No abstract

Sporopollenin is a complex biopolymer which is the main component of the pollen grain exine and is partly composed of the aromatic compounds para-coumaric acid (pCA) and ferulic acid (FA). These compounds absorb ultraviolet-B radiation (UV-B, 280-315 nm), and their abundance in pollen and spores has been shown to increase in response to increased UV-B flux. Here, we show that the relative abundance of UV-B-absorbing compounds (UACs) measured using Fourier Transform Infrared Spectrometry (FTIR) in modern pollen of autumn-pollinating Cedrus atlantica trees increases in response to summer UV-B flux. This relationship was observed in native Moroccan samples (r 2 = 0.84, p < 0.0001), but not across a larger environmental gradient including non-Moroccan samples (r 2 = 0.00, p = 0.99). For non-Moroccan samples of known provenance, the abundance of UACs is similar to the abundance of UACs found in samples from their place of origin. The FTIR spectra of these samples also closely resemble the FTIR spectra of samples from their place of origin. This unexpected finding suggests there could be a heritable component to UAC production possibly associated with epigenetic memory, an important adaptive mechanism in conifers. Our results indicate that the relative abundance of UACs in Cedrus atlantica pollen could be used as a proxy to reconstruct historic summer UV-B flux in Northwest Africa during at least the Holocene and Late Glacial period while also highlighting how UV-B proxies should be established using pollen samples from specimens growing in their native range or environment.

33 modern surface samples were collected in the environmentally and climatologically contrasting regions of the Middle and High Atlas Mountains, Morocco. Samples representing forested and steppe montane environments (1935 m to 2760 m a.s.l.) are clustered around study sites at Lake Tislit (High Atlas, semi-arid oro-Mediterranean bioclime), and Lake Sidi Ali and Michliffen (Middle Atlas, sub-humid montane Mediterranean bioclime). Good discrimination between regional pollen spectra is evident, with Middle Atlas samples reflecting higher arboreal cover (Cedrus and evergreen Quercus) and High Atlas samples with high abundances of non-arboreal taxa, including Artemisia and Fabaceae. These four taxa (Cedrus, evergreen Quercus, Artemisia and Fabaceae) are furthermore shown to be reliable indicators of local source vegetation within a 100 m 2 quadrat, taking into account threshold abundances of 7%, 20%, 4% and 10%, respectively. Deciduous Quercus, Olea and Phillyrea show long-distance pollen dispersal across both regions, contributing to non-trivial arboreal pollen (AP) values of up to 35% (typically 20-30%) in the High Atlas spectra. In the Middle Atlas, AP values of 40-50% occur in open sampling locations and >60% under forest canopy cover. These insights should be taken into account when interpreting ancient pollen spectra from regional lakes and bogs for palaeoenvironmental reconstruction.

Quantitative climate reconstructions from pollen typically rely on empirical relationships between pollen abundances or assemblages and climate, such as the modern analogue technique. However, these techniques may be problematic when applied to fossil sequences, as they cannot separate anthropogenic from climatic influence on pollen assemblages. Here, we reconstruct Mid‐ to Late Holocene summer aridity in the Middle Atlas, Morocco, using stable carbon isotope analysis of isolated fossil Cedrus pollen. This approach is based on well‐documented plant physiological responses to moisture stress and is therefore independent of vegetation composition. We find that there has been a general long‐term trend of increasing summer aridity in the region during the last 5000 years to the present day. The gradual decline of Cedrus atlantica forest in the Late Holocene follows this aridity trend. Additionally, we show how isolating a specific pollen type for carbon isotope analysis yields a robust climate signal, versus using pollen concentrates or bulk sediment. Our findings indicate that climate has become drier in the region and confirms the Mid‐ to Late Holocene aridification trend observed more widely in the western Mediterranean, using a novel proxy for this region with good potential for wider application in other environments.

The first experimental and theoretical study of the N⋅⋅⋅⋅N spin-pair radical is reported. Its high-resolution ESR (electron spin resonance) spectrum has been observed in neon matrices and interpreted on the basis of weakly interacting atoms using a model recently developed for the H⋅⋅⋅⋅H spin-pair. To fully interpret the N⋅⋅⋅⋅N radical results it was necessary to include electronic state mixing effects among all possible spin states, namely the 1Σg+, 3Σu+, 5Σg+, and 7Σu+ states. Several different trapping sites were observed which indicated the interaction of N atoms at distinctly different separation distances in the neon lattice. Calculated J values at the complete active space self-consistent field (CASSCF) level (TZP basis set) were compared with the experimental results for the various trapping site distances. The 15N⋅⋅⋅⋅15N radical in the dominant trapping site had magnetic parameters of g=2.0016(2), A(15N)=15.9(1) MHz, D=−178(1)MHz and J=468(2) MHz. Using the point dipole approximation this corresponds to a N⋅⋅⋅⋅N separation distance of 6.41 Å. A most unusual type of magnetic dipole transition was observed that involves a transition between electronic states of formally different S values where S is the total spin quantum number for a given electronic state.