Synchronous retreat of Thwaites and Pine Island glaciers in response to external forcings in the presatellite era.

Today, relatively warm Circumpolar Deep Water is melting Thwaites Glacier at the base of its ice shelf and at the grounding zone, contributing to significant ice retreat. Accelerating ice loss has been observed since the 1970s; however, it is unclear when this phase of significant melting initiated. We analyzed the marine sedimentary record to reconstruct Thwaites Glacier's history from the early Holocene to present. Marine geophysical surveys were carried out along the floating ice-shelf margin to identify core locations from various geomorphic settings. We use sedimentological data and physical properties to define sedimentary facies at seven core sites. Glaciomarine sediment deposits reveal that the grounded ice in the Amundsen Sea Embayment had already retreated to within ~45 km of the modern grounding zone prior to ca. 9,400 y ago. Sediments deposited within the past 100+ y record abrupt changes in environmental conditions. On seafloor highs, these shifts document ice-shelf thinning initiating at least as early as the 1940s. Sediments recovered from deep basins reflect a transition from ice proximal to slightly more distal conditions, suggesting ongoing grounding-zone retreat since the 1950s. The timing of ice-shelf unpinning from the seafloor for Thwaites Glacier coincides with similar records from neighboring Pine Island Glacier. Our work provides robust new evidence that glacier retreat in the Amundsen Sea was initiated in the mid-twentieth century, likely associated with climate variability.

An X-ray micro-fluorescence study to investigate the distribution of Al, Si, P and Ca ions in the surrounding soft tissue after implantation of a calcium phosphate-mullite ceramic composite in a rabbit animal model.

Synthetic calcium phosphates, despite their bioactivity, are brittle. Calcium phosphate- mullite composites have been suggested as potential dental and bone replacement materials which exhibit increased toughness. Aluminium, present in mullite, has however been linked to bone demineralisation and neurotoxicity: it is therefore important to characterise the materials fully in order to understand their in vivo behaviour. The present work reports the compositional mapping of the interfacial region of a calcium phosphate--20 wt% mullite biocomposite/soft tissue interface, obtained from the samples implanted into the long bones of healthy rabbits according to standard protocols (ISO-10993) for up to 12 weeks. X-ray micro-fluorescence was used to map simultaneously the distribution of Al, P, Si and Ca across the ceramic-soft tissue interface. A well defined and sharp interface region was present between the ceramic and the surrounding soft tissue for each time period examined. The concentration of Al in the surrounding tissue was found to fall by two orders of magnitude, to the background level, within ~35 µm of the implanted ceramic.

Structural studies of bioactivity in sol-gel-derived glasses by X-ray spectroscopy.

Extended X-ray absorption fine structure spectroscopy and X-ray absorption near edge structure, X-ray fluorescence spectroscopy, and X-ray powder diffraction have been used to study the local calcium environment in four sol-gel-derived bioactive calcium silicate glasses of the general formula (CaO)(x)(SiO(2))(1-x). The formation of a hydroxyapatite layer on the composition with the highest bioactivity (x = 0.3) with time has been studied, in an in vitro environment, by immersion in simulated body fluid (SBF) at 37 degrees C. The calcium oxygen environment in the four compositions has been shown to be six-coordinate in character. Both the extended X-ray absorption fine structure spectroscopy and X-ray absorption near edge structure show a gradual increase in coordination number and Ca--O bond distance with longer exposure to SBF. X-ray fluorescence show that calcium is quickly lost from the samples on exposure to SBF and the calcium concentration then recovers with time. There is clear evidence that the recovery of calcium content is due to the formation of a CaO-P(2)O(5)-rich layer. Annealing of samples at 650 degrees C shows the presence of what, on the length scales probed by X-ray diffraction, appears to be noncrystalline calcium phosphate after 1 h of exposure to an SBF solution, which becomes more crystalline on longer exposure.