Oceanic circulation remains one of the poorly understood elements of the global climate system, despite its importance to planetary heat redistribution and carbon cycling. The nature of deep-water formation and circulation in ancient oceans are even more poorly constrained. In order to understand climate dynamics of past and future climates we must have a better understanding of the role of deep-ocean circulation. In this dissertation I investigated changes in intermediate- to deep-water circulation in three different ocean basins during two different geologic eras. The first study focused on the late Pleistocene (~25 ? 60 ka) California margin to investigate the role of intermediate water circulation in abrupt climate fluctuations. The other two studies investigated deep-water circulation during the Late Cretaceous (~70 ? 100 Ma) greenhouse interval, to determine if deep waters formed in the southern Indian or Atlantic basins. The above studies employed neodymium isotopes preserved in biogenic apatite (fish teeth and bones) and foraminiferal calcite to reconstruct the provenance of intermediate- to deep-water masses. Here I present data from two sites located at intermediate depths on the late Pleistocene California margin as well as seven Deep Sea Drilling Project and Ocean Drilling Program Cretaceous aged sites; four in the South Atlantic Ocean, and three in the Indian Ocean. The new Pleistocene data rule out changes in the source of intermediate waters to the California margin, thus the recorded changes in seafloor oxygenation were caused by changes in sea surface productivity. In the Cretaceous, the spread of deep waters formed in the high-latitude South Atlantic was hindered by tectonic barriers until the mid Campanian when the subduction of Rio Grande Rise allowed for the continuous flow of deep waters from the Southern Ocean into the North Atlantic. The deep Cretaceous Indian Ocean was filled with deep waters formed in the high-latitude Indian Ocean, until being replaced with waters sourced in the Pacific from the late Cenomanian to early Campanian before a return to southern Indian-sourced waters for the remainder of the study interval.

No description available.

Diatom analysis performed on sediment profiles from several localities provided information on past changes in the aquatic environment related to climatic and artificial shifts since the Last Glacial-Interglacial Transition. The practicality of diatom analysis from fish guts of three European fish species in palaeolimnological studies was tested.

This doctoral dissertation presents a study of the glacial history of the North American Cordillera using numerical ice sheet modelling calibrated against field evidence. This area, characterized by the steep topography of several mountain ranges separated by large inter-montane depressions, was once covered by a large-scale ice mass: the former Cordilleran ice sheet. Because of the irregular topography on which the ice sheet formed, geological studies have often had only local or regional relevance, thus leaving the Cordilleran ice sheet least understood among Pleistocene ice sheets in terms of its extent, volume, and dynamics. Here, I present numerical simulations that allow quantitative reconstructions of the former ice sheet evolution based on approximated physics of glacier flow. These simulations show that the geometry of the Last Glacial Maximum Cordilleran ice sheet was largely controlled by sharp contrasts in regional temperature, precipitation, and daily temperature variability associated with the presence of mountain ranges. However, this maximum stage appears short-lived and out of balance with contemporaneous climate. During most of the simulated last glacial cycle, the North American Cordillera is characterized by an intermediate state of glaciation including isolated glaciers and ice caps covering major mountain ranges, the largest of which is located over the Skeena Mountains. The numerically modelled Cordilleran ice sheet appears in constant imbalance with evolving climate conditions, while the complexity of this transient response transcends that encapsulated in two-dimensional, conceptual models of ice sheet growth and decay. This thesis demonstrates the potential of numerical ice sheet modelling to inform on ice sheet history and former climate conditions over a glacial cycle, given that ice sheet models can be calibrated against field constraints. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Mansucript.</p>

In order to determine the timing of Antarctic ice sheet retreat and advance during the Late Quaternary, various tools are used to measure the age of marginal marine sediments. Carbonate 14C dating is a well-established approach, but requires foraminiferal microfossils, shells or other carbonate materials that are rare in most Antarctic regions, and may also suffer from vital effects, which can result in variability of up to 500 years in living organisms. Bulk acid insoluble organic (AIO) 14C dates are frequently as an alternative, but this approach works best where high productivity and sedimentation rates reign, and not too well in condensed sequences where high proportions of detritus are present. Compound specific dating methods have also been employed, but these may still yield an average age from a mixture of components and require very large sample sizes. Alternate methods of applying a chronology have also been used, such as magnetic intensity dating, or regional correlation with well-dated cores, but these may not always provide accurate and precise dates. Here I present work, some published with co-authors, of progressive improvements of Ramped PyrOx 14C dating, which utilizes the thermochemical degradation of components within a bulk AIO sediment sample. This dissertation focuses on the study, improvement and application of advanced Ramped PyrOx techniques. These improvements include novel techniques, such as compositing and isotope dilution that I use to date sediments where the proportion of contemporaneously deposited carbon is very small relative to other detrital components, and maximize the accuracy of resulting dates while minimizing costs in precision from utilizing ultra-small fractions of the bulk sample. Ramped PyrOx 14C dating techniques allows us to generate chronologies for cores that would otherwise go undated. Furthermore, these techniques can be used to push the limits of radiocarbon dating not only to regions where accurate core chronologies have been difficult to come by, but also further back in time, into marine sediment horizons deposited at or before the last glacial maximum (LGM), where highly detrital material has precluded radiocarbon dating in the past. Wider use of these techniques can enable more coordinated a priori coring efforts to constrain regional glacial responses to rapid warming.

Implementing oxygen isotopes (H218O, H216O) in coupled climate models provides both an important test of the individual model's hydrological cycle, and a powerful tool to mechanistically explore past climate changes while producing results directly comparable to isotope proxy records. The addition of oxygen isotopes in the University of Victoria Earth System Climate Model (UVic ESCM) is described. Equilibrium simulations are performed for preindustrial and Last Glacial Maximum (LGM) conditions. The oxygen isotope content in the model's preindustrial climate is compared against observations for precipitation and seawater. The distribution of oxygen isotopes during the LGM is compared against available paleo-reconstructions. Records of temporal variability in the oxygen isotopic composition of biogenic carbonates from ocean sediment cores inform our understanding of past continental ice volume and ocean temperatures. Interpretation of biogenic carbonate d18O variability typically neglects changes due to factors other than ice volume and temperature, equivalent to assuming constant local seawater isotopic composition. This investigation focuses on whether sea ice, which fractionates seawater during its formation, could shift the isotopic value of seawater during distinct climates. Glacial and interglacial states are simulated with the isotope-enabled UVic ESCM, and a global analysis is performed. Results indicate that interglacial-glacial sea ice variability produces as much as a 0.13 permil shift in local seawater, which corresponds to a potential error in local paleotemperature reconstruction of approximately 0.5 C. Isotopic shifts due to sea ice variability are concentrated in the Northern Hemisphere, specifically in the Labrador Sea and northeastern North Atlantic. / Graduate

Thesis (M.S.) in Earth Sciences--University of Maine, 2008. / Includes vita. Includes bibliographical references (leaves 79-88).

Thesis (M. S.)--Earth and Atmospheric Sciences, Georgia Institute of Technology, 2009. / Committee Chair: Jean Lynch-Stieglitz; Committee Co-Chair: Emanuele Di Lorenzo; Committee Member: Annalisa Bracco; Committee Member: Robert Black

We investigate the effect of a 120 meter sea level drop on transport through the Caribbean Sea and the Florida Straits during the Last Glacial Maximum (LGM) relative to the present, using the Regional Ocean Modeling System (ROMS). A geostrophic transport estimate for the Florida Straits suggests the LGM Florida Current was weaker than today by one third, inferring a likely decrease in the North Atlantic overturning circulation by 12-15 Sv. A possible impact of a shallower LGM Florida Straits sill depth on the Florida Current has been suggested. Our model results show that the volume transport through the Florida Straits is slightly reduced in a lower sea level model simulation when compared to a control sea level simulation (34.8 ± 2.0 Sv vs. 39.8 ± 2.3 Sv). The difference in transport is of the order of 5 Sv, representing a maximum limit to the LGM flow reduction due to sea level change. Therefore the change in sill depth between the LGM and the present is unlikely to have been a cause of the entire observed flow reduction.

The shape of the thermocline and the depth of the lysoline in the western tropical Pacific are both influenced by the overlying atmosphere, and both the shape of thermocline and the depth of the lysocline can be reconstructed from foraminifera-based paleo-proxies. Paleoclimate proxy evidence suggests a southward shift of the Intertropical Convergence Zone (ITCZ) during times of Northern Hemisphere cooling, including the Last Glacial Maximum (LGM), 19-23 ka before present. However, evidence for movement over the Pacific has mainly been limited to precipitation reconstructions near the continents, and the position of the Pacific marine ITCZ is less well constrained. In this study, I address this problem by taking advantage of the fact that the upper ocean density structure reflects the overlying wind field. I reconstruct changes in the upper ocean density structure during the LGM using oxygen isotope measurements on the planktonic foraminifera G. ruber and G. tumida in a transect of sediment cores from the Western Tropical Pacific. The data suggest a ridge in the thermocline just north of the present-day ITCZ persists for at least part of the LGM, and a structure in the Southern Hemisphere that differs from today. The reconstructed structure is consistent with that produced in a General Circulation Model with both a Northern and Southern Hemisphere ITCZ. I also attempt to reconstruct the upper ocean density structure for Marine Isotope Stages 5e and 6, the interglacial and glacial periods, respectively, previous to the LGM. The data show a Northern Hemisphere thermocline ridge for both of these periods. There is insufficient data to draw any conclusions about the Southern Hemisphere thermocline. Using the same set of sediment cores, I also attempt to reconstruct lysocline depth over the last 23,000 years using benthic foraminiferal carbon isotope ratios, planktonic foraminiferal masses, and sediment coarse fraction percentage. Paleoclimate proxy evidence and modeling studies suggest that the deglaciation following the LGM is associated with a deepening of the lysocline and an increase in sedimentary calcite preservation. Although my data lack the resolution to constrain the depth of the lysocline, they do show an increase in calcite preservation during the last deglaciation, consistent with lysocline deepening as carbon moves from the deep ocean to the atmosphere.