Elucidating Climate Dynamics from Holocene Reconstructions in Alaska, Greenland, and the North Atlantic

Arctic amplification is a combination of ice-ocean-atmospheric feedbacks that causes the northern high latitudes to be extremely sensitive to climate change. These processes combined with anthropogenic warming are currently driving Arctic temperatures to increase at a rate almost four times faster than global average. However, it is unclear how current and future climate change in the Arctic compares to the natural variability of the climate system over longer timescales. The Holocene (11,700 years ago to present) provides abundant well-preserved, sub-millennial to centennial-scale records of climate, and encompasses periods of relatively rapid warming and cooling which can be compared to anthropogenic warming. Despite the utility of Holocene paleoclimate records for capturing climate variability relative to present, there remains questions regarding the spatiotemporal patterns and drivers of climate change in Arctic regions. One key limitation is the lack of a dense network of high-resolution proxies that capture temperature and precipitation variability. For example, in southwestern Greenland, ice-free terrestrial areas are key to understanding how temperature and precipitation variability drives the melting of the Greenland Ice Sheet (GrIS) at is margins. In Arctic Alaska, the interplay between moisture source and sea-ice extent in the Arctic Ocean may be important to future precipitation dynamics in this region, but few records exist to examine centennial-scale linkages in the past. Broadly, existing paleoclimate records from these regions indicate that a complex combination of temperature, sea-ice extent, glacier thaw, and ocean and atmospheric circulation patterns drive climate change through time. However, disentangling these dynamic processes remains elusive. The three chapters that make up my dissertation are focused on 1) producing new climate reconstructions that contribute to our understanding of Arctic climate dynamics in the Holocene and 2) constrain the drivers of climate variability on centennial to millennial timescales in high latitude environments. The main questions of my dissertation research are as follows: Chapter 2 – Is the heterogeneity of climate trends observed over the Middle to Late Holocene in southwestern Greenland a result of disparate climate records capturing distinct aspects of the climate system? Are there complex feedbacks between ice thaw and ocean dynamics that can be elucidated from comparing paleoclimate records along a north to south gradient? Chapter 3 – What were the major drivers of hydroclimate variability in Arctic Alaska over the Late Holocene? Do regional records show comparable responses to sea-ice and atmospheric forcing, or is there local-scale variability? Chapter 4 – Does NAO-like climate variability impact the North Atlantic region on centennial timescales in the Late Holocene? What does a compilation of climate reconstructions from the North Atlantic and northern Europe suggest about the climate system in this region? For each of these chapters, I utilize isotope-based climate proxies and consider the relative information that can be obtained from site specific variability versus regional coherency among these records, with the overarching goal of highlighting the importance of understanding unique drivers of proxy variability when reconstructing the past. To address my first research questions (Chapter 2), I used oxygen isotopes from insect remains (chitin) preserved in lake sediments to reconstruct past temperature and precipitation variability from a site in southwestern Greenland and compare this new record to isotope-based reconstructions along the western coast of Greenland. This new isotope record spans the past 8,200 years and has among the highest temporal resolution of the existing paleoclimate records in the region. By examining the trends and relative magnitudes of change among these records, I elucidate the spatial variability of climate response to forcing in this region during the Middle to Late Holocene. These records suggest that sea-ice variability in the Labrador Sea likely played an important role the timing and magnitude of climate change from north to south along the western Coast of Greenland. For my second set of questions (Chapter 3), I build upon the techniques from Chapter 2 to further understand past moisture and temperature variability in the Arctic by focusing on the Alaskan North Slope. With this work, I capitalized on sediment measurements including magnetic susceptibility, elemental abundances from X-ray fluorescence, and bulk-sediment carbon isotopes to constrain how the lake catchment may have changed through time, and determine the impact of those changes on the isotopic composition of lake water. The chitin-isotope record from my site spans the past 3,000 years, capturing the broad-scale trend of Neoglacial cooling in the region as well as the Little Ice Age. I find evidence that the isotopic composition of rainfall, as recorded by insect remains, was strongly linked to the extent of sea-ice in the Arctic Ocean in the Late Holocene. Moreover, this study offers important insight into the local-scale variability that can impact the isotopic composition recorded in sedimentary records. To answer my final research questions (Chapter 4), I capitalize on empirical datasets of precipitation isotopes, including my newly generated isotope record from Greenland (Chapter 2), to generate a regional analysis for understanding dominant climate forcing mechanisms in the North Atlantic Sector over the past 3,000 years. I find that there is a robust bimodal pattern of moisture variability from the North Atlantic to eastern Europe, which is a pattern that at present is linked to the North Atlantic Oscillation (NAO). However, careful consideration of these records and spatial patterns suggests that mechanisms such as sea-ice variability and ocean circulation more directly affect precipitation isotope variability in the North Atlantic sector on centennial timescales, contrary to previous records that suggest that NAO-like climate variability was an important driver. Together, these chapters illustrate the importance of careful consideration of proxy data when drawing inferences about climate variability on multiple timescales.

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