Browsing by Author "Yeung, Laurence"

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    Evaluating changes in high altitude temperature and atmospheric circulation during the last deglaciation using clumped isotopic composition of oxygen in polar ice cores
    (2023-04-21) Banerjee, Asmita; Yeung, Laurence
    The last deglacial period, spanning 21,000 to 10,000 years before present, has been studied extensively to quantify Earth system responses to changes in climate forcings like greenhouse gas concentrations. During this time, the Earth system underwent near-synchronous changes: atmospheric greenhouse gas concentrations and surface temperatures increased, ice volume and sea ice extent decreased resulting in sea level rise while atmospheric and ocean circulation patterns underwent drastic changes. Paleoclimate archives are used extensively to understand the causes and quantify the magnitude of these past changes. However, most of these studies are focused on the surface. Little information exists about the vertical profile of the atmosphere, namely how high-altitude temperatures and stratosphere-to-troposphere transport fluxes evolve with a rapidly evolving climate. Understanding the evolution of the vertical thermal structure of the atmosphere is necessary for quantifying how temperature lapse rates change with changing climate. Furthermore, air mass exchange between the stratosphere and the troposphere governs the chemistry of both regions and is expected to accelerate in a warming world. Thus, evaluating how high-altitude temperatures and atmosphere circulation evolved in the past is crucial for predicting their changes in the future. This dissertation evaluates the potential for a novel ice core proxy record, clumped isotopic composition of molecular oxygen measured in occluded air in polar ice cores, to provide constraints on how high-altitude temperatures and stratosphere-to-troposphere transport evolved during the last deglacial period. Clumped isotopic composition of oxygen, denoted by Δ36, is the proportional abundance of two heavy isotopes of oxygen, i.e., 18O18O in O2 and its formation is sensitive to the thermal and photochemical properties of the atmosphere. Isotope exchange reactions in the stratosphere and troposphere and mass exchange between the two governs the net surface Δ36 value. Evaluation of changing clumped isotopic composition of O2 during the deglacial period can provide insights on how upper-tropospheric temperatures and/or atmospheric circulation evolved through this time. The Last Glacial Maximum (LGM) spanning 21,000 to 18,000 years before present is first studied in Chapter 2. Polar ice core clumped isotopic compositions are measured and factors affecting the measured values are evaluated. Measurements are complemented with results from a global three-dimensional chemical transport model to infer changes in upper-tropospheric temperatures during this time. Finally, computed upper-tropospheric temperatures are compared with existing records of global surface temperature change during the LGM to infer changes in the temperature lapse rate during this time. The evolution of clumped isotopic composition of O2 during the Bølling Allerød warm period and Younger Dryas cold stadial spanning 15,000-11,000 years before present is investigated in Chapter 3. Both these periods represent abrupt centennial scale changes in the Earth’s climate, notably in the Northern Hemisphere. Ice core measurements indicate that Δ36 values reach pre-Industrial levels during this time, much before global surface temperatures. Combination of the measured values with sensitivity experiments indicate the non-linear relationship between Earth’s surface and high-altitude temperatures during periods of abrupt climate change, particularly ones that alter the cryospheric extent in the Northern Hemisphere. The results presented here indicates the role of Northern Hemisphere ice cover in governing the thermal structure of the atmosphere and lapse rate feedback. Finally, in Chapter 4, changes in atmospheric circulation, specifically, air mass exchange between the stratosphere and the troposphere during Heinrich Stadial 1 (HS1) is evaluated. HS1 (18000-14700 years before present) is thought to be a consequence of slowdown of the Atlantic Meridional Overturning Circulation (AMOC), a thermohaline circulation responsible for meridional heat transport. Slowdown of the AMOC affects the meridional temperature gradient and in turn, affects atmospheric circulation. Measurements of clumped isotopic composition of O2 in the ice core record indicates an abrupt increase during Heinrich Stadial 1, that is attributed to enhanced stratosphere-to-troposphere transport fluxes of high Δ36 bearing O2. The results presented here observationally constrain the relationship between increased meridional temperature gradient and air mass exchange between the stratosphere and the troposphere. Subsequent investigations using three-dimensional chemical transport models and imposed meridional temperature gradients may provide a more cohesive understanding of the mechanisms relating the two.
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    Large contribution of light-dependent oxygen uptake to global O2 cycling
    (2021-08-31) Valerio, David Armando; Yeung, Laurence
    The oxygen triple-isotope composition (Δ′17O = δ′17O - θ × δ′18O) of tropospheric O2 is an important parameter used in mass-balance proxy estimates of gross oxygen productivity in the modern ocean and the global biosphere. We created a chemical reaction network box model of the Δ′17O budget of tropospheric O2 to examine the key controls on the oxygen triple-isotope composition of tropospheric O2 (Δ′17O O2, trop). Our model is composed of four boxes: the stratosphere, the troposphere, the terrestrial biosphere/hydrosphere, and the marine biosphere/hydrosphere. We find that including an O2 consumption flux via Mehler-like reactions in marine cyanobacteria equal to between 40 and 50% of marine gross oxygen productivity resolves three issues at once: 1) interlaboratory disagreements on the oxygen triple-isotope signature of tropospheric O2 in the present day, 2) the incompatibility of model predictions of Δ′17O O2, trop with corresponding air observations in two laboratory reference frames, and 3) puzzling discrepancies between concurrently measured Δ′17O-gross oxygen productivity and 14C-net carbon productivity rates in the ocean. We also discuss how variations in the extent of Mehler-like reactions complicate Δ′17O-based interpretations of global gross oxygen productivity since the Last Glacial Maximum and propose a series of experiments to test our hypothesized large flux of O2 being consumed by Mehler-like reactions in the global O2 budget. Fundamentally, this work questions what variations in the Δ′17O of O2 indicate about global biogeochemical cycling.
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    Oxygen Isotope Fractionation during Photosynthesis at Different Light Intensities
    (2023-07-10) Yuan, Bing; Yeung, Laurence
    This work is to study the oxygen isotopic fractionation during photosynthesis under different light intensities (60 to 1750 μE·m-2·s-1) and thylakoids extracted from spinach leaves are used as reaction material to suppress respirations and photorespiration. Triple oxygen isotope method is used to estimate the gross oxygen production. The isotope fractionation slope for the plot of ln(δ17O) versus. ln(δ18O)) of the oxygen produced from photosynthesis is 0.5242 ± 0.0039, consistent wih previous measurement. Using the substrate water and produced oxygen as reactant and product, respectively, the 18α values increases from 1.0011 to 1.0041 as light intensity increased from 60 to 1750 μE·m-2·s-1. From our analysis, both the VSMOW values and the isotope fractionation slope of respiration (λ17/18) impact the accuracy of GOP estimation. We suggest using λ17/18 = 0.522 and the VSMOW data reported by Luz et al. in 2011 to estimate GOP through the modified GOP equation by Prokopenko et al. in 2011. Clumped isotope 18O18O of oxygen from photosynthesis has a negative Δ36 which is around 0 to -0.15‰. It means that the formation of O2 during photosynthesis is a non-isotopic equilibrium process starting from the non-equivalent binding of two water molecules, consistent with previous prediction. Δ36 value maybe used to estimate GOP in future because it is not influenced by the substrate medium. From our study, as light intensities rise from 60 to 1750 μE·m-2·s-1, oxygen isotope fractionation (18ε) increases from 1 to 4‰ while the oxygen production yield decreases from 32 to 17 μmol/mg chl. Baes on our model, the increased isotope fractionation and the decreased net O2 production yield during photosynthesis are mainly caused by the excitation of O2 from ground state to the singlet state (1O2) under photoinhibition. About 15-30% oxygen is consumed in this way. Formation of 1O2 has a theoretical isotope fractionation of -6.6‰ at room temperature. When the light intensity is above 1200 μE·m-2·s-1, besides 1O2, a small percentage of O2- (1-3.3%) is formed which further increases oxygen isotope fractionation of collected oxygen.
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    The Effect of Nitrate Availability on Oxygen Isotope Fractionation During Cyanobacterial Photosynthesis
    (Rice University, 2023) Guo, Lingkun; Yeung, Laurence
    Marine primary productivity supports food webs and ecosystem health, driving large-scale animal distribution patterns in the ocean. Primary productivity is also a fundamental process in the biological pump, which sequesters inorganic carbon from the atmosphere through photosynthesis and transports it to the ocean interior where it can be stored for millennial or greater timescales. Presently, different methods for quantifying productivity disagree with each other, presenting a major research challenge. To understand how climate change may impact the biosphere, it is necessary to continuously improve methods for quantifying primary productivity. The isotopic composition of dissolved oxygen in the ocean can be used as a constraint on oxygen production and therefore, be used to quantify marine primary productivity through the carbon-oxygen stoichiometry of photosynthesis. To distinguish newly produced O2 from O2 already present in the atmosphere, the 18O/16O and 17O/16O ratios of dissolved oxygen can be used. This approach is termed the “triple-oxygen isotope (TOI) method” and it relies on laboratory studies of the behavior of oxygen isotope ratios during photosynthesis to constrain field measurements. Despite the potential of the TOI method, major knowledge gaps remain. For example, nutrient availability affects the photosynthesis activity of microorganisms and is heterogeneous across global oceans. Therefore, understanding how oxygen isotope fractionation may vary under nutrient str ess is crucial for correctly interpreting field measurements. To address this knowledge gap, this study examines the effect of nitrate availability on the isotopic composition of cyanobacterial photosynthetic oxygen. Freshwater cyanobacteria Synechocystis PCC 6803 are inoculated in media with different nitrate concentrations. The cyanobacterial photosynthetic oxygen is collected and its TOI composition is analyzed. Additionally, this study generates new data on the clumped isotope composition of photosynthetic oxygen, which is a parameter describing the degree of randomness in the distribution of rare oxygen isotopes (17O and 18O) in O2 molecules. Results suggest that there is larger oxygen isotope fractionation when cyanobacteria experience more nitrate limitation, and that the TOI and clumped isotope composition of photosynthetic oxygen can be utilized together to constrain gross oxygen production and provide information on the mechanism of cyanobacterial photosynthesis.