Complex reservoir sedimentation revealed by an unusual combination of sediment records,Kangaroo Creek Reservoir,South Australia

Despite their direct links to human use, reservoirs are not widely utilised, relative to natural lakes, for deriving sediment histories. One explanation is the complex sedimentation patterns observed in water storages. Here a highly unusual combination of sedimentary records is used to determine the sedimentation history of Kangaroo Creek Reservoir, South Australia. We compare contiguous high resolution (0.5 cm sampling interval) diatom records from an almost 1.3 m core extracted from the bottom of the reservoir and from a 0.4 m monolith of sediment perched 15 m above the reservoir bottom on a disused bridge that was submerged following initial reservoir filling in 1970. The diatom histories are supplemented by evidence provided by other indicators, most notably radionuclide concentrations and ratios. Interestingly, despite the fact that the reservoir has been >20 m deep for more than 70% of its recorded history, distinct sections of the reservoir bottom core, but not the bridge monolith, are dominated by non-planktonic diatoms. We attribute the occurrences of these phases to inflows that occur following heavy catchment rains at times when the reservoir is drawn down. These characteristic sections have, in turn, been used to refine the site’s chronology. Despite having a length of almost 1.3 m, a variety of data suggests that the core has not recovered pre-reservoir sediment, but rather spans the period from 1981 (11 years after first filling) to 2001, when the core was extracted. It is clear, therefore, that sediments in the bottom of the reservoir are accumulating rapidly (>7 cm year⁻¹), although more than 40% of this deposition occurs in less than 5% of the time. It appears that in the period 1996–2001, quiescent sedimentation rates, both in the perched bridge locality and on the reservoir bottom, slowed in response to reduced stream flow. Our findings indicate that, with caution, complex patterns of sedimentation in water storages can be disentangled. However, it was difficult to precisely correlate diatom sequences from the two records even in periods of quiescent sedimentation, suggesting that reservoir bottom diatom sequences should be interpreted with considerable caution. Furthermore, while storm-derived inflows such as those identified may deliver a substantial proportion of sediment and phosphorus load to storages, the ensuing deposition patterns may render much of the phosphorus unavailable to the overlying waters.     

Late Cenozoic uplift of the Amanos Mountains and incision of the Middle Ceyhan river gorge, southern Turkey; Ar–Ar dating of the Düziçi basalt

Using the Ar–Ar technique, we have obtained the first numerical dates for the Pleistocene volcanism along the valley of the River Ceyhan in the Düziçi area of southern Turkey, in the western foothills of the Amanos Mountains. Our six dates indicate a single abrupt episode of volcanism at  270 ka. We have identified a staircase of 7 fluvial terraces, at altitudes of up to  230 m above the present level of the Ceyhan. Using the disposition of the basalt as an age constraint, we assign these terraces to cold-climate stages between marine oxygen isotope stages 16 and 2, indicating rates of fluvial incision, equated to surface uplift, that increase upstream through the western foothills of this mountain range at between 0.25 and 0.4 mm a^− 1. Extrapolation of these uplift rates into the axis of the range suggests that the entire  2300 m of present-day relief has developed since the Mid-Pliocene, a view that we confirm using numerical modelling. Since  3.7 Ma the Amanos Mountains have formed a transpressive stepover along the northern part of the Dead Sea Fault Zone, where crustal shortening is required by the geometry. Using a physics-based technique, we have modelled the overall isostatic response to the combination of processes occurring, including crustal thickening caused by the shortening, erosion caused by orographic precipitation, and the resulting outward flow of mobile lower-crustal material, in order to predict the resulting history of surface uplift. This modelling suggests that the effective viscosity of the lower crust in this region is in the range  1–2 × 10¹⁹ Pa s, consistent with a Moho temperature of  590 ± 10 °C, the latter value being in agreement with heat flow data. This modelling shows that the nature of the active crustal deformation is now understood, to first order at least, in this key locality within the boundary zone between the African and Arabian plates, the structure and geomorphology of which have been fundamentally misunderstood in the past.     

The influence of asymmetries in overlying stratification on near-bed turbulence and sediment suspension in a partially mixed estuary

Data collected from the York River estuary demonstrate the importance of asymmetries in stratification to the suspension and transport of fine sediment. Observations collected during two 24-h deployments reveal greater concentrations of total suspended solids during the flood phase of the tide despite nearly symmetric near-bed tidal current magnitude. In both cases, tidally averaged net up-estuary sediment transport near the bed was clearly observed despite the fact that tidally averaged residual near-bed currents were near zero. Tidal straining of the along-channel salinity gradient resulted in a stronger pycnocline lower in the water column during the ebb phase of the tide and appeared to limit sediment suspension. Indirect measurements suggest that the lower, more intense, pycnocline on the ebb acted as a barrier, limiting turbulent length scales and reducing eddy diffusivity well below the pycnocline, even though the lower water column was locally well mixed. In order to more conclusively link changes in stratification to properties of near-bed eddy viscosity and diffusivity, longer duration tripod and mooring data from an additional experiment are examined, that included direct measurement of turbulent velocities. These additional data demonstrate how slight increases in stratification can limit vertical mixing near the bed and impact the structure of the eddy viscosity below the pycnocline. We present evidence that the overlying pycnocline can remotely constrain the vertical turbulent length scale of the underlying flow, limiting sediment resuspension. As a result, the relatively small changes in stratification caused by tidal straining of the pycnocline allow sediment to be resuspended higher in the water column during the flood phase of the tide, resulting in preferential up-estuary transport of sediment.Responsible Editor: Iris Grabemann     

Near-infrared CCD observations of umbral dots

Umbral fine structures have been observed at 8500 Å using a new CCD detector. Four frames with diffraction-limited seeing were obtained. Between 68 and 91 umbral dots with a brightness contrast greater than 2% were found in each frame, although no dots were found in the darkest part of the umbra. The intrinsic flux of the umbral dots varies widely, indicating that their intrinsic brightness does as well. The mean dot lifetime is estimated as 15 min, although some dots were observed to live more than 2 h. Some of the umbral dots are flowing into the umbra at speeds up to 0.5 km s^-1. These dots have higher than average contrast and are associated with penumbral grains.     

Zn-Pb mineralisation in the Silvermines district,Ireland: a product of burial diagenesis

Carbonate-sulphide cement stratigraphic relationships in the host rock and ore have been used to constrain the age of mineralisation at the Silvermines zinc-lead-barium deposit. The base-metal sulphides post-date planar dolomite and replace stylolites. Furthermore, the pre-mineralisation planar dolomites also replace stylolites. These and other diagenetic observations indicate that the base-metal sulphides formed at burial depths greater than 800 m, but probably predate the Variscan deformation (since pressure shadows overgrow base metal sulphides). This indicates that the sulphides are of epigenetic origin, constraining the age of mineralisation to between the late Chadian (347 Ma) and the late Westphalian (307 Ma). However, the most likely age for mineralisation, (based on widespread macro-stylolite development) is Asbian (339 Ma) or younger. No evidence of synsedimentary sulphides (in the form of hydrothermal chimneys, vent faunas, or sulphides intergrown with marine cements) was observed at Silvermines. Mineralised breccias (black matrix breccias), late-stage internal sediments, and dissolution zones within the carbonate cements all appear to be produced by hydrothermal karsting that occurred during the mineralisation process. Fluid inclusion homogenisation temperatures for ore-stage calcites (up to 300 °C) approach the peak temperature estimates derived from regional maturation parameters (270 to 310 °C from conodont alteration indices and vitrinite reflectance). This suggests that homogenisation temperatures represent maximum heating temperatures (probably during Variscan time) rather than mineralisation temperatures.Editorial handling: J. Mengue     

Ecological effects of low toxicity oil-based mud drilling in the Beatrice oilfield

To investigate the effects of drilling discharges on the seabed fauna, surveys were carried out in the Beatrice oilfield after drilling 13 wells with water-based muds, and then after one and five further wells had been drilled using low toxicity oil-based muds. Localized benthic effects were found after the water-based mud drilling. After the use of oil-based muds, the nature of the effects was different, although there was little increase in the area involved. Possible reasons for this are discussed and burial and organic enrichment are suggested as the major influences. It is concluded that the use of low toxicity oil-based mud at Beatrice has resulted in only limited benthic effects, suggesting that the use of these muds is environmentally acceptable.     

Stereo-Photogrammetric Mapping of Tooth Replicas Incorporating Texture

Commercial digital photogrammetric software has been applied to convergent stereoscopic photography of human tooth replicas prepared to exhibit optical texture resulting in successful generation of 3D coordinate data. Tooth replicas were imaged using a semi-metric 35 mm camera and f = 100 mm macro lens on extension bellows. Model precision was within acceptable limits of 12  μ m or better for manual target matching and 21  μ m or better for automatic image matching. Further improvement in optical texture is required to achieve automatic image matching precision comparable to that of manual target matching. Small errors in interior orientation parameters attributed to instability in the bellows as well as small errors in the relative orientation resulted in some systematic errors. The use of a fixed camera lens system is expected to reduce these errors. When combined with commercially available, moderately priced, digital SLR cameras this brings 3D model generation closer to everyday clinical dental practice.     

Nanogeochemistry of hydrothermal magnetite

Magnetite from hydrothermal ore deposits can contain up to tens of thousands of parts per million (ppm) of elements such as Ti, Si, V, Al, Ca, Mg, Na, which tend to either structurally incorporate into growth and sector zones or form mineral micro- to nano-sized particles. Here, we report micro- to nano-structural and chemical data of hydrothermal magnetite from the Los Colorados iron oxide–apatite deposit in Chile, where magnetite displays both types of trace element incorporation. Three generations of magnetites (X–Z) were identified with concentrations of minor and trace elements that vary significantly: SiO₂, from below detection limit (bdl) to 3.1 wt%; Al₂O₃, 0.3–2.3 wt%; CaO, bdl–0.9 wt%; MgO, 0.02–2.5 wt%; TiO₂, 0.1–0.4 wt%; MnO, 0.04–0.2 wt%; Na₂O, bdl–0.4 wt%; and K₂O, bdl–0.4 wt%. An exception is V₂O₃, which is remarkably constant, ranging from 0.3 to 0.4 wt%. Six types of crystalline nanoparticles (NPs) were identified by means of transmission electron microscopy in the trace element-rich zones, which are each a few micrometres wide: (1) diopside, (2) clinoenstatite; (3) amphibole, (4) mica, (5) ulvöspinel, and (6) Ti-rich magnetite. In addition, Al-rich nanodomains, which contain 2–3 wt% of Al, occur within a single crystal of magnetite. The accumulation of NPs in the trace element-rich zones suggest that they form owing to supersaturation from a hydrothermal fluid, followed by entrapment during continuous growth of the magnetite surface. It is also concluded that mineral NPs promote exsolution of new phases from the mineral host, otherwise preserved as structurally bound trace elements. The presence of abundant mineral NPs in magnetite points to a complex incorporation of trace elements during growth, and provides a cautionary note on the interpretation of micron-scale chemical data of magnetite.