Observations of semidiurnal internal tidal currents off central Chile (36.6°S)

Observations of semidiurnal internal tidal currents from three moorings deployed on the continental shelf off central Chile during summer and winter of 2005 are reported. The spectra of the baroclinic currents showed large peaks at the semidiurnal band with a dominant counterclockwise rotation, which was consistent with internal wave activity. The amplitude of the barotropic tidal currents varied according to the spring–neap cycle following the sea level fluctuations. In contrast, the amplitudes of the internal tide showed high spatial-temporal variability not directly related to the spring–neap modulation. Near the middle of the continental shelf and near the coast (San Vicente Bay) the variance of the semidiurnal baroclinic current is larger than the variance of its barotropic counterpart. The vertical structure of the baroclinic tidal current fluctuations was similar to the structure of the first baroclinic internal wave mode. In general, in the three study sites the variance of the baroclinic current was larger near the surface and bottom and tended to show a minimum value at mid depths. Kinetic energy related to semidiurnal internal waves was larger in winter when stratification of the water column was stronger. During summer, upwelling and the decrease of freshwater input from nearby rivers reduced the vertical density stratification. The amplitude of the semidiurnal internal tide showed a tendency to be enhanced with increasing stratification as observed in other upwelling areas. The continental shelf break and submarine canyons, which limit the continental shelf in the alongshore direction, represent near-critical slopes for the semidiurnal period and are suggested to be the main internal tide generation sites in the study region.     

Quartz and zircon decoupling in sandstone: Petrography and quartz cathodoluminescence of the Early Triassic continental Buntsandstein Group in Germany

This study illustrates how decoupling of quartz and zircon can be used advantageously in provenance research. Thirty‐eight fine‐grained to coarse‐grained arkose samples of the Early Triassic intracontinental Buntsandstein Group from the Central European Basin in Germany were analysed for their petrography and 1200 grains in 23 of these for their detrital quartz cathodoluminescence characteristics. The samples represent the Hessian and Thuringian sub‐basins and the Eichsfeld–Altmark Swell separating them. The Hessian Sub‐basin includes more metamorphic lithoclasts with a larger content of plutonic grains than are found further east in the Thuringian Sub‐basin. More than 90% of the detrital quartz from the eastern Thuringian Sub‐basin produce medium to bright blue cathodoluminescence colours and corresponding spectra that are typical for igneous or high‐temperature metamorphic origin. Differently, the quartz from the Hessian Sub‐basin mostly luminesces brown and dark to medium blue, typical for a low‐temperature metamorphic origin. Quartz from the Eichsfeld–Altmark Swell and the western Thuringian Sub‐basin is a mixture between these origins. The quartz indicates different catchments for the sub‐basins, possibly the Bohemian Massif and the Massif Central, with converging transport routes on and close to the eastern fringe of the swell. Taking published zircon data from the same samples into account, light mineral‐zircon grain‐size shifts are up to 2 Φ units. That can be explained by mineral decoupling due to different transport modes for quartz and zircon and different zircon‐size availability in the source areas, exaggerated by combined aqueous–aeolian transport, as well as sample preparation‐induced sorting. This study concludes that submerged highs significantly can influence continental sediment transport. Hence, vast, flat continental areas with submerged morphological highs and a seemingly straightforward transportation pattern may be more complex than expected. The results also illustrate that analysis of detritus that has been affected by different dominating transport modes, and further sorting during sampling and preparation can reveal additional source information.     

Changes in Jupiter’s Great Red Spot (1979-2006) and Oval BA (2000-2006)

We analyze velocity fields of the Great Red Spot (GRS) and Oval BA that were previously extracted from Cassini, Galileo, and Hubble Space Telescope images (Asay-Davis, X.S., Marcus, P.S., Wong, M., de Pater, I. [2009]. Icarus 203, 164-188). Our analyses use reduced-parameter models in which the GRS, Oval BA, and surrounding zonal (east-west) flows are assumed to have piece-wise-constant potential vorticity (PV), but with finite-sized transition regions between the pieces of constant PV rather than sharp steps. The shapes of the regions of constant PV are computed such that the flow is a steady, equilibrium solution of the 2D quasigeostrophic equations when viewed in a frame translating uniformly in the east-west direction. All parameter values of the models, including the magnitudes of the PV, areas of the regions with constant PV, locations of the transition regions, widths of the transition regions, and the value of the Rossby deformation radius, are found with a genetic algorithm such that the velocity produced by the equilibrium solution is a “best-fit” to the observed velocity fields. A Monte Carlo method is used to estimate the uncertainties in the best-fit parameter values.The best-fit results show that there were significant changes (greater than the uncertainties) in the PV of the GRS between Galileo in 1996 and Hubble in 2006. In particular, the shape of the PV anomaly of the GRS became rounder, and the area of the PV anomaly of the GRS decreased by 18%, although the magnitudes of PV in the anomaly remained constant. In contrast, neither the area nor the magnitude of the PV anomaly of the Oval BA changed from 2000, when its cloud cover was white, to 2006, when its cloud cover was red. The best-fit results also show that the areas of the PV anomalies of the GRS and of the Oval BA are smaller than the areas of their corresponding cloud covers at all times. Using the best-fit values of the Rossby deformation radius, we show that the Brunt-Väisälä frequency is 15% larger at 33°S than at 23°S. As expected (Marcus, 1993), the best-fit results show that the PV of the zonal flow has “jumps” at the latitudes of the maxima of the eastward-going jet streams. However, a surprising result is that a large “jump” in the PV of the zonal flow occurs at the location of a maximum of the westward going jet stream neighboring the GRS. Another surprise is that the jumps in the PV of the zonal flow do not all have the same sign, which implies that there is not a monotonic “staircase” of zonal PV from north to south as was anticipated ( [Marcus, 1993] and [McIntyre, 2008]).     

Could surface roughness be a poor proxy for landslide age? Results from the Swabian Alb,Germany

The potential of surface roughness to quantify geomorphological landforms and processes has been enhanced with the availability of high‐resolution digital terrain models (DTM). Recent studies that attempt to identify landslide features with surface roughness have suggested that this measure of topographic heterogeneity may also be applied to estimate the relative age of landslides. This is a provisional study that explores the potential of this relationship by assessing the ability of surface roughness to act as a proxy for relative landslide age. The surface roughness for a set of 12 dated landslides in the Swabian Alb that occurred between 1789 and 1985 was calculated from a 1 m² spatial resolution LiDAR DTM with three algorithms: root‐mean‐square‐height (RMSH), standard deviation of slope (SDS), and direction cosine eigenvalue ratios (DCE). Scale‐dependence was analysed by calculating surface roughness for a range of moving window sizes (3 × 3, 5 × 5, 9 × 9 and 15 × 15), and surface roughness for each landslide was summarized by the median and upper quartile. Only weak correlations (best Spearman's rho 0.58) were present between landslide age and surface roughness. This correlation becomes weaker with increasing moving window size. Given weak observed associations and discussed challenges pertaining to the complexities of landslide morphology change over time, we currently find that surface roughness alone may not be justifiable to act as a proxy for landslide age for our study region. Furthermore, we recommend future studies should focus on addressing possible natural and anthropogenic factors such as land use change that may alter surface roughness. These studies may focus on one of the three roughness measures used here as they are strongly correlated. Copyright © 2014 John Wiley & Sons, Ltd.     

Defining rates of landscape evolution in a south Tibetan graben with in situ‐produced cosmogenic 10Be

The Tangra Yum Co graben is one of the active structures that accommodate the east‐west extension of the southern Tibetan Plateau and hosts one of the largest Tibetan lakes, which experienced lake‐level changes of ~200 m during the Holocene. In this study, cosmogenic ¹⁰Be is employed to: (1) quantify catchment‐wide denudation rates in fault‐bounded mountain ranges adjacent to the Tangra Yum Co graben; (2) date palaeo‐shorelines related to the Holocene lake‐level decline; and (3) determine the age of glacial advances in this region. The fault‐bounded, non‐glaciated mountain range north of Tangra Yum Co – and presumably most other areas around the lake – erode at low rates of 10–70 mm/ka. Owing to the slow erosion of the landscape, the sediments delivered to Tangra Yum Co have high ¹⁰Be concentrations. As a consequence, accurate exposure dating of sediment‐covered terraces and beach ridges is difficult, because the pre‐depositional ¹⁰Be concentration may exceed the post‐depositional ¹⁰Be concentration from which exposure ages are calculated. This difficulty is illustrated by a rather inaccurate ¹⁰Be exposure age of 2.3 ± 1.4 ka (i.e. an error of 60%) for a terrace that is located 67 m above the lake. Nevertheless, the age is consistent with luminescence ages for a series of beach ridges and provides further evidence for the decline of the lake level in the late Holocene. At Tangra Yum Co exposure dating of beach ridges via ¹⁰Be depth profiles is not feasible, because the pre‐depositional ¹⁰Be component in these landforms varies with depth, which violates a basic assumption of this approach. ¹⁰Be ages for boulders from two moraines are much older than the early Holocene lake‐level highstand, indicating that melting of glaciers in the mountain ranges adjacent to Tangra Yum Co has not contributed significantly to the lake‐level highstand in the early Holocene. Copyright © 2015 John Wiley & Sons, Ltd.     

Food web structure in a near-pristine mangrove area of the Australian Wet Tropics

Carbon and nitrogen isotopic composition was used to identify the main sources of carbon and describe the main trophic pathways in Deluge Inlet, a near-pristine mangrove estuary in tropical north Queensland, Australia. Producers' δ¹³C varied from −28.9‰ for mangroves to −18.6‰ for seagrass. Animals were also well separated in δ¹³C (−25.4‰ to −16.3‰ for invertebrates and −25.2‰ to −17.2‰ for fish), suggesting considerable differences in ultimate sources of carbon, from a substantial reliance on mangrove carbon to an almost exclusive reliance on seagrass. In general, invertebrates had lower δ¹⁵N than fish, indicating lower trophic levels. Among fish, δ¹⁵N values reflected well the assumed trophic levels, as species from lower trophic levels had lower δ¹⁵N than species from higher trophic levels. Trophic levels and trophic length were estimated based on δ¹⁵N of invertebrate primary consumers (6.1‰), with results suggesting a food web with four trophic levels. There was also evidence of a high level of diet overlap between fish species, as indicated by similarities in δ¹³C for fish species of higher trophic levels. Stable isotope data was also useful to construct a general model for this food web, where five main trophic pathways were identified: one based on both mangrove and microphytobenthos, one on plankton, two on both microphytobenthos and seagrass, and one based mainly on seagrass. This model again suggested the presence of four trophic levels, in agreement with the value calculated based on the difference in δ¹⁵N between invertebrate primary consumers and top piscivores.     

Salinity Biases and the Variability of the Atlantic Meridional Overturning Circulation in GFDL-CM3

Ocean Science Journal - The variability of the Atlantic Meridional Overturning Circulation (AMOC) and the Meridional Freshwater Transport (Mov) at 24°N, 11°S, and 30°S, simulated...     

Palaeolithic archaeology of the Bytham River: human occupation of Britain during the early Middle Pleistocene and its European context

The Early and early Middle Pleistocene archaeological record in Britain from c. 900 to 500 ka marks a critical shift in human occupation of northwest Europe, from occasional pioneer populations with simple core and flake technology to more widespread occupation associated with the appearance of Acheulean technology. Key to understanding this record are the fluvial deposits of the extinct Bytham River in central East Anglia, where a series of Lower Palaeolithic sites lie on a 15 km stretch of the former river. In this paper we present the results of new fieldwork and a reanalysis of historical artefact collections of handaxes and scrapers to: 1) establish the chronostratigraphic context of the Bytham archaeological record; 2) examine variability in lithic artefact typology and technology through time; and 3) explore the implications for understanding variation in lithic technology in the European record. Six phases of occupation of Britain are identified from at least marine isotope stage (MIS) 21 to MIS 13, with the last three phases characterised by distinctive lithic technology. We argue that this relates to the discontinuous occupation of Britain, where each phase represents the arrival of new groups derived from different European populations with distinctive material culture.     

Tuning a Kalman filter carrier tracking algorithm in the presence of ionospheric scintillation

GPS Solutions - Strong ionospheric electron content gradients may lead to fast and unpredictable fluctuations in the phase and amplitude of the signals from Global Navigation Satellite Systems...