Identifying hazards associated with lava deltas

Lava deltas, formed where lava enters the ocean and builds a shelf of new land extending from the coastline, represent a significant local hazard, especially on populated ocean island volcanoes. Such structures are unstable and prone to collapse—events that are often accompanied by small explosions that can deposit boulders and cobbles hundreds of meters inland. Explosions that coincide with collapses of the East Lae ‘Apuki lava delta at Kīlauea Volcano, Hawai‘i, during 2005–2007 followed an evolutionary progression mirroring that of the delta itself. A collapse that occurred when the lava–ocean entry was active was associated with a blast of lithic blocks and dispersal of spatter and fine, glassy tephra. Shortly after delta growth ceased, a collapse exposed hot rock to cold ocean water, resulting in an explosion composed entirely of lithic blocks and lapilli. Further collapse of the delta after several months of inactivity, by which time it had cooled significantly, resulted in no recognizable explosion deposit. Seaward displacement and subsidence of the coastline immediately inland of the delta was measured by both satellite and ground-based sensors and occurred at rates of several centimeters per month even after the lava–ocean entry had ceased. The anomalous deformation ended only after complete collapse of the delta. Monitoring of ground deformation may therefore provide an indication of the potential for delta collapse, while the hazard associated with collapse can be inferred from the level of activity, or the time since the last activity, on the delta.     

Detection Of Soft X-Ray Emission From Comet C/1995 O1 (Hale–Bopp)

We report the detection of soft X-rays from comet C/1995 O1 (Hale-Bopp) by the Low Energy Concentrator Spectrometer (LECS) on-board the X-ray satellite, BeppoSAX. The observations took place on 1996 September 10–11 approximately 1 day after a large dust outburst (Schulz et al., 1997–1999). After correcting for the comets motion, a 7σ enhancement was found centered (2.1 ± 1.3) x 10⁵ km from the position of the nucleus, in the general solar direction. The total X-ray luminosity in the 0.1–2.0 keV energy band is 5 x 10¹⁶ erg s⁻¹ which is at least a factor of ∼ 3 greater than measured by the Extreme Ultraviolet Explorer (EUVE)4 days later and suggests that the bulk of the emission measured by the LECS is related to the dust outburst. The extracted LECS spectrum is well fit by a thermal bremsstrahlung-like distribution of temperature of 0.29 ± 0.06 keV - consistent with that observed in other comets. We find no evidence for fluorescent carbon or oxygen emission and place 95% confidence limits of 1.0 x 10¹⁵ and 7.8 x 10¹⁵ erg s⁻¹ to narrow line emission at 0.28 and 0.53 keV, respectively. We calculate that if such lines are present, they constitute at most 18% of the 0.1–2.0 keV continuum luminosity. This revised version was published online in July 2006 with corrections to the Cover Date.     

The dispersion of magnetite bedload tracer across a gravel point-bar and the development of heavy-mineral placers

Economic concentrations of heavy-minerals are often associated with fluvial point-bars but prospecting models identifying the heaviest concentrations are poorly developed. Consequently, the dispersal and storage of a heavy-mineral bedload tracer–magnetite–across a rapidly evolving point-bar was studied using magnetic susceptibility as a surrogate measure of magnetite concentration. The bar-head was a preferential area for the development of a placer owing to a lag accumulation of magnetite over an armoured bed surface. In contrast, when viewed in plan, the bar-platform and bar-tail were regions of tracer dilution owing to downstream dispersion and mixing with shale in the vertical as the bar-top rapidly aggraded. However, in section, false-bottom placers developed along bedding planes were evident. The latter consisted of thin layers of concentrated magnetite resulting from the passage of bedload sheets, consisting of a mix of shale and magnetite, moving repeatedly from the bar-head to accrete over the bar-tail. Differential density-sorting of magnetite and shale occurred during transport and deposition, such that the heavier magnetite tended to accumulate as a visible concentrated bed-layer, later to be over-run by layers of shale-sediment in which magnetite was present diffusely. However, the placer thickness was greater than that visible because finer fractions of magnetite from each concentrated layer infiltrated the interstices of the top of the shale bed below. The placer thickness, the infiltration potential of the sediment bed and the actual rate of infiltration of the tracer were determined by fitting a mathematical function to measured variation in magnetic susceptibility with depth in the sediment body. Finally, a simple mathematical model, described in the literature as reproducing the plan-view of flow and topographic patterns in river bends, was found to reproduce patterns of depth, velocity, shear stress and competence in the point-bar environment which were in accordance with the interpretation of the field data. It was concluded that such a model when linked to entrainment functions for sediments of mixed density might be suitable for prospecting for economic heavy minerals in the point-bar environment.     

Skill assessment of three earth system models with common marine biogeochemistry

We have assessed the ability of a common ocean biogeochemical model, PISCES, to match relevant modern data fields across a range of ocean circulation fields from three distinct Earth system models: IPSL-CM4-LOOP, IPSL-CM5A-LR and CNRM-CM5.1. The first of these Earth system models has contributed to the IPCC 4th assessment report, while the latter two are contributing to the ongoing IPCC 5th assessment report. These models differ with respect to their atmospheric component, ocean subgrid-scale physics and resolution. The simulated vertical distribution of biogeochemical tracers suffer from biases in ocean circulation and a poor representation of the sinking fluxes of matter. Nevertheless, differences between upper and deep ocean model skills significantly point to changes in the underlying model representations of ocean circulation. IPSL-CM5A-LR and CNRM-CM5.1 poorly represent deep-ocean circulation compared to IPSL-CM4-LOOP degrading the vertical distribution of biogeochemical tracers. However, their representations of surface wind, wind stress, mixed-layer depth and geostrophic circulations (e.g., Antarctic Circumpolar Current) have been improved compared to IPSL-CM4-LOOP. These improvements result in a better representation of large-scale structure of biogeochemical fields in the upper ocean. In particular, a deepening of 20–40 m of the summer mixed-layer depth allows to capture the 0–0.5 μgChl L^?1 concentrations class of surface chlorophyll in the Southern Ocean. Further improvements in the representation of the ocean mixed-layer and deep-ocean ventilation are needed for the next generations of models development to better simulate marine biogeochemistry. In order to better constrain ocean dynamics, we suggest that biogeochemical or passive tracer modules should be used routinely for both model development and model intercomparisons.