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• 11. [Article] Late Cenozoic Climate, Ice-sheet and Earth-surface Evolution Derived from Terrestrial and Marine Sedimentary Archives

  The goals of this dissertation are centered on understanding changes in Earth surface and climate systems through the use of geologic proxies as records of past changes in these systems. Specifically, ...

  Citation × Citation Citation Abstract Copy Title: Late Cenozoic Climate, Ice-sheet and Earth-surface Evolution Derived from Terrestrial and Marine Sedimentary Archives Author: Bill, Nicholas S. The goals of this dissertation are centered on understanding changes in Earth surface and climate systems through the use of geologic proxies as records of past changes in these systems. Specifically, this dissertation (1) establishes a new chronology for retreat of the Ross Sea sector of the West Antarctic Ice during the last deglaciation, (2) investigates the changes in the global climate system during the mid- Pleistocene transition, and (3) constrains the timing of surface uplift in Alaska, and the control that topography has on regional climate and the hydrologic cycle over the tectonic timescale. This dissertation establishes the timing of the last deglaciation of the Ross Sea Sector of the West Antarctic Ice Sheet (WAIS), addressing the question of whether grounded ice in the Ross Embayment deglaciated entirely in the Holocene or earlier. ¹⁰Be surface exposure dating of granitic glacial erratic boulders indicates that the onset of sustained retreat from the local Last Glacial Maximum started at 18.6 ± 1.1 ka, and that sustained retreat occurred into the middle Holocene. We attribute most of this retreat to temperature and radiative forcing of the ablation zone in McMurdo Sound with only the final stages of retreat possibly influenced by Holocene grounding line retreat. The mid-Pleistocene transition (MPT) represents a transition from predominantly 40-kyr climate cycles to predominantly 100-kyr cycles in the absence of any change in orbital forcing. Here I combine all existing records of SST (n=7) and δ¹³C (n=17) that span the entirety of the last 2 Ma and use principal component analysis to detect the shared global signal of these records across the MPT. I also develop stacks of ice volume, ocean basin-scale δ¹³C gradients and CO₂ reconstructions in an attempt to characterize the interaction between deep ocean circulation and climate change across the MPT. I find that the characteristic change in cyclicity of the MPT appears in SST, δ¹³C and ice-volume reconstructions. I interpret marine isotope stage (MIS) 23 (~900 ka) as a skipped interglacial that led to MIS 25- 21 as being the first 100-kyr period, potentially initiating the 100-kyr cycle during the rest of the Pleistocene. I also find that the largest global negative δ¹³C excursion in the Pleistocene occurred during the glacial periods MIS 24 and MIS 22. This excursion is likely related to a mean shift to a reduced glacial-period AMOC across the MPT that is observed in multiple AMOC strength reconstructions, and that I interpret as a key component of the MPT. The topography of southern Alaska has been shown to have likely experienced rapid exhumation during the early Pliocene stating ~5-6 Ma. Using δD measurements on OH- groups of clay minerals, I constrain the change in isotopic composition of paleo-meteoric water from the Miocene-Holocene in the interior of Alaska as a proxy for the surface uplift history of Alaskan topography via its control on the δD signal of surface water. My results suggest that there was rapid surface uplift of the several mountain ranges encompassing southern Alaska during the period from ~6.5-3 Ma. Title: Late Cenozoic Climate, Ice-sheet and Earth-surface Evolution Derived from Terrestrial and Marine Sedimentary Archives Author: Bill, Nicholas S. The goals of this dissertation are centered on understanding changes in Earth surface and climate systems through the use of geologic proxies as records of past changes in these systems. Specifically, this dissertation (1) establishes a new chronology for retreat of the Ross Sea sector of the West Antarctic Ice during the last deglaciation, (2) investigates the changes in the global climate system during the mid- Pleistocene transition, and (3) constrains the timing of surface uplift in Alaska, and the control that topography has on regional climate and the hydrologic cycle over the tectonic timescale. This dissertation establishes the timing of the last deglaciation of the Ross Sea Sector of the West Antarctic Ice Sheet (WAIS), addressing the question of whether grounded ice in the Ross Embayment deglaciated entirely in the Holocene or earlier. ¹⁰Be surface exposure dating of granitic glacial erratic boulders indicates that the onset of sustained retreat from the local Last Glacial Maximum started at 18.6 ± 1.1 ka, and that sustained retreat occurred into the middle Holocene. We attribute most of this retreat to temperature and radiative forcing of the ablation zone in McMurdo Sound with only the final stages of retreat possibly influenced by Holocene grounding line retreat. The mid-Pleistocene transition (MPT) represents a transition from predominantly 40-kyr climate cycles to predominantly 100-kyr cycles in the absence of any change in orbital forcing. Here I combine all existing records of SST (n=7) and δ¹³C (n=17) that span the entirety of the last 2 Ma and use principal component analysis to detect the shared global signal of these records across the MPT. I also develop stacks of ice volume, ocean basin-scale δ¹³C gradients and CO₂ reconstructions in an attempt to characterize the interaction between deep ocean circulation and climate change across the MPT. I find that the characteristic change in cyclicity of the MPT appears in SST, δ¹³C and ice-volume reconstructions. I interpret marine isotope stage (MIS) 23 (~900 ka) as a skipped interglacial that led to MIS 25- 21 as being the first 100-kyr period, potentially initiating the 100-kyr cycle during the rest of the Pleistocene. I also find that the largest global negative δ¹³C excursion in the Pleistocene occurred during the glacial periods MIS 24 and MIS 22. This excursion is likely related to a mean shift to a reduced glacial-period AMOC across the MPT that is observed in multiple AMOC strength reconstructions, and that I interpret as a key component of the MPT. The topography of southern Alaska has been shown to have likely experienced rapid exhumation during the early Pliocene stating ~5-6 Ma. Using δD measurements on OH- groups of clay minerals, I constrain the change in isotopic composition of paleo-meteoric water from the Miocene-Holocene in the interior of Alaska as a proxy for the surface uplift history of Alaskan topography via its control on the δD signal of surface water. My results suggest that there was rapid surface uplift of the several mountain ranges encompassing southern Alaska during the period from ~6.5-3 Ma. Close

• 12. [Article] Glacial greenhouse-gas fluctuations controlled by ocean circulation changes

  Earth's climate and the concentrations of the atmospheric greenhouse gases carbon dioxide (CO2) and nitrous oxide (N2O) varied strongly on millennial timescales during past glacial periods. Large and rapid ...

  Citation × Citation Citation Abstract Copy Title: Glacial greenhouse-gas fluctuations controlled by ocean circulation changes Author: Schmittner, Andreas, Galbraith, Eric D. Earth's climate and the concentrations of the atmospheric greenhouse gases carbon dioxide (CO2) and nitrous oxide (N2O) varied strongly on millennial timescales during past glacial periods. Large and rapid warming events in Greenland and the North Atlantic were followed by more gradual cooling, and are highly correlated with fluctuations of N2O as recorded in ice cores. Antarctic temperature variations, on the other hand, were smaller and more gradual, showed warming during the Greenland cold phase and cooling while the North Atlantic was warm, and were highly correlated with fluctuations in CO2. Abrupt changes in the Atlantic meridional overturning circulation (AMOC) have often been invoked to explain the physical characteristics of these Dansgaard–Oeschger climate oscillations, but the mechanisms for the greenhouse-gas variations and their linkage to the AMOC have remained unclear. Here we present simulations with a coupled model of glacial climate and biogeochemical cycles, forced only with changes in the AMOC. The model simultaneously reproduces characteristic features of the Dansgaard–Oeschger temperature, as well as CO2 and N2O fluctuations. Despite significant changes in the land carbon inventory, CO2 variations on millennial timescales are dominated by slow changes in the deep ocean inventory of biologically sequestered carbon and are correlated with Antarctic temperature and Southern Ocean stratification. In contrast, N2O co-varies more rapidly with Greenland temperatures owing to fast adjustments of the thermocline oxygen budget. These results suggest that ocean circulation changes were the primary mechanism that drove glacial CO2 and N2O fluctuations on millennial timescales. Title: Glacial greenhouse-gas fluctuations controlled by ocean circulation changes Author: Schmittner, Andreas, Galbraith, Eric D. Earth's climate and the concentrations of the atmospheric greenhouse gases carbon dioxide (CO2) and nitrous oxide (N2O) varied strongly on millennial timescales during past glacial periods. Large and rapid warming events in Greenland and the North Atlantic were followed by more gradual cooling, and are highly correlated with fluctuations of N2O as recorded in ice cores. Antarctic temperature variations, on the other hand, were smaller and more gradual, showed warming during the Greenland cold phase and cooling while the North Atlantic was warm, and were highly correlated with fluctuations in CO2. Abrupt changes in the Atlantic meridional overturning circulation (AMOC) have often been invoked to explain the physical characteristics of these Dansgaard–Oeschger climate oscillations, but the mechanisms for the greenhouse-gas variations and their linkage to the AMOC have remained unclear. Here we present simulations with a coupled model of glacial climate and biogeochemical cycles, forced only with changes in the AMOC. The model simultaneously reproduces characteristic features of the Dansgaard–Oeschger temperature, as well as CO2 and N2O fluctuations. Despite significant changes in the land carbon inventory, CO2 variations on millennial timescales are dominated by slow changes in the deep ocean inventory of biologically sequestered carbon and are correlated with Antarctic temperature and Southern Ocean stratification. In contrast, N2O co-varies more rapidly with Greenland temperatures owing to fast adjustments of the thermocline oxygen budget. These results suggest that ocean circulation changes were the primary mechanism that drove glacial CO2 and N2O fluctuations on millennial timescales. Close

• 13. [Article] Biogenic and terrigenous sedimetations at Ceara rise, western tropical Altantic, supports Pliocene - Pleistocene deep - water linkage between hemispheres

  Calcium carbonate percentages at five Ceara Rise sites were estimated at 1- to 2-k.y. intervals over the past 5 m.y., using reflectance spectroscopy and magnetic susceptibility proxies. From these estimates ...

  Citation × Citation Citation Abstract Copy Title: Biogenic and terrigenous sedimetations at Ceara rise, western tropical Altantic, supports Pliocene - Pleistocene deep - water linkage between hemispheres Author: Harris, Sara E., King, Terri, Mix, Alan C. Calcium carbonate percentages at five Ceara Rise sites were estimated at 1- to 2-k.y. intervals over the past 5 m.y., using reflectance spectroscopy and magnetic susceptibility proxies. From these estimates and detailed correlations between sites, gradients of calcite and terrigenous sediment accumulation rates in a depth transect of sites reveal variations in local climate and calcite dissolution related to deep-water masses. Relative to shallow sites on the southern Ceara Rise, accumulation rates of terrigenous sediments at deeper sites near the Amazon Fan were higher during glacial periods. Analogous variations in terrigenous sedimentation before the expansion of Northern Hemisphere ice sheets ~3 m.y. ago suggests that tropical climate cycles occurred independently of polar glaciation. Decreasing accumulation rates of calcite with increasing water depth reveal patterns of carbonate dissolution, which varied on orbital time-scales (10- 100 k.y. periods) throughout the Pliocene-Pleistocene. Maximum dissolution at deep relative to shallow sites occurred in the transition from interglacial to glacial conditions, and maximum preservation occurred during global warming, at all orbital periods. If the local dissolution gradient is linked to relative contributions of North Atlantic Deep Water and Antarctic Bottom Water, this phasing of events confirms a key prediction of SPECMAP that deep-water adjustments may translate climate changes between hemispheres. Dissolution and preservation events, however, may also reflect a transient response to a net flux of organic matter between the continents and the oceans during ice-age climate transitions. Title: Biogenic and terrigenous sedimetations at Ceara rise, western tropical Altantic, supports Pliocene - Pleistocene deep - water linkage between hemispheres Author: Harris, Sara E., King, Terri, Mix, Alan C. Calcium carbonate percentages at five Ceara Rise sites were estimated at 1- to 2-k.y. intervals over the past 5 m.y., using reflectance spectroscopy and magnetic susceptibility proxies. From these estimates and detailed correlations between sites, gradients of calcite and terrigenous sediment accumulation rates in a depth transect of sites reveal variations in local climate and calcite dissolution related to deep-water masses. Relative to shallow sites on the southern Ceara Rise, accumulation rates of terrigenous sediments at deeper sites near the Amazon Fan were higher during glacial periods. Analogous variations in terrigenous sedimentation before the expansion of Northern Hemisphere ice sheets ~3 m.y. ago suggests that tropical climate cycles occurred independently of polar glaciation. Decreasing accumulation rates of calcite with increasing water depth reveal patterns of carbonate dissolution, which varied on orbital time-scales (10- 100 k.y. periods) throughout the Pliocene-Pleistocene. Maximum dissolution at deep relative to shallow sites occurred in the transition from interglacial to glacial conditions, and maximum preservation occurred during global warming, at all orbital periods. If the local dissolution gradient is linked to relative contributions of North Atlantic Deep Water and Antarctic Bottom Water, this phasing of events confirms a key prediction of SPECMAP that deep-water adjustments may translate climate changes between hemispheres. Dissolution and preservation events, however, may also reflect a transient response to a net flux of organic matter between the continents and the oceans during ice-age climate transitions. Close

• 14. [Article] Southern Ocean control of glacial AMOC stability and Dansgaard-Oeschger interstadial duration

  Access to this item has been restricted by repository administrators at the request of the publisher until June 30, 2016.

  Citation × Citation Citation Abstract Copy Title: Southern Ocean control of glacial AMOC stability and Dansgaard-Oeschger interstadial duration Author: Buizert, Christo, Schmittner, Andreas Access to this item has been restricted by repository administrators at the request of the publisher until June 30, 2016. Title: Southern Ocean control of glacial AMOC stability and Dansgaard-Oeschger interstadial duration Author: Buizert, Christo, Schmittner, Andreas Access to this item has been restricted by repository administrators at the request of the publisher until June 30, 2016. Close

• 15. [Article] On the origin and timing of rapid changes in atmospheric methane during the Last Glacial Period

  This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/gb/index.shtml.

  Citation × Citation Citation Abstract Copy Title: On the origin and timing of rapid changes in atmospheric methane during the Last Glacial Period Author: Steig, Eric J., Harder, Susan, Brook, Edward J., Severinghaus, Jeff, Sucher, Cara M. This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/gb/index.shtml. Title: On the origin and timing of rapid changes in atmospheric methane during the Last Glacial Period Author: Steig, Eric J., Harder, Susan, Brook, Edward J., Severinghaus, Jeff, Sucher, Cara M. This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/gb/index.shtml. Close

• 16. [Article] Model of late Pleistocene-Holocene variations in rate of sediment accumulation : Panama Basin, eastern equatorial Pacific

  The assumption of constant quartz accumulation for the deep-sea sediment core Y69-106P, taken in the Panama Basin, has been used to date the core and construct a sedimentation rate versus time curve for ...

  Citation × Citation Citation Abstract Copy Title: Model of late Pleistocene-Holocene variations in rate of sediment accumulation : Panama Basin, eastern equatorial Pacific Author: Pisias, Nicklas G. The assumption of constant quartz accumulation for the deep-sea sediment core Y69-106P, taken in the Panama Basin, has been used to date the core and construct a sedimentation rate versus time curve for it. Stratigraphic control for the calculated time scale includes three carbon-14 measurements, the extinction of the radiolarian Stylatractus universus, and correlation with oxygen isotope curves from other dated cores. The model sedimentation rates and sediment composition data allow the determination of sediment accumulation rates for calcium carbonate, opaline silica, and remaining "detritus". Fluctuations in calcium carbonate accumulation rates correlate closely with variations in oxygen isotope ratios in biogenous carbonate from two other equatorial Pacific sediment cores. High oxygen isotope ratios correlate with high calcium carbonate accumulation rates. Opaline silica accumulation rates reflect changes in the dominance of radiolarian fossil groups which can be related to surface circulation in the Panama Basin. The successive maxima in opal and calcium accumulation rates and the oxygen isotope ratios in Y69-106P suggest that at the end of the last glacial period, the rate of calcium carbonate dissolution increased in the Panama Basin first, to be followed by a decrease in the intensity of the eastern equatorial Pacific upwelling, and finally the volume of continental glaciers decreased. Spectral analysis of the rate of accumulation of calcium carbonate in Y69-106P and of the oxygen isotope record of core V28-238 indicate the presence of a 23,000 year periodicity. Analysis of opal accumulation rate suggest a 100,000 year periodicity. These two periodicities are found in fluctuations in the earth's orbital parameters. The eccentricity of the orbit fluctuates with an average period of 98,000 years and the precession of the equinoxes changes with a 22,000 year period. Title: Model of late Pleistocene-Holocene variations in rate of sediment accumulation : Panama Basin, eastern equatorial Pacific Author: Pisias, Nicklas G. The assumption of constant quartz accumulation for the deep-sea sediment core Y69-106P, taken in the Panama Basin, has been used to date the core and construct a sedimentation rate versus time curve for it. Stratigraphic control for the calculated time scale includes three carbon-14 measurements, the extinction of the radiolarian Stylatractus universus, and correlation with oxygen isotope curves from other dated cores. The model sedimentation rates and sediment composition data allow the determination of sediment accumulation rates for calcium carbonate, opaline silica, and remaining "detritus". Fluctuations in calcium carbonate accumulation rates correlate closely with variations in oxygen isotope ratios in biogenous carbonate from two other equatorial Pacific sediment cores. High oxygen isotope ratios correlate with high calcium carbonate accumulation rates. Opaline silica accumulation rates reflect changes in the dominance of radiolarian fossil groups which can be related to surface circulation in the Panama Basin. The successive maxima in opal and calcium accumulation rates and the oxygen isotope ratios in Y69-106P suggest that at the end of the last glacial period, the rate of calcium carbonate dissolution increased in the Panama Basin first, to be followed by a decrease in the intensity of the eastern equatorial Pacific upwelling, and finally the volume of continental glaciers decreased. Spectral analysis of the rate of accumulation of calcium carbonate in Y69-106P and of the oxygen isotope record of core V28-238 indicate the presence of a 23,000 year periodicity. Analysis of opal accumulation rate suggest a 100,000 year periodicity. These two periodicities are found in fluctuations in the earth's orbital parameters. The eccentricity of the orbit fluctuates with an average period of 98,000 years and the precession of the equinoxes changes with a 22,000 year period. Close

• 17. [Article] The use of foraminiferal uranium/calcium ratios as an indicator of changes in seawater uranium content

  We examine the utility of the uranium (U) content of planktonic foraminifera tests as an indicator of past changes in seawater U content. The U/Ca ratio in foraminifera from Atlantic and Caribbean cores ...

  Citation × Citation Citation Abstract Copy Title: The use of foraminiferal uranium/calcium ratios as an indicator of changes in seawater uranium content Author: Emerson, Steven, Peterson, Larry C., Russell, Ann D., Mix, Alan C. We examine the utility of the uranium (U) content of planktonic foraminifera tests as an indicator of past changes in seawater U content. The U/Ca ratio in foraminifera from Atlantic and Caribbean cores in constant in the Holocene and decreases by ~25% during the last glacial period. Magnesium/calcium (Mg/Ca) ratios of the same samples show similar trends. While the timing of the U/Ca changes appears to be associated with glacial-interglacial changes, the magnitude of the change is too large to be caused by variations in the extent of anoxic or suboxic sediments or by changes in riverine input. We assume that the same process produced changes in both U/Ca and Mg/Ca ratios because of a strong correlation between the two ratios. Partial dissolution of the calcite is ruled out, because we observe the same changes in well-preserved cores from basins with opposite dissolution histories. We also reject exchange between foraminiferal and pore water U because of the oxic depositional environment of both cores and because of the consistency in the U/Ca trends from cores in different parts of the ocean. We suggest that the observed foraminiferal U/Ca and Mg/Ca trends may be the result of a temperature effect on the incorporation of these metals. If this is true, it introduces the possibility of a new paleotemperature indicator but complicates the use of U/Ca ratios in planktonic foraminifera tests as an indicator of past seawater U changes. Title: The use of foraminiferal uranium/calcium ratios as an indicator of changes in seawater uranium content Author: Emerson, Steven, Peterson, Larry C., Russell, Ann D., Mix, Alan C. We examine the utility of the uranium (U) content of planktonic foraminifera tests as an indicator of past changes in seawater U content. The U/Ca ratio in foraminifera from Atlantic and Caribbean cores in constant in the Holocene and decreases by ~25% during the last glacial period. Magnesium/calcium (Mg/Ca) ratios of the same samples show similar trends. While the timing of the U/Ca changes appears to be associated with glacial-interglacial changes, the magnitude of the change is too large to be caused by variations in the extent of anoxic or suboxic sediments or by changes in riverine input. We assume that the same process produced changes in both U/Ca and Mg/Ca ratios because of a strong correlation between the two ratios. Partial dissolution of the calcite is ruled out, because we observe the same changes in well-preserved cores from basins with opposite dissolution histories. We also reject exchange between foraminiferal and pore water U because of the oxic depositional environment of both cores and because of the consistency in the U/Ca trends from cores in different parts of the ocean. We suggest that the observed foraminiferal U/Ca and Mg/Ca trends may be the result of a temperature effect on the incorporation of these metals. If this is true, it introduces the possibility of a new paleotemperature indicator but complicates the use of U/Ca ratios in planktonic foraminifera tests as an indicator of past seawater U changes. Close

• 18. [Article] Augmenting and interpreting ice core greenhouse gas records

  The three studies that comprise this dissertation seek to answer significant questions in paleoclimatology through unconventional applications of ice core greenhouse gas data. These studies involve different ...

  Citation × Citation Citation Abstract Copy Title: Augmenting and interpreting ice core greenhouse gas records Author: Rosen, Julia L The three studies that comprise this dissertation seek to answer significant questions in paleoclimatology through unconventional applications of ice core greenhouse gas data. These studies involve different gases and span the interval of time between the Last Glacial Maximum and the Industrial Revolution, but are united by their nontraditional use of greenhouse gases and their attempt to realize the potential for greenhouse gases to reveal important information about Earth’s climate. Ever since their discovery, the abrupt climate changes of the last glacial period known as Dansgaard-Oeschger (D-O) events have proved challenging to explain. The dominant hypothesis involves periodic freshwater discharges into the North Atlantic, which may regulate the strength of the Meridional Overturning Circulation (MOC) and its role in transporting heat to high latitudes. These events were not restricted to the North Atlantic, and can also be recognized in paleoclimate archives around the world. However, numerous uncertainties surrounding the mechanism behind D-O events remain, including how they are communicated to low latitudes and whether other hypotheses can be definitively ruled out. To constrain the mechanism behind abrupt climate changes, we investigate the phasing of climate changes in high- and low-latitude regions at the Bølling Transition, the penultimate abrupt warming event of the last glacial period. We use methane and the ¹⁵N/¹⁴N ratio of N₂ from the North Greenland Eemian (NEEM) ice core, which serve as proxies for tropical climate and Greenland temperature, respectively. We find that these gases change synchronously in the ice core record, and use a firn air model together with a Monte Carlo approach to constrain the phase lag to within several decades. Our results indicate that the mechanism behind the Bølling Transition was capable of rapidly transmitting the climate signal across the planet in a matter of years, and must therefore involve components of the climate system that are suitably reactive. The glacial-interglacial change in atmospheric methane concentrations revealed in ice core records has spurred a decade of debate about its cause. The most likely explanations involve dramatic changes in methane emissions, which originate from both high- and low-latitude wetlands. One method of investigating the changing latitudinal distribution of methane sources is to quantify the difference in methane concentrations between Greenland and Antarctica, which changes in proportion to the fraction of methane produced at high northern latitudes. Previous attempts to determine the methane interpolar difference (IPD) abound, but many have been hampered by complications in synchronizing bipolar ice core records and analytical uncertainties. We present the first continuous estimate of the methane IPD across the termination using high-resolution methane data from the NEEM and West Antarctic Ice Sheet (WAIS) Divide ice cores. Our results reveal the dominant role of tropical sources in driving abrupt changes in atmospheric methane concentrations, and show that boreal methane sources were surprisingly insensitive to dramatic climate changes. We hypothesize that changes in Northern Hemisphere snow and ice cover exerted strong control over tropical methane emissions, while gradually increasing solar insolation and land area allowed boreal sources to grow during the termination. We also investigate the IPD across the major climate transitions of the termination, and during four centennial-scale methane variations, and find opposing trends in boreal and tropical source strengths during these transient events. We propose that temporary decoupling of the locus of interhemispheric mixing, the position of the Intertropical Convergence Zone, and tropical precipitation may explain these results. Atmospheric concentrations of nitrous oxide (N₂O) have risen by ~20% from preindustrial to modern times, but the cause of this increase is not fully understood. The change has been previously attributed to various agricultural activities which perturb microbial processes in soils, but exactly how remains an outstanding question with important implications for future mitigation of N₂O emissions. We present the first measurements of the isotopomers of tropospheric N₂O over the interval from 1450 to 1920 CE. Our results confirm that the preindustrial atmosphere was enriched in all isotopes relative to the modern atmosphere. Furthermore, we estimate that the net anthropogenic source of nitrous oxide must be depleted in all heavy isotopes and have a strong site preference, consistent with a strong role for agricultural emissions and characteristic of N₂O derived from nitrification. We also find a large oscillation in the site preference of the ¹⁵N in N₂O during the Little Ice Age between 1500 and 1700 CE. We hypothesize that this excursion may be due to changing climate conditions that led to an increase in the amount of N₂O produced by nitrification vs. denitrification. Title: Augmenting and interpreting ice core greenhouse gas records Author: Rosen, Julia L The three studies that comprise this dissertation seek to answer significant questions in paleoclimatology through unconventional applications of ice core greenhouse gas data. These studies involve different gases and span the interval of time between the Last Glacial Maximum and the Industrial Revolution, but are united by their nontraditional use of greenhouse gases and their attempt to realize the potential for greenhouse gases to reveal important information about Earth’s climate. Ever since their discovery, the abrupt climate changes of the last glacial period known as Dansgaard-Oeschger (D-O) events have proved challenging to explain. The dominant hypothesis involves periodic freshwater discharges into the North Atlantic, which may regulate the strength of the Meridional Overturning Circulation (MOC) and its role in transporting heat to high latitudes. These events were not restricted to the North Atlantic, and can also be recognized in paleoclimate archives around the world. However, numerous uncertainties surrounding the mechanism behind D-O events remain, including how they are communicated to low latitudes and whether other hypotheses can be definitively ruled out. To constrain the mechanism behind abrupt climate changes, we investigate the phasing of climate changes in high- and low-latitude regions at the Bølling Transition, the penultimate abrupt warming event of the last glacial period. We use methane and the ¹⁵N/¹⁴N ratio of N₂ from the North Greenland Eemian (NEEM) ice core, which serve as proxies for tropical climate and Greenland temperature, respectively. We find that these gases change synchronously in the ice core record, and use a firn air model together with a Monte Carlo approach to constrain the phase lag to within several decades. Our results indicate that the mechanism behind the Bølling Transition was capable of rapidly transmitting the climate signal across the planet in a matter of years, and must therefore involve components of the climate system that are suitably reactive. The glacial-interglacial change in atmospheric methane concentrations revealed in ice core records has spurred a decade of debate about its cause. The most likely explanations involve dramatic changes in methane emissions, which originate from both high- and low-latitude wetlands. One method of investigating the changing latitudinal distribution of methane sources is to quantify the difference in methane concentrations between Greenland and Antarctica, which changes in proportion to the fraction of methane produced at high northern latitudes. Previous attempts to determine the methane interpolar difference (IPD) abound, but many have been hampered by complications in synchronizing bipolar ice core records and analytical uncertainties. We present the first continuous estimate of the methane IPD across the termination using high-resolution methane data from the NEEM and West Antarctic Ice Sheet (WAIS) Divide ice cores. Our results reveal the dominant role of tropical sources in driving abrupt changes in atmospheric methane concentrations, and show that boreal methane sources were surprisingly insensitive to dramatic climate changes. We hypothesize that changes in Northern Hemisphere snow and ice cover exerted strong control over tropical methane emissions, while gradually increasing solar insolation and land area allowed boreal sources to grow during the termination. We also investigate the IPD across the major climate transitions of the termination, and during four centennial-scale methane variations, and find opposing trends in boreal and tropical source strengths during these transient events. We propose that temporary decoupling of the locus of interhemispheric mixing, the position of the Intertropical Convergence Zone, and tropical precipitation may explain these results. Atmospheric concentrations of nitrous oxide (N₂O) have risen by ~20% from preindustrial to modern times, but the cause of this increase is not fully understood. The change has been previously attributed to various agricultural activities which perturb microbial processes in soils, but exactly how remains an outstanding question with important implications for future mitigation of N₂O emissions. We present the first measurements of the isotopomers of tropospheric N₂O over the interval from 1450 to 1920 CE. Our results confirm that the preindustrial atmosphere was enriched in all isotopes relative to the modern atmosphere. Furthermore, we estimate that the net anthropogenic source of nitrous oxide must be depleted in all heavy isotopes and have a strong site preference, consistent with a strong role for agricultural emissions and characteristic of N₂O derived from nitrification. We also find a large oscillation in the site preference of the ¹⁵N in N₂O during the Little Ice Age between 1500 and 1700 CE. We hypothesize that this excursion may be due to changing climate conditions that led to an increase in the amount of N₂O produced by nitrification vs. denitrification. Close

• 19. [Article] Chronology reconstruction for the disturbed bottom section of the GISP2 and the GRIP ice cores: Implications for Termination II in Greenland

  This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/jgr/.

  Citation × Citation Citation Abstract Copy Title: Chronology reconstruction for the disturbed bottom section of the GISP2 and the GRIP ice cores: Implications for Termination II in Greenland Author: Brook, Edward J., Bender, Michael L., Landais, Amaelle, von Fischer, Joseph C., Suwa, Makoto This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/jgr/. Title: Chronology reconstruction for the disturbed bottom section of the GISP2 and the GRIP ice cores: Implications for Termination II in Greenland Author: Brook, Edward J., Bender, Michael L., Landais, Amaelle, von Fischer, Joseph C., Suwa, Makoto This is the publisher’s final pdf. The published article is copyrighted by the American Geophysical Union and can be found at: http://www.agu.org/journals/jgr/. Close

• 20. [Article] A new mechanism for Dansgaard-Oeschger cycles

  This is the publisher’s final pdf. The published article is copyrighted by American Geophysical UnionAmerican Geophysical Union and can be found at: http://publications.agu.org/journals/.

  Citation × Citation Citation Abstract Copy Title: A new mechanism for Dansgaard-Oeschger cycles Author: Clark, P. U., Schrag, D. P., Petersen, S. V. This is the publisher’s final pdf. The published article is copyrighted by American Geophysical UnionAmerican Geophysical Union and can be found at: http://publications.agu.org/journals/. Title: A new mechanism for Dansgaard-Oeschger cycles Author: Clark, P. U., Schrag, D. P., Petersen, S. V. This is the publisher’s final pdf. The published article is copyrighted by American Geophysical UnionAmerican Geophysical Union and can be found at: http://publications.agu.org/journals/. Close