ALBERT

Description: We derive a generalized theory for gravitationally self-consistent, static sea level variations on earth models of arbitrary complexity that takes into account the redistribution of sediments. The theory is an extension of previous work that incorporated, into the governing equations, shoreline migration due to local sea level variations and changes in the geometry of grounded, marine-based ice. In addition, we use viscoelastic Love number theory to present a version of the new theory valid for spherically symmetric earth models. The Love number theory accounts for the gravitational, deformational and rotational effects of the sediment redistribution. As a first, illustrative application of the new theory, we compute the perturbation in sea level driven by an idealized pulse of sediment transport into the Gulf of Mexico. We demonstrate that incorporating a gravitationally self-consistent water load in this case significantly improves the accuracy of sea level predictions relative to previous simplified treatments of the sediment redistribution.

Description: 〈span〉Hillslopes constitute the majority of Earth’s land surface area and dominate the supply of sediment to rivers. Hillslope sediment transport is commonly modeled with a rate law that depends on slope and a rate coefficient, 〈span〉D〈/span〉, that is understood to represent the intensity of transport mechanisms. Although many transport mechanisms are related to water and biota, it is unclear whether 〈span〉D〈/span〉 varies predictably with climate and life. We compiled previous estimates of 〈span〉D〈/span〉 from around the world and also made new estimates for additional sites. The compilation reveals an overall trend in which 〈span〉D〈/span〉 increases strongly with increasing moisture among relatively dry sites and less strongly with increasing moisture among relatively wet sites. Vegetation type has a secondary effect on 〈span〉D〈/span〉 among drier sites, with 〈span〉D〈/span〉 increasing from deserts to grasslands to forests, but not among wetter sites. These trends suggest that the establishment of life in a landscape substantially accelerates soil creep, whereas differences in biological communities among sites with abundant moisture have a relatively small effect on creep.〈/span〉

Description: Fluvial features on Titan have been identified in synthetic aperture radar (SAR) data taken during spacecraft flybys by the Cassini Titan Radar Mapper (RADAR) and in Descent Imager/Spectral Radiometer (DISR) images taken during descent of the Huygens probe to the surface. Interpretations using terrestrial analogs and process mechanics extend our perspective on fluvial geomorphology to another world and offer insight into their formative processes. At the landscape scale, the varied morphologies of Titan’s fluvial networks imply a variety of mechanical controls, including structural influence, on channelized flows. At the reach scale, the various morphologies of individual fluvial features, implying a broad range of fluvial processes, suggest that (paleo-)flows did not occupy the entire observed width of the features. DISR images provide a spatially limited view of uplands dissected by valley networks, also likely formed by overland flows, which are not visible in lower-resolution SAR data. This high-resolution snapshot suggests that some fluvial features observed in SAR data may be river valleys rather than channels, and that uplands elsewhere on Titan may also have fine-scale fluvial dissection that is not resolved in SAR data. Radar-bright terrain with crenulated bright and dark bands is hypothesized here to be a signature of fine-scale fluvial dissection. Fluvial deposition is inferred to occur in braided channels, in (paleo)lake basins, and on SAR-dark plains, and DISR images at the surface indicate the presence of fluvial sediment. Flow sufficient to move sediment is inferred from observations and modeling of atmospheric processes, which support the inference from surface morphology of precipitation-fed fluvial processes. With material properties appropriate for Titan, terrestrial hydraulic equations are applicable to flow on Titan for fully turbulent flow and rough boundaries. For low-Reynolds-number flow over smooth boundaries, however, knowledge of fluid kinematic viscosity is necessary. Sediment movement and bed form development should occur at lower bed shear stress on Titan than on Earth. Scaling bedrock erosion, however, is hampered by uncertainties regarding Titan material properties. Overall, observations of Titan point to a world pervasively influenced by fluvial processes, for which appropriate terrestrial analogs and formulations may provide insight.

Description: The most extreme climate transitions in Earth history are recorded by the juxtaposition of Neoproterozoic glacial deposits with overlying cap carbonate beds. Some of the most remarkable sedimentary structures within these beds are sharp-crested (trochoidal) bedforms with regular spacing of as much as several meters that are often interpreted as giant wave ripples formed under extreme wave conditions in a nonuniform postglacial climate. Here we evaluate this hypothesis using a new bedform stability diagram for symmetric oscillatory flows that indicates that the first-order control on the formation of trochoidal rather than hummocky bedforms is sediment size, not wave climate. New measurements of bedform wavelengths and particle sizes from the ca. 635 Ma Nuccaleena Formation, Australia, indicate that the giant ripples are generally composed of coarse to very coarse sand; most are within the trochoidal bedform stability phase space for normal wave climates. Moreover, numerical simulations of flow over fixed bedforms show that symmetric trochoidal ripples with a nearly vertical angle of climb may be produced over long time periods with variable wave climates in conjunction with rapid seabed cementation. These data reveal that, rather than extreme wave conditions, the giant wave ripples are a consequence of the unusual mode of carbonate precipitation during a global carbon cycle perturbation unprecedented in Earth history.

Description: 〈span〉〈div〉Abstract〈/div〉Hillslopes constitute the majority of Earth’s land surface area and dominate the supply of sediment to rivers. Hillslope sediment transport is commonly modeled with a rate law that depends on slope and a rate coefficient, 〈span〉D〈/span〉, that is understood to represent the intensity of transport mechanisms. Although many transport mechanisms are related to water and biota, it is unclear whether 〈span〉D〈/span〉 varies predictably with climate and life. We compiled previous estimates of 〈span〉D〈/span〉 from around the world and also made new estimates for additional sites. The compilation reveals an overall trend in which 〈span〉D〈/span〉 increases strongly with increasing moisture among relatively dry sites and less strongly with increasing moisture among relatively wet sites. Vegetation type has a secondary effect on 〈span〉D〈/span〉 among drier sites, with 〈span〉D〈/span〉 increasing from deserts to grasslands to forests, but not among wetter sites. These trends suggest that the establishment of life in a landscape substantially accelerates soil creep, whereas differences in biological communities among sites with abundant moisture have a relatively small effect on creep.〈/span〉

Description: Waves erode sea cliffs by various mechanisms, but the influence of wave power on bedrock coastal erosion has not been well quantified, making it difficult to predict how rocky coasts evolve in different environments. Volcanic ocean islands offer a unique opportunity to examine the influence of waves on bedrock coastal erosion because many islands have relatively homogeneous bedrock, well-constrained initial topography, and considerable differences in wave power between shorelines that face different directions and wave regimes. We used lava-flow ages and the morphology of coastal profiles on Maui, Kaho‘olawe, and the Big Island of Hawai‘i (USA) to estimate sea-cliff retreat rates at 11 sites that experience nearly eightfold differences in incident wave power. Using a range of possible sea-level histories that incorporate different trends of subsidence due to volcanic loading, we modeled the evolution of each coastal profile since its formation (12 ka to 1.4 Ma) to find the regionally consistent relative sea-level history and the site-specific sea-cliff retreat rates that best reproduce observed coastal profiles. We found a best-fit relative sea-level history prescribed by an effective elastic lithosphere thickness of 30 km, consistent with estimates from observations of total deflection beneath the Hawaiian Ridge. This suggests that coastal profiles may retain a decipherable record of sea-level change. Comparing the best-fit sea-cliff retreat rates to mean annual wave power at each site, which we calculated from 30 yr hindcast wave data, we found a positive relationship between wave power and sea-cliff erosion, consistent with theoretical predictions and measurements on unlithified coastal bluffs. These comparisons provide field evidence that bedrock coastal erosion scales with wave power, offering a basis for modeling rocky coast evolution in different wave climates.