Controls upon the Last Glacial Maximum deglaciation of the northern Uummannaq Ice Stream System, West Greenland

The Uummannaq Ice Stream System (UISS) was a convergent cross-shelf ice stream system that operated in West Greenland during the Last Glacial Maximum (LGM). This paper presents new evidence constraining the geometry and evolution of the northern sector of the UISS and considers the factors controlling its dynamic behaviour. Geomorphological mapping, 21 new terrestrial cosmogenic nuclide (TCN) exposure ages, and radiocarbon dating constrain LGM warm-based ice stream activity in the north of the system up to 1400 m a.s.l. Intervening plateaux areas either remained ice free, or were covered by cold-based icefields. Beyond the inner fjords, topography and bathymetry forced ice flow southwards into the Uummannaq Trough, where it coalesced with ice from the south, and formed the trunk zone of the UISS. Deglaciation of the UISS began at 14.9 cal. ka BP. Rapid retreat from the LGM limit was forced by an increase in air temperatures and rising sea level, enhanced by the bathymetric over-deepening of the Uummannaq and Igdlorssuit Sund troughs. Ice reached the inner fjord confines in the northern Uummannaq area by 11.6 ka and experienced an ice marginal stabilisation in Rink–Karrat Fjord for up to 5 ka. This was a function of topographic constriction and bathymetric shallowing, and occurred despite continued climatic forcing. In the neighbouring Ingia Fjord this did not occur. Following this period of stability, ice within Rink–Karrat Fjord retreated, reaching the present ice margin or beyond after 5 ka. The presence of a major ice stream within a mid-fjord setting, during the mid-Holocene and the Holocene Thermal Maximum (∼11–5 ka) is in direct contrast to records of other ice streams throughout West Greenland, which suggest ice had retreated beyond its present margin by 9–7 ka. This demonstrates the potential importance of topographic control on calving margin stability, and its ability to override climatic forcing

Peatland hydrology and carbon release: why small-scale process matters

Peatlands cover over 400 million hectares of the Earth's surface and store between one-third and one-half of the world's soil carbon pool. The long-term ability of peatlands to absorb carbon dioxide from the atmosphere means that they play a major role in moderating global climate. Peatlands can also either attenuate or accentuate flooding. Changing climate or management can alter peatland hydrological processes and pathways for water movement across and below the peat surface. It is the movement of water in peats that drives carbon storage and flux. These small-scale processes can have global impacts through exacerbated terrestrial carbon release. This paper will describe advances in understanding environmental processes operating in peatlands. Recent (and future) advances in high-resolution topographic data collection and hydrological modelling provide an insight into the spatial impacts of land management and climate change in peatlands. Nevertheless, there are still some major challenges for future research. These include the problem that impacts of disturbance in peat can be irreversible, at least on human time-scales. This has implications for the perceived success and understanding of peatland restoration strategies. In some circumstances, peatland restoration may lead to exacerbated carbon loss. This will also be important if we decide to start to create peatlands in order to counter the threat from enhanced atmospheric carbon

The geomorphological record of an ice stream to ice shelf transition in Northeast Greenland

ACKNOWLEDGEMENTS This work was funded through NERC Standard Grant NE/N011228/1. We thank the Alfred Wegner Institute, and particularly Angelika Humbert and Hicham Rafiq, for their logistic support through the iGRIFF project. Further support was provided from Station Nord (Jorgen Skafte), Nordland Air, Air Greenland, and the Joint Arctic Command. Naalakkersuisut, Government of Greenland, provided Scientific Survey (VU-00121) and Export (046/2017) licences for this work. We thank Chris Orton for help with production of figures. Finally, we would like to thank our Field Ranger Isak (after which Isakdalen is informally named) and dog Ooni for keeping us safe in the field. We thank Rob Storrar and an anonymous reviewer for their comments which helped improve the manuscript.Peer reviewedPublisher PD

Neutral top-pion and lepton flavor violating processes

In the context of topcolor-assisted techicolor(TC2) models, we study the contributions of the neutral top-pion $\pi^{0}_{t}$ to the lepton flavor violating(LFV) processes $l_{i}\to l_{j}\gamma$ and $l_{i}\to l_{j}l_{k}l_{l}$. We find that the present experimental bound on $\mu\to e\gamma$ gives severe constraints on the free parameters of $TC2$ models. Taking into account these constraints, we consider the processes $l_{i}\to l_{j}l_{k}l_{l}$ generated by top-pion exchange at the tree-level and the one loop level, and obtain $Br(\mu\to 3e)\simeq 2.87\times 10^{-14}$, $1.1\times 10^{-15}\leq Br(\tau\to 3e)\simeq Br(\tau\to 2e\mu)\leq 4.4 \times 10^{-15}$, $3.1\times 10^{-15} \leq Br(\tau\to 2\mu e)\simeq Br(\tau\to 3\mu)\leq 1.5 \times 10^{-14}$ in most of the parameter space.Comment: latex files,16 pages, 6 figures. Submitted to Phys. Rev.

Rare Charm Decays in the Standard Model and Beyond

We perform a comprehensive study of a number of rare charm decays, incorporating the first evaluation of the QCD corrections to the short distance contributions, as well as examining the long range effects. For processes mediated by the $c\to u\ell^+\ell^-$ transitions, we show that sensitivity to short distance physics exists in kinematic regions away from the vector meson resonances that dominate the total rate. In particular, we find that $D\to\pi\ell^+\ell^-$ and $D\to\rho\ell^+\ell^-$ are sensitive to non-universal soft-breaking effects in the Minimal Supersymmetric Standard Model with R-parity conservation. We separately study the sensitivity of these modes to R-parity violating effects and derive new bounds on R-parity violating couplings. We also obtain predictions for these decays within extensions of the Standard Model, including extensions of the Higgs, gauge and fermion sectors, as well as models of dynamical electroweak symmetry breaking.Comment: 45 pages, typos fixed, discussions adde

Holocene history of the 79°N ice shelf reconstructed from epishelf lake and uplifted glaciomarine sediments

Nioghalvfjerdsbrae, or 79∘ N Glacier, is the largest marine-terminating glacier draining the Northeast Greenland Ice Stream (NEGIS). In recent years, its ∼ 70 km long fringing ice shelf (hereafter referred to as the 79∘ N ice shelf) has thinned, and a number of small calving events highlight its sensitivity to climate warming. With the continued retreat of the 79∘ N ice shelf and the potential for accelerated discharge from NEGIS, which drains 16 % of the Greenland Ice Sheet (GrIS), it has become increasingly important to understand the long-term history of the ice shelf in order to put the recent changes into perspective and to judge their long-term significance. Here, we reconstruct the Holocene dynamics of the 79∘ N ice shelf by combining radiocarbon dating of marine molluscs from isostatically uplifted glaciomarine sediments with a multi-proxy investigation of two sediment cores recovered from Blåsø, a large epishelf lake 2–13 km from the current grounding line of 79∘ N Glacier. Our reconstructions suggest that the ice shelf retreated between 8.5 and 4.4 ka cal BP, which is consistent with previous work charting grounding line and ice shelf retreat to the coast as well as open marine conditions in Nioghalvfjerdsbrae. Ice shelf retreat followed a period of enhanced atmospheric and ocean warming in the Early Holocene. Based on our detailed sedimentological, microfaunal, and biomarker evidence, the ice shelf reformed at Blåsø after 4.4 ka cal BP, reaching a thickness similar to present by 4.0 ka cal BP. Reformation of the ice shelf coincides with decreasing atmospheric temperatures, the increased dominance of Polar Water, a reduction in Atlantic Water, and (near-)perennial sea-ice cover on the adjacent continental shelf. Along with available climate archives, our data indicate that the 79∘ N ice shelf is susceptible to collapse at mean atmospheric and ocean temperatures ∼ 2 ∘C warmer than present, which could be achieved by the middle of this century under some emission scenarios. Finally, the presence of “marine” markers in the uppermost part of the Blåsø sediment cores could record modern ice shelf thinning, although the significance and precise timing of these changes requires further work

D* Production in Deep Inelastic Scattering at HERA

This paper presents measurements of D^{*\pm} production in deep inelastic scattering from collisions between 27.5 GeV positrons and 820 GeV protons. The data have been taken with the ZEUS detector at HERA. The decay channel $D^{*+}\to (D^0 \to K^- \pi^+) \pi^+$ (+ c.c.) has been used in the study. The $e^+p$ cross section for inclusive D^{*\pm} production with $5<Q^2<100 GeV^2$ and $y<0.7$ is 5.3 \pms 1.0 \pms 0.8 nb in the kinematic region {$1.3<p_T(D^{*\pm})<9.0$ GeV and $| \eta(D^{*\pm}) |<1.5$}. Differential cross sections as functions of p_T(D^{*\pm}), $\eta(D^{*\pm}), W$ and $Q^2$ are compared with next-to-leading order QCD calculations based on the photon-gluon fusion production mechanism. After an extrapolation of the cross section to the full kinematic region in p_T(D^{*\pm}) and $\eta$(D^{*\pm}), the charm contribution $F_2^{c\bar{c}}(x,Q^2)$ to the proton structure function is determined for Bjorken $x$ between 2 $\cdot$ 10$^{-4}$ and 5 $\cdot$ 10$^{-3}$.Comment: 17 pages including 4 figure

Observation of Scaling Violations in Scaled Momentum Distributions at HERA

Charged particle production has been measured in deep inelastic scattering (DIS) events over a large range of $x$ and $Q^2$ using the ZEUS detector. The evolution of the scaled momentum, $x_p$, with $Q^2,$ in the range 10 to 1280 $GeV^2$, has been investigated in the current fragmentation region of the Breit frame. The results show clear evidence, in a single experiment, for scaling violations in scaled momenta as a function of $Q^2$.Comment: 21 pages including 4 figures, to be published in Physics Letters B. Two references adde

Observation of hard scattering in photoproduction events with a large rapidity gap at HERA

Events with a large rapidity gap and total transverse energy greater than 5 GeV have been observed in quasi-real photoproduction at HERA with the ZEUS detector. The distribution of these events as a function of the $\gamma p$ centre of mass energy is consistent with diffractive scattering. For total transverse energies above 12 GeV, the hadronic final states show predominantly a two-jet structure with each jet having a transverse energy greater than 4 GeV. For the two-jet events, little energy flow is found outside the jets. This observation is consistent with the hard scattering of a quasi-real photon with a colourless object in the proton.Comment: 19 pages, latex, 4 figures appended as uuencoded fil