Bottom processes, morphology, and geotechnical properties of the continental slope south of Baltimore Canyon

The continental slope south of Baltimore Canyon seaward of the coasts of Delaware and Maryland has a different morphology and sedimentary structure than adjacent portions of the continental margin. Ridges of sediment 600 m thick and transverse to the slope contain many unconformities that can be traced from ridge to ridge. The age of the sediment is inferred to be late tertiary to recent with the morphology related to a major drainage system. Physical properties of a suite of sediment cores display a pattern that varies in relationship to the morphology and depositional environment. Sedimentary structures and low shear strengths indicate instability of surficial sediments present on the upper slope and can be correlated with regions where the seismic reflection profiles show slumping has occurred. A veneer of sand overlying the general silty clay of the area is present on the upper slope and on the ridges indicating sand spillover from the shelf with a recent change in deposition pattern.     

Comparative Study of Sand Porosity and a Technique for Determining Porosity of Undisturbed Marine Sediment

Porosity is a fundamental property of marine sediment from which wet bulk density can be easily determined and used in a variety of geoacoustic, geotechnical, and sedimentological studies, analyses, and models. However, methods of sampling marine sands suffer from the common problem of core disturbance making the in situ porosity difficult to obtain. Embedding the sediment within an epoxy resin matrix will minimize the disturbance to the microfabric and preserve the in situ sedimentary structure for subsequent study. Image analysis can then be used on thin sections to study the microfabric and porometry. A comprehensive review and analysis of published data on the porosity of predominantly clean sands has been completed and a simple, accurate, and nondestructive technique is described for preparing and measuring the porosity of marine sediment (siliciclastic sand) that has been infiltrated aboard ship immediately upon sample collection and chemically fixed and infiltrated with epoxy shortly thereafter. The average porosity of 36 samples of marine sand collected offshore Fort Walton Beach, Florida, and embedded with resin was determined to be 41.30%. From the review of published data the average porosity of sand was determined to be 37.7%, 42.3%, and 46.3% for packed, natural (in situ), and loose packing conditions, respectively, for a range of sorting coefficients and grain sizes.     

A study of the angle dependence of roll damping moment

The angle dependence of the roll damping moment is investigated by analysing experimentally obtained free roll decay records. Two ship models were used with and without bilge keels, also results with forward speed were obtained. The analysis indicate strong angle dependence and explains why the quadratic and cubic velocity dependent damping moments are successful in many cases.     

Euler–Lagrange equations for the spectral element shallow water system

We present the derivation of the discrete Euler–Lagrange equations for an inverse spectral element ocean model based on the shallow water equations. We show that the discrete Euler–Lagrange equations can be obtained from the continuous Euler–Lagrange equations by using a correct combination of the weak and the strong forms of derivatives in the Galerkin integrals, and by changing the order with which elemental assembly and mass averaging are applied in the forward and in the adjoint systems. Our derivation can be extended to obtain an adjoint for any Galerkin finite element and spectral element system.We begin the derivations using a linear wave equation in one dimension. We then apply our technique to a two-dimensional shallow water ocean model and test it on a classic double-gyre problem. The spectral element forward and adjoint ocean models can be used in a variety of inverse applications, ranging from traditional data assimilation and parameter estimation, to the less traditional model sensitivity and stability analyses, and ensemble prediction. Here the Euler–Lagrange equations are solved by an indirect representer algorithm.     

Weak and strong constraint data assimilation in the inverse Regional Ocean Modeling System (ROMS): Development and application for a baroclinic coastal upwelling system

We describe the development and preliminary application of the inverse Regional Ocean Modeling System (ROMS), a four dimensional variational (4DVAR) data assimilation system for high-resolution basin-wide and coastal oceanic flows. Inverse ROMS makes use of the recently developed perturbation tangent linear (TL), representer tangent linear (RP) and adjoint (AD) models to implement an indirect representer-based generalized inverse modeling system. This modeling framework is modular. The TL, RP and AD models are used as stand-alone sub-models within the Inverse Ocean Modeling (IOM) system described in [Chua, B.S., Bennett, A.F., 2001. An inverse ocean modeling system. Ocean Modell. 3, 137–165.]. The system allows the assimilation of a wide range of observation types and uses an iterative algorithm to solve nonlinear assimilation problems. The assimilation is performed either under the perfect model assumption (strong constraint) or by also allowing for errors in the model dynamics (weak constraints). For the weak constraint case the TL and RP models are modified to include additional forcing terms on the right hand side of the model equations. These terms are needed to account for errors in the model dynamics.Inverse ROMS is tested in a realistic 3D baroclinic upwelling system with complex bottom topography, characterized by strong mesoscale eddy variability. We assimilate synthetic data for upper ocean (0–450 m) temperatures and currents over a period of 10 days using both a high resolution and a spatially and temporally aliased sampling array. During the assimilation period the flow field undergoes substantial changes from the initial state. This allows the inverse solution to extract the dynamically active information from the synthetic observations and improve the trajectory of the model state beyond the assimilation window. Both the strong and weak constraint assimilation experiments show forecast skill greater than persistence and climatology during the 10–20 days after the last observation is assimilated.Further investigation in the functional form of the model error covariance and in the use of the representer tangent linear model may lead to improvement in the forecast skill.     

Interglacial vegetation succession: A view from southern Europe

Factors influencing interglacial vegetation development in southern Europe are considered in a series of comparisons of the vegetation and climatic signatures of selected periods. Multivariate analysis provides a method for standardizing comparison of interglacial vegetation successions, and insolation values and geological evidence supply information on the climatic character of individual periods. Application of this comparative approach to a long pollen record from northwest Greece presents an opportunity to examine a series of interglacial successions under constant background site characteristics, secure chronostratigraphical positions and minimal differences in species' immigration rates. The record of four interglacial period equivalent to marine oxygen isotopic substages 5e, 7c, 9c and 11c is examined. The comparison shows that the two earliest periods are characterized by similar vegetation development despite differences in climatic regimes. Dependence on initial conditions is one of the emergent aspects of the comparisons, suggesting that the nature of surviving populations during a cold stage may be critical in determining the course of interglacial succession.     

The uranium and thorium decay series nuclides in Mt. St. Helens effusives

The concentrations of the radionuclides²³⁸U,²³⁰Th,²²⁶Ra,²¹⁰Pb,²¹⁰Po,²³²Th,²²⁸Ra and²²⁸Th and the abundances of major elements were determined in samples from all major eruptions of Mt. St. Helens from May 18, 1980 through June 21, 1981. During this time the effusives changed from plagioclase-phyric dacite to a more andesitic composition but the concentrations of U and Th series nuclides were measurably invariant. The average²³²Th/²³⁸U weight ratio in the rocks is 2.4 and the²³⁰Th/²³²Th activity ratio equals the²³⁸U/²³²Th activity ratio indicating no fractionation of U from Th during magma genesis.²²⁶Ra activity is in excess (～40% on average) of its parent²³⁰Th whereas²²⁸Ra is in radioactive equilibrium with its parent²³²Th, constraining the time of magma formation between 30 and 10⁴ years prior to eruption. The²¹⁰Pb/²²⁶Ra activity ratios in the samples average 1.0, with a 20% scatter on either side, but allowing for volatile²¹⁰Pb loss at time of eruption excess²¹⁰Pb over²²⁶Ra is inferred, indicating that the time of magma formation was within the last 150 years.²¹⁰Po was virtually absent in the samples immediately after eruption, indicating its total loss by volatilization during eruption. The quantity of²¹⁰Po volatilized during the May 18, 1980 event is estimated to be in the range of 300 Ci from the effusives and as much as 5000 Ci total including losses from heated slide material. The²²²Rn activity volatilized should have been comparable to the²¹⁰Po activity released.