Seasonal succession and microlamina formation in a meromictic lake displaying varved sediments

In a small meromictic lake near Toronto, Canada, a mass mortality of photosynthetic bacteria followed the ventilation of the chemocline during fall (autumn) and resulted in 3–8 g m^-2 day^-1 of organic matter being deposited as a dark layer in sediment traps which were suspended in the permanently anaerobic zone. This mass mortality of photosynthetic bacteria occurred in late autumn following the annual thermal destratification of the lake's mixolimnion. Wind mixing during this period of homeothermy resulted in the introduction of low levels of dissolved oxygen into the lake's chemocline. The ensuing mass mortality of photosynthetic bacteria resulted in the release of elemental sulphur as the sulphur-rich bacteria decomposed and sank to the bottom of the lake. The ferrous ions in the water below a depth of 15 m in Crawford Lake reacted with this sulphur to form black ferrous sulphides and pyrite which formed a dark microlamina on the lake floor. Each dark microlamina was overlain by a light coloured (calcite-rich) layer which was deposited each spring and summer during the 3 yr period of this study. The mechanism of microlamina formation elucidated here has been based on the examination of bi-weekly sediment trap information. This approach has permitted an explanation of the mechanisms by which specific events such as calcite precipitation and phytoplankton seasonal succession are transcribed into the sediment record.     

Sediment suspension dynamics and a new criterion for the maintenance of turbulent suspensions

The vertical component of the turbulent flow acceleration term, , is used to determine the net positive vertical force that may support a suspended sediment load. A dimensionless criterion, Λ, is proposed for the maintenance of suspension, defined as the ratio of the maximum vertical turbulent stress to immersed weight of the suspended load above a unit bed area. In order that a suspension be maintained: where v ′ is instantaneous vertical turbulent velocity, σ and ρ are solid and fluid densities, respectively and m is the suspended load dry mass. The Λ criterion is dynamic, being a ratio of stresses and is analogous in this respect to Shields dimensionless stress criterion, θ, for the initiation of bedload motion. The new criterion is successful in predicting the maintenance of steady-state suspended sediment transport in open channel shear flow and deposition from non-uniform particulate density flows of wall jet type.     

The conquering of climate: discourses of fear and their dissolution

We are living in a climate of fear about our future climate. The language of the public discourse around global warming routinely uses a repertoire which includes words such as 'catastrophe', 'terror', 'danger', 'extinction' and 'collapse'. To help make sense of this phenomenon the story of the complex relationships between climates and cultures in different times and in different places is in urgent need of telling. If we can understand from the past something of this complex interweaving of our ideas of climate with their physical and cultural settings we may be better placed to prepare for different configurations of this relationship in the future. This paper examines two earlier European discourses of fear associated with climate – one from the early-modern era (climate as judgement) and one from the modern era (climate as pathology) – and traces the ways in which these discourses formed and dissolved within a specific cultural matrix. The contemporary discourse of fear about future climate change (climate as catastrophe) is summarised and some ways in which this discourse, too, might be dissolved are examined. Conventional attempts at conquering the climatic future all rely, implicitly or explicitly, upon ideas of control and mastery, whether of the planet, of global governance or of individual and collective behaviour. These attempts at 'engineering' future climate seem a degree utopian and brash. Understanding the cultural dimensions of climate discourses offers a different way of thinking about how we navigate the climatic future. However our contemporary climatic fears have emerged – as linked, for example, to neoliberal globalism, to ecological modernisation and the emergence of a risk society, or to a deeper instinctive human anxiety about the future – they will in the end be dissipated, re-configured or transformed as a function of cultural change.     

A Modified Transformed Soil Adjusted Vegetation Index for Cropland in Jilin Province,China

正1 Introduction Vegetation indices (VIs) derived from satellite observations are an essential source of information for operational monitoring of the Earth's vegetation (Qu et al.,2018;Yan et al.,2008).However,soil background dramatically affects the performances ofⅥs (Baret and Guyot,1991;Gilabert et al.,2002;Huete,1988;Qi et al.,1994).So soil-adjustedⅥs (Baret and Guyot,1991;Gilabert et al.,2002;Huete,1988;Qi et al.,1994) are     

Tectonic and climatic controls on Holocene channel migration,incision and terrace formation by the Rio Grande in the Palomas half graben,southern Rio Grande rift,USA

In order to evaluate potential effects of tectonics and climate change on the behaviour of the axial Rio Grande in the Rio Grande rift, a 16·5 km stretch of modern floodplain and Holocene terraces were mapped in the tectonically active Palomas half graben, south‐central New Mexico, USA. In addition, 51 cores and natural exposures were logged and 20 radiocarbon dates were obtained from charcoal, bulk organic matter, mollusc shells and pedogenic calcite. The Holocene alluvium comprises four terraces above the modern floodplain, each of which formed by a period of river incision followed by stability and renewed floodplain construction to a level below that of the previous terraces. Estimated times of incision between Terraces I and II, II and III, and III and IV are after 12 400, 8040 to 5310, and 760 to 550 yr bp , respectively, whereas the incision between Terrace IV and the modern floodplain occurred within the last 260 years. Although there is some evidence for tectonic control on river behaviour in the southern part of the basin, terrace formation is interpreted as being related to climate change, with periods of incision corresponding to times of increased aridity and low sediment/water discharge ratio in the Rio Grande. This process may have resulted from a reduction in intensity and magnitude of summer storms which supply sediment to the axial river, coupled with an increase in spring discharge peak caused by snowmelt in upstream mountain catchments.     

Spatio-temporal drought analysis in the Trakya region,Turkey

Abstract

Since droughts are natural phenomena, their occurrence cannot be predicted with certainty and thus it must be treated as a random variable. Once drought duration and magnitude have been found objectively, it is possible to plan for the transport of water in known quantities to drought-stricken areas either from alternative water resources or from water stored during wet periods. The summation of deficits over a particular period is referred to as the drought magnitude. Drought intensity is the ratio of drought magnitude to its duration. These drought properties at different truncation levels provide significant hydrological and hydrometeorological design quantities. In this study, the run analysis and z-score are used for determining drought properties of given hydrological series. In addition, kriging is used as a spatial drought analysis for mapping. This study is applied to precipitation records for Istanbul, Edirne, Tekirdag and Kirklareli in the Trakya region, Turkey and then the drought period, magnitude and standardized precipitation index (SPI) values are presented to depict the relationships between drought duration and magnitude.     

Laboratory sustained turbidity currents form elongate ridges at channel mouths

The turbulent flow structure, suspended sediment dynamics and deposits of experimental sustained turbidity currents exiting a channel across a break in slope into a wide tank are documented. The data shed light on the flow evolution and deposit geometry of analogous natural channel‐fed submarine fans. Flows generated in a 0·3 m wide, sloping (0°, 6°, 9° or 20°) channel crossed an angular slope break and spread onto a horizontal tank floor. Flow development comprised: (i) channelized phase (unsteady channel flow developing into steady channel flow); (ii) initial lateral expansion phase (unsteady‐spreading wall jet phase); (iii) constant lateral expansion phase (steady‐spreading wall jet phase); and (iv) rapid waning phase. Phases (i) and (iv) are similar to laterally constrained turbidity currents, but phases (ii) and (iii) are considerably different from such two‐dimensional currents. Steeper channel slopes produced greater flow velocities and turbulence intensities, but these effects diminished markedly with distance from the channel mouth. Flow velocity vectors in the tank had similar patterns for all channel slopes, with a central core of faster velocity and narrow vector dispersion and slower flow with larger dispersion at the jet margins. Suspended sediment concentrations were higher within flow heads and dense basal layers in flow bodies. Time‐averaged acoustic backscatter data showed vertical concentration gradients, confirmed by siphon samples. The deposits comprised a thick central ridge, of similar order width to the channel mouth, with abrupt margins and a surrounding, very thin, fan‐like sheet. The ridge was coarser grained and better sorted than the original sediment, with grain‐size fining downstream, particularly over the fan‐like sheet. The formation of a central ridge suggests that, in the tank, vertical turbulent momentum exchange is more significant for sediment dynamics than spanwise momentum exchange due to lateral expansion. The streamwise elongate geometry of the ridge contrasts with conventional fan‐like geometry found with surge‐type turbidity flows, a result that has widespread stratigraphic and economic implications.