Investigating Bering Sea oceanographic response to the Milankovitch orbital cycle climatic shift during the middle Pleistocene

The transition of Earth’s glacial-interglacial cycles from 41 kyr to 100 kyr periodicity during the middle Pleistocene (the Mid-Pleistocene Transition (MPT); ~1.2–0.6 Ma) marks one of the largest climate events of the Cenozoic, but the causes of this cooling transition remain unclear, as the emergence of the 100 kyr Milankovitch orbital ‘eccentricity’ in climate records occurred without a long term change in external orbital forcing. Hypotheses for this transition have so far remained largely untested due to a lack of detailed, high resolution climate proxy information from critical regions on the planet. Major hypotheses infer changes to North American Ice Sheet dynamics, an early expansion of subpolar sea ice, and decreasing atmospheric CO2. Using sediment geochemistry and palaeontological proxies, this thesis assesses how the variability in sea ice, nutrient upwelling and primary productivity in the Northern Bering Sea impacted regional and global climate through the MPT, via their impact on North Pacific Intermediate Water expansion, regional carbon cycling and the subpolar biological pump. Through calculation of a semi-quantitative nutrient upwelling index (based on nitrogen isotopes and opal accumulation), key findings of this thesis indicate that sea ice played a dominant role on orbital scale variability in nutrient upwelling at the Bering slope, following global changes in atmospheric pCO2, continental ice sheet accumulation and sea level fluctuations. This is supported by fossil diatom assemblages which distinguish how sea ice dynamics evolved through the MPT, including high resolution variability in response to atmospheric teleconnections in the early Pleistocene. Principally, results support the notion that enhanced glacial formation of NPIW since the 900 kyr event (0.9 Ma) acted to cause region-wide suppression of deep water CO2 ventilation in the subarctic Pacific Ocean. Preliminary assessment of diatom silicon isotopes also suggests that sea ice delivery of iron, in combination with changes to nutrient cycling, may have additionally contributed to lowering glacial pCO2 which promoted increased duration of post-MPT glacials. Overall, this thesis calls for increased attention to subarctic Pacific palaeoceanography in Quaternary climate studies

Charge Transport and Transfer at the Nanoscale Between Metals and Novel Conjugated Materials

Abstract Organic semiconductors (OSCs) and graphene are two classes of conjugated materials that hold promise to create flexible electronic displays, high speed transistors, and low-cost solar cells. Crucial to understanding the behavior of these materials is understanding the effects metallic contacts have on the local charge environment. Additionally, characterizing the charge carrier transport behavior within these materials sheds light on the physical mechanisms behind transport. The first part of this thesis examines the origin of the low-temperature, high electric field transport behavior of OSCs. Two chemically distinct OSCs are used, poly-3(hexylthiophene) (P3HT) and 6,13- bis(triisopropyl-silylethynyl) (TIPS) pentacene. Several models explaining the low-temperature behavior are presented, with one using the Tomonaga-Luttinger liquid (TLL) insulator-to-metal transition model and one using a field-emission hopping model. While the TLL model is only valid for 1-dimensional systems, it is shown to work for both P3HT (1D) and TIPS-pentacene (2D), suggesting the TLL model is not an appropriate description of these systems. Instead, a cross-over from thermally-activated hopping to field-emission hopping is shown to explain the data well. The second part of this thesis focuses on the interaction between gold and platinum contacts and graphene using suspended graphene over sub-100 nanometer channels. Contacts to graphene can strongly dominate charge transport and mobility as well as significantly modify the charge environment local to the contacts. Platinum electrodes are discovered to be strong dopants to graphene at short length scales while gold electrodes do not have the same effect. By increasing the separation distance between the electrodes, this discrepancy is shown to disappear, suggesting an upper limit on charge diffusion from the contacts. Finally, this thesis will discuss a novel technique to observe the high-frequency behavior in OSCs using two microwave sources and an organic transistor as a mixer. A theoretical model motivating this technique is presented which suggests the possibility of retrieving gigahertz charge transport phenomena at kilohertz detection frequencies. The current state of the project is presented and discrepancies between devices made with gold and platinum electrodes measured in the GHz regime are discussed

Local charge transfer doping in suspended graphene nanojunctions

We report electronic transport measurements in nanoscale graphene transistors with gold and platinum electrodes whose channel lengths are shorter than 100 nm, and compare them with transistors with channel lengths from 1 \textmu{}m to 50 \textmu{}m. We find a large positive gate voltage shift in charge neutrality point (NP) for transistors made with platinum electrodes but negligible shift for devices made with gold electrodes. This is consistent with the transfer of electrons from graphene into the platinum electrodes. As the channel length increases, the disparity between the measured NP using gold and platinum electrodes disappears.Comment: 11 pages, 3 figures, to appear in Appl. Phys. Let

Interfacial charge transfer in nanoscale polymer transistors

Interfacial charge transfer plays an essential role in establishing the relative alignment of the metal Fermi level and the energy bands of organic semiconductors. While the details remain elusive in many systems, this charge transfer has been inferred in a number of photoemission experiments. We present electronic transport measurements in very short channel (L < 100 nm) transistors made from poly(3-hexylthiophene) (P3HT). As channel length is reduced, the evolution of the contact resistance and the zero gate voltage conductance are consistent with such charge transfer. Short channel conduction in devices with Pt contacts is greatly enhanced compared to analogous devices with Au contacts, consistent with charge transfer expectations. Alternating current scanning tunneling microscopy (ACSTM) provides further evidence that holes are transferred from Pt into P3HT, while much less charge transfer takes place at the Au/P3HT interface

Induced CNS expression of CXCL1 augments neurologic disease in a murine model of multiple sclerosis via enhanced neutrophil recruitment.

Increasing evidence points to an important role for neutrophils in participating in the pathogenesis of the human demyelinating disease MS and the animal model EAE. Therefore, a better understanding of the signals controlling migration of neutrophils as well as evaluating the role of these cells in demyelination is important to define cellular components that contribute to disease in MS patients. In this study, we examined the functional role of the chemokine CXCL1 in contributing to neuroinflammation and demyelination in EAE. Using transgenic mice in which expression of CXCL1 is under the control of a tetracycline-inducible promoter active within glial fibrillary acidic protein-positive cells, we have shown that sustained CXCL1 expression within the CNS increased the severity of clinical and histologic disease that was independent of an increase in the frequency of encephalitogenic Th1 and Th17 cells. Rather, disease was associated with enhanced recruitment of CD11b+ Ly6G+ neutrophils into the spinal cord. Targeting neutrophils resulted in a reduction in demyelination arguing for a role for these cells in myelin damage. Collectively, these findings emphasize that CXCL1-mediated attraction of neutrophils into the CNS augments demyelination suggesting that this signaling pathway may offer new targets for therapeutic intervention

MicroRNA-155 enhances T cell trafficking and antiviral effector function in a model of coronavirus-induced neurologic disease

BackgroundMicroRNAs (miRNAs) are noncoding RNAs that modulate cellular gene expression, primarily at the post-transcriptional level. We sought to examine the functional role of miR-155 in a model of viral-induced neuroinflammation.MethodsAcute encephalomyelitis and immune-mediated demyelination were induced by intracranial injection with the neurotropic JHM strain of mouse hepatitis virus (JHMV) into C57BL/6 miR-155 (+/+) wildtype (WT) mice or miR-155 (-/-) mice. Morbidity and mortality, viral load and immune cell accumulation in the CNS, and spinal cord demyelination were assessed at defined points post-infection. T cells harvested from infected mice were used to examine cytolytic activity, cytokine activity, and expression of certain chemokine receptors. To determine the impact of miR-155 on trafficking, T cells from infected WT or miR-155 (-/-) mice were adoptively transferred into RAG1 (-/-) mice, and T cell accumulation into the CNS was assessed using flow cytometry. Statistical significance was determined using the Mantel-Cox log-rank test or Student's T tests.ResultsCompared to WT mice, JHMV-infected miR-155 (-/-) mice developed exacerbated disease concomitant with increased morbidity/mortality and an inability to control viral replication within the CNS. In corroboration with increased susceptibility to disease, miR-155 (-/-) mice had diminished CD8(+) T cell responses in terms of numbers, cytolytic activity, IFN-γ secretion, and homing to the CNS that corresponded with reduced expression of the chemokine receptor CXCR3. Both IFN-γ secretion and trafficking were impaired in miR-155 (-/-) , virus-specific CD4(+) T cells; however, expression of the chemokine homing receptors analyzed on CD4(+) cells was not affected. Except for very early during infection, there were not significant differences in macrophage infiltration into the CNS between WT and miR-155 (-/-) JHMV-infected mice, and the severity of demyelination was similar at 14 days p.i. between WT and miR-155 (-/-) JHMV-infected mice.ConclusionsThese findings support a novel role for miR-155 in host defense in a model of viral-induced encephalomyelitis. Specifically, miR-155 enhances antiviral T cell responses including cytokine secretion, cytolytic activity, and homing to the CNS in response to viral infection. Further, miR-155 can play either a host-protective or host-damaging role during neuroinflammation depending on the disease trigger

Sea-ice response to climate change in the Bering Sea during the Mid-Pleistocene Transition

This is the final version. Available on open access from Elsevier via the DOI in this recordData availability: All data presented in this paper is available in the Supplementary Materials.Sea-ice is believed to be an important control on climatic changes through the Mid-Pleistocene Transition (MPT; 0.6–1.2 Ma). However, the low resolution/short timescale of existing reconstructions prevents a full evaluation of these dynamics. Here, diatom assemblages from the Bering Sea are used to investigate sea-ice evolution on millennial timescales. We find that sea-ice was primarily controlled by ice-sheet/sea level fluctuations that modulated warm water flow into the Bering Sea. Facilitated by an amplified Walker circulation, sea-ice expansion began at ∼1.05 Ma with a step-increase during the 900 kyr event. Maximal pack ice was simultaneous with glacial maxima, suggesting sea-ice was responding to, rather than modulating ice-sheet dynamics, as proposed by the sea-ice switch hypothesis. Significant pack ice, coupled with Bering Strait closure at 0.9 Ma, indicates that brine rejection played an integral role in the glacial expansion/deglacial collapse of intermediate waters during the MPT, regulating subarctic ocean-atmospheric exchanges of CO2.Natural Environment Research Council (NERC)British Geological Surve

Silicic acid cycling in the Bering Sea during the Mid‐Pleistocene Transition

This is the final version. Available from Wiley via the DOI in this record. Data Availability Statement: The data presented in this paper are stored in the Pangaea data repository (https://doi.pangaea.de/10.1594/ PANGAEA.933139)The rate of deep-ocean carbon burial is considered important for modulating glacial-interglacial atmospheric CO2 concentrations and global climate during the Quaternary. It has been suggested that glacial iron fertilization and increased efficiency of the biological pump in the Southern Ocean since the Mid-Pleistocene Transition (MPT) was key in lowering atmospheric pCO2 and facilitating rapid land ice accumulation. There is growing evidence that a similar mechanism may have existed in the subarctic Pacific Ocean, although this has not yet been assessed. Here, the silicon isotope composition of diatoms (δ30Sidiatom) from the Bering Sea upwelling region is used to assess the role of nutrient cycling on the subarctic Pacific biological pump during the MPT. Results show that during and after the “900 kyr event,” the high productivity green belt zone was characterized by low silicic acid utilization but high supply, coincident with the dominance of diatom resting spores. We posit that as nutrient upwelling was suppressed following pack ice growth and expansion of glacial North Pacific Intermediate Water (GNPIW), primary productivity became nitrate-limited and enhanced opal remineralization caused a relative increase in silicic acid supply. However, preferential preservation and higher cellular carbon content of diatom resting spores, as well as increased supply of iron from expanded sea ice, likely sustained the net efficiency of the Bering Sea biological pump through the MPT. Remnant iron and silicic acid may also have propagated into the lower subarctic Pacific Ocean through GNPIW, aiding a regionally efficient biological pump at 900 kyr and during post-MPT glacials.Natural Environment Research CouncilBritish Geological SurveyNatural Environment Research CouncilNatural Environment Research Counci

Assessment of bias in carbon isotope composition of organic leaf matter due to pre‐analysis milling methods

Rationale Stable isotope analysis of leaf material has many applications including assessment of plant water-use efficiency and paleoclimatology. To facilitate interpretations of small shifts in the carbon isotope composition (δ13C) of leaves, accurate and repeatable results are required. Pre-sample homogenisation is essential to ensure a representative sample is analysed, but can also introduce error. Methods We investigate how different grinding methods (freezer-milling and ball-milling) affect the carbon content and δ13C of tree leaves from a wetland in Queensland, Australia, commenting on how increased temperature, sample contamination, sample loss, or poor homogenisation may impact results. Results No alteration of leaf δ13C is observed due to different milling methods, although there may be a significant increase in %C of samples processed using ball-milling. Conclusions We suggest %C variability is possibly due to contamination from abraded plastic vials or insufficient homogenisation during ball-milling, with no significant impact on δ13C. Overall, we suggest that intermittent ball-milling may be the best solution to reduce costs, preparation time and use of liquid nitrogen, aiming to achieve complete homogenisation using the shortest possible duration of milling