Hypervelocity impacts into ice‐topped layered targets: Investigating the effects of ice crust thickness and subsurface density on crater morphology

Many bodies in the outer solar system are theorized to have an ice shell with a different subsurface material below, be it chondritic, regolith, or a subsurface ocean. This layering can have a significant influence on the morphology of impact craters. Accordingly, we have undertaken laboratory hypervelocity impact experiments on a range of multilayered targets, with interiors of water, sand, and basalt. Impact experiments were undertaken using impact speeds in the range of 0.8–5.3 km s^?1, a 1.5 mm Al ball bearing projectile, and an impact incidence of 45°. The surface ice crust had a thickness between 5 and 50 mm, i.e., some 3–30 times the projectile diameter. The thickness of the ice crust as well as the nature of the subsurface layer (liquid, well consolidated, etc.) have a marked effect on the morphology of the resulting impact crater, with thicker ice producing a larger crater diameter (at a given impact velocity), and the crater diameter scaling with impact speed to the power 0.72 for semi‐infinite ice, but with 0.37 for thin ice. The density of the subsurface material changes the structure of the crater, with flat crater floors if there is a dense, well‐consolidated subsurface layer (basalt) or steep, narrow craters if there is a less cohesive subsurface (sand). The associated faulting in the ice surface is also dependent on ice thickness and the substrate material. We find that the ice layer (in impacts at 5 km s^?1) is effectively semi‐infinite if its thickness is more than 15.5 times the projectile diameter. Below this, the crater diameter is reduced by 4% for each reduction in ice layer thickness equal to the impactor diameter. Crater depth is also affected. In the ice thickness region, 7–15.5 times the projectile diameter, the crater shape in the ice is modified even when the subsurface layer is not penetrated. For ice thicknesses, <7 times the projectile diameter, the ice layer is breached, but the nature of the resulting crater depends heavily on the subsurface material. If the subsurface is noncohesive (loose) material, a crater forms in it. If it is dense, well‐consolidated basalt, no crater forms in the exposed subsurface layer.     

The effects of trophic interactions and spatial competition on algal community composition on Hawaiian coral reefs

Much of coral reef ecology has focused on how human impacts change coral reefs to macroalgal reefs. However, macroalgae may not always be a good indicator of reef decline, especially on reefs with significant sea urchin populations, as found in Kenya and Hawaii. This study tests the effects of trophic interactions (i.e. herbivory by fishes and sea urchins) and spatial competition (between algae and coral) on algal community structure of reefs surrounding two Hawaiian Islands that vary in their level of human impacts. Reef‐building organisms (corals and crustose coralline algae) were less abundant and turf algae were more abundant on Maui as compared to Lanai, where human impacts are lower. In contrast to previous studies, we found no evidence that macroalgae increased with human impacts. Instead, low turf and macroalgal abundance were best explained by the interactive effects of coral cover and sea urchin abundance. Fishing sea urchin predators appeared to have cascading effects on the benthic community. The absence of sea urchin predators and high sea urchin densities correspond to a disproportionately high abundance of turf and crustose coralline algae. We propose that high turf algal abundance is a better indicator of reef decline in Hawaii than high macroalgal abundance because turf abundance was highest on reefs with low coral cover and few fish. The results of this study emphasize that understanding changes in community composition are context‐dependent and that not all degraded reefs look the same.     

Balancing deformation in NW Borneo: Quantifying plate-scale vs. gravitational tectonics in a delta and deepwater fold-thrust belt system

Recent GPS measurements demonstrate that NW Borneo undergoes 4–6 mm of plate-scale shortening a year, which is not accommodated by plate-scale structures. The only geological structure in NW Borneo described to accommodate on-going shortening is the Baram Delta System located on the outer shelf to basin floor. Delta toe fold-thrust belts are commonly thought to be caused by margin-normal compressional stresses generated by margin-parallel upslope gravitational extension.     

Active mud volcanoes on the upper slope of the western Nile deep-sea fan—first results from the P362/2 cruise of R/V Poseidon

In February 2008, cruise P362/2 was undertaken aboard R/V Poseidon to the Giza and North Alex mud volcanoes (MVs) on the upper slope of the western Nile deep-sea fan. Emitted fluids were strongly depleted in chloride and rich in hydrocarbons, predominantly of thermogenic origin. In-situ sediment temperature measurements indicate extremely high and moderate levels of activity for the North Alex MV and Giza MV, respectively, and suggest rapid changes from dormant to active stages. Both the physical properties of core sediments (e.g., color and magnetic susceptibility), and their assemblages of micro- and nannofossils point to different sources for the two mud volcanoes. Biostratigraphic dating suggests source depths of 2,100–2,450 mbsf for the Giza MV and 1,150–1,550 mbsf for the North Alex MV. Very high temperatures of up to 70°C in shallow sediments at the North Alex MV can be explained only if the fluid source were warmer and deeper than the sediment source.     

Guiding ecological principles for marine spatial planning

The declining health of marine ecosystems around the world is evidence that current piecemeal governance is inadequate to successfully support healthy coastal and ocean ecosystems and sustain human uses of the ocean. One proposed solution to this problem is ecosystem-based marine spatial planning (MSP), which is a process that informs the spatial distribution of activities in the ocean so that existing and emerging uses can be maintained, use conflicts reduced, and ecosystem health and services protected and sustained for future generations. Because a key goal of ecosystem-based MSP is to maintain the delivery of ecosystem services that humans want and need, it must be based on ecological principles that articulate the scientifically recognized attributes of healthy, functioning ecosystems. These principles should be incorporated into a decision-making framework with clearly defined targets for these ecological attributes. This paper identifies ecological principles for MSP based on a synthesis of previously suggested and/or operationalized principles, along with recommendations generated by a group of twenty ecologists and marine scientists with diverse backgrounds and perspectives on MSP. The proposed four main ecological principles to guide MSP—maintaining or restoring: native species diversity, habitat diversity and heterogeneity, key species, and connectivity—and two additional guidelines, the need to account for context and uncertainty, must be explicitly taken into account in the planning process. When applied in concert with social, economic, and governance principles, these ecological principles can inform the designation and siting of ocean uses and the management of activities in the ocean to maintain or restore healthy ecosystems, allow delivery of marine ecosystem services, and ensure sustainable economic and social benefits.     

Benthic habitat quality change as measured by macroinfauna community in a tidal flat on the west coast of Korea

Yellow Sea tidal flats are internationally recognised for their contribution to biological diversity and yet are under enormous pressure from reclamation, pollution and overexploitation. The benthic macroinfauna community is the dominant community on these tidal flats and a reliable indicator of benthic environmental changes. We surveyed the current benthic macroinfauna community of the Ganghwa Southern Tidal Flat, the largest remaining Korean mud flat in the Yellow Sea, in order to examine changes in the environmental situation of this benthic ecosystem. The results show a significant decline in species diversity from the last survey made in 2003, and a shift in species composition with appearances of polychaetes indicative of pollution and physical disturbances and other opportunistic species becoming dominant in both density and biomass. The benthic community shift observed during the two study periods may be associated with increased nutrient pollution as well as increased physical disturbances in this area. However, we recognise the limitations of the data both in frequency and scope but believe the significant changes to the composition of the benthic fauna are sufficient to warrant concern. Observations are required to examine the extent to which these human activities induce benthic community shift in this tidal flat.     

Dynamic propagation criteria for catastrophic failure in planar landslides

Quantitative assessment of the risk of submarine landslides is an essential part of the design process for offshore oil and gas developments in deep water, beyond the continental shelf. Landslides may be triggered by a reduction in shear strength of subsea sediments over a given zone, caused for example by seismic activity. Simple criteria are then needed to identify critical conditions whereby the zone of weakness could grow catastrophically to cause a landslide. A number of such criteria have been developed over the last decade, based either on ideas drawn from fracture mechanics, or considering the equilibrium of the initial weakened zone and adjacent process zones of gradually softening material. Accounting for the history of the weak zone initiation is critical for derivation of reliable propagation criteria, in particular considering dynamic effects arising from accumulating kinetic energy of the failing material, which will allow the failure to propagate from a smaller initial zone of weakened sediments. Criteria are developed here for planar conditions, taking full account of such dynamic effects, which are shown to be capable of reducing the critical length of the softened zone by 20% or more compared with criteria based on static conditions. A numerical approach is used to solve the governing dynamic equations for the sliding material, the results from which justify assumptions that allow analytical criteria to be developed for the case where the initial softening occurs instantaneously. The effect of more gradual softening is also explored. Copyright © 2016 John Wiley & Sons, Ltd.     

The multiple chronological techniques applied to the Lake Suigetsu SG06 sediment core,central Japan

The varved sediment of Lake Suigetsu (central Japan) provides a valuable opportunity to obtain high‐resolution, multi‐proxy palaeoenvironmental data across the last glacial/interglacial cycle. In order to maximize the potential of this archive, a well‐constrained chronology is required. This paper outlines the multiple geochronological techniques being applied – namely varve counting, radiocarbon dating, tephrochronology (including argon–argon dating) and optically stimulated luminescence (OSL) – and the approaches by which these techniques are being integrated to form a single, coherent, robust chronology. Importantly, we also describe here the linkage of the floating Lake Suigetsu (SG06) varve chronology and the absolute (IntCal09 tree‐ring) time scale, as derived using radiocarbon data from the uppermost (non‐varved) portion of the core. This tie‐point, defined as a distinct (flood) marker horizon in SG06 (event layer B‐07–08 at 1397.4 cm composite depth), is thus derived to be 11 255 to 11 222 IntCal09 cal. years BP (68.2% probability range).     

Hydrologic simulation of clay‐settling areas in the phosphate mining district,Florida

Clay‐settling areas (CSAs) are one of the most conspicuous and development‐limiting landforms remaining after phosphate mining. Many questions are asked by the mining and regulatory communities with regard to the correct modelling (predictive) methods and assumptions that should be used to yield viable hydrologic post‐reclamation landforms within CSAs. Questions as to the correct methodology to use in modelling/predicting long‐term CSA hydrologic performance have historically been difficult to answer because the data and analysis to support popular hypotheses did not exist. The goal of this paper was to substantially improve the data, analysis and predictive methodology necessary to return CSAs to viable hydrologic units, and moreover, to develop better understanding of the hydrology of CSAs and their ability to support wetlands. The study site is located at the Fort Meade Mine in Polk County, Florida. In this paper, continuous model simulation and calibration of study site were conducted for the hydrologic model, Hydrological Simulation Program – FORTRAN, which was generally selected on the basis of its popularity in predicting the hydrologic behaviour of CSAs. The objective of this study was to simulate streamflow discharges and stage to estimate runoff response from these areas on the basis of the observed rainfall within the CSA. A set of global hydrologic parameters was selected and tested during the calibration by the parameter estimation software PEST. A comparison of the simulated and observed flow data indicates that the model calibration adequately reproduces the hydrologic response of the CSAs. The estimated parameters can be used as references for future application of the model. Copyright © 2012 John Wiley & Sons, Ltd.