Constraining bedrock erosion during extreme flood events

The importance of high-magnitude, short-lived flood events in controlling the evolution of bedrock landscapes is not well understood. During such events, erosion processes can shift from one regime to another upon the passing of thresholds, resulting in abrupt landscape changes that can have a long lasting legacy on landscape morphology. Geomorphological mapping and topographic analysis document the evidence for, and impact of, extreme flood events within the Jökulsárgljúfur canyon (North-East Iceland). Surface exposure dating using cosmogenic 3He of fluvially sculpted bedrock surfaces determines the timing of the floods that eroded the canyon and helps constrain the mechanisms of bedrock erosion during these events. Once a threshold flow depth has been exceeded, the dominant erosion mechanism becomes the toppling and transportation of basalt lava columns and erosion occurs through the upstream migration of knickpoints. Surface exposure ages allow identification of three periods of rapid canyon cutting during erosive flood events about 9, 5 and 2 ka ago, when multiple active knickpoints retreated large distances (> 2 km), each leading to catastrophic landscape change within the canyon. A single flood event ~9 ka ago formed, and then abandoned, Ásbyrgi canyon, eroding 0.14 km3 of rock. Flood events ~5 and ~2 ka ago eroded the upper 5 km of the Jökulsárgljúfur canyon through the upstream migration of vertical knickpoints such as Selfoss, Dettifoss and Hafragilsfoss. Despite sustained high discharge of sediment-rich glacial meltwater (ranging from 100 to 500 m3 s-1); there is no evidence for a transition to an abrasion-dominated erosion regime since the last erosive flood: the vertical knickpoints have not diffused over time and there is no evidence of incision into the canyon floor. The erosive signature of the extreme events is maintained in this landscape due to the nature of the bedrock, the discharge of the river, large knickpoints and associated plunge pools. The influence of these controls on the dynamics of knickpoint migration and morphology are explored using an experimental study. The retreat rate of knickpoints is independent of both mean discharge, and temporal variability in the hydrograph. The dominant control on knickpoint retreat is the knickpoint form which is set by the ratio of channel flow depth to knickpoint height. Where the knickpoint height is five times greater than the flow depth, the knickpoints developed undercutting plunge pools, accelerating the removal of material from the knickpoint base and the overall retreat rate. Smaller knickpoints relative to the flow depth were more likely to diffuse from a vertical step into a steepened reach or completely as the knickpoint retreated up the channel. These experiments challenge the established assumption in models of landscape evolution that a simple relationship exists between knickpoint retreat and discharge/drainage area. In order to fully understand how bedrock channels, and thus landscapes, respond and recover to transient forcing, further detailed study of the mechanics of erosion processes at knickpoints is required

Increased population use of medications for male lower urinary tract symptoms/benign prostatic hyperplasia correlates with changes in indications for transurethral resection of the prostate.

To access publisher's full text version of this article click on the hyperlink at the bottom of the pageThe aim of this study was to determine whether there are correlations between medication use for lower urinary tract symptoms/benign prostate hypertrophy (LUTS/BPH) and alteration in incidence and indications for transurethral resection of the prostate (TURP).The number of TURP patients between 1984 and 2008 in Iceland was obtained from hospital registries. The number of defined daily doses (DDDs) of 5-alpha-reductase inhibitors (5aRIs) and alpha-blockers (ABs) sold was obtained from the Icelandic Medicines Control Agency. Charts of all surgical BPH patients in Iceland from 1998 to 2008 were retrospectively reviewed. The main outcomes measures were: DDDs sold of 5aRIs and ABs, total numbers of TURP, indications for TURP and complications.After the introduction of ABs and 5aRIs, sales increased annually at a near linear rate. TURP rates peaked in 1992, then declined. In 2008, 81 and 3.4 of 1000 men over the age of 50 used LUTS/BPH medications or underwent TURP, respectively. There was an inverse correlation between LUTS/BPH medication use and (i) overall TURP (R(2) = 0.85), (ii) TURP done for absolute indications (R(2) = 0.91), and (iii) LUTS with (R(2) = 0.77) and (iv) without previous medical therapy (R(2) = 0.75). As medication use rose, fewer TURPs were performed for previous history of urinary retention, and more for recurrent urinary tract infections.Increased use of ABs and 5aRIs in the Icelandic population correlated with decreasing incidences of TURP procedures for both LUTS and absolute indications. The sequelae of BPH and indications for TURP are changing as medication use increases, although a clear causative link is hard to establish.Icelandic Urology Associatio

Adenocarcinoma of the prostate in Iceland: a population-based study of stage, Gleason grade, treatment and long-term survival in males diagnosed between 1983 and 1987

To access publisher full text version of this article. Please click on the hyperlink in Additional Links fieldOBJECTIVE: To investigate adenocarcinoma of the prostate in a single population with an extended follow-up period. MATERIAL AND METHODS: Using the Icelandic Cancer Registry, we identified all Icelandic men diagnosed with prostate cancer between 1983 and 1987. Disease stage, initial treatment and follow-up information were obtained from hospital records and death certificates. A critical evaluation was made of the accuracy of the death certificates regarding prostate cancer. All available histology information was reviewed and graded according to the Gleason grading system. RESULTS: A total of 414 men were diagnosed with adenocarcinoma of the prostate. Of these, 370 were alive at the time of diagnosis and stage could be determined. Four stage groups were defined: focal incidental (n=50); localized (n=164); local advanced (n=32); and metastatic disease (n=124). The mean age at diagnosis was 74.4 years (range 53-94 years). The combined Gleason score was 2-5 in 89, 6-7 in 117, 8-10 in 117 and unknown in 47 cases. The median follow-up period for the group was 6.15 years (range 0.3-19.8 years). Thirty men received treatment with curative intent: radiation therapy, n=20; and radical prostatectomy, n=10. A total of 334 patients died during the follow-up period, of whom 168 (50%) died of prostate cancer. Prostate cancer-specific survival at 10 and 15 years was 100% and 90.6%, respectively for focal incidental cancer; 73.1% and 60.8% for men with localized disease; 23.4% and 11.7% for local advanced disease; and 6.81% and 5.45% for metastatic disease. A Cox multivariate analysis showed age, stage and Gleason score to be independent predictors of prostate cancer death. A total of 104 patients with localized disease and a Gleason score o

Volcanic sands of Iceland - Diverse origins of aeolian sand deposits revealed at Dyngjusandur and Lambahraun

International audienceThe origin, formation and evolution of volcanic sands are less well known than the formation of the much more common quartz-rich sand sheets. Combining active volcanism and a cold climate, Iceland is covered for about 21% of its surface by sandy areas. The sands were analyzed in detail at two sites and results reveal their diverse origins. The first site is Dyngjusandur, located north of Vatnajokull, and the second site is the Lambahraun area, located south of Langjokull. At both sites, the sand origin is determined from field observations (wind directions from ventifacts), chemical and mineralogical analyses of rocks and sands. At Dyngjusandur, the sand is dominated by glass grains, a situation typical of sand plains in Iceland. Hyaloclastite ridges presently buried beneath Vatnajokull are the dominant source of the sand, and only large size plagioclase crystals (0.5 cm) in sands seem to be derived from the lava flows. Hyaloclastite ridges were crushed by glaciers and mechanically eroded sediments were washed out by melt-water onto flood plains. The sand chemical composition is spatially homogeneous and similar to the average composition of neighboring sub-aerial lava flows, reflecting efficient mixing of distinct sources below the glacier. The presence of sand north of Dyngjujokull can be taken as a way to explore the average chemical composition of non-exposed volcanic material beneath the glacier. In the case of Lambahraun, prevailing winds indicate several potential sources of sand at the north of the sand sheet. Comparison of chemical and mineralogical analyses of sands and rock samples helped to refine the exact origin. In contrast with the first site, the sand is dominated by crystals and is chemically consistent with a mixture of material derived from the lava flows of Eldborgir and Skersli shield volcanoes. Analysis of the contact between the lava flows and the glacier reveals that basaltic sand grains formed as the result of recent advances of the glacier abrading the rocks. The direct interaction of glacial and fluvio-glacial activity with basaltic plains appears to be necessary to produce a large amount of sands in a relatively short period of time (<4000years). This site appears to be an excellent natural laboratory for further studies concerning the sand evolution and physical sorting processes in basaltic material, which have important implications for understanding aeolian processes on Mars. Copyright (C) 2011 John Wiley & Sons, Ltd

Volcanic Sand in Iceland: Diverse origins of aeolian sand deposits revealed at Dyngjusandur and Lambahraun, Iceland

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