Holocene Glacier and Ice Cap Fluctuations in Southern Greenland: Investigations Using Lacustrine Archives and Geospatial Methods

Larocca, Laura Jennifer

doi:https://doi.org/10.21985/n2-j4tj-cx02

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Holocene Glacier and Ice Cap Fluctuations in Southern Greenland: Investigations Using Lacustrine Archives and Geospatial Methods

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In recent decades, mass loss from glaciers and ice caps (GICs) worldwide has accelerated, contributing significantly to the observed sea–level rise. Their continued melt is also expected to alter local hydrological systems, affecting freshwater availability and quality. Despite these problematic impacts, little is known about GIC variations prior to the satellite era—a limitation that hinders our understanding of their response to periods of sustained warming. A longer–term perspective on GIC fluctuations also offers an invaluable window into the past, informing our knowledge of the magnitude and timing of Holocene temperature changes, and of the climate dynamics that govern a warmer world. This dissertation investigates the recent (20th and 21st century) and long–term (Holocene) history of GIC fluctuations in south Greenland and, in one chapter, the latter in the broader Arctic. The research within employs a novel approach which combines geospatial methods and analysis of glacial lake sediment records to constrain (1) the timing of Arctic Holocene climate shifts, and (2) the magnitude of summer season temperature change by modeling past GIC equilibrium–line altitudes (ELAs). The work also places recent glacier retreat in south Greenland into a century–long frame of reference by documenting glacier frontal positions over the last ~120 years using a rare archive of historical air photos. Combined, these data provide new Holocene records of climate and GIC variability in southern Greenland and a long–term framework for anticipating the consequences of future Arctic warming. Chapters 2 and 3 present five new continuous lake sediment records of glacier fluctuations from south and southwest Greenland. Three records from the far south show that mountain glaciers were present in the early Holocene, from ~10.75 ka (ka = thousands of years before 1950 CE) until ~7.3–7.1 ka. These records suggest that summer temperatures remained cooler than ~1.2–1.8°C above present during that time interval and provide strong evidence that south Greenland experienced a delayed thermal maximum relative to summer insolation forcing and to climate at sites farther north. Roughly 500 km to the northwest, two records from southwest Greenland show that some large, high elevation, ice caps likely survived the Holocene Thermal Maximum (HTM), and that smaller, lower elevation, glaciers likely melted away completely by at least ~8.6 ka. In the late Holocene, the glacial lake records from the south and southwest show sustained glacier regrowth between ~3.1 and ~1.2 ka, reflecting a broad Arctic pattern of declining summer insolation and cooling summer air temperatures. In the southwest, regional GIC records indicate progressive lowering of glacier ELAs and a summer cooling of ~2.7°C from the middle to late Holocene, assuming no change in precipitation. Reconstructed ELAs based on maximum historical moraine extents from glaciers in the south suggest that summer temperatures were at least ~0.4–0.9°C cooler than present during the second, more severe portion of a recent cool period called the Little Ice Age (~1300-1850 CE). To date, there are relatively few continuous records of GIC variations inferred from glacial lake sediments over the Holocene across the Arctic—a vast area, which hosts a wide range of modern climates. Chapter 4 presents a compilation of all available lacustrine GIC records (n = 66) from seven Arctic regions. The work summarizes evidence for when GICs were smaller than today or absent altogether, indicating warmer than present summers, and evidence for when GICs regrew in lake catchments, indicating summer cooling. Together, the pan–Arctic records suggest that the majority of GICs were smaller than present or absent by ~10 ka, which is consistent with orbitally driven high boreal summer insolation in the early Holocene. Importantly, the work finds that in the first half of the Holocene, most of the Arctic’s small GICs became significantly reduced or melted away completely in response to summer temperatures which, on average, were only moderately warmer than today. In the middle Holocene, the compilation shows that GICs began to regrow, and summers began to cool across the Arctic by ~6 ka. The pan–Arctic records also indicate two periods of enhanced GIC growth and cooling in the mid–to–late Holocene between ~4.5–3 ka and after ~2 ka. Finally, Chapter 5 investigates the length change of sixty–one land–terminating glaciers in South Greenland over ~120–years using a set of 20th century historical air photos along with modern satellite imagery. The study confirms that glaciers have sensitively responded to changes in summer air temperature over the past century. South Greenland’s glaciers experienced their highest rates of retreat in the first half of the 20th century and in the first two decades of the 21st century. Between these two intervals, a period of cooler summers and lower retreat occurred in the second half of the 20th century. Comparison with comparable glacier length change studies in other regions of Greenland show overall similar trends. Notably, glacier retreat rates have accelerated in all regions since ~2000 CE, and, in comparison to other regions, glaciers in the south show the smallest magnitude of retreat over the last two decades.

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