Advances in Air–Sea \hbox {CO}_2 Flux Measurement by Eddy Correlation

Eddy-correlation measurements of the oceanic \(\hbox {CO}_2\) flux are useful for the development and validation of air–sea gas exchange models and for analysis of the marine carbon cycle. Results from more than a decade of published work and from two recent field programs illustrate the principal interferences from water vapour and motion, demonstrating experimental approaches for improving measurement precision and accuracy. Water vapour cross-sensitivity is the greatest source of error for \(\hbox {CO}_2\) flux measurements using infrared gas analyzers, often leading to a ten-fold bias in the measured \(\hbox {CO}_2\) flux. Much of this error is not related to optical contamination, as previously supposed. While various correction schemes have been demonstrated, the use of an air dryer and closed-path analyzer is the most effective way to eliminate this interference. This approach also obviates density corrections described by Webb et al. (Q J R Meteorol 106:85–100, 1980). Signal lag and frequency response are a concern with closed-path systems, but periodic gas pulses at the inlet tip provide for precise determination of lag time and frequency attenuation. Flux attenuation corrections are shown to be \(<\) 5 % for a cavity ring-down analyzer (CRDS) and dryer with a 60-m inlet line. The estimated flux detection limit for the CRDS analyzer and dryer is a factor of ten better than for IRGAs sampling moist air. While ship-motion interference is apparent with all analyzers tested in this study, decorrelation or regression methods are effective in removing most of this bias from IRGA measurements and may also be applicable to the CRDS.     

The vesiculated layer in the anomalous diogenite Northwest Africa 12973

Diogenites, which are part of the howardite–eucrite–diogenite (HED) group, are considered to represent rocks from the lower crust and mantle of a differentiated planetary body, likely the asteroid 4 Vesta. The Northwest Africa 12973 (NWA 12973) meteorite was classified as an anomalous diogenite due to the occurrence of a vesiculated layer. This work reports on the petrographic and geochemical study of two fragments of this meteorite, aiming to better constrain the origin of the vesiculated layer. Whereas the interior of NWA 12973 (here called host) presents the typical characteristics of an olivine diogenite, that is, coarse-grained pyroxene, olivine ribbons, chromite, and accessory phases, the vesiculated layer presents a fine-grained pyroxene groundmass with local rounded relics of olivine and interstitial chromite and metal, and is characterized by abundant large vesicles. The contact between the vesiculated layer and the host is sharply defined. The composition of individual minerals does not show any significant differences between the host and the vesiculated layer, suggesting in situ melting. Geothermobarometry indicates a slightly higher crystallization temperature at lower pressure for the vesiculated layer, consistent with melting and crystallization under lower crustal conditions upon exhumation. The trigger for the local melting was likely a large impact event, which was responsible for adiabatic decompression in the central area or deep faulting and frictional melting.     

A hitherto unrecognised band in the Raman spectra of silica rocks: influence of hydroxylated Si–O bonds (silanole) on the Raman moganite band in chalcedony and flint (SiO<Subscript>2</Subscript>)

Chalcedony is a spatial arrangement of hydroxylated nanometre-sized α-quartz (SiO₂) crystallites that are often found in association with the silica mineral moganite (SiO₂). A supplementary Raman band at 501 cm⁻¹ in the chalcedony spectrum, attributed to moganite, has been used for the evaluation of the quartz/moganite ratio in silica rocks. Its frequency lies at 503 cm⁻¹ in sedimentary chalcedony, representing a 2 cm⁻¹ difference with its position in pure moganite. We present a study of the 503 cm⁻¹ band’s behaviour upon heat treatment, showing its gradual disappearance upon heating to temperatures above 300 °C. Infrared spectroscopic measurements of the silanole (SiOH) content in the samples as a function of annealing temperature show a good correlation between the disappearance of the 503 cm⁻¹ Raman band and the decrease of structural hydroxyl. Thermogravimetric analyses reveal a significant weight loss that can be correlated with the decreasing of this Raman band. X-ray powder diffraction data suggest the moganite content in the samples to remain stable. We propose therefore the existence of a hitherto unknown Raman band at 503 cm⁻¹ in chalcedony, assigned to ‘free’ Si–O vibrations of non-bridging Si–OH that oscillate with a higher natural frequency than bridging Si–O–Si (at 464 cm⁻¹). A similar phenomenon was recently observed in the infrared spectra of chalcedony. The position of this Si–OH-related band is nearly the same as the Raman moganite band and the two bands may interfere. The actually observed Raman band in silica rocks might therefore be a convolution of a silanole and a moganite vibration. These findings have broad implications for future Raman spectroscopic studies of moganite, for the assessment of the quartz/moganite ratio, using this band, must take into account the contribution from silanole that are present in chalcedony and moganite.     

A history of tourism at the Mer de Glace: Adaptations of glacier tourism to glacier fluctuations since 1741

Climate change induces profound changes in mountain territories that affect the activities pursued there. Tourism is one activity that is strongly impacted by these changes. Most research on tourism in mountain regions has focused on winter sports tourism, but glacier tourism, a summer activity, is also severely affected by climate change-induced phenomena, including glacier retreat and other geomorphological processes. Given that Alpine glacier tourism has been pursued since the 18 th century and glaciers have been changing throughout this period, this article aims to examine the adaptations implemented by glacier tourism operators since the activity's beginnings. Through analysis of historical documents, glaciological surveys and semi-structured interviews, this paper reveals how tourism activities around the Mer de Glace(the largest French glacier) have been impacted by glacier fluctuations since the first visit to the area in 1741. The study reveals that these developments have mainly been driven by glacier retreat and associated paraglacial dynamics. More recently, the shortening of the period of snow cover on the glacier has also impacted tourism activities. Most of the adaptation strategies implemented by tourism operators during the period in question are found to be reactive, consisting in the installation of safety equipment, the renovation of access points or the building of new structures to allow tourist activities to continue despite the consequences of glacier retreat. Other strategies, such as the adoption of new activities by workers previously specialised in another, are identified that could be considered transformative strategies, although these were only adopted by workers when their activity was no longer feasible. The analysis of materials from 1741 to the present also reveals that the recent impacts of climate change on glacier tourism are much more significant and diverse than those produced by past fluctuations. Therefore, it is important that stakeholders take suitable measures to ensure the sustainable future of the Mer de Glace and other glacier tourism destinations.