京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第19511号 / 理博第4171号 / 新制||理||1599(附属図書館) / 32547 / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)准教授 河上 哲生, 教授 平島 崇男, 教授 山 明 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

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Much of the world’s aquatic food webs and nutritional relationships have been blurred by the ever-increasing evidence that many phytoplankton are not exclusively heterotrophic or autotrophic, but instead mixotrophic. Mixotrophy is a continuum of different energy and carbon-acquisition mechanisms utilizing both autotrophy and heterotrophy which distorts the concept of single trophic tier modality. This makes mixotrophs flexible to adapt to environmental pressures and is becoming more the rule than the exception in many aquatic ecosystems. One unique environmental setting where mixotrophy could be highly beneficial to food web stability is in seasonally occurring ephemeral pools – aka vernal pools. Mid-Atlantic vernal pools are biodiverse biogeochemical hotspots and critical breeding habitats for a diverse number of endemic taxa including many endangered amphibian species. Vernal pools are not permanent standing bodies of water and have fluxes in hydrology, temperatures, nutrients, and irradiance to name a few. These extremes make vernal pools an ideal setting for mixotrophic phytoplankton, however it’s never been investigated. Our survey found mixotrophy in every vernal pool sampled, as well as elevated grazing rates in pools experiencing nontypical seasonal conditions. From these small-scale forest pools to the world’s oceans mixotrophy is a widespread nutritional strategy. The Southern Ocean is essential for powering worldwide ocean circulation, regional biogeochemical cycles, and global climate. One of the major hurdles with understanding mixotrophy is identifying the phytoplankton capable of shifting nutritional strategies. While many Southern Ocean plankters have been properly identified as mixotrophic, one such keystone species has gone mislabeled until now. Phaeocystis antarctica is a well-studied Haptophyte algae that plays major roles in the global carbon and sulfur cycles. This species has been historically labeled as an obligate phototroph, but contradictory survives the long dark Antarctic winter without any known evidence of encystment. We suspect that this highly abundant species is in fact mixotrophic, capable of phagocytosis to supplement the irradiance shortcomings of the Antarctic dark. We experimented with varying degrees of light and nutrient limitations to determine possible triggers for P. antarctica grazing. Our results showed P. antarctica ingesting in every treatment, but its highest grazing rates corresponded with limitations to its primary photosynthetic mode. Apart from the newly realized complexity P. antarctica brings to the Southern Ocean food web, it is an environment that suffers from microplastic pollution that can impede these mixotrophic species. Mismanaged plastic waste around the world, especially microfiber discharge from laundered synthetic textiles, escape into the natural environment, and eventually concentrate in the oceans. The Southern Ocean can become disproportionately polluted in regions due to microfibers becoming sequestered once crossing the Antarctic circumpolar current and even becoming trapped in sea-ice formations. While it is easy to see the devastation plastic waste has on megafauna (i.e. turtles, fish, birds, and whales), its microscopic devastation is less obvious. Plastic waste comes in many forms and one less researched form is buoyant polyester microfibers <1mm that interact with colony forming algae. We utilized different concentrations of polyester microfibers and mixing speeds to determine if microfiber interactions with colony formations increases or decreases overall colony buoyancy. Smaller concentrations of polyester microfibers can impart a positively buoyant effect onto P. antarctica colonies regardless of mixing speed, however larger concentrations negatively affected colony buoyancy regardless of mixing speed. / Biology

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Several thousand coastal perennially ice-covered oligotrophic lakes and ponds have been identified on the Antarctic continent. To date, most hydrochemical studies on Antarctica’s ice-covered lakes have been undertaken in the McMurdo Dry Valleys (more than 20 lakes/ponds studied since 1957) because of their proximity to the McMurdo research station and the New Zealand station Scott Base. Yet, little attention has been given to coastal ice-covered lakes situated in Antarctica’s central Queen Maud Land region, and more specifically in the Untersee Oasis: a polar Oasis that encompasses two large perennially ice-covered lakes (Lake Untersee & Lake Obersee), and numerous small ice-covered morainic ponds. Consequently, this PhD research project aims to describe and understand the distribution, ice cover phenology, and contemporary hydrochemistry of perennially ice-covered lakes and ponds located in the Untersee Oasis and their effect on the activity of the benthic microbial ecosystem. Lake Untersee, the largest freshwater coastal lake in central Queen Maud Land, was the main focus of this study. Its energy and water mass balance was initially investigated to understand its current equilibrium and how this perennially well-sealed ice-covered lake may evolve under changing climate conditions. Results suggest that Lake Untersee’s mass balance was in equilibrium between the late 1990s and 2018, and the lake is mainly fed by subglacial meltwater (55-60%) and by subaqueous melting of glacier ice (40-45%). A recursive stable water isotope (δD-δ18O) evolution model for well-sealed perennial ice-covered lakes that takes into account the effect of changing chemistry in residual waters on δD-δ18O values was then developed and determined that Lake Untersee is in isotopic steady-state. Modeling results also showed that Untersee most likely did not receive additional inputs from surface streams during the last 300–500 years at the time of sampling, in November-December 2017. However, in mid-January 2019, Untersee experienced a glacial lake outburst flood (GLOF) that increased the water level by 2 m (contributing 1.75×107 m3 of water), modifying its water chemistry and inorganic carbon load. High-resolution grain size and carbon isotope analyses of the benthic microbial mats suggest that GLOFs occurred periodically over the Holocene and that those events sporadically increased the primary productivity of its benthic microbial ecosystem. Finally, ice-covered ponds in the Oasis were identified and sampled to compare their morphometric properties, hydrochemical properties, and microbial mat activity with Lake Untersee. It was discovered that the Untersee Oasis ponds offer the full spectrum of ice cover types (i.e., perennial well-sealed, perennial and moat forming, and seasonally ice-covered) and that their hydrochemical properties depend on ice cover type. Empirical pond data was used to determine how Lake Untersee and the ponds themselves will evolve as they transition under a warming climate from well-sealed to moat forming and from moat forming to seasonally ice-covered.

The Antarctic ice sheet is a geophysically - and in an age of growing concern about global warming, geopolitically - important portion of Earth. The composition and dynamics of the Antarctic ice sheet influence global climate patterns, global sea level and the planet's radiation budget. Recent evidence also suggests that the long term stability of portions of the ice sheet may be in jeopardy. In this thesis I use data from three Ku-band space-borne scatterometers to monitor changes in the backscatter signature of the Antarctic ice sheet from 1978 through 2007. Significant changes in backscatter, which result from geophysical changes in the ice sheet itself, are found over much of the Antarctic continent, especially in West Antarctica and along much of the coasts. Less drastic changes, including regular seasonal variations, are observed over much of the ice shelf. Possible scattering mechanisms are proposed and discussed. A secondary result is the demonstration of the stability of NASA's QuikSCAT scatterometer, data from which is used extensively in this thesis and in many other publications. It is shown that QuikSCAT's observation geometry and backscatter instrumentation have remained consistent to great precision throughout its nearly nine-year long mission.

Western Antarctica has been experiencing significant warming for at least the past fifty years. While higher Net Surface Mass Balance (SMB) over West Antarctica during this period of warming is expected, SMB reconstructions from ice cores reveal a more complex pattern during the period of warming. The mechanisms giving rise to SMB variability over the West Antarctic Ice Sheet (WAIS) are not well understood due to lack of instrumental data. The Southern Annular Mode (SAM) and El Niño Southern Oscillation (ENSO) are believed to contribute to WAIS SMB variability but the assumption has not been rigorously tested. SMB during years where SAM and ENSO are in extreme phases is compared to average SMB from the period 1979-2010. Additionally, atmospheric circulation anomalies are used to assess what circulation patterns accompany extreme modes of climate during the same period. The results suggest that significantly lower SMB occurs when SAM is in an extremely positive phase or ENSO is in an extremely negative phase. Additionally, atmospheric circulation anomalies show that certain circulation patterns accompany extreme modes of climate, which contribute to SMB variability over the WAIS. Ultimately, the location of low and high pressure cells is the best predictor for extreme accumulation events over the WAIS. These results are verified by assessing observed net SMB trends from a network of firn cores located from the central WAIS. Seven new firn cores are added to improve the spatial network of regional net SMB measurements. Reconstructed net SMB is calculated from new firn core records, and compared to the existing cores. The new suite of preliminary firn core records show the same significant decreasing trend that is observed in existing cores. This represents a negative region-wide SMB trend that is likely in part due to trends in SAM and ENSO.

<p>  </p> <p>Reconstructing past glacier and ice-sheet extents is important to better understand how glacial systems have responded to past climate changes in hope of constraining predictions of their responses to ongoing anthropogenic climate warming. As such, the most recent period of climatic variations, from the Last Glacial Maximum (LGM, ca. 21 ka) through today, is of great interest as a prominent example of how ice has reacted to past climatic warming events. Surface exposure dating utilizing cosmogenic nuclides can directly constrain when past ice deglaciated in current and former glacial landscapes. Numerous studies have utilized long-lived cosmogenic radionuclides (i.e., 10Be, 26Al) in polar regions to reconstruct glacial systems. However, due to prevalent non-erosive cold-based ice, prior nuclides from pre-LGM can be preserved. </p> <p>The research described in this dissertation applies <em>in situ </em>cosmogenic 14C (<em>in situ</em> 14C), an emerging geochronometer, to polar glacial landscapes in Sweden and Antarctica to constrain the timing and rate of glacial ice retreat. <em>In situ </em>14C more closely reflects the post-LGM deglacial signal in polar regions because it is less likely to preserve prior nuclides (inheritance) under minimally erosive ice. Our cosmogenic 10Be–26Al–14C concentrations near the Riukojietna ice cap, the last remaining ice cap in Sweden, combined with a sedimentary record from a proximal proglacial lake, indicate the ice cap likely survived during a warm period in the Holocene, but was less extensive than today. The <em>in situ</em> 14C exposure data from nunataks in western Dronning Maud Land (DML), East Antarctica indicate significant coastal thickening (up to 850 m) not predicted by models to date. In addition, this work dates the timing of post-LGM ice surface lowering in two drainage basins in western DML. These results demonstrate the significant contribution of <em>in situ</em> 14C in polar regions.</p> <p>In addition to applications of <em>in situ</em> 14C in polar regions, this work also describes the development of a compositionally dependent <em>in situ</em> 14C production rate calculator. The ability to extract <em>in situ</em> 14C from samples which quartz cannot be separated (either quartz-poor or fine-grained) would allow new avenues of research. The computational framework will be a useful tool in efforts to broaden the utility of <em>in situ</em> 14C to quartz-poor and fine-grained rock types. </p>

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