Environmental controls on branched tetraether lipid distributions in tropical East African lake sediments

Quantifying past continental temperature changes is an important aspect of paleoclimate research as it allows us to constrain the amplitude of natural variability, test predictive climate models, and provide a proper context for changes that may arise in response to anthropogenically-induced climate change. The recently developed biomarker-based methylation index of branched tetraethers/cyclization ratio of branched tetraethers (MBT/CBT) proxy shows potential as a new method for continental temperature reconstruction, but thus far it has only been applied successfully in ocean margin sediments. To assess whether this proxy is also applicable to the sedimentary record in tropical lacustrine systems, we investigated the distribution of branched glycerol dialkyl glycerol tetraethers (GDGTs) in recently deposited sediments from 46 lakes in tropical East Africa. These lakes span a substantial range in surface elevation (770-4500 m above sea level), and thus also a wide gradient of mean annual temperature. We find that, saline lakes excepted, branched GDGTs are universally abundant in the lakes investigated and can be used to predict mean annual air temperature (MAAT) with a high degree of accuracy. However, the existing global MBT/CBT calibration for MAAT based on soils predicts inaccurate temperatures when applied to our African lake dataset. This observation, together with the fact that surface water pH, and to lesser extent, lake depth appear to influence the distribution of branched GDGTs among sites, leads us to conclude that in situ production of branched GDGTs in lakes is likely. The robust relationship between branched GDGT distribution and the temperature and pH of African freshwater lakes makes these compounds suitable for paleoenvironmental reconstruction, however we urge caution in using branched GDGTs in lake sediments to infer past temperatures, unless their exact origin can be determined.     

Isotope Ratio Determination in the Earth and Environmental Sciences: Developments and Applications in 2003

This annual review documents developments and applications in the field of isotope ratio determination, as reflected in the literature for the Earth and environmental sciences for the year 2003. Particular emphasis is placed upon the relationship between the two dominant analytical techniques-thermal ionisation mass spectrometry (TIMS) and multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS)-and the rapidly changing nature of their roles in isotope analysis. Additionally, the review covers developments in single-collector ICP-MS and TOF technologies, new sample preparation procedures and the characterisation of isotopic reference materials, together with fundamental investigations of mass spectrometer performance.     

Environmental conditions in the Late Cretaceous African Tethys: conclusions from a microscopic-microchemical study of ooidal ironstones from Egypt,Sudan and Nigeria

Two types of ooidal ironstone deposits, all of Late Cretaceous age, were recognised in an area trending from Nigeria in a northeasterly direction via Sudan to Egypt. The two types of ironstones are of the kaolinite-type deposits (Agbaja area, Bida area, Sokoto and Potiskum, Nigeria; Shendi and Wadi Haifa, Sudan; and Kalabscha, Egypt) and of the chamosite-type deposits (Aswan, Egypt and Okigwe, Nigeria). Post-diagenetic ferruginisation of these deposits is reflected in only small variations in the chemical composition of the two types. Obvious differences include the varying MgO concentrations, which are considerably higher in the chamosite-type (in the range from ∼0.5–6.75 wt%). In the kaolinite-type, MgO varies from 0.0 to ∼0.4 wt%.One of the principal characteristics of the chamosite-type deposits is the occurrence of fossils, especially of brecciated shell material and bioturbation. These features are unknown in the kaolinite-type. In both types, pyrite and siderite are common constituents. These minerals are of diagenetic origin and were formed under reducing conditions in the presence of either chamositic clay minerals or kaolinite. The protoliths of both the kaolinite and the chamositic types were lateritic weathered rocks of the hinterland, transported via fluvial drainage systems and deposited in marine basins. The differences of the two types have been explained by the attendant environmental conditions. The environment of the chamosite-type is compatible with fully marine conditions and normal salinity, resulting in the availability of Mg leading to the formation of chamositic clay during diagenesis. The environment of the kaolinite-type is thought to be a marginal basin with high river discharge and thus lower salinity with zero or negligibly low Mg concentration. In this environment, a diagenetic transformation of the sedimentary kaolinite precorsur into chamositic clay failed to occur. The model is supported by the distribution patterns of the two ironstone types. Within the study area, the chamositic-types are located at the extreme positions and are thus closest to the open sea where fully marine conditions are most likely to have occurred.     

Compressional reworking of the East African Orogen in the Uluguru Mountains of eastern Tanzania at c. 550 Ma: implications for the final assembly of Gondwana

The Uluguru Mountains of eastern Tanzania represent a key area to unravel the tectonic history of Gondwana assembly along the Neoproterozoic East African Orogen. On the basis of combined structural and ⁴⁰Ar/³⁹Ar geochronological investigations, we have re‐interpreted the structural architecture of the region. Texturally‐late, ductile to cataclastic low‐angle tectonic contacts are recognized, which delimit the contacts between the main lithological units that form a post‐metamorphic, NW‐verging nappe stack. ⁴⁰Ar/³⁹Ar geochronology places this nappe‐forming event at c. 550 Ma, post‐dating tonalitic dyke emplacement at c. 580 Ma. Results from this study (i) demonstrate that a distinct and later episode of convergence occurred in the region at c. 550 Ma, when previous structural elements of the East African Orogen were finally juxtaposed; and (ii) extends models involving the polyphase assembly of Gondwana to East Africa.