UV-B absorbing pigments in spores: biochemical responses to shade in a high-latitude birch forest and implications for sporopollenin-based proxies of past environmental change

Current attempts to develop a proxy for Earth’s surface ultraviolet-B (UV-B) flux focus on the organic chemistry of pollen and spores because their constituent biopolymer, sporopollenin, contains UV-B absorbing pigments whose relative abundance may respond to the ambient UV-B flux. Fourier transform infrared (FTIR) microspectroscopy provides a useful tool for rapidly determining the pigment content of spores. In this paper, we use FTIR to detect a chemical response of spore wall UV-B absorbing pigments that correspond with levels of shade beneath the canopy of a high-latitude Swedish birch forest. A 27% reduction in UV-B flux beneath the canopy leads to a significant (p<0.05) 7.3% reduction in concentration of UV-B absorbing compounds in sporopollenin. The field data from this natural flux gradient in UV-B further support our earlier work on sporopollenin-based proxies derived from sedimentary records and herbaria collections

Some unusual features in the behaviour of cyclohexane in confined geometry studied by neutron scattering

Cyclohexane, C6H12, is one of the simplest organic molecules due to its sixfold symmetry and weak van der Waals interaction. The dynamic characteristics have been extensively studied in the liquid and plastic crystal phases by spectroscopic means and have been analysed in terms of an unhindered rotator model. The transition from the liquid to the plastic crystal at 6.55oC involves a low entropy change and leads to a fcc crystal with a large defect concentration. At lower temperatures, -87oC, the rotational motion is frozen out to give a brittle crystal phase. One of the more interesting aspects of cyclohexane is revealed when the material is confined in a meso-porous solid. Neutron and NMR studies of cyclohexane in a porous sol-gel silica [1] showed that the phase transition temperatures were substantially depressed. Further neutron diffraction studies [2] indicated that the plastic crystal phase exhibited structural changes over a wide temperature range and that there was a hysteresis effect in the temperature sequence. The large depression of the nucleation point was shown to be inversely proportional to pore size over most of the range and subsequently used for pore size characterisation by NMR cryoporometry [3]. Comparison with other organic liquids [4] has shown that cyclohexane exhibits one of the largest effects and this behaviour has lead to a new interest in the properties. Another unexpected phenomenon has been observed when cyclohexane liquid is added to the top of a granular powder of porous sol-gel silica in a sealed container. After a few minutes, an orange coloration develops in transmitted light (blue in reflected), which persists for up to a day. This behaviour indicates that there are spatial density variations on a scale comparable with that of light. Some test experiments using small-angle neutron scattering [5] showed that these features were changing on a time-scale of minutes; the effect could be observed in C6D12 but not in C6H12, confirming that the effect was due to the liquid behaviour in the pores. As a consequence of these recent studies, it is now interesting to make a more detailed study of the phase behaviour of cyclohexane in confined geometry and the following sections outline some of the research which is currently in progress

Ozone measurements at British Antarctic Survey stations

Ozone measurements have been made regularly at Argentine Islands and Halley Bay since 1957. A recent critical review of the data revealed inconsistencies in the extraterrestrial constants adopted for the various spectrophotometers used. These have been resolved and the data recalculated to give consistent series. The variations in total ozone on differing time-scales, and their relation to variations in the temperature of the lower stratosphere, are discussed. A major increase in total ozone occurs in the course of the breakdown of the winter polar stratospheric vortex. It is often preceded by large quasi-periodic fluctuations associated with baroclinic waves in the vortex. The response of the vortex to these waves is discussed and comparisons made with conventional data from lower latitude stations and with satellite data on Southern Hemisphere sudden warmings. Long-term trends in total ozone are shown to be small at both stations. A recent global analysis of ozone trends is noted

Recent measurements of total ozone at British Antarctic Survey stations

The deep minimum in total ozone, the ‘ozone hole’, which is now observed each year in early spring in Antarctica, appears to be confined to the cold core of the polar vortex. The breakdown of the vortex in 1986 followed an unusual course, and produced an atypical variation of total ozone at Halley Bay. Upper-air data show that the cold core in this year was subjected to large displacement and rapid deformations. Despite these dynamic perturbations, the ozone amount within the core seems to have been scarcely affected. This is consistent with the observed behaviour of other tracers at high latitudes. The observations are most readily explained by supposing that the core of the polar vortex is maintained as a material entity or isolated air mass, even when it suffers erosion of material from its edge (as happens throughout the period leading up to the final warming). It has recently been suggested, solely on dynamical grounds, that the process of erosion is essentially one sided; the eroded material is readily mixed into middle latitudes, but the main vortex is remarkably impervious to even small-scale incursions of the surrounding air. Formerly, the timing of the final warming was very variable from year to year. Since 1979, no early final warming has occurred. The effect on monthly mean temperatures has been described by some workers as a cooling of the lower stratosphere. W hat should be said is that temperature in the core of the vortex rises more slowly now than formerly. This is not unexpected, because the heating rate in the lower stratosphere is strongly dependent on ozone amount. It is suggested that this slower warming can, by its influence on the diabatic mass circulation, affect ozone amounts outside the vortex. The limited evidence available suggests that no significant depletion of ozone occurs in darkness. However, by the time that the Sun is high enough to permit frequent measurements of ozone the loss rate is high and remains so until the equinox. It then diminishes rapidly, and the minimum value of ozone is attained by mid-October, well before the final warming of the lower stratosphere. This suggests a greater degree of solar control than is evinced by other aspects of vortex behaviour. It is inferred that the depletion of ozone arises largely from chemical sinks, and some reactions likely to be important are discussed. Attention is drawn to the photochemically labile reservoirs HOCl and NO3. Competitive reactions between their photoproducts determine critically the persistence, or otherwise, of an HOx, catalytic cycle in early spring

Measurements of total ozone using Dobson spectrometers: Some comments on their history

The Dobson ozone spectrometer has been used for studies of the ozone layer for about 60 years. It remains indispensable for assessing the accuracy of satellite-borne instruments. The steps which led to the present understanding of global ozone distribution and variations are reviewed