Agents Of Erosion Research Articles

Weighted linear combination method versus grid based overlay operation method – A study for potential soil erosion susceptibility analysis of Malda district (West Bengal) in India

Soil erosion is a natural process. The factors which are responsible for soil erosion are rainfall, flowing water, ice, temperature changes, gravity i.e. mostly the physical agents of erosion along with the anthropogenic agents. The present paper focuses on the delineation of soil erosion susceptible areas in Malda district (west Bengal) using weighted linear combination method and grid based overlay operation method and their effectiveness. The Arc GIS 9.3 software were used to complete the process of Geo-referencing, digitization of drainage network, and the generation of various maps like; Rainfall, Soil texture, Slope, Land use/cover, Vegetation and distance from river maps etc, of Malda district. The surface lowering rate map and the determination of rate of soil erosion of fifteen blocks in Malda district has been prepared and determined using field based empirical pegging operation data for the years 2015 and 2016 through Arc GIS 9.3 software. Potential soil erosion susceptibility zonation map has been prepared based on weighted linear combination (WLC) method and grid based overlay operation method with selected eight soil erosion parameters in ARC-GIS 9.3 software and ERDAS environment. The validation of the both methods has been done based on accuracy assessment and ROC curve.

On the pre-history of the turbidite concept: an Alpine perspective on occasion of the 70th anniversary of Kuenen’s 1948 landmark talk

The recognition of turbidity currents as an important geological agent of sediment transport, sedimentation, and erosion 70 years ago, initiated a bonanza in clastic sedimentology and led to new interpretations of both recent marine sediments and ancient sedimentary rocks. Only scarce attention had been paid to graded bedding, a hallmark of turbidites, before ca. 1930 and almost nobody had linked it to episodic mass flow sedimentation. This article is an addition to the pre-history of the turbidite revolution from an Alpine, and more specifically, a Swiss perspective. It focuses on two time periods, the early and middle part of the eighteenth century and the turn from the nineteenth to the twentieth century. In the former period, Swiss naturalists such as Scheuchzer and Gruner had not only recognized the ubiquity of graded bedding in some Alpine flysch successions, but also proposed hypotheses as to its likely origin. In doing so, Gruner invoked in 1773 periodic sediment mobilization due to bottom currents on the floor of an ante-diluvian sea, resulting in distinct episodes of sediment settling giving rise to normally graded layers. Gruner’s model was inspired by several contemporaneous pioneers of experimental clastic sedimentology. This interest in physical sedimentology declined during the nineteenth century and it was only with the start of the second period discussed herein that geologists began to re-appreciate the importance of sediment movement at the bottom of the oceans. This time, inspiration came from the practical experience of Swiss geologists with shore collapse events and subsequent sedimentary mass flows in lakes. Building on that, they were able to better interpret parts of the Alpine sedimentary record, especially flysch successions. However, it was only with Bailey, Migliorini, Kuenen and others during the 1930s and 1940s that the link between mass flows and graded bedding was finally established.

Rainfall and inflow effects on soil erosion for hillslopes dominated by sheet erosion or rill erosion in the Chinese Mollisol region

Erosion agents and patterns profoundly affect hillslope soil loss characteristics. However, few attempts have been made to analyze the effects of rainfall and inflow on soil erosion for hillslopes dominated by sheet erosion or rill erosion in the Chinese Mollisol region. The objective of this study was to discuss the erosive agent (rainfall or inflow), hillslope erosion pattern (sheet erosion or rill erosion) and slope gradient effects on runoff and soil losses. Two soil pans (2.0 m long, 0.5 m wide and 0.5 m deep) with 5° and 10° slopes were subjected to rainfall (0 and 70 mm h–1) and inflow (0 and 70 mm h–1) experiments. Three experimental combinations of rainfall intensity (RI) and inflow rate (IR) were tested using the same water supply of 70 mm by controlling the run time. A flat soil surface and a soil bed with a straight initial rill were prepared manually, and represented hillslopes dominated by sheet erosion and rill erosion, respectively. The results showed that soil losses had greater differences among treatments than total runoff. Soil losses decreased in the order of RI70+IR70 > RI70+IR0 > RI0+IR70. Additionally, soil losses for hillslopes dominated by rill erosion were 1.7–2.2 times greater at 5° and 2.5–6.9 times greater at 10° than those for hillslopes dominated by sheet erosion. The loss of <0.25 mm soil particles and aggregates varying from 47.72%–99.60% of the total soil loss played a dominant role in the sediment. Compared with sheet erosion hillslopes, rill erosion hillslopes selectively transported more microaggregates under a relatively stable rill development stage, but rills transported increasingly more macroaggregates under an active rill development stage. In conclusion, eliminating raindrop impact on relatively gentle hillslopes and preventing rill development on relatively steep hillslopes would be useful measures to decrease soil erosion and soil degradation in the Mollisol region of northeastern China.

Glacial Steady State Topography Controlled by the Coupled Influence of Tectonics and Climate.

Glaciers and rivers are the main agents of mountain erosion. While in the fluvial realm empirical relationships and their mathematical description, such as the stream power law, improved the understanding of fundamental controls on landscape evolution, simple constraints on glacial topography and governing scaling relations are widely lacking. We present a steady state solution for longitudinal profiles along eroding glaciers in a coupled system that includes tectonics and climate. We combined the shallow ice approximation and a glacial erosion rule to calculate ice surface and bed topography from prescribed glacier mass balance gradient and rock uplift rate. Our approach is inspired by the classic application of the stream power law for describing a fluvial steady state but with the striking difference that, in the glacial realm, glacier mass balance is added as an altitude‐dependent variable. From our analyses we find that ice surface slope and glacial relief scale with uplift rate with scaling exponents indicating that glacial relief is less sensitive to uplift rate than relief in most fluvial landscapes. Basic scaling relations controlled by either basal sliding or internal deformation follow a power law with the exponent depending on the exponents for the glacial erosion rule and Glen's flow law. In a mixed scenario of sliding and deformation, complicated scaling relations with variable exponents emerge. Furthermore, a cutoff in glacier mass balance or cold ice in high elevations can lead to substantially larger scaling exponents which may provide an explanation for high relief in high latitudes.

Wind Waves and Cliff Shaking on Macrotidal Shore Platforms: A Case-study from North Yorkshire, U.K.

ABSTRACT Kennedy, D.M.; Vann Jones, E.C.; Dickson, M.E., and Rosser, N.J., 2018. Wind waves and cliff shaking on macrotidal shore platforms: A case-study from North Yorkshire, U.K. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 436–440. Coconut Creek (Florida), ISSN 0749-0208. Wind waves are a major erosive agent on the rocky coast. The physical impact of waves is also important in driving erosion above the splash zone. This occurs through the transferral of energy into the bedrock by the action of wave breaking causing microseismic shaking. In this study we concurrently measure wave energy and resultant microseismic shaking on a macrotidal shore platform at Staithes, North Yorkshire, U.K. It is found that infragravity wave generation is limited when compared to microtidal coasts and where it does occur it is confined to the wave breaking zone. Cliff shaking ...

Substrate controls on valley formation by groundwater on Earth and Mars

Valleys with amphitheater-shaped headwalls on Mars have been used to constrain early martian hydrology and, importantly, have been interpreted as eroded from groundwater-fed springs, which might have constituted hospitable environments for life on ancient Mars. Groundwater-fed springs have carved valleys in rare examples on Earth; however, these valleys are in loose sandy sediments and weakly cemented sandstones, and it is unclear whether groundwater is also an effective erosion agent in the basaltic bedrock and boulders within martian valleys. Here we develop a theoretical model for the efficiency of valley formation by groundwater-seepage erosion, and we show that valley formation by groundwater is limited to narrow ranges in aquifer permeabilities and sediment sizes that are characteristic of loose or weakly consolidated sand. The model is validated against groundwater-carved valleys in loose sand in physical experiments and natural valleys on Earth. Applied to valleys near Echus Chasma, Mars, our model precludes a formation by seepage erosion due to the inferred basaltic bedrock; instead, the model implies that surface flows of water were required to form the valleys, with significant implications for the hydrology, climate, and habitability of ancient Mars.

Modelling Soil Erosion for Land Management in Ungauged Golole Catchment in Marsabit County, Kenya

This study modeled soil erosion between January 2016 and September 2018 for land management in Golole catchment. The Revised Universal Soil Loss Equation (RUSLE) constituting the main agents of soil erosion was modeled in a Geographical Information System (GIS) environment. The objective of this study was to model soil erosion for land management in the ungauged Golole catchment. The Golole catchment soil erosion map reveals that within the catchment the soil loss was not homogeneous and erosion risk was not the same. The catchment experiences an annual mean score soil loss rate of 279 t/ha that is above the recommended maximum allowable annual soil loss rate of 4 t/ha. The catchment’s soil loss rate is described as high and severe representing 70% and 30% of landmass respectively. This study found the need to decelerate the above soil loss rates to moderate and low levels by adopting soil erosion mitigation measures such as stone contour ridges, manure, strip cropping, and terracing in the cultivated areas and controlled grazing in the lowland rangeland. The study strongly felt the need to protect the forest reserve from tree cutting and further human encroachment. This study concludes that there is the need for further research 1) in the forest reserve areas that showed the greatest rates of soil erosion menace to determine the underlying causes, and 2) to assess the temporal trends of the soil erosion hazard using high-resolution data.

Meso-Cenozoic exhumation and relevant isostatic process: The Barents and Kara shelves

Fennoscandia and Arctic shelves are known to have experienced several periods of Meso-Cenozoic uplift and erosion, but their magnitude and timing are still debated. The aim of our study is to estimate erosion volumes and distinguish between different erosional episodes, including glacial and pre-glacial ones. Our estimation approach is based on a combination of geological-geomorphological analysis, computer simulations of different erosion processes in time and space, mass-balance, burial control, and other methods Isostatic modelling is an obligatory component in the geomorphological reconstructions, since the erosion and subsequent isostatic uplift change the inclinations of various surfaces. Isostatic modelling is especially of significant importance in cases of poor or lacking age control.We find that the role of Neogene erosion might previously have been strongly exaggerated in some areas. Intensive Triassic-Jurassic deformations mark exhumation of positive structures (i.e. Novaya Zemlya, Pay-Khoy, Fedynsky). Novaya Zemlya − Pay-Khoy − Polar Ural belt was likely strongly exhumed during Neocomian (Berriasian). Upper Cretaceous erosion was noticeable in some structural zones, and strong Rupelian erosion is evident in Cenozoic. Changes of depositional provinces complicated the pattern, especially in Eocene.The role of climatic changes with global cooling could be discussed for Oligocene. Neogene erosion, with final glacial substages completed the erosional history, creating several prominent troughs and basins.We present a high-resolution model of Quaternary glacial and fluvioglacal erosion. Many landforms in the area were modified by glacial and periglacial processes, in particular by variable glacial erosion due to lithological, structural and topographic diversity. Glacial erosion of relative highs was often of minor importance, but other erosional agents could be active locally. We suggest a scenario of typical ice sheet development in the Barents Region, for the use in erosion computations. Radial glacial erosion pattern and significant basal ice velocities seem to be expected at the beginning of the early ice-age stage, resulting in widening of pre-glacial drainage elements. Pre-glacial lowlands and river pattern could provoke early stage onset of topographic ice-streams, especially in permafrost condition.Major ice streams were predefined by geological structures and tectonics.Different values of the elastic properties of the lithosphere dramatically impact isostatic movements and influence mass-balance computations, landscape features and delineation of watersheds.

CALCULATING BEDLOAD TRANSPORT IN RIVERS: CONCEPTS, CALCULUS ROUTINES AND APPLICATION

Rivers are immensely important to human activities such as water supply, navigation, energy generation, and agriculture. They are also an important morphodynamic agent of erosion, transport and deposition. Their capacity to transport sediment depends on their hydraulic characteristics and can be predicted by mathematical models. Several mathematical models can be used to compute bed-load transport. Each one is appropriately better for certain conditions. In this paper, we present an application built in Microsoft Excel to compute the bed-load transport in rivers based on the Van Rijn mathematical model. The Van Rijn model is appropriate for rivers transporting sandy sediments in conditions of subcritical flow. Hydraulic parameters such as channel slope, stream power, and Reynolds and Froude numbers can be calculated using the application proposed here. The application was tested in the Paraná River and results from the calculations are consistent with data obtained from fieldwork surveys. The error of the application was only 20%, which shows good agreement of the model with survey values.

An Analysis of the Collapse Potential of Slope using the Rom Scale: A Case Study of Sultan Azlan Shah Campus, Sultan Idris Education University, Malaysia

Slope failure is a geological process that is closely related to the agents of erosion, such as water, wind, temperature and vegetation cover in the vicinity of the slope. Therefore, this study was conducted to analyse the current status of the slope stability within the new Sultan Idris Education University campus using the ROM scale. A total of 20 slopes or soil sampling points were included in this study. The results obtained from the ROM scale shows that about 16 of the 20 slopes with critical status have experienced landslides. This is due to the land fragility, lack of binder (clay) between the rocks, sand and silt. In fact, the proportion of sand in the soil exceeded 90%. Thus, additional protective measures should be taken to increase the level of soil moisture and improve the bond between soil compositions and, ultimately, the slope stability in Sultan Azlan Shah Campus can be improved. This is due the soil moisture of the critical slopes is only at 25% to 30%. And with the implementation of slope protection measures by engineering and bioengineering techniques, landslide occurrence can be avoided and reduced in the future.

Modelling Soil Detachment of Different Management Practices in the Red Soil Region of China

AbstractSoil erosion from cropland is a primary cause of soil degradation in the hilly red soil region of China. Soil characteristics and the resistance of soil to erosion agents can be improved with appropriate management practices. In this study, hydraulic flume experiments were conducted to investigate the effects of five management practices [manure fertilizer (PM), straw mulch cover (PC), peanut–orange intercropping (PO), peanut–radish rotation (PR) and traditional farrow peanut (PF)] on soil detachment. Based on the results, three conservation management practices (PC, PM and PO) increased the resistance of soil to concentrated flow erosion. The rill erodibility of different treatments was ranked as follows: PC (0·001 s m−1) &lt; PM (0·004 s m−1) &lt; PO (0·007 s m−1) &lt; PF (0·01 s m−1) &lt; PR (0·027 s m−1). The rill erodibility was affected by soil organic content, aggregate stability and bulk density. The soil detachment rate was closely correlated with the flow discharge and slope gradient, and power functions for these two factors were developed to evaluate soil detachment rates. Additionally, the shear stress, stream power and unit stream power were compared when estimating the soil detachment rate. The power functions of stream power and shear stress were equivalent, and both are recommended to predict detachment rates. Local soil conservation can benefit from the results of this study with improved predictions of erosion on croplands in the red soil region of China. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Joint-bounded crescentic scars formed by subglacial clast-bed contact forces: Implications for bedrock failure beneath glaciers

Glaciers and ice sheets are important agents of bedrock erosion, yet the precise processes of bedrock failure beneath glacier ice are incompletely known. Subglacially formed erosional crescentic markings (crescentic gouges, lunate fractures) on bedrock surfaces occur locally in glaciated areas and comprise a conchoidal fracture dipping down-ice and a steep fracture that faces up-ice. Here we report morphologically distinct crescentic scars that are closely associated with preexisting joints, termed here joint-bounded crescentic scars. These hitherto unreported features are ca. 50–200mm deep and involve considerably more rock removal than previously described crescentic markings. The joint-bounded crescentic scars were found on abraded rhyolite surfaces recently exposed (<20years) beneath a retreating glacier in Iceland, as well as on glacially sculpted Precambrian gneisses in NW Scotland and various Precambrian rocks in Ontario, glaciated during the Late Pleistocene. We suggest a common formation mechanism for these contemporary and relict features, whereby a boulder embedded in basal ice produces a continuously migrating clast-bed contact force as it is dragged over the hard (bedrock) bed. As the ice-embedded boulder approaches a preexisting joint in the bedrock, stress concentrations build up in the bed that exceed the intact rock strength, resulting in conchoidal fracturing and detachment of a crescentic wedge-shaped rock fragment. Subsequent removal of the rock fragment probably involves further fracturing or crushing (comminution) under high contact forces. Formation of joint-bounded crescentic scars is favoured by large boulders at the base of the ice, high basal melting rates, and the presence of preexisting subvertical joints in the bedrock bed. We infer that the relative scarcity of crescentic markings in general on deglaciated surfaces shows that fracturing of intact bedrock below ice is difficult, but that preexisting weaknesses such as joints greatly facilitate rock failure. This implies that models of glacial erosion need to take fracture patterns of bedrock into account.

Slope Processes, Mass Movement and Soil Erosion: A Review

Soil erosion and land degradation are global problems and pose major issues in many countries. Both soil erosion and mass movement are two forms of land degradation and humans play important roles in these geomorphological processes. This paper reviews slope processes associated with mass movement and soil erosion and contributory factors, including physical and human agents. Acting together, these cause diverse geomorphological features. Slope processes are illustrated by reference to case studies from Brazil and UK. The causes and impacts of erosion are discussed, along with appropriate remedial bioengineering methods and the potential of the measures to prevent these types of environmental degradation. Although there are several agents of erosion, water is the most important one. Cultivation can promote soil erosion, due to ploughing and harvesting, which moves soil down slopes. Soil erosion and mass movement data would inform the viability of soil conservation practices. Integrated management of drainage basins offers a promising way forward for effective soil conservation and soil remedial bioengineering in Brazil and UK.

Whole genome sequencing of Fowl aviadenovirus A - a causative agent of gizzard erosion and ulceration, in adult laying hens

Gizzard erosion and ulceration (GEU) caused by fowl aviadenovirus serotype 1 (FAdV-1) of the species Fowl aviadenovirus A (FAdV-A) represents an economically important problem in poultry production. The disease affects mostly young chicken broilers or layers before production.In this study, an unusual GEU outbreak in a flock of laying hens at 38weeks of age is described. The affected flock showed elevated mortality rates, with the highest number of dead birds appearing between the 39th and 40th week of life, with a subsequent reduction in laying performance and decreased total egg weight. Post-mortem examination showed the presence of erosion in multiple areas of the gizzard, with wall perforation in the proximity of the interventriculus. FAdV antibodies were detected in all examined sera with an ELISA assay. The virus was isolated from pathologically altered gizzards. PCR, subsequent sequencing and phylogenetic analysis of the partial hexon gene confirmed the presence of FAdV-A DNA.To investigate the molecular background of FAdV-A which causes GEU in adult hens, whole genome sequencing was performed on two FAdV-A strains - strain W-15, obtained from the outbreak described in this study and strain 61/11z, isolated from a GEU outbreak in 3-week-old broiler chickens in 2011.The genome size of FAdV-A W-15 is 43,849bp. Genome sequence and genome organization resembles those of the reference, apathogenic CELO strain and the newly sequenced GEU strain, 61/11z.Most amino acid changes, between CELO and GEU strains, were observed in ORF0, ORF1, ORF14, IVa2, polymerase, pIIIa, penton base and fiber-2. Analysis conducted on the translated ORFs revealed that W-15 and 61/11z are nearly identical, with the highest rate of amino acid mutations in pTP, 100K, ORF9 and ORF10.In this study, the occurrence of GEU, caused by FAdV-1 infection, in adult layer chickens and the effects of such infection on egg production parameters are described in detail. Moreover, the whole genome sequences of two pathogenic, GEU inducing FAdV-A strains have been provided and characterized for the first time, which in the future will help to pinpoint the viral factors involved in pathogenicity.

10Be measurements in bedrock constrain erosion beneath the Greenland Ice Sheet margin

AbstractGlacial erosion is a key process driving landscape evolution, but it remains unclear what factors dictate the rate at which subglacial erosion occurs. Moreover, estimates of subglacial erosion that do not rely on sediment flux techniques are rare. Here, we present in situ 10Be measurements from bedrock surfaces in western Greenland with well‐constrained ice‐cover histories to quantify the erosion rate beneath the Greenland Ice Sheet margin during historical times. We calculate an abrasion rate of 0.72 ± 0.35 mm yr−1 and a likely total basin‐wide erosion rate (abrasion + quarrying) of ~1–1.8 mm yr−1, which are at least 1 order of magnitude higher than typical subglacial erosion rates in other polar landscapes. A compilation of published 10Be data suggests that the southwestern Greenland Ice Sheet acts as a particularly effective erosional agent within the broader Baffin Bay‐Greenland region over millennial to glacial‐interglacial timescales, suggestive of a basal ice sheet thermal regime controlled by regional climate.

Glacial landscape evolution by subglacial quarrying: A multiscale computational approach

AbstractQuarrying of bedrock is a primary agent of subglacial erosion. Although the mechanical theory behind the process has been studied for decades, it has proven difficult to formulate the governing principles so that large‐scale landscape evolution models can be used to integrate erosion over time. The existing mechanical theory thus stands largely untested in its ability to explain postglacial topography. In this study we relate the physics of quarrying to long‐term landscape evolution with a multiscale approach that connects meter‐scale cavities to kilometer‐scale glacial landscapes. By averaging the quarrying rate across many small‐scale bedrock steps, we quantify how regional trends in basal sliding speed, effective pressure, and bed slope affect the rate of erosion. A sensitivity test indicates that a power law formulated in terms of these three variables provides an acceptable basis for quantifying regional‐scale rates of quarrying. Our results highlight the strong influence of effective pressure, which intensifies quarrying by increasing the volume of the bed that is stressed by the ice and thereby the probability of rock failure. The resulting pressure dependency points to subglacial hydrology as a primary factor for influencing rates of quarrying and hence for shaping the bedrock topography under warm‐based glaciers. When applied in a landscape evolution model, the erosion law for quarrying produces recognizable large‐scale glacial landforms: U‐shaped valleys, hanging valleys, and overdeepenings. The landforms produced are very similar to those predicted by more standard sliding‐based erosion laws, but overall quarrying is more focused in valleys, and less effective at higher elevations.

Erosion under climate and human pressures: An alpine lake sediment perspective

We review the scientific efforts over the last decades to reconstruct erosion from continuous alpine lake sediment records. We focused both on methodological issues, showing the growing importance of non-destructive high resolution approaches (XRF core-scanner) as well as progresses in the understanding of processes leading to the creation of an “erosion signal” in lakes. We distinguish “continuous records” from “event-records”. Both provide complementary information but need to be studied with different approaches. Continuous regionally-relevant records proved to be particularly pertinent to document regional erosion patterns throughout the Holocene, in particular applying the source to sink approach. Event-based approaches demonstrated and took advantage of the strong non-linearity of sediment transport in high altitude catchment areas. This led to flood frequency and intensity reconstructions, highlighting the influence of climate change upon flood dynamics in the mountain. The combination of different record types, both in terms of location (high vs. low elevation), sedimentology (high vs. low terrigenous contribution) and significance (local vs. regional) is one of the main outputs of this paper. It allows the establishment of comprehensive histories of NW French Alps erosion, but also and consequently, soil dynamics and hydrological patterns throughout the Holocene. We also discuss the influence of glacier dynamics, one of the major agents of erosion in the Alps. A major feature is the growing human influence upon erosion at a local scale since at least the middle of the Bronze Age (3500 cal. BP). However and according to the regional record from Lake Bourget, only few periods of rising erosion at local scales generated a regional record that can be discriminated from wetter climatic periods. Among them, the period between 200 BCE and 400 AD appeared to be marked by a generalised rise in human-triggered erosion at local scales in the northern French Alps. This review highlights the importance of modern high-resolution and interdisciplinary studies of lake sediments, in order to better understand the complex relationships between humans, climate and the Earth system in general. We strongly argue that regional integration of data is now required to move a step further. Such an integration is easier with cost- and time-effective methods as well as after a better definition of approaches and their limits. This should lead to a stronger collaboration between paleo-data producers and modellers in the near future.

Formation mechanisms of channels on Titan through dissolution by ammonium sulfate and erosion by liquid ammonia and ethane

Data obtained from the Cassini Visual and Infrared Mapping Spectrometer (VIMS), Imaging Science Subsystem (ISS), and Synthetic Aperture Radar (SAR) instruments have revealed an array of fluvial channels on Titan's surface, often several hundreds of kilometers in length. The paucity of impact craters on Titan's surface suggests a formation by fluvial erosion into the water-ice bedrock. Additionally, at the landing site, the Huygens Probe Descent Imager and Spectral Radiometer (DISR) imaged Earth-like rounded cobbles 0.3–15cm in diameter composed of water ice, reminiscent of rounded stream clasts on Earth. In this paper we examine different fluvial features on Titan, identified by the Cassini spacecraft, and evaluate the possibilities of channel formation by dissolution of ice by a concentrated solution of ammonium sulfate, and by mechanical erosion by flow of liquid ammonia and liquid ethane. We find that chemical erosion of Titan's channels could be completed in 280 to 1100 years (all units of time in this paper are Terrestrial, not Titanian), much shorter than the period of about 84,000 years that a concentrated (NH4)2SO4-H2O solution could exist as a liquid on the Titan surface. Mechanical erosion of Titan's channels is generally a much slower process, on the order of 102 to 105 years to completion, and is also slower than mechanical erosion of a model river on Earth, averaging 103 to 104 years. The erosional sequence of the channels on Titan may have started after the formation of water-ice on the surface by the process of chemical dissolution by (NH4)2SO4-H2O, overlapping, or followed by, a period of mechanical erosion by liquid NH3. A final stage on the cooling surface of Titan might have been characterized by liquid C2H6 as an agent of mechanical erosion.

Effects of Shoreline Dynamics on Saltmarsh Vegetation.

We evaluated the impact of shoreline dynamics on fringing vegetation density at mid- and low-marsh elevations at a high-energy site in the northern Gulf of Mexico. Particularly, we selected eight unprotected shoreline stretches (75 m each) at a historically eroding site and measured their inter-annual lateral movement rate using the DSAS method for three consecutive years. We observed high inter-annual variability of shoreline movement within the selected stretches. Specifically, shorelines retrograded (eroded) in year 1 and year 3, whereas, in year 2, shorelines advanced seaward. Despite shoreline advancement in year 2, an overall net erosion was recorded during the survey period. Additionally, vegetation density generally declined at both elevations during the survey period; however, probably due to their immediate proximity with lateral erosion agents (e.g., waves, currents), marsh grasses at low-elevation exhibited abrupt reduction in density, more so than grasses at mid elevation. Finally, contrary to our hypothesis, despite shoreline advancement, vegetation density did not increase correspondingly in year 2 probably due to a lag in response from biota. More studies in other coastal systems may advance our knowledge of marsh edge systems; however, we consider our results could be beneficial to resource managers in preparing protection plans for coastal wetlands against chronic stressors such as lateral erosion.

Living on the edge: Diadema mexicanum in the upper Gulf of California

Sea urchins play a crucial role in the health and dynamics of reef ecosystems. Diadema mexicanum is a dominant grazer and erosive agent of the substratum in reef environments in the eastern tropical Pacific. Its reported distribution extends from the middle of the Gulf of California (26° N) to northern Peru (6°23′ S), including oceanic islands. Here, we report the occurrence of Diadema mexicanum in Isla San Jorge (31°0′38.53″ N, 113°14′34.84″ W), the northernmost island in the Gulf of California, which extends its range an additional 600 km northward. Sea urchins, ranging in test size from 4.5 to 12.4 cm, were present at 2–6 m in October 2015. This test size was one of the largest reported for this species in the eastern tropical Pacific. Spine length in sea urchins in the upper gulf ranged from 3.3 to 15.6 cm. Variation in body size of sea urchin may reflect variation in more structurally complex reefs from isolated islands that provide shelter from predation. The reef structure of Isla San Jorge is formed by high coral cover of the scleractinian coral Porites panamensis, with an average colony height of 26.27 cm (standard error, SE ±1.58, n = 60), similar to coral reef communities of the southern Gulf of California. Although D. mexicanum is considered a great force of erosion to the substratum in reef environments in the eastern tropical Pacific, no evidence of erosion was observed at Isla San Jorge, indicating a balanced dynamic between herbivores, macroalgae, and corals.