It has been commonly recognized that a series of global glaciation events occurred during the late Neoproterozoic Era (800 - 540 million years ago (Ma)). However, the extent of these glaciations continues to be hotly debated, namely whether the whole Earth was ice covered (ie. a “hard snowball”) or only the continents were fully ice covered but the oceans were not (“slushball/soft snowball”). Through a combination of climate modeling and carbon cycle modeling, I have investigated the plausibility of the “soft snowball” Earth hypothesis. It is demonstrated that the flow of land ice is critical to the formation of a “soft snowball”, such that low latitude land ice must be generated through ice transported from high latitudes. In order for a climate state of this kind to be realizable, continental fragments at low latitude must be well connected to those at high latitude, and the high latitude continents must be sufficiently extensive that a large ice sheet may initiate and subsequently flow to low latitude. It is found that these constraints are satisfied by the most accurate available continental reconstruction for both the initial Sturtian glaciation of the late Neoproterozoic and the subsequent Marinoan event. It is furthermore proposed that the alternative “hard snowball” hypothesis would have been prevented by a negative feedback due to the enhanced remineralization of dissolved organic carbon (DOC) in the ocean due to increased oxygen solubility in seawater at lower temperature. This process would release CO2 to the atmosphere, thus counteracting the initial climate cooling. I have also carried out detailed simulations in which an explicit model of the carbon cycle is coupled to the ice-sheet coupled climate model to investigate this feedback quantitatively. It is found that the remineralization of the DOC does indeed provide a strong negative feedback that counteracts climate cooling. The action of this feedback not only prevents the descent of the climate into a hard snowball state, but also enables the model to re-produce the δ13C carbon isotopic anomalies observed to accompany Neoproterozoic glacial events. The resistance of this carbon cycle coupled climate system to descent into a “hard snowball” state is further tested against stochastic perturbations, and shown to be robust in the presence of such influence.

It has been commonly recognized that a series of global glaciation events occurred during the late Neoproterozoic Era (800 - 540 million years ago (Ma)). However, the extent of these glaciations continues to be hotly debated, namely whether the whole Earth was ice covered (ie. a “hard snowball”) or only the continents were fully ice covered but the oceans were not (“slushball/soft snowball”). Through a combination of climate modeling and carbon cycle modeling, I have investigated the plausibility of the “soft snowball” Earth hypothesis. It is demonstrated that the flow of land ice is critical to the formation of a “soft snowball”, such that low latitude land ice must be generated through ice transported from high latitudes. In order for a climate state of this kind to be realizable, continental fragments at low latitude must be well connected to those at high latitude, and the high latitude continents must be sufficiently extensive that a large ice sheet may initiate and subsequently flow to low latitude. It is found that these constraints are satisfied by the most accurate available continental reconstruction for both the initial Sturtian glaciation of the late Neoproterozoic and the subsequent Marinoan event. It is furthermore proposed that the alternative “hard snowball” hypothesis would have been prevented by a negative feedback due to the enhanced remineralization of dissolved organic carbon (DOC) in the ocean due to increased oxygen solubility in seawater at lower temperature. This process would release CO2 to the atmosphere, thus counteracting the initial climate cooling. I have also carried out detailed simulations in which an explicit model of the carbon cycle is coupled to the ice-sheet coupled climate model to investigate this feedback quantitatively. It is found that the remineralization of the DOC does indeed provide a strong negative feedback that counteracts climate cooling. The action of this feedback not only prevents the descent of the climate into a hard snowball state, but also enables the model to re-produce the δ13C carbon isotopic anomalies observed to accompany Neoproterozoic glacial events. The resistance of this carbon cycle coupled climate system to descent into a “hard snowball” state is further tested against stochastic perturbations, and shown to be robust in the presence of such influence.

In order to understand and predict climate in a world driven by anthropogenic influences, increased understanding of natural climate variability is vital. The doctoral dissertation presented here focuses on multi-decadal climate modes, and in particular, the Atlantic Multi-decadal Oscillation (AMO), and how it is expressed in the tropical to sub-tropical western Atlantic Ocean over the last ~700 years. Multi-decadal modes have been linked to weather around the Atlantic, and have also been correlated with the occurrence of anomalous climatic events (in both temperature and precipitation), although the relationship of these modes to anthropogenic impacts is still undefined. The study area for this project encompasses South Florida, the Bahamas, and the Caribbean; however, the dissertation begins with a review article covering the current published records and reconstructions of multi-decadal variability from the Atlantic sector. Beyond this, the research turns to the reconstruction of multi-decadal variability from biogenic carbonates, including corals and sclerosponges. Because Atlantic Multi-decadal Variability (AMV) is still poorly understood and current reconstructions diverge beyond the instrumental period, the use of long-lived coral and sclerosponge specimens allows for an examination of multi-decadal variability over the last several centuries. Two multi-century records of temperature and salinity are reconstructed from the geochemistry of a coral from coastal Florida and sclerosponge collected in the Bahamas. Additional analyses from corals collected throughout the Lesser Antilles are also examined to determine their sensitivity to Atlantic AMV for future work. The final chapter of this dissertation presents a study investigating the reproducibility and fidelity of some of the geochemical proxy techniques utilized in biogenic carbonates.

Marine sediment samples were collected from Quebrada la Higuera, Chile to provide a record of planktonic and benthic foraminifera as a means of reconstructing the paleo-bathymetry and paleo-environment of the Quebrada la Higuera basin. The research area is located in northern Chile, south of Caldera in Quebrada la Higuera (S 27° 16.443, W 70° 54.887). The Quebrada la Higuera basin rests unconformably atop Mesozoic gabbroic basement, which is locally exposed, representing fault-bounded horsts and grabens. At the base of the section, diatomaceous sediment within the sequence suggests intervals of high productivity. The co-occurrence of the shallower water benthic species Nonionella miocenica and Epistominella pulchella with deep-water species Bolivina tumida, Uvigerina peregrina, Globobulimina auriculata, and Bolivina subfusiformis, at 10-23 m above the base of the section and again at 44-62 m, within fining-upward sedimentary packages suggests possible turbidites caused by transgressive-regressive cycles. Additionally, the occurrence and high abundance of these species are indicative of high productivity. A 30-40 cm Late Miocene phosphatic bed consisting of vertebrate and invertebrate fossils lies unconformably atop a poorly consolidated sand ~42 m from the base of the section. This phosphatic bed suggests that upwelling was occurring off the coast of Chile earlier than previously believed. The hyperaridity of Chile's Atacama Desert has been the result of the cold upwelling Peru-Chile current, which is further intensified by the rain-shadow effect of the Andes and the cold, dry descending Hadley cell. The occurrence of planktonic foraminifer Globigerina bulloides at the base of the section establishes the maximum possible lower age limit of 15.97 Ma (Middle Miocene). At 47 m, the co-occurrence of Globorotalia margaritae, Globorotalia crassaformis, and Dentogloboquadrina altispira establishes a maximum age limit for that horizon of ~3.6 Ma (Early Pliocene), providing an age range for the Quebrada la Higuera section from Middle Miocene through Early Pliocene. This study provides a record of planktonic and benthic foraminifera as a means of reconstructing the paleohistory of Quebrada la Higuera and more broadly through the Caldera region. The analyses address interactions between uplift, atmospheric conditions, sea level, and upwelling within the region from interpretations of the composite section, including facies relationship between sediments and occurring foraminifera species. These observations, along with the foraminiferal record, help broaden the understanding and development of the Quebrada la Higuera Neogene record.

My study was designed to identify the relationships between climate variables, such as precipitation and temperature, and δ 18 O values of tree ring &alpha-cellulose extracted from exactly dated tree rings of Pterocarpus angolensis growing in the arid to semiarid Mzola region of western Zimbabwe. This species is known to be sensitive to seasonal variation in rainfall. In this region, the wet season occurs during the austral summer from mid November to early April followed by a dry winter season from around June through October. Overall, the total annual rainfall exhibits a high degree of spatial and temporal variation with a mean of less than 600 mm per year. I applied the Modified Brendel technique to isolate &alpha-cellulose from raw wood samples extracted from two P. angolensis trees and measured the α-cellulose δ 18 O values using continuous flow isotope ratio mass spectrometry. I developed a 30-year (1955-1984) &alpha-cellulose δ 18 O chronology and correlated it with tree-ring width, meteoric water δ 18 O values, monthly and seasonal precipitation totals, and mean seasonal temperature. The δ 18 O values of meteoric water for this region were obtained from the Global Network of Isotopes in Precipitation (GNIP) and correlated with the δ 18 O values of tree ring &alpha-cellulose. The strongest correlations were observed between &alpha-cellulose δ 18 O values and February total precipitation (r = -0.49, p = 0.006) and to a lesser degree total wet season (NDJFMA) precipitation, In particular, unusually rainy wet seasons were significantly correlated (r = -0.79, P = 0.007) with &alpha-cellulose δ 18 O. This relationship is consistent with 18 O-depleted values measured in summer precipitation during periods of high rainfall, which is most likely the result of the isotopic amount effect reported in tropical regions. I also identified a positive correlation (r = 0.49, p = 0.03) between &alpha-cellulose δ 18 O and the δ 18 O values of meteoric water, and investigated the possibility of an isotopic temperature effect for δ 18 O in meteoric water, which also may be reflected in the δ 18 O values in tree ring &alpha-cellulose. The strongest correlation with mean temperature was observed during the wet summer season (r = 0.56, p = 0.01). My results suggest that the δ 18 O values of P. angolensis tree rings can be used as natural indicators of paleoclimate in southern Africa.

The middle Miocene thermal maximum (~16 Ma) was a period of global climate unusually warm and wet for the Neogene and is of interest as a paleo-analog for future climate change from anthropogenic global warming. In Australia, paleosols of the Oligocene-Miocene Etadunna and Pliocene Tirari Formations formed in arid palaeoclimates and include pedogenic gypsum. The Middle Miocene paleosol has shallow calcareous nodules and stout root traces suggesting vegetation like dry woodland. Comparable mallee vegetation now grows no closer than 1200 km to the southwest, so middle Miocene warm-wet climate enabled range extension of mallee and woody thickening of plants in the Australia outback. There is no evidence in the outback of middle Miocene rain forest, which may have expanded its range to form kaolinitic Ultisols near Sydney, Mudgee, and Gulgong, in New South Wales. Nor is there evidence so far inland of swamp woodlands and heaths like those producing brown coals in the Latrobe Valley, Victoria. In Argentina, the Santa Cruz Formation, in addition to its rich fossil vertebrate assemblages, contains a similar paleosol record of middle Miocene climate change. The early Miocene in the Santa Cruz Formation is characterized by a thick sequence of weakly to strongly developed grassland paleosols (Orthents and Ustolls) as well as paleosols with root traces and profile forms of open shrubland vegetation (Cambid). The middle Miocene thermal maximum is characterized by paleosols (Udepts, Udalfs) developed under open woodland-shrubland vegetation, during a period of climatic warmth and humidity. The late middle Miocene is characterized by a greater diversity of pedotypes, all suggestive of an arid and cooler environment (Argids, Cambids, and Ustepts). Middle Miocene soil maps compiled for this study show extension of tropical soils (Oxisols, Ultisols) into northern and southern mid-latitudes, accompanied by thermophilic flora and fauna. Peats, lignites, and Histosols of wetlands are also more abundant at higher latitudes, especially in the northern hemisphere, during the middle Miocene. The expansion of such soils is an expected result of greater precipitation associated with higher water vapor content of a warmer atmosphere during the Middle Miocene. This dissertation includes previously published co-authored material in chapter II.

Paleo data-model comparison is the process of comparing output from model simulations of past periods with paleoenvironmental data. It enables us to understand both the paleoclimate mechanism and responses of the earth environment to the climate and to evaluate how models work. This dissertation has two parts that each involve the development and application of approaches for data-model comparisons. In part 1, which is focused on the understanding of both past and future climatic changes/variations, I compare paleoclimate and historical simulations with future climate projections exploiting the fact that climate-model configurations are exactly the same in the paleo and future simulations in the Coupled Model Intercomparison Project Phase 5. In practice, I investigated large-scale temperature responses (land-ocean contrast, high-latitude amplification, and change in temperature seasonality) in paleo and future simulations, found broadly consistent relationships across the climate states, and validated the responses using modern observations and paleoclimate reconstructions. Furthermore, I examined the possibility that a small set of common mechanisms controls the large-scale temperature responses using a simple energy-balance model to decompose the temperature changes shown in warm and cold climate simulations and found that the clear-sky longwave downward radiation is a key control of the robust responses. In part 2, I applied the equilibrium terrestrial biosphere models, BIOME4 and BIOME5 (developed from BIOME4 herein), for reconstructing paleoclimate. I applied inverse modeling through the iterative forward-modeling (IMIFM) approach that uses the North American vegetation data to infer the mid-Holocene (MH, 6000 years ago) and the Last Glacial Maximum (LGM, 21,000 years ago) climates that control vegetation distributions. The IMIFM approach has the potential to provide more accurate quantitative climate estimates from pollen records than statistical approaches. Reconstructed North American MH and LGM climate anomaly patterns are coherent and consistent between variables and between BIOME4 and BIOME5, and these patterns are also consistent with previous data synthesis. This dissertation includes previously published and unpublished coauthored material.

Lake sediment cores collected from four lakes (Upper Fly Lake 61.04°N, 138.09°W, 1326 m a.s.l.; Jenny Lake 61.04°N, 138.36°W, 817 m. a.s.l.; Donjek Kettle 61.69°N, 139.76°W, 732 m a.s.l.; Lake WP02 61.48°N, 139.97°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°-16.6°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 ∼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°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.