Mud Origin, Characterisation and Human Activities (MOCHA): Final report

The cohesive sediments, which are frequently found in the Belgian nearshore zone (southern North Sea), are of different age such as tertiary clays and Holocene, modern and recently deposited muds. The area is characterised by a turbidity maximum. The source areas of the recently deposited muds and the effect of human impact vs. natural processes on the distribution and/or erosion of these sediments have been investigated using historic and recent bottom samples, in situ and remote sensing (satellite images) SPM concentration measurements, numerical modelling, GIS and clay mineral and microfossil analysis. The Schelde estuary, the potential erosion areas of cohesive sediments on the BCS and adjacent areas and the SPM transport through the Dover Strait have been considered as possible source areas.The historic bottom samples have been collected in the beginning of the 20th century, the quality of these samples and the meta-information is very high and they have proven to be a major reference to understand the evolution of the cohesive sediment distribution. The recent bottom samples consist of box core, Reineck core and Van Veen grab samples collected during the last 10 years. The processing of the historic and recent data on cohesive sediments was mainly based on field descriptions of the samples (consolidation, thickness) and morphological evolution. On some of the recent samples radioactive and gamma densitometric measurements have been carried out. During the processing the emphasis was put on the occurrence of thick layers (>30 cm) of freshly deposited to very soft consolidated mud and of clay and mud pebbles, because these sediments are witnesses of changes.Satellite images, in situ measurements and a 2D hydrodynamic numerical model have been combined to calculate the long term SPM transport through the Dover Strait and in the southern North Sea. The satellite images (SeaWiFS) provide synoptic views of SPM concentration. The representativness of SPM concentration maps derived from satellites for calculating long term transports has been investigated by comparing the SPM concentration variability from the in situ measurements with those of the satellite data. It is underlined that SPM concentration measurements should be carried out during at least one tidal cycle in high turbidity areas to obtain representative values of SPM concentration.Areas where the thickness of the Quaternary cover is less than 2.5 m were defined as potential erosion areas of Palaeogene clay containing deposits. In the framework of this project, the geological data related to the BCS have been reviewed and the relevant information was compiled into a GIS. This also included a small part of the French continental shelf. Additionally information was added from vibrocores analysis and Dutch geological data. Approximately 20 % of the BCS, 6 % of the small part of the French area and only 3% of the Dutch study area could possibly serve as a source for fine suspended sediments. Quaternary muds are mostly presented in the 2 eastern nearshore area; on the Dutch part they occur more offshore. Their occurrence represents 11% of the BCS and approximately 35% of the Rabsbank area Cretaceous microfossils are present in all samples and have been transported into the area with the residual water transport. Material from the east, in particular from the Eocene-Oligocene transitional strata, has been found in the eastern nearshore area up to about Oostende. This zone coincides with the extension of the Holocene mud and could indicate an erosion of these sediments and/or a transport of clay minerals from the Schelde estuary.Clay mineral analysis has been carried by two approaches in order to determine source areas. The results of the second approach show that no systematic differences in the clay mineralogy depending on geographic location could have been found within the samples. The results clearly prove the necessity of using more elaborate sample preparation procedures in examining the provenance of the mud deposits.Thick layers of fresh mud were deposited in the beginning of the 20th century mainly in a narrow band along the coast from about Nieuwpoort up to the mouth of the Westerschelde. These deposits were mainly the result of natural morphological changes. Today, most of the depositions of thick layers of fresh mud have been induced by anthropogenic operations, such as dumping, deepening of the navigation channels and construction and extension of the port of Zeebrugge. Comparing the actual situation with the situation 100 years ago reveals that the area around Zeebrugge where fresh mud is deposited extends more offshore today

Predicting effective diffusion coefficients in mudrocks using a fractal model and small-angle neutron scattering measurements

The determination of effective diffusion coefficients of gases or solutes in the water‐saturated pore space of mudrocks is time consuming and technically challenging. Yet reliable values of effective diffusion coefficients are important to predict migration of hydrocarbon gases in unconventional reservoirs, dissipation of (explosive) gases through clay barriers in radioactive waste repositories, mineral alteration of seals to geological CO2 storage reservoirs, and contaminant migration through aquitards. In this study, small‐angle and very small angle neutron scattering techniques have been utilized to determine a range of transport properties in mudrocks, including porosity, pore size distributions, and surface and volume fractal dimensions of pores and grains, from which diffusive transport parameters can be estimated. Using a fractal model derived from Archie's law, we calculate effective diffusion coefficients from these parameters and compare them to laboratory‐derived effective diffusion coefficients for CO2, H2, CH4, and HTO on either the same or related mudrock samples. The samples include Opalinus Shale from the underground laboratory in Mont Terri, Switzerland, Boom Clay from a core drilled in Mol, Belgium, and a marine claystone cored in Utah, USA. The predicted values were compared to laboratory diffusion measurements. The measured and modeled diffusion coefficients show good agreement, differing generally by less than factor 5. Neutron or X‐ray scattering analysis is therefore proposed as a novel method for fast, accurate estimation of effective diffusion coefficients in mudrocks, together with simultaneous measurement of multiple transport parameters including porosity, pore size distributions, and surface areas, important for (reactive) transport modeling

Calcite cementation of screes: palaeoclimatic implications

Calcite cements that consolidated scree deposits are investigated to reconstruct the paleoclimate of the area near the ancient city of Sagalassos (southwest Turkey). The mineralogical and geochemical characteristics of the calcite cements indicate that after deposition of the screes, the climate changed from less to more humid conditions and again to a more dry environment.status: publishe