Enhanced Surface Melting of the Fennoscandian Ice Sheet during Stadials

Unexpected melting of Northern Hemisphere ice sheets during periods of regional cooling characterizes the climate of the last glacial period. While the Heinrich Events are the most well-studied example of this phenomenon, Samuel Toucanne and colleagues recently documented evidence of Fennoscandian Ice Sheet melting during Heinrich Stadials, the cold periods during which Heinrich Events occur. In this dissertation, I use the geographic provenance of sediments in the Bay of Biscay, a proxy for Fennoscandian Ice Sheet melting, along with other paleoclimate records to: (1) demonstrate the persistence of abrupt Fennoscandian Ice Sheet melting as a feature of the Pleistocene climate system, (2) develop a self-consistent explanation for the synchronous melting of ice sheets in the North Atlantic region, and (3) elucidate the timing of abrupt climate change in the Northern Hemisphere. I begin by introducing a framework for inferring the subglacial transport distance of fine sediments from coupled provenance and grain size analyses. This chapter untangles the relationships between the source, size, transport history, and geochemical signature of glacigenic sediments in northern Europe, clarifying the geographical significance of sediment provenance in the Bay of Biscay. I then develop a new method for the spectral analysis of unevenly sampled time series. In the following chapter, I apply the new spectral method to time series of Fennoscandian Ice Sheet melting, Laurentide Ice Sheet melting, and solar activity changes during the last glacial period. Doing so reveals a coherence between ice sheet melting and solar activity and helps explain the quasi-periodic melting of ice sheets on millennial timescales. I then extend the neodymium isotope provenance record of Fennoscandian Ice Sheet melting through Marine Isotope Stage 6, demonstrating that enhanced summertime melting of the FIS during Heinrich Stadials is a recurring feature of glacial periods. In the final chapter, I document a relationship between the occurrence of abrupt ice sheet melting in the Northern Hemisphere and the precession of Earth’s spin axis to reveal an astronomical forcing of millennial-scale climate change

Enhanced surface melting of the Fennoscandian Ice Sheet during periods of North Atlantic cooling

Heinrich events (HEs) are dramatic episodes of marine-terminating ice discharge and sediment rafting during periods of cold North Atlantic climate. However, the causal chain of events leading to their occurrence is unresolved. Here, we demonstrate that enhanced surface melting of land-terminating margins of the southern Fennoscandian Ice Sheet (FIS) is a recurring feature of Heinrich stadials (HSs), the cold periods during which HEs occur. We use neodymium isotopes to show that the Channel River transported detrital sediments from the interior of eastern Europe to the Bay of Biscay in the northeast Atlantic Ocean at ca. 158–154 ka. Based on similar evidence from the last glacial period, we infer that this interval corresponds to the melting and retreat of the southern FIS margin despite contemporaneous cooling in the North Atlantic and central Europe. The FIS melting episode occurred just prior to a HE, consistent with findings from the more recent HSs 1, 2, and 3. Based on this evidence, we clarify a sequence of events that precedes HEs. Precursor melting of North Atlantic–adjacent ice sheets induces an initial Atlantic meridional overturning circulation (AMOC) slowdown. Atmospheric changes during the resulting HS cause summertime warming in northern Europe that drives enhanced FIS melting. Subsequent meltwater discharge to the North Atlantic further weakens the AMOC and warms the intermediate water masses that contribute to HEs

A flexible mathematical model platform for studying branching networks : experimentally validated using the model actinomycete, Streptomyces coelicolor

Branching networks are ubiquitous in nature and their growth often responds to environmental cues dynamically. Using the antibiotic-producing soil bacterium Streptomyces as a model we have developed a flexible mathematical model platform for the study of branched biological networks. Streptomyces form large aggregates in liquid culture that can impair industrial antibiotic fermentations. Understanding the features of these could aid improvement of such processes. The model requires relatively few experimental values for parameterisation, yet delivers realistic simulations of Streptomyces pellet and is able to predict features, such as the density of hyphae, the number of growing tips and the location of antibiotic production within a pellet in response to pellet size and external nutrient supply. The model is scalable and will find utility in a range of branched biological networks such as angiogenesis, plant root growth and fungal hyphal networks

Lynx Mission Concept Status

Lynx is a concept under study for prioritization in the 2020 Astrophysics Decadal Survey. Providing orders of magnitude increase in sensitivity over Chandra, Lynx will examine the first black holes and their galaxies, map the large-scale structure and galactic halos, and shed new light on the environments of young stars and their planetary systems. In order to meet the Lynx science goals, the telescope consists of a high-angular resolution optical assembly complemented by an instrument suite that may include a High Definition X-ray Imager, X-ray Microcalorimeter and an X-ray Grating Spectrometer. The telescope is integrated onto the spacecraft to form a comprehensive observatory concept. Progress on the formulation of the Lynx telescope and observatory configuration is reported in this paper

Using observational data to emulate a randomized trial of dynamic treatment switching strategies

BACKGROUND: When a clinical treatment fails or shows suboptimal results, the question of when to switch to another treatment arises. Treatment switching strategies are often dynamic because the time of switching depends on the evolution of an individual's time-varying covariates. Dynamic strategies can be directly compared in randomized trials. For example, HIV-infected individuals receiving antiretroviral therapy could be randomized to switching therapy within 90 days of HIV-1 RNA crossing above a threshold of either 400 copies/ml (tight-control strategy) or 1000 copies/ml (loose-control strategy).METHODS: We review an approach to emulate a randomized trial of dynamic switching strategies using observational data from the Antiretroviral Therapy Cohort Collaboration, the Centers for AIDS Research Network of Integrated Clinical Systems and the HIV-CAUSAL Collaboration. We estimated the comparative effect of tight-control vs. loose-control strategies on death and AIDS or death via inverse-probability weighting.RESULTS: Of 43 803 individuals who initiated an eligible antiretroviral therapy regimen in 2002 or later, 2001 met the baseline inclusion criteria for the mortality analysis and 1641 for the AIDS or death analysis. There were 21 deaths and 33 AIDS or death events in the tight-control group, and 28 deaths and 41 AIDS or death events in the loose-control group. Compared with tight control, the adjusted hazard ratios (95% confidence interval) for loose control were 1.10 (0.73, 1.66) for death, and 1.04 (0.86, 1.27) for AIDS or death.CONCLUSIONS: Although our effective sample sizes were small and our estimates imprecise, the described methodological approach can serve as an example for future analyses

In situ multiple sulfur isotope analysis by SIMS of pyrite, chalcopyrite, pyrrhotite, and pentlandite to refine magmatic ore genetic models

With growing interest in the application of in situ multiple sulfur isotope analysis to a variety of mineral systems, we report here the development of a suite of sulfur isotope standards for distribution relevant to magmatic, magmatic-hydrothermal, and hydrothermal ore systems. These materials include Sierra pyrite (FeS2), Nifty-b chalcopyrite (CuFeS2), Alexo pyrrhotite (Fe(1 −x)S), and VMSO pentlandite ((Fe,Ni)9S8) that have been chemically characterized by electron microprobe analysis, isotopically characterized for δ33S, δ34S, and δ36S by fluorination gas-source mass spectrometry, and tested for homogeneity at the micro-scale by secondary ion mass spectrometry. Beam-sample interaction as a function of crystallographic orientation is determined to have no effect on δ34S and Δ33S isotopic measurements of pentlandite. These new findings provided the basis for a case study on the genesis of the Long-Victor nickel-sulfide deposit located in the world class Kambalda nickel camp in the southern Kalgoorlie Terrane of Western Australia. Results demonstrate that precise multiple sulfur isotope analyses from magmatic pentlandite, pyrrhotite and chalcopyrite can better constrain genetic models related to ore-forming processes. Data indicate that pentlandite, pyrrhotite and chalcopyrite are in isotopic equilibrium and display similar Δ33S values + 0.2‰.This isotopic equilibrium unequivocally fingerprints the isotopic signature of the magmatic assemblage. The three sulfide phases show slightly variable δ34S values (δ34Schalcopyrite = 2.9 ± 0.3‰, δ34Spentlandite = 3.1 ± 0.2‰, and δ34Spyrrhotite = 3.9 ± 0.5‰), which are indicative of natural fractionation. Careful in situ multiple sulfur isotope analysis of multiple sulfide phases is able to capture the subtle isotopic variability of the magmatic sulfide assemblage, which may help resolve the nature of the ore-forming process. Hence, this SIMS-based approach discriminates the magmatic sulfur isotope signature from that recorded in metamorphic- and alteration-related sulfides, which may not be resolved during bulk rock fluorination analysis. The results indicate that, unlike the giant dunite-hosted komatiite systems that thermo-mechanically assimilated volcanogenic massive sulfides proximal to vents and display negative Δ33S values, the Kambalda ores formed in relatively distal environments assimilating abyssal sulfidic shales

Incidence of AIDS-Defining Opportunistic Infections in a Multicohort Analysis of HIV-infected Persons in the United States and Canada, 2000–2010

Background. There are few recent data on the rates of AIDS-defining opportunistic infections (OIs) among human immunodeficiency virus (HIV)–infected patients in care in the United States and Canada

Evaluating the Effects of SARS-CoV-2 Spike Mutation D614G on Transmissibility and Pathogenicity.

Global dispersal and increasing frequency of the SARS-CoV-2 spike protein variant D614G are suggestive of a selective advantage but may also be due to a random founder effect. We investigate the hypothesis for positive selection of spike D614G in the United Kingdom using more than 25,000 whole genome SARS-CoV-2 sequences. Despite the availability of a large dataset, well represented by both spike 614 variants, not all approaches showed a conclusive signal of positive selection. Population genetic analysis indicates that 614G increases in frequency relative to 614D in a manner consistent with a selective advantage. We do not find any indication that patients infected with the spike 614G variant have higher COVID-19 mortality or clinical severity, but 614G is associated with higher viral load and younger age of patients. Significant differences in growth and size of 614G phylogenetic clusters indicate a need for continued study of this variant