			Home About us Contact

Sediment Grain Size (sediment + grain_size)

Distribution by Scientific Domains

Earth and Environmental Science	50%

Selected Abstracts

Relation between flow, surface-layer armoring and sediment transport in gravel-bed rivers

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 8 2008
John Pitlick
Abstract This study investigates trends in bed surface and substrate grain sizes in relation to reach-scale hydraulics using data from more than 100 gravel-bed stream reaches in Colorado and Utah. Collocated measurements of surface and substrate sediment, bankfull channel geometry and channel slope are used to examine relations between reach-average shear stress and bed sediment grain size. Slopes at the study sites range from 0·0003 to 0·07; bankfull depths range from 0·2 to 5 m and bankfull widths range from 2 to 200 m. The data show that there is much less variation in the median grain size of the substrate, D50s, than there is in the median grain size of the surface, D50; the ratio of D50 to D50s thus decreases from about four in headwater reaches with high shear stress to less than two in downstream reaches with low shear stress. Similar trends are observed in an independent data set obtained from measurements in gravel-bed streams in Idaho. A conceptual quantitative model is developed on the basis of these observations to track differences in bed load transport through an idealized stream system. The results of the transport model suggest that downstream trends in total bed load flux may vary appreciably, depending on the assumed relation between surface and substrate grain sizes. Copyright © 2007 John Wiley & Sons, Ltd. [source]

The importance of meiofauna to lotic ecosystem functioning

FRESHWATER BIOLOGY, Issue 1 2000
Christine C. Hakenkamp
Summary 1Although meiofauna occur in large numbers in many streams, almost nothing is known about their functional role. 2In other systems, meiofauna influence microbial and organic matter dynamics through consumption and bioturbation. Given that these are important processes in streams, meiofauna have the potential to influence lotic function by changing the quality and availability of organic matter as well as the number and biotic activity of benthic microbes. Selective feeding by meiofauna has the potential to alter the availability of nutrients and organic carbon. 3Meiofauna generally contribute only a small amount to metazoan production and biomass in streams, although exceptions occur. Within a stream, the relative importance of meiofauna may reflect whether the temporary or permanent meiofauna dominate the meiobenthos as well as the season when samples are collected. 4We suggest stream conditions (small sediment grain size, restricted interstitial flow) under which meiofauna have the greatest likelihood of influencing stream ecosystem function. 5Important areas for future research include addressing whether meiofauna feed selectively, whether meiofauna are links or sinks for carbon in streams, and whether bioturbation by meiofauna influences stream ecosystem processes in a predictable manner. [source]

LOWER SILURIAN "HOT SHALES" IN JORDAN: A NEW DEPOSITIONAL MODEL

JOURNAL OF PETROLEUM GEOLOGY, Issue 3 2009
D. K. Loydell
Data are presented from the Batra Formation (also known as the Mudawwara Shale Formation) of a core from well BG-14 in the Batna el Ghoul area, southern Jordan, which enable a new depositional model to be proposed for the middle Rhuddanian (lower Llandovery, Silurian) "hot shale" which may be applicable to other Arabian and North African "hot shales" of similar stratigraphical age. This "hot shale" probably results from rapid early burial of organic carbon associated with a minor regression during which anoxic bottom conditions were maintained for most, but not all, of the time. Evidence for regression comes from (1) increased sediment grain size within the "hot shale" by comparison with underlying shales; (2) palynological changes including a decrease in acritarch species diversity; an increase in the relative abundance of sphaeromorphs, veryhachiids with three processes and acritarchs with short, simple processes; and a decrease in the relative abundance of acanthomorphs; (3) a positive ,13Corg excursion (other Late Ordovician and Silurian positive ,13Corg excursions occur during regressions); and (4) very brief intervals of oxygenation (associated with sediment influx) reflected in the preservation of graptolites as three-dimensional pyrite internal moulds, rather than as flattened periderm. The minor regression reflects a eustatic sea-level fall, evidence for which has recently been presented from several regions, including Arctic Canada, Bohemia and Scotland. The BG-14 "hot shale" is shown to be thicker than estimated in previous studies. Previous TOC measurements from the upper part of the "hot shale" were affected by the weathering of overlying strata in the BG-14 core. ICP-MS measurements show that uranium content is high in these weathered levels, extending the stratigraphical extent of the "hot shale" interval into the middle Rhuddanian. Depositional models such as that presented here rely on a robust biostratigraphical framework; in the Ordovician and Silurian of Arabia and North Africa, this can be provided by graptolites and chitinozoans. [source]

Multiproxy evidence of an early Holocene (8.2 kyr) climate oscillation in central Nova Scotia, Canada

JOURNAL OF QUATERNARY SCIENCE, Issue 7 2002
Professor Ian Spooner
Abstract An early Holocene lake sediment record from central Nova Scotia contains a minerogenic oscillation that is closely correlative with the 8.2 kyr event (ca. 8200 cal. yr BP), an event that has not been reported elsewhere in Atlantic Canada. A variety of biological and sedimentological indicators have been examined to characterise autochthonous and allochthonous changes that occurred during this time. The minerogenic upper oscillation (UO, ca. 8400 cal. yr BP) is marked by an increase in the chrysophyte:diatom ratio. Following the oscillation, the diatom community reflects a shift to more productive, less acidic conditions. The pollen record shows no major response to this short-lived event. Lithostratigraphic analyses indicates that the UO is characterised by an increase in clastic content, magnetic susceptibility and mean sediment grain size, all indicators of changing environmental conditions, most likely the result of regional cooling. The Taylor Lake record adds to a growing body of evidence for a widespread, hemispheric climate oscillation at 8.2 kyr. Copyright © 2002 John Wiley & Sons, Ltd. [source]