Cretaceous-Tertiary crossroads of migration in the Sahel

A series of expeditions undertaken over the last three years by a team of geologists from Kingston Polytechnic and the British Museum (Natural History) has increased our understanding of the Cretaceous and Tertiary palaeogeography of Central West Africa. The latest expedition discovered a dinosaur graveyard of Lower Cretaceous (Wealden) age. Studies of these and other fossils within the sedimentary rocks of the Iullemmeden Basin in Niger and Mali have shed new light on the Trans-Saharan Seaway migration route during the Cretaceous and Tertiary periods.     

Dimethylsulphide production in the subantarctic southern ocean under enhanced greenhouse conditions

Dimethylsulphide (DMS) is an important sulphur‐containing trace gas produced by enzymatic cleavage of its precursor compound, dimethylsulphoniopropionate (DMSP), which is released by marine phytoplankton in the upper ocean. After ventilation to the atmosphere, DMS is oxidised to form sulphate aerosols which in the unpolluted marine atmosphere are a major source of cloud condensation nuclei (CCN). Because the micro‐physical properties of clouds relevant to climate change are sensitive to CCN concentration in air, it has been postulated that marine sulphur emissions may play a rôle in climate regulation. The Subantarctic Southern Ocean (41–53°S) is relatively free of anthropogenic sulphur emissions, thus sulphate aerosols will be mainly derived from the biogenic source of DMS, making it an ideal region in which to evaluate the DMS‐climate regulation hypothesis. We have extended a previous modelling analysis of the DMS cycle in this region by employing a coupled general circulation model (CGCM) which has been run in transient mode to provide a more realistic climate scenario. The CGCM output provided meteorological data under the IPCC/IS92a radiative forcing scenario. A DMS production model has been forced with the CGCM climate data to simulate the trend in the sea‐to‐air DMS flux for the period 1960 to 2080, corresponding to equivalent CO₂ tripling relative to pre‐industrial levels. The results confirm a minor but non‐negligible increase in DMS flux in this region, in the range +1% to +6% predicted over the period simulated. Uncertainty analysis of the DMS model predictions have confirmed the positive sign for the change in DMS flux, that is a negative DMS feedback on warming.     

KRISP 85 — An international seismic investigation of the Kenya Rift

British, German, American and Kenyan seismologists undertook a preliminary crustal seismic programme in the Kenya Rift in the summer of 1985. Thirteen explosions, totalling 7 tonnes, were fired in lakes and boreholes, and were recorded by 42 three-component stations. Preliminary results show that the deep structure along the axis of the Rift is more complex than previous models have suggested.     

Submarine-spring controlled calcification and growth of large Rivularia bioherms, Late Pleistocene (MIS 5e), Gulf of Corinth, Greece

The cyanobacterium Rivularia haematites has calcified to form unusually large (up to 10 m high) bioherms in the Pleistocene Gulf of Corinth. Today R. haematites calcifies only in freshwater environments but these Gulf of Corinth bioherms have a brackish affinity, limited areal extent, and occur within marine deposits. Field relations and preliminary U-series dating suggest a marine isotope stage (MIS) 5e age for the bioherms. This age is compatible with published MIS 5e ages for corals in the marine sediments above the bioherms and is consistent with their current elevation based on average uplift rates. Bioherm growth during MIS 5e constrains their formation during a time of near sea-level highstand when the Gulf of Corinth was marine. Growth cavities in the bioherms are encrusted by brackish tolerant coralline algae. Field mapping of the MIS 5e highstand palaeoshoreline shows the bioherms grew in water <16 m deep. Mg contents of the bioherm calcites, and associated coralline algal skeletons, are both much lower than expected for marine MIS 5e carbonates. They are best explained if the calcites precipitated from brackish fluids with Mg/Ca ratios below 2, implying at least 60% input of freshwater with low Mg/Ca ratio. Sr isotopes confirm a strong input of groundwater that had partially equilibrated with Mesozoic limestones. The limited areal extent of the bioherms and their close association with karstified fault scarps suggest that they formed in shallow sea water where freshwater submarine springs delivered CaCO₃ saturated water that promoted rapid calcification of cyanobacteria. Rapid calcification and strong degassing of CO₂ from the spring water resulted in disequilibrium stable isotope compositions for the calcites.     

Bedload path length and point bar development in gravel-bed river models

Low-sinuosity meandering gravel-bed flume experiments were employed to investigate spatial patterns of deposition, which point to patterns of channel development related to the pool and bar morphology. At channel-forming discharges, fluorescent bedload tracers indicate that deposition is typically focused around the point bar apex, downstream of the apex (contributing to downstream bar migration), and at the bar head/riffle surface. Seven flume experimental runs illustrate a sequence of point bar development related to the spatial patterns of tracer deposition, and the related path length distribution. At early stages of bar formation, transport is from the scour zone across the point bar head to the bar apex and bar margin downstream of the apex. As the point bar develops, bedload transport across the bar decreases, as transport along the channel thalweg increases and sediment is deposited along the bar margin. Deposition cells appear to move from downstream to upstream of the bar apex as this sequence of bar formation progresses. At low (non-channel-forming) discharges, transport occurs to the bar head/riffle surface with very little material being transported to the apex region or point bar interior. The implication is that there is an inherent connection between the loci of particle deposition and point bar formation, largely controlled by the morphology of the channel.     

Modelling ancient tides: the Upper Carboniferous epi-continental seaway of Northwest Europe

Due to a lack of modern analogues, debate surrounds the importance of tides in ancient epi-continental seas. However, numerical modelling can provide a quantitative means of investigating palaeo-tidality without recourse to analogues. Finite element modelling of the European Upper Carboniferous epi-continental seaway predicts an exceedingly low Lunar tidal range (ca 5 cm in the open water regions of the UK and Southern North Sea). The Imperial College Ocean Model (ICOM) uses finite element methods and an unstructured tetrahedral mesh that is computationally very efficient. The accuracy and sensitivity of ICOM tidal range predictions were tested using bathymetric data from the present-day Mediterranean Sea. The Mediterranean Sea is micro-tidal and varies in depth up to 5·4 km with an average depth of 1–2 km. ICOM accurately predicts the tidal range given both a realistic, but smoothed, bathymetry and a straight sided basin with a uniform depth of 1 km. Variation in uniform depth from 100 to 3000 m with and without islands consistently predicts micro-tidality, demonstrating that the model is robust and the effect of bathymetric uncertainty on model output is relatively small. The extremely low tidal range predicted for the European Upper Carboniferous is thus deemed robust. Putative Upper Carboniferous tidal deposits have been described in the UK and southern North Sea, but are represented by cyclic rhythmites and are limited to palaeo-estuaries. Calculations based on an embayed coast model show that the tidal range could have been amplified to ca 1 m in estuaries and that this is sufficient to form cyclic rhythmites. Without tidal mixing, the tropical equatorial heat and salinity enhancement would promote stratification in the open water body. The introduction of organic matter probably caused anoxia, biotic mortality and carbon accumulation, as evidenced by numerous black ‘marine-band’ shales.     

Hindered settling of sand grains

The sedimentation rate of sand grains in the hindered settling regime has been considered to assess particle shape effects. The behaviour of various particulate systems involving sand has been compared with the widely used Richardson–Zaki expression. The general form of the expression is found to hold, in as much as remaining as a suitable means to describe the hindered settling of irregular particles. The sedimentation exponent n in the Richardson–Zaki expression is found to be significantly larger for natural sand grains than for regular particles. The hindered settling effect is therefore greater, leading to lower concentration gradients than expected. The effect becomes more pronounced with increasing particle irregularity. At concentrations around 0·4, the hindered settling velocity of fine and medium natural sands reduces to about 70% of the value predicted using existing empirical expressions for n. Using appropriate expressions for the fluidization velocity and the clear water settling velocity, a simple method is discussed to evaluate the sedimentation exponent and to determine the hindered settling effect for sands of various shapes.