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Creators/Authors contains: "Yeung, Laurence Y."

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  1. Abstract

    The history of tropospheric O3, an important atmospheric oxidant, is poorly constrained because of uncertainties in its historical budget and a dearth of independent records. Here, we estimate the mean tropospheric O3burden during the Last Interglacial period (LIG; 115 to 130 thousand years ago) using a record of the clumped isotopic composition of O2(i.e., Δ36values) preserved in Antarctic ice. The measured LIG Δ36value is 0.03 ± 0.02‰ (95% CI) higher than the late pre‐industrial Holocene (PI; 1,590–1,850 CE) value and corresponds to a modeled 9% reduction in LIG tropospheric O3burden (95% CI: 3%–15%), caused in part by a substantial reduction in biomass burning emissions during the LIG relative to the PI. These results are consistent with the hypothesis that late‐Pleistocene megafaunal extinctions caused woody and grassy fuels to accumulate on land, leading to enhanced biomass burning in the preindustrial Holocene.

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  2. Abstract

    The isotopic composition of dissolved oxygen offers a family of potentially unique tracers of respiration and transport in the subsurface ocean. Uncertainties in transport parameters and isotopic fractionation factors, however, have limited the strength of the constraints offered by18O/16O and17O/16O ratios in dissolved oxygen. To improve our understanding of oxygen cycling in the ocean's interior, we investigated the systematics of oxygen isotopologues in the subsurface Pacific using new data and a 2‐D isotopologue‐enabled isopycnal reaction‐transport model. We measured18O/16O and17O/16O ratios, as well as the “clumped”18O18O isotopologue in the northeast Pacific, and compared the results to previously published data. We find evidence that oxygen consumption in the northeast Pacific follows different mass‐dependent fractionation exponents from those typically used in oceanographic studies. These fractionation factors imply that an elevated proportion of17O compared to18O in dissolved oxygen—that is, its triple‐oxygen isotope composition—may not uniquely reflect only gross primary productivity and mixing. For all oxygen isotopologues, transport, respiration, and photosynthesis comprise important parts of their respective budgets. Mechanisms of oxygen removal in the subsurface ocean are discussed.

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  3. Abstract

    Ice cores and other paleotemperature proxies, together with general circulation models, have provided information on past surface temperatures and the atmosphere's composition in different climates. Little is known, however, about past temperatures at high altitudes, which play a crucial role in Earth's radiative energy budget. Paleoclimate records at high‐altitude sites are sparse, and the few that are available show poor agreement with climate model predictions. These disagreements could be due to insufficient spatial coverage, spatiotemporal biases, or model physics; new records that can mitigate or avoid these uncertainties are needed. Here, we constrain the change in upper‐tropospheric temperature at the global scale during the Last Glacial Maximum (LGM) using the clumped‐isotope composition of molecular oxygen trapped in polar ice cores. Aided by global three‐dimensional chemical transport modeling, we exploit the intrinsic temperature sensitivity of the clumped‐isotope composition of atmospheric oxygen to infer that the upper troposphere (effective mean altitude 10–11 km) was 6–9°C cooler during the LGM than during the late preindustrial Holocene. A complementary energy balance approach supports a minor or negligible steepening of atmospheric lapse rates during the LGM, which is consistent with a range of climate model simulations. Proxy‐model disagreements with other high‐altitude records may stem from inaccuracies in regional hydroclimate simulation, possibly related to land‐atmosphere feedbacks.

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  4. Abstract

    The movement of tropical cyclones (TCs), particularly around the time of landfall, can substantially affect the resulting damage. Recently, trends in TC translation speed and the likelihood of stalled TCs such as Harvey have received significant attention, but findings have remained inconclusive. Here, we examine how the June-September steering wind and translation speed of landfalling Texas TCs change in the future under anthropogenic climate change. Using several large-ensemble/multi-model datasets, we find pronounced regional variations in the meridional steering wind response over North America, but―consistently across models―stronger June-September-averaged northward steering winds over Texas. A cluster analysis of daily wind patterns shows more frequent circulation regimes that steer landfalling TCs northward in the future. Downscaling experiments show a 10-percentage-point shift from the slow-moving to the fast-moving end of the translation-speed distribution in the future. Together, these analyses indicate increases in the likelihood of faster-moving landfalling Texas TCs in the late 21stcentury.

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  5. Abstract

    Carbon isotope (δ13C) records from marine sediments and sedimentary rocks have been extensively used in Cenozoic chemostratigraphy. The early Paleogene interval in particular has received exceptional attention because negative carbon isotope excursions (CIEs) documented in the sedimentary record, for example, at the Paleocene Eocene Thermal Maximum (PETM), ca ∼56 Ma, are believed to reflect significant global carbon cycle perturbations during the warmest interval of the Cenozoic era. However, while bulk carbonate δ13C values exhibit robust correlations across widely separated marine sedimentary basins, their absolute values and magnitude of CIEs vary spatially, especially over time intervals characterized by major deviations in global carbon cycling. Moreover, bulk carbonates in open‐marine environments are an ensemble of different components, each with a distinct isotope composition. Consequently, a comprehensive interpretation of the bulk‐δ13C record requires an understanding of co‐evolution of these components. In this study, we dissect sediments, from the late Paleocene‐early Eocene interval, at ODP Site 1209 (Shatsky Rise, Pacific Ocean) to investigate how a temporally varying bulk carbonate ensemble influences the overall carbon isotope record. A set of 45 samples were examined for δ13C and δ18O compositions, as bulk and individual size fractions. We find a significant increase in coarse‐fraction abundance across the PETM, driven by a changing community structure of calcifiers, modulating the size of the CIE at Site 1209 and thus making it distinct from those recorded at other open‐marine sites. These results highlight the importance of biogeography in the marine stable isotope record, especially when isotope excursions are driven by climate‐ and/or carbon cycle changes. In addition, community composition changes will alter the interpretation of weight percent coarse fraction as proxy for carbonate dissolution.

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  6. Abstract

    Tropospheric18O18O is an emerging proxy for past tropospheric ozone and free‐tropospheric temperatures. The basis of these applications is the idea that isotope‐exchange reactions in the atmosphere drive18O18O abundances toward isotopic equilibrium. However, previous work used an offline box‐model framework to explain the18O18O budget, approximating the interplay of atmospheric chemistry and transport. This approach, while convenient, has poorly characterized uncertainties. To investigate these uncertainties, and to broaden the applicability of the18O18O proxy, we developed a scheme to simulate atmospheric18O18O abundances (quantified as ∆36values) online within the GEOS‐Chem chemical transport model. These results are compared to both new and previously published atmospheric observations from the surface to 33 km. Simulations using a simplified O2isotopic equilibration scheme within GEOS‐Chem show quantitative agreement with measurements only in the middle stratosphere; modeled ∆36values are too high elsewhere. Investigations using a comprehensive model of the O‐O2‐O3isotopic photochemical system and proof‐of‐principle experiments suggest that the simple equilibration scheme omits an important pressure dependence to ∆36values: the anomalously efficient titration of18O18O to form ozone. Incorporating these effects into the online ∆36calculation scheme in GEOS‐Chem yields quantitative agreement for all available observations. While this previously unidentified bias affects the atmospheric budget of18O18O in O2, the modeled change in the mean tropospheric ∆36value since 1850 CE is only slightly altered; it is still quantitatively consistent with the ice‐core ∆36record, implying that the tropospheric ozone burden increased less than 40% over the twentieth century.

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