We present a continuous high-resolution precisely dated multiproxy record of hydroclimate variability at Anjohibe in northwestern Madagascar using speleothem AB13. The record spanned ~4,484 to ~2,863 years BP and showed general agreement with previously published speleothem records from the same approximate location. However, a speleothem record from Rodrigues Island, located ~1,600 km to the east of Madagascar, did not align, suggesting that paleoclimate records from Rodrigues Island may not serve as accurate proxies for northwestern Madagascar, as has been previously suggested. Stalagmite AB13 also provides a detailed record of rainfall variability during the 4.2 ka event, the abrupt climate disturbance associated with the collapse of several early human civilizations. Between ~3,900 – 4,300 years BP, Anjohibe experienced two periods of moderate drying. The most significant climate perturbation in the record was a drought that lasted ~300 years with peak dryness at ~3,000 years BP. This extended drought may have contributed to the reduction of the local perennial wetland environments and thus may have implications for the extirpation of Malagasy pygmy hippopotamuses in this part of the dry deciduous forest.

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<p>Northern peatlands (>45°N) mostly initiated during the Holocene and have been a large C sink to the atmosphere. Northern peatland formation prefers wet and cold condition where the productivity persistently exceeds decomposition and thereby C accumulates. As the northern high latitude region is likely to be warmer in the future, whether northern peatlands will continue being C sinks or switch to C sources is uncertain. To address this issue, I revise and apply a process-based model designed for describing peatland biogeochemical processes, Peatland Terrestrial Ecosystem Model (PTEM), to simulate the C dynamics at both site and regional level, from 15 ka BP-2300. For the site-level simulation, PTEM 1.0 is substantially revised into PTEM 2.0 in terms of peat accumulation process, plant functional types, productivity and decomposition, and soil thermal properties. A simulation from peat initiation to 2300 is conducted for three northern peatland sites. I found PTEM 2.0 can effectively capture the historical C accumulation progress, when compared with the observation. The future simulation indicates northern peatlands have reduced C sink capacity or switch to a C source under N insufficiency and water table deepening. </p> <p>Afterwards, a historical pan-Arctic simulation during 15ka BP-1990 is conducted. PTEM 2.0 is revised into PTEM 2.1 by adding spatially-explicit run-on and run off processes. The spatially-explicit peat initiation dataset is derived from neural network approach and a spatially-explicit peat expansion trend is established on top of it. My estimated pan-Arctic peatland C storage is 396-421 Pg C with the long-term C accumulation rate (CAR) of 22.9 g C∙m-2 yr-1. The simulated spatial distribution of peat C and the temporal pattern of CAR both agree with literature values. I analyzed northern peatlands’ response to historical climate change since 0.5 ka BP and found decreased CAR in the warmer non-permafrost and permafrost-thaw region, while the opposite was found in the colder permafrost region. The results indicate warmer southern peatlands will first switch to a C source under warming while more northern peatlands will become larger sinks. </p> <p>Based on the result of historical simulation, a future simulation is conducted for 1990-2300 with peatland expansion/shrinkage considered. PTEM 2.1 is revised into PTEM 2.2 such that the water table depth, run-on and run-off are estimated from a TOPMODEL approach. In the 21st century, northern peatlands are projected to be a C source of 1.2-13.3 Pg C under five out of six climate scenarios. During 2100-2300, northern peatlands under all scenarios are a C source under all climate scenarios. Northern peatlands switch to C sources due to deepening water table depth, insufficient N availability, and plant functional type shift. I found that northern peatlands remain as a C sink until a mean pan-Arctic peatlands annual temperature reaches -2.09 - -2.89°C. This study predicts a northern peatland sink to a source shift around 2050, earlier than previous estimates of after 2100, and emphasizes the vulnerability of northern peatlands to climate change. </p>

Lakes sediments from the Lofoten Islands, Norway, can be used to generate well resolved records of past climate and environmental change. This dissertation presents three lacustrine paleoenvironmental reconstructions that show evidence for Holocene climate changes associated with North Atlantic climate dynamics and relative sea-level variations driven by glacio-isostatic adjustment. This study also uses distal tephra deposits (cryptotephra) from Icelandic volcanic eruptions to improve the chronologies of these reconstructions and explores new approaches to crypto-tephrochronology. Past and present conditions at Vikjordvatnet, Fiskebølvatnet, and Heimerdalsvatnet were studied during four field seasons conducted from 2007-2010. Initially, each lake was characterized by measuring water column chemistry, logging annual temperature fluctuations, and conducting bathymetric and seismic surveys. Sediment cores were then collected and analyzed using multiple techniques, including: sediment density, magnetic susceptibility, loss-on-ignition, total carbon and nitrogen, δ13C and δ15N of organic matter, and elemental compositions acquired by scanning X-ray fluorescence. Chronologies were established using radiocarbon dating and tephrochronology. A 13.8 cal ka BP record from Vikjordvatnet provides evidence for glacial activity during the Younger Dryas cold interval and exhibits trends in Ti, Fe, and organic content during the Holocene that correlate with regional millennial-scale climate trends and provide evidence for more rapid events. A 9.7 cal ka BP record from Fiskebølvatnet shows a strong signal of sediment inwashing likely driven by local geomorphic conditions, although there is evidence that increased inwashing at the onset of the Neoglacial could have been associated with increased precipitation. Heimerdalsvatnet provides a record of relative sea-level change. A 7.8 cal ka BP sedimentary record reflects changes in salinity and water column conditions as the lake was isolated and defines sea-level regression following the Tapes transgression. Cryptotephra horizons were identified in sediments of Heimerdalsvatnet, Vikjordvatnet, and Sverigedalsvatn. They were also found in a Viking-age boathouse excavated along the shore of Inner Borgpollen. These include the GA4-85, BIP-24a, SILK-N2, Askja, 860 Layer B, Hekla 1158, Hekla 1104, Vedde Ash, and Saksunarvatn tephra. This research project also explored the use of scanning XRF to locate cryptotephra in lacustrine sediments and presents experimental results of XRF scans of tephra-spiked synthetic sediment cores.

A 7.09 m lacustrine sediment core was taken from Mariana Lake, Alberta (55º57'N, 112º01'W) to determine if the regional vegetational complex had been affected by the mid-Holocene Hypsithermal. Dating control was provided by 6 radiocarbon dates, with a second degree polynomial fitted to the dates to give an age to depth curve. The basal date of the core is 11 300 ± 110 yr BP. Fossil pollen analysis of the core revealed five distinct pollen zones. Between 11 856 and 10 434 yr BP the study site supported a sparse herb dominated vegetation. A spruce and shrub birch assemblage followed, from 10 434 to 9 100 yr BP, with increased vegetation density. The climate was likely similar to today. This was replaced by a paper birch and spruce complex from 9 100 to 7 638 yr BP likely as a result of warming climate. A mild Hypsithermal effect is recorded between 7 638 and 5 623 yr BP. A forest of decreased crown density was created, with spruce, paper birch and poplar being the major vegetational components. Modern conditions have existed at the site from 5 623 yr BP onwards, though it is hypothesised the regional water budget may have increased slightly around 2 228 yr BP; increased peat development appears to have occurred at this time. Through the use of difference diagrams the site is compared to three other published sites which, along with Mariana Lake, form a north-south transect from the southern Boreal Forest to the northern Alberta Boreal Forest. The hypsithermal vegetation changes varies with distance to the Parkland-Boreal Forest ecotone boundary. It is concluded the parkland did not reach Mariana Lake during the mid-Holocene, though conditions at the site became similar to those at the southern edge of the Boreal Forest that is proximal to the parkland. / Thesis / Bachelor of Arts (BA)

<p> Isostatic rebound and climatic changes throughout the Holocene have resulted in several periods of lowered water-levels in the Great Lakes watershed. The Early Holocene lowstand phase corresponds with the Paleoindian period in the Great Lakes Region (11,000-9000 YBP), and subsequent water-level rise has inundated Paleoindian archaeological sites. This research sought to reconstruct the water-level history of Rice Lake (located north of Lake Ontario) in order to identify areas of high potential for submerged prehistoric sites. </p> <p> Over 750 line km of detailed geophysical data (single-beam bathymetry) and 16 sediment cores were collected over a 30 km2 area of Rice Lake. Sediment cores were visually logged and analyzed for particle size, microfossils and microdebitage. Water-level reconstructions accounting for sediment infill and isostatic rebound of the lake record a post Lake Iroquois (after 12 ka BP) Early Holocene lowstand (~10-12 m below present level (bpl)) (EH-1). At 10 ka BP, gradually rising water-levels and establishment of wetlands as indicated by thecamoebian assemblages coincide with a newly discovered Paleoindian occupation of the Mcintyre basin, where quartz microdebitage was found. Waterlevel continued to rise to almost 2 m bpi until 6.5 ka BP, when warmer and drier temperatures caused a sudden drop in water-levels as recorded by a pollen hiatus. After 4 ka BP, water-levels quickly recovered and stabilized as shown by rapid recovery of pre-hiatus thecamoebian biofacies and the establishment of wild rice stands. </p> <p> The small number of known, well-preserved Great Lakes Paleoindian sites has limited analysis of Early Holocene population densities, migration patterns, cultural diffusion, or the chronology of settlement. The method of modelling water-level fluctuations and associated archaeological potential developed in this thesis represents a substantial advance in our understanding of Early Holocene archaeology in the Great Lakes. These methods will have broader application to exploration of submerged terrestrial landscapes elsewhere in the Great Lakes and will allow for future regional synthesis of archaeological site distribution and characteristics. </p> / Thesis / Doctor of Philosophy (PhD)

Effective coastal adaptation to sea-level rise requires an understanding of how much and how fast glaciers and ice sheets will melt in the coming decades, together with an understanding of the provenance of that ice melt. When land ice is lost to the oceans, sea-levels do not rise uniformly across the globe, but exhibit a “sea-level fingerprint” specific to the source of ice melt, posing an important question motivating this thesis: Which ice mass(es) will contribute the first 1m/3 feet of sea-level rise? The glacial-geologic record archives the vulnerability of ice sheets and their sub-sectors to past warming. To analyze this record of past glacial change, I develop and apply cosmogenic-nuclide techniques for investigating the climate sensitivity of four key ice sheets. The novel geochemical techniques described here also allow me to investigate processes of landscape evolution, including subglacial and subaerial erosion. Subglacial erosion dictates landscape development in glaciated and formerly glaciated settings, which in turn influences ice-flow dynamics and the climate sensitivity of ice masses, making it an important input in ice-sheet models. In Chapter 1, I use 10Be measurements in surficial bedrock and a 4-m-long bedrock near Jakobshavn Isbræ, to constrain the erosion rate beneath the Greenland Ice Sheet (GrIS) on historical and orbital timescales. 10Be concentrations measured below ~2 m depth in a 4-m-long bedrock core are greater than what is predicted by an idealized production-rate depth profile and I develop a model to utilize this excess 10Be at depth to constrain orbital-scale erosion rates. I find that erosion rates beneath GrIS were 0.4–0.8 mm yr-1 during historical times and 0.1–0.3 mm yr-1 on Pleistocene timescales. The broad similarity between centennial- and orbital-scale erosion rates suggests that subglacial erosion rates adjacent to Jakobshavn Isbræ have remained relatively uniform throughout the Pleistocene. In Chapter 2, I present cosmogenic 10Be and 3He data from Ferrar dolerite pyroxenes in surficial rock samples and a bedrock core from the McMurdo Dry Valleys, Antarctica, opening new opportunities for exposure dating in mafic rocks. I describe scalable laboratory methods for isolating beryllium from pyroxene, estimate a spallation production rate for 10Be in this mineral phase, referenced to 3He, of 3.6 ± 0.2 atoms g-1 yr-1, and present initial estimates for parameters associated with 10Be and 3He production by negative muon capture. I also demonstrate that the 10Be-3He pair in pyroxene can be used to simultaneously resolve exposure ages and subaerial erosion rates, and that the precision of my 10Be measurements in pyroxene enable exposure dating on Last Glacial Maximum to Late Holocene surfaces, including moraines, on a global scale. In Chapter 3, I apply exposure dating locally to investigate the Last Glacial Maximum (LGM) and initial deglaciation of the Laurentide Ice Sheet (LIS), the most dynamic continental ice sheet, in southern New England and New York City. I synthesize new and existing exposure age chronologies from moraines and other glacial deposits that span ~26 to 20.5 ka, and quantify retreat rates for the southeastern LIS margin. Initial retreat at <5 to 30 m yr-1 started within the canonical LGM period, representing the slowest LIS retreat rates of the entire New England deglacial record, which I relate to a slow rise in modeled local summer temperatures through the LGM. Employing similar exposure dating techniques in Chapter 4, I describe the first 10Be ages from nunataks of the Juneau Icefield (JIF), Alaska, that I collected through the Juneau Icefield Research Program (JIRP) in order to evaluate icefield thinning during the Late Glacial and Holocene. I find that the JIF was smaller-than-present under warm climate conditions during the early-to-mid Holocene, elucidating the sensitivity of the icefield to warming. Tackling the climate crisis more broadly and in turn, addressing pressing Earth science questions like those posed in this dissertation, requires diverse perspectives. Yet, the Earth sciences have historically been among the least diverse of the STEM disciplines. As one contribution to a comprehensive effort through JIRP to increase diversity in the geosciences pipeline, Chapter 5 details the curriculum for a two-week course titled ‘A Virtual Expedition to the Juneau Icefield’ that I co-designed and co-taught in 2021 to bring accessible polar science experiences to high school students.

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