| Summary: | As our global community grapples with the ongoing challenges of climate change mitigation and adaptation, records of past climate changes provide important benchmarks for climate models and insights into the interactions between climate changes, human societies, and the landscape. In this dissertation, I reconstruct past changes in climate and redefine the human settlement history of the North Atlantic region. Understanding the human and climate history of the North Atlantic is particularly important because the region is especially sensitive to changes in climate, and climate and oceanographic changes in the region have global ramifications. I focus on the Faroe Islands and Iceland. These islands are close to oceanographic and atmospheric fronts and are sensitive to small changes in the climate system. Furthermore, the Faroes and Iceland were not settled by humans until the the first millennium CE, and as such, provide climate records that are unaltered by human land use activities for most of the Holocene, and are important test cases for understanding human-climate-landscape interactions after human settlement. In this work, I use lake sediment cores to reconstruct continuous climate records for the Holocene period, and in some cases, snapshots of previous interglacials. Lipid biomarkers are a group of molecules that accumulate in lake sediment and can be traced back to organisms in the lake or its watershed. I use these molecules and their isotopic compositions to quantify past changes in temperature and precipitation isotopes, which provide constraints on past climate and atmospheric circulation. I use fecal biomarkers, which are produced by the gut bacteria of humans and livestock, to determine when humans first settled in a lake’s watershed, and the impact they had on the environment. Ancient DNA stored in lake sediment can also be used to trace past changes in vegetation and to determine if livestock were present in the watershed, enhancing our understanding of agricultural impacts on the landscape. By combining these molecular tools, in Chapter 2, I reconstruct the Holocene and Last Interglacial climate in the Faroe Islands using the hydrogen and carbon isotopes of leaf waxes. In this case, I found that the Faroe Islands slowly transitioned over the Holocene from a climate that was warmer and wetter than present conditions, to a climate that is cooler and drier. In the Faroes, the Last Interglacial climate was similar to the early Holocene. In Chapter 3, I use fecal biomarkers and sedimentary ancient DNA to determine that humans arrived 300 years prior than previously thought. Human settlement caused erosion in the catchment and a shift in the vegetation community. In Chapter 4, I use the hydrogen isotopic composition of leaf waxes in conjunction with a biomarker-based temperature reconstruction from a lake in Iceland to determine past shifts in regional atmospheric circulation. I found that a shift towards more northerly-sourced precipitation, consistent with conditions similar to a positive NAO, occurred at the same time as regional neoglaciation and expansion of sea ice. All together, these records advance our understanding of Holocene and Last Interglacial climate change, the human settlement history
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