Anticipated Improvements to River Surface Elevation Profiles From the Surface Water and Ocean Topography Mission

Existing publicly available digital elevation models (DEMs) provide global-scale data but are often not precise enough for studying processes that depend on small-scale topographic features in rivers. For example, slope breaks and knickpoints in rivers can be important in understanding tectonic processes, and riffle-pool structures are important drivers of riverine ecology. More precise data (e.g., lidar) are available in some areas, but their spatial extent limits large-scale research. The upcoming Surface Water and Ocean Topography (SWOT) satellite mission is planned to launch in 2021 and will provide measurements of elevation and inundation extent of surface waters between 78° north and south latitude on average twice every 21 days. We present a novel noise reduction method for multitemporal river water surface elevation (WSE) profiles from SWOT that combines a truncated singular value decomposition and a slope-constrained least-squares estimator. We use simulated SWOT data of 85–145 km sections of the Po, Sacramento, and Tanana Rivers to show that 3–12 months of simulated SWOT data can produce elevation profiles with mean absolute errors (MAEs) of 5.38–12.55 cm at 100–200 m along-stream resolution. MAEs can be reduced further to 4–11 cm by averaging all observations. The average profiles have errors much lower than existing DEMs, allowing new advances in riverine research globally. We consider two case studies in geomorphology and ecology that highlight the scientific value of the more accurate in-river DEMs expected from SWOT. Simulated SWOT elevation profiles for the Po reveal convexities in the river longitudinal profile that are spatially coincident with the upward projection of blind thrust faults that are buried beneath the Po Plain at the northern termination of the Apennine Mountains. Meanwhile, simulated SWOT data for the Sacramento River reveals locally steep sections of the river profile that represent important habitat for benthic invertebrates at a spatial scale previously unrecognizable in large-scale DEMs presently available for this river

Paleo-denudation rates suggest variations in runoff drove aggradation during last glacial cycle, Crete, Greece

Fluvial aggradation and incision are often linked to Quaternary climate cycles, but it usually remains unclear whether variations in runoff or sediment supply or both drive channel response to climate variability. Here we quantify sediment supply with paleo-denudation rates and provide geochronological constraints on aggradation and incision from the Sfakia and Elafonisi alluvial-fan sequences in Crete, Greece. We report seven optically stimulated luminescence and ten radiocarbon ages, eight 10Be and eight 36Cl denudation rates from modern channel and terrace sediments. For five samples, 10Be and 36Cl were measured on the same sample by measuring 10Be on chert and 36Cl on calcite. Results indicate relatively steady denudation rates throughout the past 80 kyr, but the aggradation and incision history indicates a link with climate shifts. At the Elafonisi fan, we identify four periods of aggradation coinciding with Marine Isotope Stages (MIS) 2, 4, 5a/b, and likely 6, and three periods of incision coinciding with MIS 1, 3, and likely 5e. At the Sfakia fan, rapid aggradation occurred during MIS 2 and 4, followed by incision during MIS 1. Nearby climate and vegetation records show that MIS 2, 4, and 6 stadials were characterized by cold and dry climates with sparse vegetation, whereas forest cover and more humid conditions prevailed during MIS 1, 3, and 5. Our data thus suggest that past changes in climate had little effect on landscape-wide denudation rates but exerted a strong control on the aggradation–incision behaviour of alluvial channels on Crete. During glacial stages, we attribute aggradation to hillslope sediment release promoted by reduced vegetation cover and decreased runoff; conversely, incision occurred during relatively warm and wet stages due to increased runoff. In this landscape, past hydroclimate variations outcompeted changes in sediment supply as the primary driver of alluvial deposition and incision

The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics

Morphometric comparisons between automated and manual karst depression inventories in Apalachicola National Forest, Florida, and Mammoth Cave National Park, Kentucky, USA

Karst depression catalogs are critical to assessing the hydrology and geohazards of an area; yet, the delineation of these features within a landscape can be a difficult, time-consuming and subjective task. This study evaluates the efficacy of karst depression inventorying using an automated fill-difference method operating on high-resolution lidar-derived digital elevation models (DEMs). The resulting catalog is compared with existing karst depression inventories for two low-development areas of the USA, Mammoth Cave National Park (MACA) and Apalachicola National Forest (ANF), where karst depressions have been mapped previously using a manual closed-contour approach. The automated fill method captures 93 and 85 % of these previously mapped karst depressions at MACA and ANF, respectively. Field observations and topographic analysis suggest that the omitted features were likely misclassified within the existing catalogs. The automated routine returns 797 and 3377 additional topographic depressions, at MACA and ANF, respectively, which are not included in the existing catalogs. While the geology of ANF is mostly homogenous Quaternary deposits, the newly identified, typically smaller-scale depressions found within MACA tend to be disproportionally located in non-carbonate-dominated formations, where the development of karst may be restricted by geologic heterogeneity. Within both areas, the size distributions of the two inventories are statistically identical for features larger than ~103 m2 in area or ~3 m in depth. For individual depressions captured by both methods at MACA, the automated fill-difference routine tends to return a slightly larger estimate of depression size and aggregate small depressions into larger ones. Conversely, at ANF, some low-relief depressions may be disaggregated by the fill-difference technique, with a trend toward smaller estimated depression areas when the automated method is employed. The automated fill-difference method, operating on high-resolution lidar-derived DEMs, can reproduc

Stochastic alluvial fan and terrace formation triggered by a high-magnitude Holocene landslide in the Klados Gorge, Crete

Alluvial fan and terrace formation is traditionally interpreted as a fluvial system response to Quaternary climate oscillations under the backdrop of slow and steady tectonic activity. However, several recent studies challenge this conventional wisdom, showing that such landforms can evolve rapidly as a geomorphic system responds to catastrophic and stochastic events, like large-magnitude mass wasting. Here, we contribute to this topic through a detailed field, geochronological, and numerical modelling investigation of thick (>50 m) alluvial sequences in the Klados catchment in southwestern Crete, Greece. The Klados River catchment lies in a Mediterranean climate, is largely floored by carbonate bedrock, and is characterised by well-preserved alluvial terraces and inset fans at the river mouth that exceed the volumes of alluvial deposits in neighbouring catchments of similar size. Previous studies interpreted the genesis and evolution of these deposits to result from a combination of Pleistocene sea-level variation and the region's long-term tectonic activity. We show that the >20 m thick lower fan unit, previously thought to be late Pleistocene in age, unconformably buries a paleoshoreline uplifted in the first centuries CE, placing the depositional age of this unit firmly in the late Holocene. The depositional timing is supported by seven new radiocarbon dates that indicate middle to late Holocene ages for the entire fan and terrace sequence. Furthermore, we report new evidence of a previously unidentified valley-filling landslide deposit that is locally 100 m above the modern stream elevation, and based on cross-cutting relationships, it predates the alluvial sequence. Observations indicate the highly erodible landslide deposit as the source of the alluvial fill sediment. We identify the likely landslide detachment area as a large rockfall scar at the steepened head of the catchment. A landslide volume of 9.08 x 10(7) m(3) is estimated based on volume reconstructions of the mapped landslide deposit and the inferred scar location. We utilise landslide runout modelling to validate the hypothesis that a high-magnitude rockfall would pulverise and send material downstream, filling the valley up to similar to 100 m. This partial liquefaction is required for the rockfall to form a landslide body of the extent observed in the valley and is consistent with the sedimentological characteristics of the landslide deposit. Based on the new age control and the identification of the landslide deposit, we hypothesise that the rapid post-landslide aggradation and incision cycles of the alluvial deposits are not linked to long-term tectonic uplift or climate variations but rather stochastic events such as mobilisation of sediment in large earthquakes, storm events, or ephemeral blockage in the valley's narrow reaches. The Klados case study represents a model environment for how stochastically driven events can mimic climate-induced sedimentary archives and lead to deposition of thick alluvial sequences within hundreds to thousands of years, and it illustrates the ultrasensitivity of mountainous catchments to external perturbations after catastrophic events.ISSN:2196-632XISSN:2196-631

Pleistocene terrace formation, Quaternary rock uplift rates and geodynamics of the Hellenic Subduction Zone revealed from dating of paleoshorelines on Crete, Greece

Quaternary paleoshorelines are common landforms on the island of Crete, a forearc high above the Hellenic Subduction Zone. These geomorphic markers are useful on Crete and elsewhere in determining coastal uplift rates, the identification of active geologic structures, and to constrain geodynamic models and seismic hazards. Controversy exists in the literature regarding the formation mechanisms and age of late Pleistocene paleoshorelines on Crete that has led to competing models of the uplift history, tectonic evolution, and seismic hazards of the Hellenic forearc. We present new mapping and results from luminescence and radiocarbon geochronology of paleoshoreline deposits that constrain the spatial and temporal pattern of rock uplift around the Cretan coastline. Existing and new radiocarbon data are variable and show no obvious age-elevation trends within individual terrace sequences. By contrast, nearly all luminescence ages, some from shorelines dated with radiocarbon, show positive age-elevation trends and range from 60–220 ka suggesting that all dated paleoshorelines are beyond the limits of radiocarbon. We propose that the inconsistencies between the different geochronological methods are the result of secondary contamination of young carbonate, possibly from meteoric waters, that bias radiocarbon in Cretan Pleistocene marine fossils. Most luminescence ages closely correlate with the timing of mid-to-late Pleistocene relative sea level highstands, consistent with stratigraphic observations. Calculated coastal uplift rates using a Monte-Carlo error analysis range from ∼0–1.2 mm/yr; the lowest uplift rates are found along the northern and eastern coasts of the island, while the most rapid are focused along the southern and western coasts where active normal faults are observed offsetting paleoshoreline sequences. Based on this new data, we favor a tectonic model where slip along upper crustal normal faults acts to locally augment a steady regional signal of uplift along the south and west coast, interpreted to result from the deep underplating of rock at the base of the subduction wedge beneath Crete. Arcward of the contact between the upper plate Moho and the top of the subducting slab, crustal thinning will occur in the orogenic wedge resulting in subsidence along the north coast of Crete