The glacial geomorphology of the Loch Lomond (Younger Dryas) Stadial in Britain: a review

This paper systematically reviews the glacial geomorphological evidence of the Loch Lomond Stadial (LLS; Younger Dryas) glaciation in Britain (12.9–11.7 ka). The geomorphology of sub‐regions within Scotland, England and Wales is assessed, providing the most comprehensive synthesis of this evidence to date. The contrasting nature of the evidence at the local scale is reviewed and conceptual themes common to multiple sub‐regions are examined. Advancements in glaciological theory, mapping technologies, numerical modelling and dating have been applied unevenly to localities across Britain, inhibiting a holistic understanding of the extent and dynamics of the LLS glaciation at a regional scale. The quantity and quality of evidence is highly uneven, leading to uncertainties regarding the extent of glaciation and inhibiting detailed analysis of ice dynamics and chronology. Robust dates are relatively scarce, making it difficult to confidently identify the limits of LLS glaciers and assess their synchroneity. Numerical models have allowed the glacier–climate relationships of the LLS to be assessed but have, thus far, been unable to incorporate local conditions which influenced glaciation. Recommendations for future research are made that will allow refined reconstructions of the LLS in Britain and contribute to a more comprehensive understanding of glacier–climate interactions during the Younger Dryas.     

The complexity of sediment recycling as revealed by common Pb isotopes in K-feldspar

Detrital zircon U-Pb geochronology has become the gold standard in evaluating source to sink relationships in sedimentary basins. However, the physical and chemical robustness of zircon, which make it such a useful mineral for provenance studies, is also a hindrance as zircon can be recycled through numerous sedimentary basins, thus obscuring the first cycle source to sink relationship. An elegant approach to addressing this potential issue is to compare the Pb isotope composition of detrital K-feldspar, a mineral which is unlikely to survive more than one erosion-transport-deposition cycle, with that of magmatic K-feldspar from potential basement source terranes. Here we present new in situ Pb isotope data on detrital K-feldspar from two Proterozoic arkosic sandstones from Western Australia, and magmatic K-feldspar grains from potential igneous source rocks, as inferred by the age and Hf isotope composition of detrital zircon grains. The data indicate that the detrital zircon and K-feldspar grains could not have been liberated from the same source rocks, and that the zircon has most likely been recycled through older sedimentary basins. These results provide a more complete understanding of apparently simple source to sink relationships in this part of Proterozoic Western Australia.     

Retreat dynamics of the eastern sector of the British–Irish Ice Sheet during the last glaciation

The findings of BRITICE-CHRONO Transect 2 through the North Sea Basin and eastern England are reported. We define ice-sheet marginal oscillation between ~31 and 16 ka, with seven distinctive former ice-sheet limits (L1–7) constrained by Bayesian statistical analysis. The southernmost limit of the North Sea Lobe is recorded by the Bolders Bank Formation (L1; 25.8–24.6 ka). L2 represents ice-sheet oscillation and early retreat to the northern edge of the Dogger Bank (23.5–22.2 ka), with the Garret Hill Moraine in north Norfolk recording a significant regional readvance to L3 at 21.5–20.8 ka. Ice-marginal oscillations at ~26–21 ka resulted in L1, L2 and L3 being partially to totally overprinted. Ice-dammed lakes related to L1–3, including Lake Humber, are dated at 24.1–22.3 ka. Ice-sheet oscillation and retreat from L4 to L5 occurred between 19.7 and 17.3 ka, with grounding zone wedges marking an important transition from terrestrial to marine tidewater conditions, triggered by the opening of the Dogger Lake spillway between 19.9 and 17.5 ka. L6 relates to ice retreat under glacimarine conditions and final ice retreat into the Firth of Forth by 15.8 ka. L7 (~15 ka) represents an ice retreat from Bosies Bank into the Moray Firth.     

Kinetic biomineralization through microfluidic chip tests

A homogeneous microfluidic chip was used to investigate the pore-scale characteristics during the process of microbially induced calcium carbonate precipitation (MICP). An image-processing scheme was developed to measure the projecting areas of the precipitated calcium carbonate. Calcium carbonate first precipitated on the bacterium side before spreading to the rest of the chip. The distribution of calcium carbonate was more uniform along the length of the microchip than along the width. Raman back-scattering spectroscopy was used to examine the chemical composition of the precipitate, identifying calcite and vaterite as the main mineral phases. Bacterium traces were noted on crystal surfaces in SEM images, suggesting a higher adsorptive capacity for irregular precipitates than well-shaped crystals.

     

A numerical model for one‐dimensional simulation of stratospheric chemistry

Abstract

We describe a one‐dimensional (1‐D) numerical model developed to simulate the chemistry of minor constituents in the stratosphere. The model incorporates most of the chemical species presently found in the upper atmosphere and has been used to investigate the effect of increasing chlorofluorocarbon (CFC) emissions on ozone (O₃).

Our calculations confirm previous results that O₃ depletions in the 20–25 km region, the region of the O₃ maximum, are very sensitive to the relative abundances of Cl_x and NO_y in the lower stratosphere for high Cl_x amounts. The individual abundances of lower stratospheric Cl_x and NO_y amounts are very sensitive to upper tropospheric mixing ratios, which, in turn, are determined largely by surface input fluxes and heterogeneous loss processes. Thus the behaviour of column O₃ depletions at high Cl_x levels is greatly affected, albeit indirectly, by tropospheric processes. For high Cl_x levels the O_x flux from the stratosphere to the troposphere is dramatically reduced, leading to a large reduction in tropospheric O₃. Some of the variation between different published 1‐D model results is most likely due to this critical dependence of O₃ depletion on NO_y‐Cl_x ratios.

Model simulations of time‐dependent CFC effects on ozone indicate that if CFCs were to remain at constant 1980 emission rates while N₂O increased at 0.25% a^?1 and CH₄ increased at 1% a^?1, we could expect a 2.2% decrease in total column O₃ (relative to the 1980 atmosphere) by the year 2000. However, if CFC emission rates were to increase by 3% a^?1 (current estimates are 5–6% a^?1), we would predict a depletion of 2.7% by the year 2000. The calculations for times beyond the year 2000 suggest that the effects on total O₃ will begin to accelerate. If methyl chloroform emissions are added at 7% a^?1 (current estimates are 7–9% a^?1) to the above CFC‐N₂O‐CH₄ scenario we calculate total O₃ depletions by the year 2000 that are 41% larger than those calculated without. This suggests that if the emissions of methyl chloroform continue to increase at their present rate then methyl chloroform could have a significant effect upon total O₃.     

Impact of tropical instability waves on nutrient and chlorophyll distributions in the equatorial Pacific

This work quantifies the role of tropical instability waves (TIWs) in modulating nutrient and chlorophyll distributions in the equatorial Pacific through an analysis of satellite data and a case study of in situ observations. A TIW index is constructed to differentiate periods of strong and weak TIW activity. TIW impacts are first examined in monthly averaged satellite SST and chlorophyll data for three distinct regions north of the equator where TIWs are most active. The chlorophyll data are high-pass filtered to preserve the seasonal cycle and remove long-term trends. Although SST follows a predictable relationship with the TIW index, chlorophyll concentrations do not. Periods of high TIW activity are characterized by cooler SSTs but consistently low chlorophyll. A case study of individual TIW vortices demonstrates that their impact on nutrients and chlorophyll is a function of intensity. Strong TIWs drive reductions in nutrients and chlorophyll due to the subduction of nutrient-replete water north of the equator and the advection of nutrient-poor water toward the equator from adjacent to the upwelling zone. Weak TIWs do not drive these advective processes to the same degree, so retain elevated nutrients that fuel chlorophyll increases. The most positive effect on nutrients and chlorophyll by TIWs was observed during boreal winter, likely owing to thermocline topography. A shallower thermocline in combination with weak TIWs results in elevated nutrients and chlorophyll north of the equator. Given the variability associated with TIW intensity and season, generalizing TIW effects has proven difficult, but targeted Lagrangian studies will better characterize these dynamic features and their impact on elemental fluxes.     

Theoretical Simulation of Solar Ultraviolet Fluxes Measured on the 8 July 1974 Stratoprobe I flights

Solar ultraviolet fluxes in the wavelength region 1900 to 2300Å were measured during the first balloon flight of Project stratoprobe on July 8, 1974, over a range of solar zenith angles at a float altitude of 28.6 km. High‐resolution computer simulations of the measured spectra were made for a range of solar zenith angles of 40° to 80° over the wavelength region 2050 to 2150A, and comparisons made with the observed spectra at 40° and 47 °. Qualitative agreements were obtained but there were significant discrepancies between the simulated and experimentally obtained fluxes. Sources of the discrepancies are discussed.     

Carbon stars

In this paper, the present state of knowledge of the carbon stars is discussed. Particular attention is given to issues of classification, evolution, variability, populations in our own and other galaxies, and circumstellar material.