ALBERT

Description: © The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Cryosphere 11 (2017): 2439-2462, doi:10.5194/tc-11-2439-2017.

Description: Methanesulfonic acid (MSA; CH3SO3H) in polar ice is a unique proxy of marine primary productivity, synoptic atmospheric transport, and regional sea-ice behavior. However, MSA can be mobile within the firn and ice matrix, a post-depositional process that is well known but poorly understood and documented, leading to uncertainties in the integrity of the MSA paleoclimatic signal. Here, we use a compilation of 22 ice core MSA records from Greenland and Antarctica and a model of soluble impurity transport in order to comprehensively investigate the vertical migration of MSA from summer layers, where MSA is originally deposited, to adjacent winter layers in polar ice. We find that the shallowest depth of MSA migration in our compilation varies over a wide range (∼ 2 to 400 m) and is positively correlated with snow accumulation rate and negatively correlated with ice concentration of Na+ (typically the most abundant marine cation). Although the considered soluble impurity transport model provides a useful mechanistic framework for studying MSA migration, it remains limited by inadequate constraints on key physico-chemical parameters – most notably, the diffusion coefficient of MSA in cold ice (DMS). We derive a simplified version of the model, which includes DMS as the sole parameter, in order to illuminate aspects of the migration process. Using this model, we show that the progressive phase alignment of MSA and Na+ concentration peaks observed along a high-resolution West Antarctic core is most consistent with 10−12 m2 s−1 〈 DMS 〈 10−11 m2 s−1, which is 1 order of magnitude greater than the DMS values previously estimated from laboratory studies. More generally, our data synthesis and model results suggest that (i) MSA migration may be fairly ubiquitous, particularly at coastal and (or) high-accumulation regions across Greenland and Antarctica; and (ii) can significantly change annual and multiyear MSA concentration averages. Thus, in most cases, caution should be exercised when interpreting polar ice core MSA records, although records that have undergone severe migration could still be useful for inferring decadal and lower-frequency climate variability.

Description: Matthew Osman acknowledges government support awarded by DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a. This work was supported by the US NSF (ANT-0632031 and PLR-1205196 to Sarah B. Das, and NSF-MRI-1126217 to Matthew J. Evans) and a Woods Hole Oceanographic Institution Interdisciplinary Research award to Sarah B. Das and Olivier Marchal.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Oceanography and Applied Ocean Science and Engineering at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2019.

Description: The Common Era (A.D. 1– present) represents a crucial period for climatic studies, documenting the timespan over which human activities have become an increasingly domineering force in shaping Earth’s landscape, climate, and ecology. Direct, quantifiable records of climatic phenomena are severely limited over much of the Common Era, necessitating high-resolution, naturally-derived proxies to extend climatic insights beyond the satellite and instrumental era, particularly across remote high-latitude and maritime regions of the North Atlantic. Here, I use modern, data-driven and physically-based modeling approaches to gain new insights into North Atlantic climate variability from the Greenlandic ice core archive. First, I investigate the climatic fidelity of ice core glaciochemical climate proxies at the microphysical-scale. I show that several soluble chemical species – key among them methanesulfonic acid (MSA) – undergo rapid vertical migration through a super-cooled liquidadvection process along ice crystal grain-boundaries. I demonstrate that significant multi-year MSA changes occur only under low snow-accumulation and high-impurity-content conditions, thus mitigating the phenomenon over much of Greenland. Building upon these findings, I then investigate the cause of declining 19th and 20th-century MSA concentrations across the interior Greenland Ice Sheet. My results illustrate that Greenlandic MSA records provide a new proxy for North Atlantic planktonic biomass changes, illuminating a 10 ± 7% decline in marine productivity over the Industrialera. I next present a new climate record from a previously-unexplored coastal ice cap in west-central Greenland. Using a physically-constrained ice cap flowline inversion model, I identify marked centennial-scale changes in coastal precipitation during the last millennium, including a ~40% increase in coastal precipitation since the industrial-onset. These changes are drastically larger than those observed from inland Greenland records, revealing enhanced sensitivity in west Greenlandic hydroclimates to regional Atlantic and Arctic-wide temperature variability. Finally, leveraging a compilation of nearly 30 annual-resolution Greenland water-isotope records, I isolate coherent signatures of atmospheric circulation variability to reconstruct changes in the North Atlantic eddydriven jet-stream over the last millennium, exposing progressively enhanced variability during the past two-centuries consistent with amplified Arctic thermal-wind forcing. This thesis thus illuminates new Common Era climatic and ecologic changes, and expands the scope of the Greenlandic ice archive as proxies of the coupled North Atlantic climate system.

Description: To my various funders: scientific research – especially the polar variety – is wicked expensive, and this dissertation wouldn’t have been possible without the big-bucks graciously provided by the U.S. Department of Defense Office of Naval Research (National Defense Science and Engineering Graduate fellowship), the National Science Foundation’s Office of Polar Programs (1205196, 1418256), as well as generous support from the Woods Hole Oceanographic Institution. An Ocean Outlook Fellowship from the Bjerknes Centre (Bergen, Norway) bred much creativity, and is also much-appreciated.

Description: © The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Atmospheric Measurement Techniques 10 (2017): 4459-4477, doi:10.5194/amt-10-4459-2017.

Description: Insoluble aerosol particles trapped in glacial ice provide insight into past climates, but analysis requires information on climatically relevant particle properties, such as size, abundance, and internal mixing. We present a new analytical method using a time-of-flight single-particle mass spectrometer (SPMS) to determine the composition and size of insoluble particles in glacial ice over an aerodynamic size range of  ∼  0.2–3.0 µm diameter. Using samples from two Greenland ice cores, we developed a procedure to nebulize insoluble particles suspended in melted ice, evaporate condensed liquid from those particles, and transport them to the SPMS for analysis. We further determined size-dependent extraction and instrument transmission efficiencies to investigate the feasibility of determining particle-class-specific mass concentrations. We find SPMS can be used to provide constraints on the aerodynamic size, composition, and relative abundance of most insoluble particulate classes in ice core samples. We describe the importance of post-aqueous processing to particles, a process which occurs due to nebulization of aerosols from an aqueous suspension of originally soluble and insoluble aerosol components. This study represents an initial attempt to use SPMS as an emerging technique for the study of insoluble particulates in ice cores.

Description: This work was supported by an internal Reed Grant from MIT and National Science Foundation award PLR-1205196 to Sarah B. Das. Matthew Osman acknowledges government support awarded by DoD, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a. Maria A. Zawadowicz acknowledges the support of NASA Earth and Space Science Fellowship. Daniel J. Cziczo acknowledges the support of the Victor P. Starr Career Development Chair at MIT.

Description: Reconstruction of the North Atlantic jet stream (NAJ) presents a critical, albeit largely unconstrained, paleoclimatic target. Models suggest northward migration and changing variance of the NAJ under 21st-century warming scenarios, but assessing the significance of such projections is hindered by a lack of long-term observations. Here, we incorporate insights from an ensemble of last-millennium water isotope–enabled climate model simulations and a wide array of mean annual water isotope (δ18O) and annually accumulated snowfall records from Greenland ice cores to reconstruct North Atlantic zonal-mean zonal winds back to the 8th century CE. Using this reconstruction we provide preobservational constraints on both annual mean NAJ position and intensity to show that late 20th- and early 21st-century NAJ variations were likely not unique relative to natural variability. Rather, insights from our 1,250 year reconstruction highlight the overwhelming role of natural variability in thus far masking the response of midlatitude atmospheric dynamics to anthropogenic forcing, consistent with recent large-ensemble transient modeling experiments. This masking is not projected to persist under high greenhouse gas emissions scenarios, however, with model projected annual mean NAJ position emerging as distinct from the range of reconstructed natural variability by as early as 2060 CE.

Description: The Atlantic Meridional Overturning Circulation (AMOC) is a key feature of the North Atlantic with global ocean impacts. The AMOC's response to past changes in forcings during the Holocene provides important context for the coming centuries. Here, we investigate AMOC trends using an emerging set of transient simulations using multiple global climate models for the past 6,000 years. Although some models show changes, no consistent trend in overall AMOC strength during the mid-to-late Holocene emerges from the ensemble. We interpret this result to suggest no overall change in AMOC, which fits with our assessment of available proxy reconstructions. The decadal variability of the AMOC does not change in ensemble during the mid- and late-Holocene. There are interesting AMOC changes seen in the early Holocene, but their nature depends a lot on which inputs are used to drive the experiment.