The role of hydrothermal fluids in sedimentation in saline alkaline lakes: Evidence from Nasikie Engida,Kenya Rift Valley

Saline alkaline lakes that precipitate sodium carbonate evaporites are most common in volcanic terrains in semi‐arid environments. Processes that lead to trona precipitation are poorly understood compared to those in sulphate‐dominated and chloride‐dominated lake brines. Nasikie Engida (Little Magadi) in the southern Kenya Rift shows the initial stages of soda evaporite formation. This small shallow (<2 m deep; 7 km long) lake is recharged by alkaline hot springs and seasonal runoff but unlike neighbouring Lake Magadi is perennial. This study aims to understand modern sedimentary and geochemical processes in Nasikie Engida and to assess the importance of geothermal fluids in evaporite formation. Perennial hot‐spring inflow waters along the northern shoreline evaporate and become saturated with respect to nahcolite and trona, which precipitate in the southern part of the lake, up to 6 km from the hot springs. Nahcolite (NaHCO₃) forms bladed crystals that nucleate on the lake floor. Trona (Na₂CO₃·NaHCO₃·2H₂O) precipitates from more concentrated brines as rafts and as bottom‐nucleated shrubs of acicular crystals that coalesce laterally to form bedded trona. Many processes modify the fluid composition as it evolves. Silica is removed as gels and by early diagenetic reactions and diatoms. Sulphate is depleted by bacterial reduction. Potassium and chloride, of moderate concentration, remain conservative in the brine. Clastic sedimentation is relatively minor because of the predominant hydrothermal inflow. Nahcolite precipitates when and where pCO₂ is high, notably near sublacustrine spring discharge. Results from Nasikie Engida show that hot spring discharge has maintained the lake for at least 2 kyr, and that the evaporite formation is strongly influenced by local discharge of carbon dioxide. Brine evolution and evaporite deposition at Nasikie Engida help to explain conditions under which ancient sodium carbonate evaporites formed, including those in other East African rift basins, the Eocene Green River Formation (western USA), and elsewhere.     

The evolution of the terrestrial-terminating Irish Sea glacier during the last glaciation

Here we reconstruct the last advance to maximum limits and retreat of the Irish Sea Glacier (ISG), the only land-terminating ice lobe of the western British Irish Ice Sheet. A series of reverse bedrock slopes rendered proglacial lakes endemic, forming time-transgressive moraine- and bedrock-dammed basins that evolved with ice marginal retreat. Combining, for the first time on glacial sediments, optically stimulated luminescence (OSL) bleaching profiles for cobbles with single grain and small aliquot OSL measurements on sands, has produced a coherent chronology from these heterogeneously bleached samples. This chronology constrains what is globally an early build-up of ice during late Marine Isotope Stage 3 and Greenland Stadial (GS) 5, with ice margins reaching south Lancashire by 30 ± 1.2 ka, followed by a 120-km advance at 28.3 ± 1.4 ka reaching its 26.5 ± 1.1 ka maximum extent during GS-3. Early retreat during GS-3 reflects piracy of ice sources shared with the Irish-Sea Ice Stream (ISIS), starving the ISG. With ISG retreat, an opportunistic readvance of Welsh ice during GS-2 rode over the ISG moraines occupying the space vacated, with ice margins oscillating within a substantial glacial over-deepening. Our geomorphological chronosequence shows a glacial system forced by climate but mediated by piracy of ice sources shared with the ISIS, changing flow regimes and fronting environments.     

An abnormal pygidium of the trilobite Brachymetopus ornatus Woodward from the Lower Carboniferous of Derbyshire

A pygidium of Brachymetopus ornatus Woodward shows an abnormally developed left side affecting the whole of the pleural area and its rib development. Such an extensive abnormality, which does not extend beyond the axial furrow and shows none of the signs of resulting from injury, is currently unique. It is interpreted as a teratological condition in which a genetic or developmental malfunction resulted in the partial mimicking of thoracic pleurae rather than the construction of the normal pygidial pleural ribs. The effects of this are progressively greater from the posterior to the anterior parts of the affected pleural region. © 1996 John Wiley & Sons, Ltd.     

The quaternary glacial history of the Lahul Himalaya,northern India

This paper presents the first glacial chronology for the Lahul Himalaya, Northern India. The oldest glaciation, the Chandra Glacial Stage, is represented by glacially eroded benches at altitudes greater than 4300 m above sea-level. This glaciation was probably of a broad valley type. The second glaciation, the Batal Glacial Stage, is represented by highly weathered and dissected lateral moraines, which are present along the Chandra valley and some of its tributaries. This was an extensive valley glaciation. The third major glaciation, the Kulti Glacial Stage, is represented by well-preserved moraines in the main tributary valleys of the Chandra valley. This represents a less extensive valley glaciation. Two minor glacial advances, the Sonapani I and II, are represented by small sharp-crested moraines, which are within a few hundred metres or few kilometres of the present-day glaciers. The change in style and extent of glaciation is attributed to an increase in aridity throughout the Quaternary, due either to global climatic change or uplift of the Pir Panjal mountains to the south of Lahul, which restricted the northward penetration of the south Asian summer monsoon. © 1996 John Wiley & Sons, Ltd.     

Geological interpretation of a deep seismic‐reflection transect across the boundary between the Delamerian and Lachlan Orogens,in the vicinity of the Grampians,western Victoria

A deep seismic‐reflection transect in western Victoria was designed to provide insights into the structural relationship between the Lachlan and the Delamerian Orogens. Three seismic lines were acquired to provide images of the subsurface from west of the Grampians Range to east of the Stawell‐Ararat Fault Zone. The boundary between the Delamerian and Lachlan Orogens is now generally considered to be the Moyston Fault. In the vicinity of the seismic survey, this fault is intruded by a near‐surface granite, but at depth the fault dips to the east, confirming recent field mapping. East of the Moyston Fault, the uppermost crust is very weakly reflective, consisting of short, non‐continuous, west‐dipping reflections. These weak reflections represent rocks of the Lachlan Orogen and are typical of the reflective character seen on other seismic images from elsewhere in the Lachlan Orogen. Within the Lachlan Orogen, the Pleasant Creek Fault is also east dipping and approximately parallel to the Moyston Fault in the plane of the seismic section. Rocks of the Delamerian Orogen in the vicinity of the seismic line occur below surficial cover to the west of the Moyston Fault. Generally, the upper crust is only weakly reflective, but subhorizontal reflections at shallow depths (up to 3 km) represent the Grampians Group. The Escondida Fault appears to stop below the Grampians Group, and has an apparent gentle dip to the east. Farther east, the Golton and Mehuse Faults are also east dipping. The middle to lower crust below the Delamerian Orogen is strongly reflective, with several major antiformal structures in the middle crust. The Moho is a slightly undulating horizon at the base of the highly reflective middle to lower crust at 11–12 s TWT (approximately 35 km depth). Tectonically, the western margin of the Lachlan Orogen has been thrust over the Delamerian Orogen for a distance of at least 25 km, and possibly over 40 km.     

Water chemistry and sedimentological observations in littlefield lake,michigan: Implications for lacustrine marl deposition

A combination of both water chemistry and sedimentological information was used to investigate the carbonate-producing mechanism in Littlefield Lake, a small lake located in Isabella County, central Michigan. Data on temperature, dissolved oxygen, pH, calcium carbonate (CaCO₃) saturation, alkalinity, calcium, and magnesium were obtained on a monthly basis over a 13-month period, with each parameter determined at 1m intervals over a depth range of 20m. The loss of dissolved carbon dioxide (CO₂) from warm surface waters during direct degassing, and to a lesser extent during photosynthetic uptake by lacustrine macrophytes and phytoplankton during the summer, results in massive precipitation of the low-magnesium calcite which predominates in all Littlefield Lake sedimentary facies However, despite the fact that carbonate precipitation in this rather typical temperate-region marl lake is directly related to, and may be driven by, seasonal variation in these physiochemical parameters, most calcite forms as encrustations around cyanophytic and chlorophytic macrophytes. Such relationships demonstrate that carbonate precipitation in marl lakes may result from complex interactions between both biochemical and physiochemical processes. As such, marl formation in this, and probably many other calcareous lake systems, can not be simply ascribed to one or the other of these two general mechanisms.     

OCCURRENCE OF VOLATILE ORGANIC CHEMICALS IN NEBRASKA GROUND WATER

Abstract. This report summarizes results of monitoring programs for volatile synthetic organic chemicals (VOCs) in Nebraska's private and public water-supply wells conducted by two State agencies. Of 97 community water-supply systems sampled as of June 1984, 16.5 percent (16 systems) showed quantifiable levels of at least one VOC. Detectable amounts of one or more VOCs were measured in samples from 15.9 percent (10) of 63 private wells sampled in 1982. These percentages are consistent with results of other State and national surveys.     

Contrasting climatic histories for the Snake River Plain, Idaho, resulting from multiple thermal maxima

Ten sites near the Snake River Plain have consistent differences in their climatic histories. Sites at low elevation reflect the “early Holocene xerothermic” of the Pacific Northwest, whereas most climatic chronologies at high elevation indicate maximum warmth or aridity somewhat later, ca. 6000 yr ago. This elevational contrast in climatic histories is duplicated at three sites from the central Snake River Plain. For sites in such close proximity, the different chronologies cannot be explained by changes in atmospheric circulation during the late Quaternary. Rather, the differences are best explained by the autecology of the plants involved and the changing seasonal climate. The seasonal climatic sequence predicted by multiple thermal maxima explains the high- and low-elevation chronologies. During the early Holocene, maximum insolation and intensified summer drought in July forced low-elevation vegetation upward. However, moisture was not a limiting factor at high elevation, where vegetation moved upward in response to increased length of growing season coincident with maximum September insolation 6000 yr ago.     

Spatial and temporal distribution of diatoms in sediments of Lake Malawi,Central Africa,and ecological implications

Lake Malawi lies in a major rift valley in Central Africa that is some 700 m deep and 550 km long. A total of 242 cores and 111 grab samples were recovered between 1984 and 1989 and form the data base for a study of diatom distribution in this lake basin. The sediments consist of homogeneous diatomites, pelagic diatomaceous muds, varved diatomite-mud couplets, turbidites, littoral sand sheets and extensive deposits of ferro-manganous nodules.Fossil diatoms show major temporal and regional contrasts between the southern, central and northern areas of the lake. A wide variety of Aulacoseira species occur in the southern lake sediments. To the north, the Nkhotakota region is generally characterised by Stephanodiscus and Aulacoseira, with occasional diatomite laminae composed of Aulacoseira or Nitzschia. The central parts of the lake show the greatest variation, with Stephanodiscus, Nitzschia and Aulacoseira all being prominent. The northern region is dominated by Aulacoseira nyassensis throughout most core sequences.Variability in these assemblages appears to be controlled by Si:P ratios, Si concentrations, turbulence and light penetration. These factors themselves are influenced by differences in the depth and duration of lake mixing due to variations in wind strength, seiches and bottom topography among different regions of the lake and from lakewide circulation patterns.