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Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
Biogeochemistry | Climate | Geochemistry | Geology | Glaciology | Sedimentology | Stratigraphy
ARCTIC AND NORTH ATLANTIC PALEO-ENVIRONMENTAL RECONSTRUCTIONS FROM LAKE SEDIMENTS
GREGORY A. DE WET, B.Sc., BATES COLLEGE
M.Sc., UNIVERSITY OF MASSSCHUSETTS, AMHERST
Ph.D., UNIVERSITY OF MASSACHUSETTS, AMHERST
Directed by: Drs. Raymond S. Bradley and Isla S. Castañeda
There are few fields in the discipline of Earth Science that hold more relevancy in 2017 than studies of earth’s climate. Called the “perfect problem” considering its complexity and magnitude, climate change will continue to be one of the greatest challenges humanity will face in the 21st century. And while numerical models provide valuable information on conditions in the future, the results from these simulations must be contextualized by the past. Climate reconstructions from paleo-environmental archives, even from periods colder or different from what we are experiencing today, provide that context. Every piece of information gleaned from these studies informs our collective knowledge of the climate system. In some cases, environmental reconstructions may include proxies for anthropogenic as well as climatological information, directly addressing one of the most important questions in climate science: how does changing climate affect humans?
The following chapters of this dissertation are exercises in trying to understand climate change in one of the most climatically sensitive regions on earth – the high northern latitudes. While my doctoral studies cover a wide range of timescales, it is broadly unified by the focus on the Arctic. In some cases, my research spans multiple glacial/interglacial cycles, in others the concentration is on the past few thousand years. In all cases, however, the goal is to utilize lacustrine sedimentary archives to inform our knowledge of climatic change in this important region.
One of the most rewarding aspects of this Ph.D. has been the creativity I have been afforded in working towards that goal. Chapter One of this dissertation involves the analysis of organic molecules, specifically bacterial membrane lipids called branched glycerol dialkyl glycerol tetraethers (brGDGTs), to sediments from Lake El’gygytgyn. These biomarkers allow for a quantitative reconstruction of temperature from multiple interglacial periods over a million years ago. Our data suggests that “super interglacial” Marine Isotope Stage 31 may have in fact lasted much longer than previously thought in the Arctic, with implications for Antarctic ice sheet extent and CO2 concentrations highly relevant to our future. Though the extraction and analysis of these biomarkers is expensive and time-consuming, the data is highly valuable and informative.
Conversely in some cases more “quick and dirty” techniques are sufficient to reconstruct important processes or factors back through time (e.g. the presence of a glacier within a lake catchment) at relatively low cost, and therefore are highly useful. The chapters dealing with Lake Gjøavatnet in Svalbard (Chapter Two) or the application of Fourier Transform Infrared (FTIR) spectroscopy to lake sediments (Chapter Five) exhibit how such techniques can also be highly informative. In Chapter Two, using a combination of core-scanning data and relatively simple destructive analyses (%loss-on-ignition, bulk carbon isotopes), we reconstructed past fluctuations in glacier extent over the Holocene, as well as identified intervals that may have been related to freshwater pulses in Fram Strait. Chapter Five is focused mainly on expanding and confirming the use of FTIR spectroscopy to Arctic lake sediments to reconstruct biogenic silica and organic matter concentrations through time. Though potentially a less direct climate proxy than paleotemperature from brGDGTs, this new technique allows for more rapid analyses using less sediment than previous methods, a valuable advance.
Chapters Three and Four are in many ways the confluence of these earlier stages of my Ph.D., where we apply a wide range of proxies to answer questions related to climate and human population dynamics. The use of biomarkers is expanded in these projects, where a broad suite of organic molecules are used to reconstruct both climate and other paleo-environmental conditions, including vegetation changes, variations in pH, and potentially anthropogenic influences. Coupled with some of the more “basic” techniques described above, we characterize an environmental disturbance in the Faroe Islands ~2200 years ago that may be evidence for the first appearance of humans in the archipelago (Chapter Three). In Chapter Four, we explore the application of brGDGTs (among other proxies) to reconstruct temperature change in southwest Greenland during the period of Norse settlement and subsequent abandonment. This work is part of an ongoing investigation into the efficacy and calibration of this promising proxy in a locale where climate change likely had a dramatic impact on the fragile communities living there.
In summary, I have not attempted to unify these chapters into a single climatological context (though some of my work, such as in the Faroes and Greenland, is highly related). Instead I present them as they are, individual projects that each have their own goals and merits within the broad framework of paleoclimatology. As I mentioned above, one of my favorite things about this field is the creativity we are afforded in our attempts to answer questions about the past. This Ph.D. has been an exercise in that creativity, focused in the high northern latitudes, and centered around the archive of lake sediments.
de Wet, Gregory A., "Arctic and North Atlantic paleo-environmental reconstructions from lake sediments" (2017). Doctoral Dissertations. 1034.
Available for download on Saturday, September 01, 2018