Inter- and intra-specimen variability masks reliable temperature control on shell Mg/Ca ratios in laboratory and field cultured Mytilus edulis and Pecten maximus (bivalvia).

yesThe Mg/Ca ratios of biogenic calcite is commonly seen as a valuable palaeo-proxy for reconstructing past ocean temperatures. The temperature dependence of Mg/Ca ratios in bivalve calcite has been the subject of contradictory observations. The palaeoceanographic use of a geochemical proxy is dependent on initial, rigorous calibration and validation of relationships between the proxy and the ambient environmental variable to be reconstructed. Shell Mg/Ca ratio data are reported for the calcite of two bivalve species, Mytilus edulis (common mussel) and Pecten maximus (king scallop), which were grown in laboratory culturing experiments at controlled and constant aquarium seawater temperatures over a range from 10 to 20 C. Furthermore, Mg/Ca ratio data of laboratory- and fieldgrown M. edulis specimens were compared. Only a weak, albeit significant, shell Mg/Ca ratio¿temperature relationship was observed in the two bivalve species: M. edulis (r2=0.37, p<0.001 for laboratory-cultured specimens and r2=0.50, p<0.001 for field-cultured specimens) and P. maximus (r2=0.21, p<0.001 for laboratory-cultured specimens only). In the two species, shell Mg/Ca ratios were not found to be controlled by shell growth rate or salinity. The Mg/Ca ratios in the shells exhibited a large degree of variability among and within species and individuals. The results suggest that the use of bivalve calcite Mg/Ca ratios as a temperature proxy is limited, at least in the species studied to date. Such limitations are most likely due to the presence of physiological effects on Mg incorporation in bivalve calcite. The utilization is further limited by the great variability both within and among shells of the same species that were precipitated under the same ambient condition

Statistical Mechanics of Broadcast Channels Using Low Density Parity Check Codes

We investigate the use of Gallager's low-density parity-check (LDPC) codes in a broadcast channel, one of the fundamental models in network information theory. Combining linear codes is a standard technique in practical network communication schemes and is known to provide better performance than simple timesharing methods when algebraic codes are used. The statistical physics based analysis shows that the practical performance of the suggested method, achieved by employing the belief propagation algorithm, is superior to that of LDPC based timesharing codes while the best performance, when received transmissions are optimally decoded, is bounded by the timesharing limit.Comment: 14 pages, 4 figure

Statistical Mechanics of Broadcast Channels Using Low Density Parity Check Codes

We investigate the use of Gallager's low-density parity-check (LDPC) codes in a broadcast channel, one of the fundamental models in network information theory. Combining linear codes is a standard technique in practical network communication schemes and is known to provide better performance than simple timesharing methods when algebraic codes are used. The statistical physics based analysis shows that the practical performance of the suggested method, achieved by employing the belief propagation algorithm, is superior to that of LDPC based timesharing codes while the best performance, when received transmissions are optimally decoded, is bounded by the timesharing limit.Comment: 14 pages, 4 figure

Transiting Exoplanets with JWST

The era of exoplanet characterization is upon us. For a subset of exoplanets -- the transiting planets -- physical properties can be measured, including mass, radius, and atmosphere characteristics. Indeed, measuring the atmospheres of a further subset of transiting planets, the hot Jupiters, is now routine with the Spitzer Space Telescope. The James Webb Space Telescope (JWST) will continue Spitzer's legacy with its large mirror size and precise thermal stability. JWST is poised for the significant achievement of identifying habitable planets around bright M through G stars--rocky planets lacking extensive gas envelopes, with water vapor and signs of chemical disequilibrium in their atmospheres. Favorable transiting planet systems, are, however, anticipated to be rare and their atmosphere observations will require tens to hundreds of hours of JWST time per planet. We review what is known about the physical characteristics of transiting planets, summarize lessons learned from Spitzer high-contrast exoplanet measurements, and give several examples of potential JWST observations.Comment: 22 pages, 11 figures. In press in "Astrophysics in the Next Decade: JWST and Concurrent Facilities, Astrophysics & Space Science Library, Thronson, H. A., Tielens, A., Stiavelli, M., eds., Springer: Dordrecht (2008)." The original publication will be available at http://www.springerlink.co

Expansion in perfect groups

Let Ga be a subgroup of GL_d(Q) generated by a finite symmetric set S. For an integer q, denote by Ga_q the subgroup of Ga consisting of the elements that project to the unit element mod q. We prove that the Cayley graphs of Ga/Ga_q with respect to the generating set S form a family of expanders when q ranges over square-free integers with large prime divisors if and only if the connected component of the Zariski-closure of Ga is perfect.Comment: 62 pages, no figures, revision based on referee's comments: new ideas are explained in more details in the introduction, typos corrected, results and proofs unchange

Using Geometry to Evaluate Strategic Road Proposals in Orbital-Radial Cities

This paper uses geometry to evaluate major road proposals in cities with road networks consisting of orbital and radial routes. The type of geometry used is a development of the Karlsruhe or Moscow metric after the cities where it was identified, although the results have wider applicability. The paper begins with a detailed consideration of the relationship between route speeds, junction access and service areas. New urban patterns are presented using optimal space filling techniques in which the aim is to maximise drive-time coverage with the minimum number of junctions. The method is then refined to allow for effects such as congestion and interstitial access. The results are then used in a case study to evaluate a well-known strategic road plan for London first proposed in the 1940s. There follows a general discussion about the policy and planning implications for London and further possible developments of the techniques presented