A Bayesian approach to inverse modelling of stratigraphy, part 1: method

The inference of ancient environmental conditions from their preserved response in the sedimentary record still remains an outstanding issue in stratigraphy. Since the 1970s, conceptual stratigraphic models (e.g. sequence stratigraphy) based on the underlying assumption that accommodation space is the critical control on stratigraphic architecture have been widely used. Although these methods considered more recently other possible parameters such as sediment supply and transport efficiency, they still lack in taking into account the full range of possible parameters, processes, and their complex interactions that control stratigraphic architecture. In this contribution, we present a new quantitative method for the inference of key environmental parameters (specifically sediment supply and relative sea level) that control stratigraphy. The approach combines a fully non‐linear inversion scheme with a ‘process–response’ forward model of stratigraphy. We formulate the inverse problem using a Bayesian framework in order to sample the full range of possible solutions and explicitly build in prior geological knowledge. Our methodology combines Reversible Jump Markov chain Monte Carlo and Simulated Tempering algorithms which are able to deal with variable‐dimensional inverse problems and multi‐modal posterior probability distributions, respectively. The inverse scheme has been linked to a forward stratigraphic model, BARSIM (developed by Joep Storms, University of Delft), which simulates shallow‐marine wave/storm‐dominated systems over geological timescales. This link requires the construction of a likelihood function to quantify the agreement between simulated and observed data of different types (e.g. sediment age and thickness, grain size distributions). The technique has been tested and validated with synthetic data, in which all the parameters are specified to produce a ‘perfect’ simulation, although we add noise to these synthetic data for subsequent testing of the inverse modelling approach. These tests addressed convergence and computational‐overhead issues, and highlight the robustness of the inverse scheme, which is able to assess the full range of uncertainties on the inferred environmental parameters and facies distributions.     

A Bayesian approach to inverse modelling of stratigraphy, part 2: Validation tests

A novel inverse modelling method is applied to the problem of constraining the environmental parameters (e.g. relative sea level, sediment supply) that control stratigraphic architecture. This technique links forward modelling of shallow-marine wave/storm-dominated stratigraphy to a combination of inverse methods formulated in a Bayesian framework. We present a number of examples in which relative sea-level and sediment-supply curves were inferred from synthetic vertical successions of grain size (e.g. wells) and synthetic thickness curves (e.g. seismically derived isopachs) extracted from a forward model simulation. These examples represent different scenarios that are designed to test the impact of data distribution, quantity and quality on the uncertainty of the inferred parameters. The inverse modelling approach successfully reproduces the gross stratigraphic architectures and relative sea level and sediment-supply histories of the synthetic forward model simulation, within the constraints of the modelled data quality. The relative importance of the forcing parameters can be evaluated by their sensitivity and impact on the inverted data. Of equal importance, the inverse results allow complete characterisation of the uncertainties inherent to the stratigraphic modelling tool and to the data quality, quantity and distribution. The numerical scheme also successfully deals with the problem of non-uniqueness of the solution of the inverse problem. These preliminary results suggest that the inverse method is a powerful tool in constraining stratigraphic architecture for hydrocarbon reservoir characterisation and modelling, and it may ultimately provide a process-based geological complement to standard geostatistical tools.     

A parallel mesh-free contaminant transport model based on the Analytic Element and Streamline Methods

This paper introduces a new method for simulating large-scale subsurface contaminant transport that combines an Analytic Element Method (AEM) groundwater flow solution with a split-operator Streamline Method for modeling reactive transport. The key feature of the method is the manner in which the vertically integrated AEM flow solution is used to construct three-dimensional particle tracks that define the geometry of the Streamline Method. The inherently parallel nature of the algorithm supports the development of reactive transport models for spatial domains much larger than current grid-based methods. The applicability of the new approach is verified for cases with negligible transverse dispersion through comparisons to analytic solutions and existing numerical solutions, and parallel performance is demonstrated through a realistic test problem based on the regional-scale transport of agricultural contaminants from spatially distributed sources.     

Sr-gorceixite,a weathering product in rich iron ores from the Córrego do Feijão Mine,Minas Gerais,Brazil

A sulfate-bearing Sr-Ba phosphate (Sr-gorceixite) formed by supergene alteration in rich iron ores from Minas Gerais, Brazil, has been studied by means of microprobe techniques. Analytical data and the structural formula are presented.     

New aromatic biomarkers and possible maturity indicators found in New Albany Shale extracts

Aromatic hydrocarbons from benzene extracts of New Albany Shale were characterized. A biomarker that has a molecular weight of 546 and a structural configuration consistent with that of an alkyl-aromatic hydrocarbon (C₄₀H₆₆) was tentatively identified. It was found that the relative concentrations of the biomarker are indicative of differing levels of thermal maturity of the shale organic matter. A 40-carbon bicyclic carotenoid (C₄₀H₄₈) is proposed as the geochemical precursor of this biomarker. Thermal maturity of the shale organic matter can also be differentiated by observing differences in “fingerprints” as obtained by field-ionization mass spectrometry on the aromatic hydrocarbon fraction. Using this technique, we found that the more mature shale samples from southeastern Illinois contain more low molecular weight extractable aromatic hydrocarbons and the less mature shale samples from northwestern Illinois contain more high molecular weight extractable aromatic hydrocarbons. It was demonstrated that field-ionization and tandem mass spectrometric techniques through fingerprint and individual compound identification, are useful for shale aromatic hydrocarbon fraction characterization and for thermal maturation interpretation.     

Response of single benthic metrics and multi-metric methods to anthropogenic pressure gradients, in five distinct European coastal and transitional ecosystems

In recent times many benthic indices have been proposed to assess the ecological quality of marine waters worldwide. In this study we compared single metrics and multi-metric methods to assess coastal and transitional benthic status along human pressure gradients in five distinct environments across Europe: Varna bay and lake (Bulgaria), Lesina lagoon (Italy), Mondego estuary (Portugal), Basque coast (Spain) and Oslofjord (Norway). Hence, 13 single metrics (abundance, number of taxa, and several diversity and sensitivity indices) and eight of the most common indices used within the European Water Framework Directive (WFD) for benthic assessment were selected: index of size spectra (ISS), Benthic assessment tool (BAT), Norwegian quality index (NQI), Multivariate AMBI (M-AMBI), Benthic quality index (BQI), (Benthic ecosystem quality index (BEQI), Benthic index based on taxonomic sufficiency (BITS), and infaunal quality index (IQI). Within each system, sampling sites were ordered in an increasing pressure gradient according to a preliminary classification based on professional judgement. The different indices are largely consistent in their response to pressure gradient, except in some particular cases (i.e. BITS, in all cases, or ISS when a low number of individuals is present). Inconsistencies between indicator responses were most pronounced in transitional waters (i.e. IQI, BEQI), highlighting the difficulties of the generic application of indicators to all marine, estuarine and lagoonal environments. However, some of the single (i.e. ecological groups approach, diversity, richness) and multi-metric methods (i.e. BAT, M-AMBI, NQI) were able to detect such gradients both in transitional and coastal environments, being these multi-metric methods more consistent in the detection than single indices. This study highlights the importance of survey design and good reference conditions for some indicators. The agreement observed between different methodologies and their ability to detect quality trends across distinct environments constitutes a promising result for the implementation of the WFD’s monitoring plans. Moreover, these results have management implications, regarding the dangers of misclassification, uncertainty in the assessment, use of conflicting indices, and testing and validation of indices.     

Delimiting the eastern extent of the Altyn Tagh Fault: Insights from structural analyses of seismic reflection profiles

The Altyn Tagh Fault (ATF) serves as a key continental‐scale controlling structural element of the Tibetan Plateau. However, its eastward extent remains controversial. Here we use high‐resolution seismic reflection profiles to investigate the subsurface structures of the easternmost ATF and use these to delimit the easternmost extent of the fault. The structural analyses show an eastward geometric change from transpressional positive flower structures to compressional thrusts, with transpression‐induced shortening magnitudes decreasing eastwards from a maximum of ~5.3 km to being absent. Stratigraphic controls indicate that the deformation took place over the last ~<1.2 Ma. Our wider findings lead us to: (a) reject the suggestion that the ATF previously extended beyond the Kuantan Shan‐Hei Shan to link with the Alxa‐East Mongolia Fault; and (b) propose that the rigid block model used to describe the Tibetan Plateau crust is not consistent with the extent and structural details of the easternmost ATF.     

Differences in vegetation characteristics with shoreline position on a tropical pocket beach

ABSTRACT

Composition and richness of coastal vegetation differs with distance from the water and sheltering by topography. Transition from pioneer beach plants to mature forests is expected to be narrow on low wave energy coasts with a tropical climate favorable to vegetation growth. The goal of this paper is to determine whether vegetation on the beach and foredune will be denser and have a greater number of species and more woody shrubs near the water as wave and wind stresses diminish because of favorable shoreline orientation within a pocket beach. Field data on beach width, beach mobility, dune height, vegetation species, vegetation height and percent vegetation cover were collected in Puerto Rico along six cross-shore transects. Beaches are more stable at transects in the lee of an eolianite barrier and a tombolo. The vegetation gradient there is compressed, with denser, taller, more diverse vegetation and more tree species close to the waterline than at more exposed sites. The lack of mobility of the beaches and dunes and lack of geomorphically significant disturbance events is in contrast to the conspicuous feedbacks between overwash, topography, and vegetation on exposed mid-latitude coasts, revealing the need for more study of low-energy beach environments.     

Liminosity distribution function for visual binary stars

The cosmic scatter of 147 of the best known visual binary stars on the Main Sequence is discussed and a new estimation of the luminosity distribution function for multiple star systems is presented. As long as the mass ratioq of a close binary is not smaller than 0.5, the distribution of close binary components is identical to the van Rhijn liminosity-function. For smaller mass ratios (q<0.5) the number of close companions decreases rapidly. It appears that less than 13% of visual binaries in our sample are simple binary systems.Communication presented at the International Conference on Astrometric Binaries, held on 13–15 June, 1984, at the Remeis-Sternwarte Bamberg, Germany, to commemorate the 200th anniversary of the birth of Friedrich Wilhelm Bessel (1784–1846).     

A low-resolution spectrophotometer for measuring the integrated colour of galaxies

The spectrophotometer presented here combines the advantage of aperture photometry with spectrophotometry to obtain colours of extended objects over the range of 3500–6000 Å. The spectrophotometric resolution matches the resolution of intermediate-band interference filters of about 140 Å FWHM.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.