Holocene Climate Change in Greenland: Investigations Using Oxygen Isotopes of Lacustrine Organic Materials

Lasher, Gregory Everett

doi:https://doi.org/10.21985/n2-vjmb-7405

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Holocene Climate Change in Greenland: Investigations Using Oxygen Isotopes of Lacustrine Organic Materials

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This dissertation explores Holocene climate change along the west coast of Greenland and late Holocene climate variability in the subpolar North Atlantic region using paleolimnology. Lake sediment archives and the climate proxies contained within them provide a valuable window into past temperature change on Greenland, home to one of Earth’s two remaining ice sheets. The studies presented here primarily use oxygen isotopes (δ18O) measured in sub fossil midges (Diptera: Chironomidae) to infer climate change over the last 10,000 years. Holocene temperature records from three lakes on the west coast of Greenland, and a late Holocene record from a south Greenland lake are presented and discussed. Chapter 2 provides a review of chironomid δ18O proxy advances over the last two decades and presents new data demonstrating the fidelity of these subfossils as indicators of past variations in lake water δ18O. An investigation of chironomid taphonomy using a novel Fourier Transform Infrared Spectroscopy (FT-IR) approach indicates that these subfossils are molecularly stable in Holocene age sediments, and possibly stable in Eemian sediments over 100,000 years old. Chironomids have the potential to provide reliable and rare records of paleo lake water where last interglacial sediments are preserved. Chapters 3 and 4 investigate millennial scale climate change from lake sediment cores in northwest and southwest Greenland. In Chapter 3, I present chironomid δ18O from Secret Lake in northwest Greenland near the Thule Air Base and find that temperatures were warmer than present for much of the last 8000 years due to high Northern Hemisphere summer insolation in the early Holocene. Temperatures were between 2.5 and 4 °C warmer than present between 7.7 (start of the record) and 6 ka (ka = thousands of years ago) and cooled by ~ 6 °C over the next 5000 years. Chapter 4 focuses on sub-Arctic Holocene climate change in southwest Greenland coast near Nuuk. Chironomid δ18O values from two lakes show that peak warmth occurred in the early Holocene, between at least 9 and 7 ka (though possibly earlier) and was followed by gradual cooling over the next 7000 years. The magnitude of likely temperature change at this southwest Greenland site is less than in northwest Greenland, demonstrating different regional responses to insolation trends, and regional modulation of climate via other mechanisms (e.g. ocean circulation). Chapters 5 and 6 focus on late Holocene climate change at a higher temporal resolution, investigating centennial scale variability in south Greenland, Baffin Bay and the subpolar North Atlantic area. Chapter 5 presents a 3000-year temperature record from Scoop Lake in south Greenland lake near Narsaq. This study provided the first quantitative temperature estimates showing Medieval warmth within an area of Norse settlement on Greenland. Evidence for coeval cold conditions elsewhere on Greenland and Baffin Island cited a hypothesized positive North Atlantic Oscillation (NAO) anomaly to explain glacier advance, however the new isotope record from Scoop Lake argues against such an anomaly. Instead there is growing evidence that enhanced delivery of warm sea surface waters to south Greenland via a strong subpolar gyre (SPG) caused the warmth between 900 and 1400 CE. These conclusions are further explored in Chapter 6, which employs principal component analysis (PCA) of 29 temperature sensitive proxy records from the high latitude North Atlantic region, Baffin Bay and Greenland to explore spatial – temporal climate heterogeneity over the last 3000 years.

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