<p>Lake sediments and glacier extents were used to reconstruct late Holocene climate changes from Kurupa Lake (N 68.35&deg;, W 154.61&deg;; 920 m asl) in the north-central Brooks Range, Arctic Alaska. The lake is fed by several tributaries, including meltwater from eight rapidly disappearing cirque glaciers. An age-depth model for the 622-cm-long sediment core was developed using five radiocarbon (¹⁴C) ages and a 239+240Pu profile. Sedimentary chlorin (chlorophyll a and derivatives) content was analyzed at high resolution (0.2 cm = ~2 yr intervals) with visible reflectance spectroscopy for the past 5.7 kyr. Sedimentary bulk density was analyzed with gamma-ray attenuation (1.0 cm = ~10 yr intervals). </p><p> High-resolution measurements of chlorin content from the last 122 yr (top 16 cm of surface sediment) were correlated with a suite of instrumental climate records from Barrow and Bettles, National Center for Environmental Prediction reanalysis data (grid = N&deg;68 to N&deg;70 &times; W&deg;156 to W&deg;154) and with published climate indices for the Arctic (e.g., Arctic Oscillation, North Atlantic Oscillation). Chlorin content correlates most strongly to average June + September temperature (r = 0.68) and a linear inference model was developed using this regression to quantitatively reconstruct summer temperatures for the past 5.7 kyr. The temperature reconstruction (50-year smoothed) shows minimum summer temperature (5.6 &plusmn; 0.9&deg;C) at 650 AD and maximum summer temperature (10.7 &plusmn; 0.9&deg;C) 2.9 ka. </p><p> Fluctuations in bulk density over the past 5.7 kyr were interpreted to reflect changes in up-valley glacier extent. The accumulation-area ratio (AAR) method was used to estimate former equilibrium-line altitudes (ELAs) for the maximum Neoglacial and modern extents of eight cirque glaciers in the study area. Independently dated moraines enable the history of glacier fluctuations to be linked to the lake sediment sequence (r2 = 0.74), and this integrated approach provides a 5.7 kyr quantitative record of centennial-scale ELA variability for Kurupa River valley. The reconstruction supports the previously established onset of Neoglaciation around 4.2 ka, as well as maximum and minimum glacier extents around 700 AD and the present, respectively. The calculated ELA lowering at 700 AD and the LIA, relative to 1982, was 63 and 55 m, respectively. </p>

Lake sediment cores collected from four lakes (Upper Fly Lake 61.04&deg;N, 138.09&deg;W, 1326 m a.s.l.; Jenny Lake 61.04&deg;N, 138.36&deg;W, 817 m. a.s.l.; Donjek Kettle 61.69&deg;N, 139.76&deg;W, 732 m a.s.l.; Lake WP02 61.48&deg;N, 139.97&deg;W, 1463 m a.s.l.) in the southwest Yukon provide records of postglacial climatic variability in the region. A 13,000 year pollen record from Upper Fly Lake indicated that herbaceous tundra existed on the landscape from 13.6 to 11 ka, followed by birch shrub tundra until 10 ka, when Picea forests were established in the region. Pollen-, chironomid-, and ostracode-inferred paleoclimate reconstructions showed a long-term cooling with increasing moisture from the late glacial through the Holocene. The early and mid-Holocene were warm and dry, with cool, wet conditions after 4 ka, and warm, dry conditions over the last 100 years. Chironomid accumulation rates provided evidence of millennial-scale climate variability, and the chironomid community responded to rapid climate changes. Late Holocene environmental variability was investigated through the analysis of paleoproduction indices (sediment loss-on-ignition, biogenic silica) and chironomid and ostracode communities. Coherent trends were revealed among the four lakes and pairs of sites located closer together showed more similarities than more distant sites located in similar environments (alpine tundra or boreal forest). Chironomid-inferred paleotemperature estimates are inconsistent with other data from the region, however certain fluctuations in paleoproduction indices and changes in abundance and composition of the chironomid and ostracode communities compare well with interpretations based on independent paleoclimate records. The White River Ash event (1147 cal yrs BP and 1953 cal yrs BP) impacted three of the four aquatic ecosystems studied, with a greater impact occurring at sites with greater ash thickness. Interannual variability in the lake environment is of lesser concern when deriving inference models relating organisms to environmental variables, and the results presented here provide guarded optimism that the sampling methodology applied in paleolimnological studies is appropriate in this region.

The purpose of this thesis was to study long-term climatic variations and their impacts on aquatic ecosystem functioning in the Canadian Arctic. Sub-fossil remains of chironomids preserved in lacustrine sediment cores, and the organic, biogenic silica and carbonate fraction of the sediment matrix were analyzed to infer past lake production and provide new evidence of the impact of past climate changes in the Arctic. The modern relationship between chironomids, biological production indices and the physical environment was studied to permit the interpretation and quantification of past conditions from sediment cores. Bedrock composition exerts a very strong influence on sediment organic, biogenic silica and carbonate content of lake sediments, and an inference model for reconstructing lake water pH based on these parameters was developed. July air temperatures also affect aquatic and terrestrial production in the Arctic, but this effect is secondary to the effect of nutrient input. An inference model for reconstructing past mean July air temperatures based on the chironomid assemblages was developed using 88 Arctic lakes, and further improved by combining it with previously published data from across North America. A new inference model for July air temperatures, based on 379 Arctic and Boreal lakes was developed. This training set was based on sites covering a large temperature gradient (2&deg;-16.6&deg;C) and containing a great diversity of chironomid assemblages, making it appropriate for reconstructing past July air temperatures for Arctic and boreal lakes. Holocene climatic conditions were evaluated for the Arctic at Lake WB02 (Northern Victoria Island), Lake KR02 (Western Victoria Island) and Lake JR01 (Boothia Peninsula). Prior to &sim;6.5-5.5ka the Arctic was relatively warm, and overall lake production was high. Mean July air temperatures remained colder during the mid- to late-Holocene until -1.0ka, when temperatures again cooled rapidly during the Little Ice Age. A recent (last 150 years) warming is seen at all three lakes as primary and chironomid production increased. At all locations, however, inferred temperatures for the last 150 years remain up to 2-3&deg;C cooler than those inferred for the early Holocene.

The cryosphere and its interactions with other components of the climate system are considered to be major influences on global climate change through the Cenozoic and into the future. However, fundamental dynamics and secondary feedbacks that drive long-term ice sheet variability on Greenland remain poorly understood. Here, a numerical climate-ice sheet modeling study is conducted with the aim of reconstructing most likely locations, timing and variability of continental ice in the mid to late Pliocene and the transition into the Pleistocene. Simulations using the GENESIS v3 General Circulation Model coupled to the Penn State Ice Sheet-Shelf Model are compared with a range of independent numerical ice sheet model simulations under Pliocene boundary conditions and validated against available proxy reconstructions. This study aims at investigating the sensitivity of an ice-free and glaciated Greenland to changes in climate forcings, and the modulation of those forcings through internal feedbacks with focus on the dynamical thresholds involved in the growth and retreat of continental ice on Greenland. Orbital changes of latitudinal and seasonal solar radiation, in combination with prevalent atmospheric pCO2 levels, are found to pace the timing of the cryospheric response. Internal feedbacks invoked though local surface characteristics on Greenland in concert with far field changes in Arctic sea surface temperature and sea ice conditions control the energy and moisture budget on Greenland with consequences for its mass balance. In the Pliocene, inception of Greenland ice is inhibited during interglacials and ice volume is limited even when orbits are favorable for ice sheet growth. During Pliocene warmth, a present-day Greenland Ice Sheet cannot be maintained and ice was most likely restricted to the highest elevations in the East and South, contributing ∼6m of equivalent sea level rise. This assessment of the sensitivity and survivability of Greenland Ice Sheet in a warmer-than-modern world implies the potential for a long-term commitment to future sea level rise from a smaller Greenland Ice Sheet.

Two lakes were studied in detail for rock magnetic properties: Lake El'gygytgyn, a crater lake formed 3.6Ma in the Far Eastern Russian Arctic, and Heimerdalsvatnet, a Holocene coastal lake located in the Lofotens off the coast of northern Norway. These two lakes have vastly different environmental histories, the former a terrestrial lake formed from a meteor impact and never covered by continental ice sheets whereas the latter went from a coastal marine setting to a completely lacustrine environment due to isostatic rebound and sea level fluctuations. Their differences are considerable, however they provide the opportunity to compare Arctic lake systems to discern similarities and differences in their magnetic properties for application to future climatic investigations. Paleomagnetic measurements and down-core magnetic susceptibility were performed at the GFZ German Research Centre for Geosciences in Potsdam for Lake El'gygytgyn and at the Laboratoire de paleomagnétisme sédimentaire at ISMER for Heimerdalsvatnet. Rock magnetic properties were measured at the University of Massachusetts Amherst, Institute of Rock Magnetism, and/or Trinity College. These measurements included: magnetic susceptibility, hysteresis parameters, Curie temperatures, and low-temperature magnetic behavior. Imaging of magnetite grains was also performed. Magnetic susceptibility measurements in Lake El'gygytgyn suggested a correlation between glacials (interglacials) and low (high) susceptibility. The large range in susceptibility indicated there could be magnetite dissolution. The first study supported this hypothesis with evidence at low temperatures (10-35K) of minerals such as siderite, rhodochrosite, and/or vivianite which could form from iron released during dissolution. Marine Isotope Stage 31 was investigated for rock magnetic properties that could continue to support or oppose findings from the first study. It was determined the presence of siderite only occurred in interglacial periods whereas its absence (and probably presence of vivianite) related to glacial periods, indicating more reduced environments during glacials versus interglacials. Heimerdalsvatnet paleomagnetic data from the marine environment (lower part of the core) revealed scattered directions whereas data from the upper part of the core (lacustrine environment) showed better consistency. Rock magnetic measurements showed some variation downcore, however the measurements are not dependable since the amount of paramagnetic material was overwhelming compared to any ferromagnetic mineral present.

Inaccessibility due to harsh weather conditions and perennial sea ice has left the Weddell Sea embayment (WSE) vastly under-studied in comparison to other regions of Antarctica. Yet understanding its deglacial history since the Last Glacial Maximum (LGM) is vital for understanding the dynamics and stability of the Antarctic ice sheet. Additionally, the debate continues as to the magnitude and timing of West Antarctic Ice Sheet (WAIS), Antarctic Peninsula Ice Sheet (APIS) and East Antarctic Ice Sheet (EAIS) advance during the LGM. Here we present geologic and geophysical evidence from the southern and eastern continental shelves of the WSE that show diachronous retreat by the WAIS and EMS. Detailed analysis of sediment cores display a retreat stratigraphy in the WSE with distal glacimarine sediments overlying proximal glacimarine deposits and till. These results, in combination with AMS radiocarbon ages, demonstrate that the grounding line of the EAIS was very near that of present day as early as 30,476 cal yr BP and indicate little, if any, advance of the EMS during the LGM. In contrast, multibeam swath bathymetry data show mega-scale glacial lineations, indicative of grounded, flowing ice in two troughs on the southern continental shelf, which drain ice from the WAIS. Although there are no radiocarbon ages to absolutely constrain the timing of this grounding event on the southern continental shelf, we interpret the lineations as LGM age based on their pristine nature. Further, there are similar geomorphic features on the western continental margin where the LGM timing of APIS advance has been demonstrated. Thus, during the LGM, the Antarctic ice sheets behaved independently in the WSE.

Studying the mean state and variability of past climate provides important insight into the dynamically coupled climate system, directly aiding projections of future climate. Reconstruction of past climate conditions can be achieved using geochemical proxies including the novel clumped isotope paleothermometer. In this thesis I use multiple proxies to study climate variability during the last glacial period and at the onset of Antarctic glaciation. Greenland ice cores record repeated millennial-scale fluctuations in climate during the last glacial period known as Dansgaard-Oeschger (DO) cycles. We measure 18O of bulk sediment and planktonic foraminifera (Neogloboquadrina Pachyderma) in sediment cores from the North Atlantic to investigate fluctuations in sediment properties on the timescale of DO cycles. We find evidence of episodic deposition of carbonate ice-rafted debris near Iceland. Integrating these observations with published data and modeling studies, we propose a new hypothesis to explain DO cycles. We suggest that a large ice shelf in the Nordic Seas acted in concert with sea ice to set the slow and fast timescales of DO cycles. The ice shelf was periodically removed by subsurface warming with the timescale of shelf regrowth setting the duration of each interstadial. We utilize the clumped isotope proxy to reconstruct the climate history during a key period of the Cenozoic - the onset of Antarctic glaciation. To facilitate this work, a new inlet is developed to streamline sample preparation and reduce sample size requirements. We decrease the required sample size from 5-8mg to 1-2mg per replicate, while still achieving external precision of 0.005-0.010o/oo, equivalent to previous methods. This new capability increases the range of possible applications for the clumped isotope paleothermometer, specifically in the field of paleoceanography. We apply the clumped isotope paleothermometer to thermocline-dwelling foraminifera (Subbotina angiporoides and Subbotina utilisindex) from the Southern Ocean core ODP689 across the Eocene-Oligocene transition. With the clumped isotope paleothermometer we separate the contributions of near- surface temperature change and ice sheet growth on the ~1o/oo increase in 18O observed in planktonic foraminifera from this site. We measure no change in temperature, and 0.8±0.2o/oo change in 18Osw, equivalent to 124-140% of the modern Antarctic ice sheet volume. / Earth and Planetary Sciences

Stable isotopes of water preserved in glaciers and icesheets have revolutionized our understanding of terrestrial paleoclimate. Post-deposition alteration of the stable water isotope ratios in snow and ice can obscure the original meteoric signal, therefore altering the interpretation of δ18O and δD as records of paleo-temperatures in ice. The effects of sublimation on δ18O and δD are not well-understood for massive (non-snow) ice and have been largely overlooked, particularly within the experimental literature. We present results from a series of environmental chamber experiments investigating alteration of the in-situ signal following sublimation. Our data suggest that sublimation enriches the ice remaining after sublimation in 18O and D. This is observed both in surface ice and in the signal of the ice at depth. These results could have important implications for studies utilizing surface ice δ18O and δD for reconstructions of paleoclimate.

No description available.

No description available.