Early fill of the Western Irish Namurian Basin: a complex relationship between turbidites and deltas

The Western Irish Namurian Basin developed in Early Carboniferous times as a result of extension across the Shannon Lineament which probably coincides with the lapetus Suture. During the late Dinantian, axial areas of the NE-SW elongate trough became deep, whilst shallow-water limestones were deposited on the flanks. This bathymetry persisted into the Namurian when carbonate deposition ceased. In axial areas, a relatively thick mudstone succession spans earliest Namurian to Chokierian whilst on the northwestern marginal shelf, a thin, condensed Namurian mudstone sequence, in which pre-Chokierian sediments are apparently absent, rests unconformably on the Dinantian. From late Chokierian to early Kinderscoutian, the basin was filled by sand-dominated clastic sediments. Sand deposition began in the axial area with deposition of a thick turbidite sequence, the Ross Formation, which is largely equivalent to the condensed mudstone succession on the flanks. Turbidity currents flowed mainly axially towards the north-east and deposited a sequence lacking well-defined patterns of vertical bed-thickness change. Channels and slide sheets occur towards the top of the formation. The turbidite system seems to have lacked well-defined lobes and stable distributary channels. Overlying the Ross Formation, the Gull Island Formation shows a decreasing incidence of turbidite sandstones at the expense of increasing siltstones. This formation is characterized by major slides and slumps interbedded with undisturbed strata. In the flanking areas of the basin, the formation is thinner, has only a few turbidites in the sequence above the condensed mudstones and contains only one slide sheet. Overall the formation is interpreted as the deposit of a major prograding slope, the lower part representing a ramp upon which turbidites were deposited, the upper part a highly unstable muddy slope lacking any conspicuous feeder channels through which sand might have been transferred to deeper water. Progradation of the slope appears to have been increasingly from the northwestern flank of the trough which is similar to the direction deduced for the overlying deltaic Tullig cyclothem which completes the initial basin fill. Whilst several features of the succession can be explained by envisaging the whole sequence as the product of one linked depositional system, the shifting directions of palaeocurrents and palaeoslope raise problems. The switch from axial to lateral supply casts doubt on the strict application of Walther's Law to the total sequence and seems to demand large avulsive shifts of the delta system on the shelf area to the west.     

Mapping nonlinear shallow-water tides: a look at the past and future

Overtides and compound tides are generated by nonlinear mechanisms operative primarily in shallow waters. Their presence complicates tidal analysis owing to the multitude of new constituents and their possible frequency overlap with astronomical tides. The science of nonlinear tides was greatly advanced by the pioneering researches of Christian Le Provost who employed analytical theory, physical modeling, and numerical modeling in many extensive studies, especially of the tides of the English Channel. Le Provost’s complementary work with satellite altimetry motivates our attempts to merge these two interests. After a brief review, we describe initial steps toward the assimilation of altimetry into models of nonlinear tides via generalized inverse methods. A series of barotropic inverse solutions is computed for the M tide over the northwest European Shelf. Future applications of altimetry to regions with fewer in situ measurements will require improved understanding of error covariance models because these control the tradeoffs between fitting hydrodynamics and data, a delicate issue in coastal regions. While M can now be robustly determined along the Topex/Poseidon satellite ground tracks, many other compound tides face serious aliasing problems. In memory of Christian Le Provost     

What controls the extreme flow through the Kerama Gap: a global HYbrid Coordinate Ocean Model reanalysis point of view

Ocean Dynamics - The temporal variability of volume transport from the North Pacific Ocean to the East China Sea (ECS) through the Kerama Gap (between Okinawa Island and Miyakojima Island—a...     

Sulfide intrusion in the tropical seagrasses Thalassia testudinum and Syringodium filiforme

Sulfur and oxygen dynamics in the seagrasses Thalassia testudinum and Syringodium filiforme and their sediments were studied in the US Virgin Islands (USVI) in order to explore sulfide intrusion into tropical seagrasses. Four study sites were selected based on the iron concentration in sediments and on proximity to anthropogenic nutrient sources. Meadow characteristics (shoot density, above- and below-ground biomass, nutrient content) were sampled along with sediment biogeochemistry. Sulfide intrusion was high in T. testudinum, as up to 96% of total sulfur in the plant was derived from sediment-derived sulfides. The sulfide intrusion was negatively correlated to the turnover of sulfides in the sediments regulated by both plant parameters and sediment sulfur pools. Sediment iron content played an indirect role by affecting sulfide turnover rates. Leaf production was negatively correlated with sulfide intrusion suggesting that active growth reduced sulfide intrusion. Sulfide intrusion was lower in S. filiforme (up to 44%) compared to T. testudinum consistent with a higher internal nighttime oxygen concentrations found for S. filiforme. When S. filiforme can take advantage of its ability to maintain high internal oxygen concentrations, as was the case on the USVI, it could increase its success in colonizing unvegetated disturbed sediments with potentially high sulfide concentrations.     

Detecting lateral resistivity inhomogeneities with the Schlumberger array

A method of detecting lateral resistivity inhomogeneities with a multi-electrode cable for the Schlumberger array is presented. Using such a cable system, two dipole-dipole soundings are gathered in addition to the Schlumberger sounding at each point. Offset differences calculated from the dipole-dipole data give qualitative information about lateral resistivity inhomogeneities. Results from 1D modelling can give quantitative information about the 2D resistivity distribution since the dipole-dipole soundings are made to either side of the Schlumberger sounding point. Examples from two different locations in Norway are shown. At Reinøya, northern Norway, a dipping layer, confirmed by refraction seismic data, was identified. In a sedimentary basin with a contamination plume at Haslemoen, southern Norway, the method has revealed lateral variations in resistivity. In both cases, the Schlumberger soundings could be fitted with a 1D resistivity model.     

Continuity and change: Spatiotemporal land use dynamics on Bellona Island, Solomon Islands

Traditional agricultural land use systems in the humid tropics of the Southwest Pacific are, as elsewhere, affected by globalization processes. This paper analyzes the directions of change in the land use system of Bellona, a small outer island in the Solomon Islands. We focus on the human–environmental interaction that shapes land use patterns and practices in the context of theoretical lines of thought concerning intensification of agricultural systems in the tropics. Aerial photography from 1966 and satellite imagery from 2006 in conjunction with studies from the 1960s and a contemporary household survey reveal only minor changes in the agricultural system. Land use and land cover dynamics are related to agricultural strategies, demographic factors, institutional actors as well as biophysical drivers or constraints. Local agricultural production still contributes significantly to local subsistence but imported food has also become a major food source. Hence, land use has become partially disconnected from the local population pressure and therefore remains relatively stable while the larger livelihood portfolio has undergone significant diversification. At present, the agricultural system is a supplement to a range of strategies supporting the increasing number of people on the island. This explains why land use patterns continue relatively unchanged while livelihood and food supply strategies have changed.     

Computing cross-isotherm volume transports from ocean temperature observations and surface heat fluxes,with application to the Barents Sea inflow

A method for determining the cross-isotherm ocean transport from surface heat flux and ocean temperature data is derived. By computing the volume flux through the isotherm that extend from 19°E, 74°N to the eastern part of the Kola Peninsula, the flow through the western entrance of the Barents Sea south of 74°N is estimated. Using three different surface heat flux datasets, the inflow is found to range from 2.9 to 4.5 Sv in winter (October–March) and from 0.4 to 1.4 Sv in summer (April–September; 1 Sv=10⁶ m³ s⁻¹). The seasonal variations are stronger than indicated by results from direct current measurements, probably because the seasonal cycle of the surface heat fluxes is overestimated along the considered isotherm. The annual mean inflow ranges from 1.9 to 2.2 Sv during a cold period (1986–1988), and from 2.4 to 3.0 Sv during a warm period (1990–1992), close to reported observations.