Reassessing southern African silcrete geochemistry: implications for silcrete origin and sourcing of silcrete artefacts

A synthesis of the geochemistry of silcretes and their host sediments in the Kalahari Desert and Cape coastal zone, using isocon comparisons, shows that silcretes in the two regions are very different. Kalahari Desert silcretes outcrop along drainage-lines and within pans, and formed by groundwater silicification of near-surface Kalahari Group sands. Silicification was approximately isovolumetric. Few elements were lost; silicon (Si) and potassium (K) were gained as microquartz precipitated in the sediment porosity and glauconite formed in the sub-oxic groundwater conditions. The low titanium (Ti) content reflects the composition of the host sands. Additional elements in the Kalahari Desert silcretes were supplied in river water and derived from weathering of silicates in basement rocks. Evaporation under an arid climate produced high-pH groundwater that mobilized and precipitated Si; this process is still occurring. In the Cape coastal zone, pedogenic silcretes cap hills and plateaus, overlying deeply weathered argillaceous bedrock. Silicification resulted from intensive weathering that destroyed the bedrock silicates, almost completely removing most elements and causing a substantial volume decrease. Some of the silica released formed a microcrystalline quartz matrix, and most Ti precipitated as anatase, so the Cape silcretes contain relatively high Ti levels. The intense weathering that formed the Cape silcretes could have occurred in the Eocene, during and after the Palaeocene-Eocene Thermal Maximum, when more acidic rainfall and high temperatures resulted in intensified silicate weathering worldwide. This could have been responsible for widespread formation of pedogenic silcretes elsewhere in Africa and around the globe. Trace element sourcing of silcrete artefacts to particular outcrops has most potential in the Cape, where differences between separate bedrock areas are reflected in the silcrete composition. In the Kalahari Desert, gains of some elements can override compositional differences of the parent material, and sourcing should be based on elements that show the least change during silicification. © 2020 John Wiley & Sons, Ltd.     

Multiple calcrete profiles in the Tabernas Basin,southeast Spain: their origins and geomorphic implications

The paper describes a sequence of Pliocene(? ) to Quaternary age calcretes developed within alluvial fan and fluvial gravels in the Tabernas Basin, Almería Province, southeast Spain. Calcrete profiles are described from sites adjacent to major tributaries of the Rambla de Tabernas. Six distinct calcrete units are identified within the basin. These have variable distributions but have developed in an identifiable evolutionary sequence. Two pairs of calcrete units are widely present across the basin preserving two former land surfaces. Each of the former land surfaces has been planated and subsequently buried by alluvial fan or fluvial gravels. A massive calcrete unit is present at the base of each gravel sequence, immediately in contact with the underlying bedrock, with a less well developed calcrete unit situated at the top of the gravel sequence. The lowest two calcrete units within the basin are more spatially restricted and are confined to the floors and flanks of incised drainage lines. The geochemistry, macro- and micromorphological properties and geomorphological positions of the calcrete units are outlined and, on the basis of this information, their mode of origin identified. Two main modes of calcrete genesis appear to be present: massive calcretes situated in direct contact with bedrock are suggested to have formed by groundwater processes, whilst calcretes situated at the top of gravel sequences are likely to have developed by pedogenic processes. Calcrete genesis is subsequently considered in the context of the reconstruction of the early phases of landscape development, and is suggested to have been controlled by phases of uplift and stability within the Tabernas Basin. © 1998 John Wiley & Sons, Ltd.     

Distribution of foraminifera and ostracods in the Onkaparinga Estuary,South Australia

Analysis of sediment and aquatic vegetation samples collected along the Onkaparinga estuary, South Australia, revealed the distribution patterns of benthic and epi-benthic micro-organisms, specifically foraminifera and ostracods. The total assemblage provided an assemblage ‘snap shot,’ contemporaneous with the environmental conditions at the time of sampling, as well as seasonally and time-averaged distribution information. Species richness is low in the upper reaches of the estuary and favours species with a wide salinity tolerance. Observed species richness increases seawards. Species richness and abundance within species is greater where there is conspicuous aquatic vegetation. Thus, species richness and distribution appear to be related to the proximity to the sea and the provision of diverse habitable substrate. Foraminiferal species, Ammonia beccarii and Elphidium excavatum, and ostracods belonging to the genera Paracypria, Xestolebris and Leptocythere dominate the total assemblage. These species are characteristic of euryhaline conditions. The mutual maximum occurrences of Reophax barwonensis, Ammobaculites barwonensis and Trochammina inflata signify mid-estuarine conditions. In the lower regions of the estuary, the appearance of shallow marine species of foraminifera such as Elphidium crispum and Quinqueloculina poeyana, and ostracods Bairdoppilata sp., Hemicytherura spp. and Paranesidea spp. signifies interactions with the adjacent inner shelf coastal waters. Throughout the entire length of the estuary, the presence of vegetal substrate created a strong numerical bias towards live occurrences of the ostracods Paracypria sp. and Xestolebris cedunaensis. These species are potentially useful as proxies for paleoenvironmental interpretations of estuarine sediments.     

Local sensitivity analysis for observed hydrocarbons in a Jupiter photochemistry model

A box model sensitivity analysis was applied to output from a version of the 1-D JPL/Caltech KINETICS photochemistry-transport model of Jupiter's atmosphere. Results quantify the controlling chemical reaction parameters for the variety of observable hydrocarbons, and suggest changes to explore and new observations and rate measurements to pursue. High sensitivities are found to photolysis steps and to several hydrogen atom recombination steps and product branches. Complexity ranges from the relatively simple scheme seen for the methyl radical, to the rich variety of reactions tested by diacetylene.     

Tracing the Seepage of Subsurface Sinkhole Vent Waters into Lake Huron Using Radium and Stable Isotopes of Oxygen and Hydrogen

Exchange of water between groundwater and surface water could alter water quality of the surface waters and thereby impact its ecosystem. Discharges of anoxic groundwater, with high concentrations of sulfate and chloride and low concentrations of nitrate and oxygen, from three sinkhole vents (El Cajon, Middle Island and Isolated) in Lake Huron have been recently documented. In this investigation, we collected and analyzed a suite of water samples from these three sinkhole vents and lake water samples from Lake Huron for Ra, radon-222, stable isotopes of oxygen and hydrogen, and other ancillary parameters. These measurements are among the first of their kind in this unique environment. The activities of Ra are found to be one to two orders of magnitude higher than that of the lake water. Isotopic signatures of some of the bottom lake water samples indicate evidences for micro-seeps at distances farther from these three vents. A plot of δD versus δ¹⁸O indicates that there are deviations from the Global Meteoric Line that can be attributed to mixing of different water masses and/or due to some subsurface chemical reactions. Using the Ra isotopic ratios, we estimated the transit times of the vent waters from the bottom to the top of the vent (i.e., sediment–water interface) to be 4–37 days. More systematic studies on the distribution of the radioactive and stable isotope studies are needed to evaluate the prevalence of micro-seeps in Lake Huron and other Great Lakes system.     

Potential transport of plaice eggs and larvae between two apparently self-contained populations in the Irish Sea

A coupled physics particle-tracking model, driven by realistic meteorological forcing was used to examine the dispersal and transport of plaice eggs and larvae in the year 2000 from two spawning grounds in the Irish Sea. The model included passive transport of eggs and early stage larvae, diel vertical movements for larvae between 7 and 10.5 mm in body-length and tidally synchronised, vertical movements for larger larvae (>9 mm body-length). The year 2000 was chosen because of the availability of ichthyoplankton data with which to initialise the model. The majority of larvae originating from spawning in the eastern Irish Sea settled into nursery grounds along the Scottish, English and Welsh coasts, in agreement with previous findings. In contrast, a significant portion of larvae originating from spawning in the western Irish Sea was transported eastwards to these same nursery grounds. Transport across the Irish Sea resulted from the onset of tidally synchronised vertical behaviour encoded in the model for older larvae. Settlement of larvae into local nursery grounds along the Irish coast was limited. Because of the prevailing winds and currents in the region, plaice eggs and larvae are unlikely to be transported from east to west; in most years spawning in the western Irish Sea probably acts as an additional source of juveniles for nursery grounds along the Scottish, English and Welsh coasts.