Chris Jenkins

Fission-track dating of British Ordovician and Silurian stratotypes

Fission-track dating of zircons and apatites from tuffs and bentonites has produced the first isotopic ages for the type sections of the Ordovician and Silurian Systems. In the Ordovician the following ages have been determined: lower Arenig 493 Ma, lower Llanvirn 487 Ma, lower Llandeilo 477 Ma, upper Caradoc 463 Ma and upper Ashgill 434 Ma. In the Silurian, the following: lower Llandovery 437 Ma, lower Wenlock 422 Ma, upper Wenlock 414 Ma and Ludlow 407 Ma. The Ordovician-Silurian boundary is interpreted as occurring at about 436 Ma. Three North American Rocklandian bentonites yielded zircons whose ages average 453 Ma. This is about 10 Ma younger than supposedly correlative units in the British type sections.

Carbonate sandwaves in Bass Strait

Side‐scan sonar, seabed photography and surficial sediment sampling document a field of sandwaves at 40–46 m water depth in eastern Bass Strait, Australia. The sandwaves are formed of coarse sand and gravel consisting of 50–92% biogenic carbonate, derived largely from the remains of molluscs, bryozoans and echinoderms. Four different scales of bedform are identified: ripples (0.1–0.3 m wavelength, 0.02–0.04 m high), small crest megaripples (4–5 m, 0.2 m), large trough megaripples (3–12 m, 0.1 ‐0.5 m) and sandwaves (55–1730 m, 2–12 m). The ripples are superimposed on both the megaripples and sandwaves. Large megaripples are present in the troughs between the sandwaves, while small megaripples are superimposed on the sandwave crests. Local tidal currents are non‐rectilinear. Sandwaves and their superimposed smaller bedforms are formed transverse to the ebb flow (from 255°) but subparallel to the flood flow, and trough megaripples are aligned transverse to the flood flow (from 135°). Minor deviations of the ...

Maximum a posteriori resampling of noisy, spatially correlated data

In any geologic application, noisy data are sources of consternation for researchers, inhibiting interpretability and marring images with unsightly and unrealistic artifacts. Filtering is the typical solution to dealing with noisy data. However, filtering commonly suffers from ad hoc (i.e., uncalibrated, ungoverned) application. We present here an alternative to filtering: a newly developed method for correcting noise in data by finding the “best” value given available information. The motivating rationale is that data points that are close to each other in space cannot differ by “too much,” where “too much” is governed by the field covariance. Data with large uncertainties will frequently violate this condition and therefore ought to be corrected, or “resampled.” Our solution for resampling is determined by the maximum of the a posteriori density function defined by the intersection of (1) the data error probability density function (pdf) and (2) the conditional pdf, determined by the geostatistical kriging algorithm applied to proximal data values. A maximum a posteriori solution can be computed sequentially going through all the data, but the solution depends on the order in which the data are examined. We approximate the global a posteriori solution by randomizing this order and taking the average. A test with a synthetic data set sampled from a known field demonstrates quantitatively and qualitatively the improvement provided by the maximum a posteriori resampling algorithm. The method is also applied to three marine geology/geophysics data examples, demonstrating the viability of the method for diverse applications: (1) three generations of bathymetric data on the New Jersey shelf with disparate data uncertainties; (2) mean grain size data from the Adriatic Sea, which is a combination of both analytic (low uncertainty) and word-based (higher uncertainty) sources; and (3) side-scan backscatter data from the Marthas Vineyard Coastal Observatory which are, as is typical for such data, affected by speckle noise. Compared to filtering, maximum a posteriori resampling provides an objective and optimal method for reducing noise, and better preservation of the statistical properties of the sampled field. The primary disadvantage is that maximum a posteriori resampling is a computationally expensive procedure.

Abyssal sediment drifts, erosion and history of bottom water flow in the Tasman Sea southwest of New Zealand

Deep‐sea sediments in the southeastern Tasman Sea underwent extensive bottom current erosion and transport during the late Cenozoic. Analysis of seismic facies shows that a northward flow caused profound erosion in narrow scoured deeps and over wide areas in the southeastern Tasman Sea southwest of New Zealand. At 47°S the flow diverted north‐northwest into the central Tasman Sea and a large lobate laminated sediment drift was constructed at the turn. The flow directions are established from mega‐bedform geometries in and on the drift and from the asymmetry of sediment remnants in the regions of profound erosion. Current activity and erosion began late in the Eocene or early in the Oligocene, probably due to both oceanographic and geographic changes at that time. Oceanographic data show that flows with similar erosional strength do not operate today. To explain the severity of past erosion by northward flows at this very eastern location in the basin, it is proposed that at times in the late Cenozoic, nor...

Offshore Sediment Character and Sand Resource Assessment of the Northern Gulf of Mexico, Florida to Texas

Abstract The Gulf of Mexico (GOM) continental shelf, extending approximately 1600 km from the Florida west coast to the U.S.–Mexico border, is a large sedimentary basin that has been the focus of much geologic study and surveys during the past 70 years, related mostly to oil and gas exploration. Relatively little attention has been focused on mapping and assessing offshore sediment character and resources, such as sand. It is increasingly recognized, however, that baseline scientific information on seafloor sediment character and composition is needed for managing and protecting natural resources and for providing information on sand availability and quality for potential use in a variety of coastal restoration and protection projects in all five of the states from Florida to Texas. The geomorphologic character and shallow sedimentary stratigraphy of the GOM shelf has been determined over geologic time by sediment inputs from rivers; sea-level fluctuations up to 120 m, resulting in transgressions and regressions of the shore; and frequent storms. These processes have resulted in deposition, reworking, and preservation of a variety of sand bodies, both on the seafloor and in buried, ancestral stream channels. Sand bodies of highly varying grain size, sorting, color, and composition are present throughout parts of the GOM inner shelf, varying greatly in size and number and often overlain or admixed with finer-grained, muddy sediment. The shelf sand bodies tend to be fine grained and are often mixed with muddy or organic detritus as well as carbonate shell material. The GOM shelf is mantled with sand mostly off the Florida shore, and sediments become progressively finer and muddier westward across the Alabama, Mississippi, Louisiana, and Texas shelf regions. The shelf off each state contains shoals that represent drowned paleoshoreline and buried, ancestral, stream-channel features that originated when sea level was lower than at present and the shore was farther seaward. These shoals offer the best promise as potential sand resources; however, further study is needed to refine these findings based on reconnaissance-scale work.

Multigrain seabed sediment transport modelling for the south-west Australian Shelf

With increasing concerns about climate change and sea-level rise, there is a need for a comprehensive under- standing of the sedimentary processes involved in the erosion, transport and deposition of sediment on the continental shelf. In the present paper, long-term and large-scale seabed morphological changes on the south-west Australian con- tinental shelf were investigated by a comprehensive sediment transport model, Sedsim. The investigated area covers the continental shelf and abyssal basins of the south-western region. The regional seabed is sensitive to environmental forces and sediment supply, and most terrigenous sediment carried down by major rivers is trapped in inland lakes or estuaries. Only a small fraction of fine-grain sediment reaches the continental shelf. The simulation has also confirmed that the Leeuwin Current and high-energy waves play the most important roles in regional long-term seabed evolution. Although the numerical implementation only approximates some forcing and responses, it represents a significant step forward in understanding the nature of potential long-term seabed change as a response to possible climate change scenarios. The 50-year forecast on the seabed morphological changes provides a reference for the management of coastal and offshore resources, as well as infrastructure, in a sustainable way.

Linking Oceanographic Modeling and Benthic Mapping with Habitat Suitability Models for Pink Shrimp on the West Florida Shelf

Abstract Research was undertaken to model and map the spatial distributions and abundances of pink shrimp Farfantepenaeus duorarum on the West Florida Shelf (WFS) using habitat suitability modeling (HSM). Data loggers and electronic logbook systems on three shrimp boats were used to gather catch and effort data along with bottom temperature, salinity, and depth data at the fishing locations. Vessel monitoring system (VMS) data supplied by the fishing company helped delineate areas with high fishing activity. For the vessels participating in this study, significantly higher mean catch per unit effort (CPUE) of pink shrimp was realized on the WFS during June–September 2004 and October–December 2004 than during January–March 2005 and April–June 2005. Suitability functions were created to predict CPUE in relation to depth, aspect, bottom type, bottom temperature, current speed, current direction, and VMS zone. Oceanographic modeling was conducted monthly from March 2004 to June 2005. Bottom current speed and direction indicated marked upwelling onto the WFS during 2004 and downwelling during 2005. The HSM linked to GIS was used to predict the spatial distributions and abundances of pink shrimp monthly from March 2004 to June 2005. While seven factors contributed to the HSM, current speed and current direction appeared to be most important during June–December 2004. The areas with the most pronounced upwelling were also the areas that the HSM predicted would have the highest mean CPUEs. This relationship was verified by overlaying the observed CPUE from the fishing vessels onto the suitability zones predicted by the HSM.

Integration of the stratigraphic aspects of very large sea-floor databases using information processing

Abstract Information-processing methods are described that integrate the stratigraphic aspects of large and diverse collections of sea-floor sample data. They efficiently convert common types of sea-floor data into database and GIS (geographical information system) tables, visual core logs, stratigraphic fence diagrams and sophisticated stratigraphic statistics. The input data are held in structured documents, essentially written core logs that are particularly efficient to create from raw input datasets. Techniques are described that permit efficient construction of regional databases consisting of hundreds of cores. The sedimentological observations in each core are located by their downhole depths (metres below sea floor — mbsf) and also by a verbal term that describes the sample ‘situation’ — a special fraction of the sediment or position in the core. The main processing creates a separate output event for each instance of top, bottom and situation, assigning top-base mbsf values from numeric or, where possible, from word-based relative locational information such as ‘core catcher’ in reference to sampler device, and recovery or penetration length. The processing outputs represent the sub-bottom as a sparse matrix of over 20 sediment properties of interest, such as grain size, porosity and colour. They can be plotted in a range of core-log programs including an in-built facility that better suits the requirements of sea-floor data. Finally, a suite of stratigraphic statistics are computed, including volumetric grades, overburdens, thicknesses and degrees of layering.

Herring supports Northeast Pacific predators and fisheries: Insights from ecosystem modelling and management strategy evaluation

This paper analyzes the trophic role of Pacific herring, the potential consequences of its depletion, and the impacts of alternative herring fishing strategies on a Northeast Pacific food web in relation to precautionary, ecosystem-based management. We used an Ecopath with Ecosim ecosystem model parameterized for northern British Columbia (Canada), employing Ecosim to simulate ecosystem effects of herring stock collapse. The ecological impacts of various herring fishing strategies were investigated with a Management Strategy Evaluation algorithm within Ecosim, accounting for variability in climatic drivers and stock assessment errors. Ecosim results suggest that herring stock collapse would have cascading impacts on much of the pelagic food web. Management Strategy Evaluation results indicate that herring and their predators suffer moderate impacts from the existing British Columbia harvest control rule, although more precautionary management strategies could substantially reduce these impacts. The non-capture spawn-on-kelp fishery, traditionally practiced by many British Columbia and Alaska indigenous peoples, apparently has extremely limited ecological impacts. Our simulations also suggest that adopting a maximum sustainable yield management strategy in Northeast Pacific herring fisheries could generate strong, cascading food web effects. Furthermore, climate shifts, especially when combined with herring stock assessment errors, could strongly reduce the biomasses and resilience of herring and its predators. By clarifying the trophic role of Pacific herring, this study aims to facilitate precautionary fisheries management via evaluation of alternative fishing strategies, and thereby to inform policy tradeoffs among multiple ecological and socioeconomic factors.

Naval Mine Impact Burial Prediction using Seafloor Database, Experiment, and GIS Technologies

The possibility of naval mines buried in the seafloor poses difficulties for navies concerned with port and seaway operations. To devise countermeasures, predictions of degrees of impact burial over wide areas of seabed must be made. Under ideal conditions, this is done with a knowledge of local seabed shear strengths, but in practice, such data are rarely available. We describe an alternative prediction method. Probabilistic predictions of mine impact burial are made across areas of variable seafloor by combining data on sedimentary character directly with experimental impact burial results. The most useful seafloor characteristics are mud content and consolidation. The predictions are relatively accurate (SD 1–22%), and are computable in detail over wide geographic areas. They are of a form immediately useful for naval operations (including calculations of risk) and are easily displayed in geographic information systems (GIS). An example is shown for the northern Gulf of Mexico.