accuracy-biased ODETLAP to complete the missing depth data from the given samples. After that, the vigorous depth fluctuations along the tracklines are removed by applying a smoothing-biased ODETLAP on the completed depth grid. To decide the optimal smoothing factor automatically, the procedure computes the areas of the individual bumps on the reconstructed surface. A surface suffering heavily from the artifacts has many small bumps but few big ones. Smoothing reduces such skewness. We find that for many datasets, the artifact is mostly gone when the coefficient of variation of the areas drops to around 1.3. Using that value to gauge the smoothing factor, the automated scheme successfully generates artifact-free seafloor surfaces within a limited error budget.

Approximation of surfaces with fault(s) and/or rapidly varying data, using a segmentation process, D m -splines and the finite element method

In many problems of geophysical interest, one has to deal with data that exhibit complex fault structures. This occurs, for instance, when describing the topography of seafloor surfaces, mountain ranges, volcanoes, islands, or the shape of geological entities, as well as when dealing with reservoir characterization and modelling. In all these circumstances, due to the presence of large and rapid variations in the data, attempting a fitting using conventional approximation methods necessarily leads to instability phenomena or undesirable oscillations which can locally and even globally hinder the approximation. As will be shown in this paper, the right approach to get a good approximant consists, in effect, in applying first a segmentation process to precisely define the locations of large variations and faults, and exploiting then a discrete approximation technique. To perform the segmentation step, we propose a quasi-automatic algorithm that uses a level set method to obtain from the given (gridded or scattered) Lagrange data several patches delimited by large gradients (or faults). Then, with the knowledge of the location of the discontinuities of the surface, we generate a triangular mesh (which takes into account the identified set of discontinuities) on which a D m -spline approximant is constructed. To show the efficiency of this technique, we will present the results obtained by its application to synthetic datasets as well as real gridded datasets in Oceanography and Geosciences.

The Helmholtz–Kirchhoff approach to modeling penetration of acoustic waves into rough seabeds

The Helmholtz–Kirchhoff integral is applied to model the penetration of sound waves into sandy seafloors at grazing angles above and below the critical angle. Although the conditions for the validity of the Kirchhoff approximation can be limiting, this approximation should be valid at high frequency for gently undulating seafloor surfaces even at moderate to low grazing angles, providing that the self-shadowing effect is carefully removed. The analytical development of the method is first presented, followed by numerical examples and comparisons with the experimental data of Maguer et al. [SACLANTCEN report, SR-287, April 1998]. The model predicts, in agreement with the 2- to 15-kHz acoustic data, the frequency where the contributions due to roughness effects begin to dominate those due to the evanescent wave. Secondary effects such as Bragg interference patterns and the loss of signal coherence with grazing angle or depth are correctly predicted. The model simulations strongly suggest that roughness of the sediment interface is most likely the cause of anomalous sound penetration into the seabed.

Shape from shading—a method of integration of sonar images and bathymetric data for ocean mapping

Abstract Shape from shading is one of the methods that derive geometric information of objects from analysis of monocular images. Application of this technique to underwater sonar images enables the conversion of imposed reflectance characteristics in sonar images to shape information, namely, slopes, about the seafloor surface. A combination of this shape information and available sparse distributed depth points results in improved dense bathymetric data. The reconstruction of shape models of seafloor surfaces from sonar images is treated as an inverse problem and is solved by the regularization theory. Sparse gridded points are used for boundary constraints. The regularization is implemented as a relaxation procedure with a hierarchical structure of multiresolution grids.

Development of a Comprehensive Oxygen-Deficient Marine Biofacies Model: Evidence from Santa Monica, San Pedro, and Santa Barbara Basins, California Continental Borderland

An extensive data base of box core x-radiographs, bottom photographs, and box core faunal data has been employed to establish the relationships between dissolved oxygen content and (1) sediment fabric, (2) biogenic structure distribution and size trends, and (3) faunal composition and species richness in three anoxic silled basins of the California continental borderland. Sediments of anaerobic environments are generally well laminated, whereas those of aerobic and dysaerobic environments are completely bioturbated. Intermediate, partially bioturbated sediments are produced near the present dysaerobic-anaerobic boundary by short-term temporal fluctuations in the position of the dysaerobic-anaerobic boundary. Although distribution of burrow types observed in x-radiographs does not show significant control by variations in dissolved oxygen, average size of burrows decreases markedly with decreasing oxygen. In terms of surface biogenic structures, most aerobic and upper dysaerobic sea floors are dominated by tracks and trails, but lower dysaerobic sea floors are characterized by burrow openings. No surface biogenic structures were observed on anaerobic sea floors. Regular and irregular echinoids dominate sea-floor surfaces in aerobic and upper dysaerobic environments, whereas polychaetes, ophiuroids, holothurians, small arthropods, and gastropods are common on lower dysaerobic sea floors. No surface-dwelling organisms were visible on anaerobic bottoms. Box core faunal analysis indicates that species richness generally decreases with increased depth and decreasing oxygen until it reaches near zero at the dysaerobic-anaerobic boundary. The most abrupt decrease in species richness occurs at the shelf-slope transition. These observations are highly supportive of criteria cited by Rhoads and Morse (1971) and Byers (1977) for distinguishing anaerobic facies from more oxygenated facies in ancient strata. However, their criteria for distinguishing between ancient aerobic and dysaerobic facies are not as definitive. Paleontologic criteria, biogenic structure size trends, and other rock characteristics are useful for establishing paleo-oxygen gradients but not for determining specific paleo-oxygen concentrations as there are no abrupt oxygen-controlled changes in these parameters above the anaerobic-dysaerobic boundary.

Mathematical Simulation of Sediment Organism Community Interactions with an IBM 7090 Computer: ABSTRACT

An ALGOL-58 computer program has been written by the authors for IBM 7090 or 7094 computers for simulating the interactions of combined sediment-organism communities through geologic time. Although the program is based on a mathematical model applicable to various sea-floor communities, the program has been developed with carbonate sediment-organism communities in mind. The mathematical model considers the sea floor at a given in to consist of a large number of discrete elements arranged on a grid of arbitrary dimensions. Each grid element may be thought of as a square containing a single community and is symbolized by an integer number in a two-dimensional array. Successive time planes (sea-floor surfaces) are generated in which the distribution of communities for each discrete instant is printed out using various symbols to identify different communities. The program employs feedback control loops in which the geographic distribution of communities on preceding sea-floor surfaces influences, but does not rigidly control, the geographic distribution of communities that develop subsequently. The selection of a community element occupying a particular square at a particular moment is treated as a random process influenced by conditional probabilities. The program makes extensive use of pseudo-random number generation methods for selection of individual elements. The program is being used to study the development of Devonian coral-stromatoporoid reefs, as well as idealized modern sediment-organism communities. Adaptive learning techniques are used to adapt the model to real data. Mathematical simulation should be a powerful oil-finding tool in some regions by yielding greater insight into the behavior of reefs and other sedimentary features and providing improved means of prediction. End_of_Article - Last_Page 343------------