On the relationship between flow and suspended sediment transport over the crest of a sand dune, Río Paraná, Argentina

The links between large‐scale turbulence and the suspension of sediment over alluvial bedforms have generated considerable interest in the last few decades, with past studies illustrating the origin of such turbulence and its influence on flow resistance, sediment transport and bedform morphology. In this study of turbulence and sediment suspension over large sand dunes in the Río Paraná, Argentina, time series of three‐dimensional velocity, and at‐a‐point suspended sediment concentration and particle‐size, were measured with an acoustic Doppler current profiler and laser in situ scattering transmissometer, respectively. These time series were decomposed using wavelet analysis to investigate the scales of covariation of flow velocity and suspended sediment. The analysis reveals an inverse relationship between streamwise and vertical velocities over the dune crest, where streamwise flow deceleration is linked to the vertical flux of fluid towards the water surface in the form of large turbulent fluid ejections. Regions of high suspended sediment concentration are found to correlate well with such events. The frequencies of these turbulent events have been assessed from wavelet analysis and found to concentrate in two zones that closely match predictions from empirical equations. Such a finding suggests that a combination and interaction of vortex shedding and wake flapping/changing length of the lee‐side separation zone are the principal contributors to the turbulent flow field associated with such large alluvial sand dunes. Wavelet analysis provides insight upon the temporal and spatial evolution of these coherent flow structures, including information on the topology of dune‐related turbulent flow structures. At the flow stage investigated, the turbulent flow events, and their associated high suspended sediment concentrations, are seen to grow with height above the bed until a threshold height (ca 0·45 flow depth) is reached, above which they begin to decay and dissipate.     

Paper II – Dirt,dates and DNA: OSL and radiocarbon chronologies of perennially frozen sediments in Siberia,and their implications for sedimentary ancient DNA studies

Arnold, L. J., Roberts, R. G., MacPhee, R. D. E., Haile, J. S., Brock, F., Möller, P., Froese, D. G., Tikhonov, A. N., Chivas, A. R., Gilbert, M. T. P. & Willerslev, E. 2010: Paper II – Dirt, dates and DNA: OSL and radiocarbon chronologies of perennially frozen sediments in Siberia, and their implications for sedimentary ancient DNA studies. Boreas, Vol. 40, pp. 417–445. 10.1111/j.1502‐3885.2010.00181.x. ISSN 0300‐9483 The sedimentary ancient DNA (sedaDNA) technique offers a potentially invaluable means of investigating species evolution and extinction dynamics in high‐latitude environments. An implicit assumption of the sedaDNA approach is that the extracted DNA is autochthonous with the host deposit and that it has not been physically transported from older source deposits or reworked within the sedimentary profile by postdepositional mixing. In this paper we investigate whether these fundamental conditions are upheld at seven perennially frozen wetland sites across the Taimyr Peninsula and coastal lowlands of north‐central Siberia. Optically stimulated luminescence (OSL) and radiocarbon (¹⁴C) dating are used to constrain the ages of both the inorganic and organic fractions of perennially frozen deposits from which sedaDNA of extinct and extant species have been recovered. OSL and ¹⁴C age/depth profiles, as well as single‐grain equivalent dose (D_e) distribution characteristics, are used to assess the stratigraphic integrity of these sedaDNA sequences by (i) identifying the presence of primary or reworked organic and inorganic material, and (ii) examining the types of depositional and postdepositional processes that have affected specific sedimentary facies. The results of this study demonstrate that even though DNA preservation and stratigraphic integrity are commonly superior in perennially frozen settings, this does not, in itself, guarantee the suitability of the sedaDNA approach. The combined OSL and ¹⁴C chronologies reveal that certain perennially frozen sites may be poorly suited for sedaDNA analysis, and that careful site selection is paramount to ensuring the accuracy of any sedaDNA study – particularly for ‘latest appearance date’ estimates of extinct taxa.     

Evolution of an early Proterozoic foreland basin carbonate platform, lower Pethei Group, Great Slave Lake, north-west Canada

The Taltheilei, Utsingi, McLean and Blanchet formations form a 175–390 m thick carbonate platform-to-basin succession in the lower part of the PaleoProterozoic Pethei Group, preserved in the eastern arm of Great Slave Lake. Carbonates accumulated along the south-east margin of the Slave Craton within a foredeep formed during the collision of the Slave and Churchill Cratons. The rocks include eight, predominantly microbial, carbonate facies that comprise five facies associations representing (1) shallow-water rimmed shelf, (2) shallow-water open shelf, (3) shallow-water ramp, (4) upper slope and deep ramp, and (5) lower slope and basin plain environments. Microbialite facies grew by organically mediated precipitation of spar and micritic cement and trapping and binding of lime mud. These wholly subtidal facies typically reflect progressive shallowing and changing geometry of the lower Pethei sea floor, from ramp, to open shelf, to shallow rimmed shelf, with associated slope and basin plain deposition. Repeated relative sea-level changes influenced platform growth. This resulted in five shallowing upward packages; each separated by an incipient drowning event of varying magnitude. Antecedent topography and the size of the preceding drowning event strongly influenced the initial growth of each interval. This repeated pattern is attributed to interaction between (a) the inherent tendency of microbial carbonates to aggrade vertically, (b) changing sedimentation rates and (c) readjustments of relative base level. The lower Pethei succession is one of few PaleoProterozoic examples of carbonate platform growth within a foreland basin. It has (1) a low gradient profile, (2) extensive slope and basin plain carbonate production and sedimentation, (3) no ooids, (4) minor terrigenous clastic sediments, and (4) a mobile, submergent shelf rim lacking substantial carbonate sand shoals.     

The ability of debris, heavily freighted with coarse clastic materials, to flow on gentle slopes

Observations of many debris-flow deposits on gently-sloping alluvial fans have disclosed that debris commonly is heavily loaded with coarse clastic material and contains large isolated blocks. The paper describes how debris charged with coarse granular material can transport large blocks, yet flow on gentle slopes. Experimental results of mixing sand-sized particles with a slurry of clay plus water indicate that 45–55 vol. % of a single size, and up to 64% of two selected sizes, can be added before interlocking occurs. Theoretical analysis of multi-size classes suggest that 89 to more than 95 vol. % debris can be clastic materials without significant particle interlocking. The clay fraction, even if minor, plays a critical role in determining strength properties of debris. The mixture of clay plus water provides a cohesive slurry that supports fine-grained particles within the debris, as well as reduces the effective normal stresses between the particles. The increased unit weight of the clay plus water plus fine-grained particles allows the support of coarser grained particles. The pyramiding upon the clay-water slurry continues until the entire debris mass is supported in a virtually frictionless position because of the reduced effective normal stress and the lack of particle interlocking. Thus, the ability of debris flows to support large blocks can be understood in terms of the high unit weight of the displaced debris plus the strength of the fluid phase; that is, the blocks float in the debris as a result of a small density difference between the blocks and the debris, plus the cohesive strength of the clay-water slurry. Also, the ability of coarse clastic debris to flow on gentle slopes probably is a result of poor sorting of debris-flow materials which contain minor amounts of clay. The poor sorting allows the debris to have a high density yet have essentially no interlocking of clasts. The high density of the debris reduces effective normal stresses between clasts, thereby reducing apparent friction of the mixture.     

Reactions in Amphibolite, Greenschist and Blueschist

Mineral assemblages in which chlorite [CHL], epidote [EPI],clinoamphibole [AMP], plagioclase [PLG] and quartz [QTZ] aremajor phases are characteristic of many low-grade mafic schists.The possible heterogeneous reactions in such an assemblage maybe separated into two types, exchange reactions and net-transferreactions. Only the latter alter significantly the modal proportionsof the minerals. A set of linearly independent reactions defines a reaction spaceof as many dimensions as there are independent reactions. Thespace defined by the net-transfer reactions alone is a sub-spacethat can be portrayed in three dimensions for the above assemblage.A procedure is presented herein that gives a set of independentreactions that may be taken as basis reactions for definingsuch a reaction space. All other reactions that can be writtenfor this assemblage, as well as observed whole-rock reactions,can be portrayed as vectors in these reaction spaces. Thesevectors connect the region (mineral facies) accessible to theabove assemblage. The whole-rock reactions of Laird (1980) relatinggreenschist, blueschist and various low-grade amphibolites fromVermont, provide informative examples, as do the whole-rockexperiments of Liou et al. (1974). Although reaction spaces apply to both equilibrium and disequilibriumassemblages the reactions selected as basis vectors correspondone-for-one to the chemical conditions for equilibrium thatmust obtain in any fully equilibrated assemblage. The set selectedis one that provides maximum sensitivity for geothermometric,geobarometric and geohygrometric purposes.     

Chlorite-Biotite Equilibrium Reactions in a Carbonate-Free System

In a system open to both potassium and water, the conditionsfor two different chloritebiotite equilibrium reactions maybe depicted on an isobaric temperature–(K)/(H) diagram.At moderate values of the (K)/(H) ratio in the fluid phasepervading the system, muscovite is involved in the equilibriumreaction: chloritequartzKOH= biotitemuscoviteH₂O. At high values of (K)/(H) K-feldspar is involved in the equilibriumreaction: chlorite quartzKOH = biotiteK-feldsparH₂O. When the same system is closed to potassium, assemblages formingat lower values of (K/(H) resemble those developed on progressivemetamorphism, whereas higher (K)/ (H assemblages in the closedsystem resemble certain retrogressive mineral assemblages. Theassemblage muscovitebiotite is characteristic of higher temperaturesand lowyer values of µ_K/µ_S than the assemblageK-feldsparchlorite in carbonate-free rocks where both potassiumand water were perfectly mobile components and SiO₂ was presentin excess.     

Geochronology and geological evolution of metamorphic rocks in the Field Islands area, East Antarctica

Detailed geochronological, structural and petrological studies reveal that the geological evolution of the Field Islands area, East Antarctica, was substantially similar to that of the adjacent Archaean Napier Complex, though with notable differences in late and post Archaean times. These differences reflect the area's proximity to the Proterozoic Rayner Complex and consequent vulnerability to tectonic process involved in the formation of the latter. Distinctive structural features of the Field Islands are (1) consistent development of a discordant, pervasive S₃ axial-plane foliation; (2) re-orientation of S₃ axial planes to approximate to the subsequent E-W tectonic trend of the nearby Rayner Complex; (3) selective retrogression by a post-D₃ static thermal overprint; and (4) relatively common development of retrogressive, E-W-trending, mylonitic shear zones. Peak metamorphic conditions in excess of 800°C at 900 ± 100 M Pa (9 kbar) were attained at one locality following, but probably close to the time of D₂ folding. D₃ took place in late Archaean times when metamorphic temperatures were about 650°C and pressures were about 600 MPa (6 kbar). Later, temperatures of 600 ± 50°C and pressures of 700 MPa (7kbar) were attained in an amphibolite-facies event, presumably associated with the widespread granulite to amphibolite-facies metamorphism and intense deformation involved in the formation of the Rayner Complex at about 1100 Ma. The area was subsequently subjected to near-isothermal uplift. Rb-Sr isotopic data indicate that the pervasive D₃ fabric developed at about 2400–2500 Ma, and this age can be further refined to 2456⁺⁸_-5 Ma by concordant zircon analyses from a syn-D₃ pegmatite. All zircons were affected by only minor (<7–10%) Pb loss and/or new zircon growth during the Rayner event at about 1100Ma. Thus the 450–850 μg/gU concentrations of these zircons were too low to cause sufficient lattice damage over the 1350 Ma (from 2450 Ma) for excessive Pb to be lost during the 1100 Ma event. The emplacement of pegmatite at 522 ± 10 Ma substantially changed the Rb-Sr systematics of the only analysed rock that developed a penetrative fabric during the 1100 Ma event. Monazite in this pegmatite contains an inherited Pb component, which probably resides in small opaque inclusions. A good correlation is found between Rb-Sr total-rock ages and rock fabric. U-Pb zircon intercepts with concordia also mostly correspond to known events. However, in one example a near perfect alignment of zircon analyses, probably developed by mixing of unrelated components, produced concordia intercepts that appear to have no direct geochronological significance.     

GATEKEEPERS AND KEYMASTERS: DYNAMIC RELATIONSHIPS OF ACCESS IN GEOGRAPHICAL FIELDWORK*

ABSTRACT. This article contributes to a recent and growing body of literature exploring the nature of fieldwork in human geography. Specifically, we critically examine the role of gatekeepers in providing access to “the field,” based on existing conceptualizations of gatekeepers in the literature and on our own experiences with gatekeepers. We argue that the concept of gatekeepers has been oversimplified, in that relationships between researchers and gatekeepers are often assumed to be unidirectional—with gatekeepers controlling or providing access by researchers—and predominantly static in form and time. Although we accept the necessity and advantages of working through gatekeepers, our experiences suggest that relationships with them are highly complex and evolve over time, with sometimes unexpected implications for research. In gathering and analyzing data, researchers become gatekeepers themselves, what we are calling “keymasters.” Reconceptualizing the gatekeeper‐researcher relationship will contribute to ongoing efforts to more fully understand field‐workers as undertaking a practice inherently political, personal, and linked to the production of knowledge.